Nomenclature

Naming Organic Compounds

The increasingly large number of organic compounds identified with each passing day, together with the fact that many of these compounds are isomers of other compounds, requires that a systematic nomenclature system be developed. Just as each distinct compound has a unique molecular structure which can be designated by a structural formula, each compound must be given a characteristic and unique name.
As organic chemistry grew and developed, many compounds were given trivial names, which are now commonly used and recognized. Some examples are:

Name	Methane	Butane	Acetone	Toluene	Acetylene	Ethyl Alcohol
Formula	CH₄	C₄H₁₀	CH₃COCH₃	CH₃C₆H₅	C₂H₂	C₂H₅OH

Such common names often have their origin in the history of the science and the natural sources of specific compounds, but the relationship of these names to each other is arbitrary, and no rational or systematic principles underly their assignments.

The IUPAC Systematic Approach to Nomenclature

A rational nomenclature system should do at least two things. First, it should indicate how the carbon atoms of a given compound are bonded together in a characteristic lattice of chains and rings. Second, it should identify and locate any functional groups present in the compound. Since hydrogen is such a common component of organic compounds, its amount and locations can be assumed from the tetravalency of carbon, and need not be specified in most cases.
The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature. Knowing these rules and given a structural formula, one should be able to write a unique name for every distinct compound. Likewise, given a IUPAC name, one should be able to write a structural formula. In general, an IUPAC name will have three essential features:

• A root or base indicating a major chain or ring of carbon atoms found in the molecular structure.
• A suffix or other element(s) designating functional groups that may be present in the compound.
• Names of substituent groups, other than hydrogen, that complete the molecular structure.

As an introduction to the IUPAC nomenclature system, we shall first consider compounds that have no specific functional groups. Such compounds are composed only of carbon and hydrogen atoms bonded together by sigma bonds (all carbons are sp³ hybridized).

This guide to IUPAC nomenclature of organic compounds provides an outline of the main principles of organic nomenclature as described in the 1979 edition of the IUPAC Nomenclature of Organic Chemistry and includes important changes agreed upon since its publication. Differences from the 1979 edition have not been specifically highlighted. However, in many cases a name used in the 1979 edition, preceded by a word such as "formerly" or "previously", appears in parentheses following a name recommended herein.

General Principles of Organic Nomenclature

Systematic naming of an organic compound generally requires the identification and naming of a parent structure. This name may then be modified by prefixes, infixes, and, in the case of a parent hydride, suffixes, which convey precisely the structural changes required to generate the actual compound from the parent structure.

Most commonly, a parent structure is a parent hydride, i.e., a structure containing, besides hydrogen, either a single atom of an element, for example, phosphane; a number of atoms (alike or different) linked together to form an unbranched chain, for example, cyclohexane, pyridine, naphthalene, and quinoline. It is sometimes convenient to employ parent hydrides, of more complex structure such as ring assemblies or ring/chain systems, for example, biphenyl, styrene, ferrocene, and cyclophanes, and to include structures with implied stereochemistry (stereoparents), for example, 5-cholestane .

Phosphane	Pentane	Cyclohexane	Pyridine	Disiloxane	Naphthalene
5α-Cholestane,	Quinoline	Biphenyl	Styrene	Acetic acid	Phosphinic acid

In order to generate the parent structure from a molecule to be named, various formal operations must be carried out. For example, in naming the structure below,

the parent hydride "pentane" is formally derived by replacing the oxygen and chlorine atoms by the appropriate number of hydrogen atoms. For constructing a name, this formal operation is reversed; the prefix "chloro-" and the suffix "-one" indicating substitution of hydrogen atoms of pentane are attached to the parent hydride name, giving the name 5-chloropentan-2-one. Prefixes and suffixes can represent a number of different types of formal operations on the parent structure. Frequently, the prefix or suffix denotes the attachment of a characteristic group, for example, "oxo-" or "-one" for =O. A prefix may describe a group which is derived from a parent hydride, for example, pentan-1-yl or pentyl for (from pentane).

The substitutive operation is the operation used most extensively in organic nomenclature. Indeed, the comprehensive nomenclature system based largely on the

application of this operation to parent structures is, for convenience, termed "substitutive nomenclature".

In constructing the names (formerly called "radicofunctional names"), the characteristic group of the compound is expressed as a functional class name, and is usually cited as a separate word rather than as a suffix. In these recommendations, however, names obtained by a substitutive operation are preferred.

The replacement operation can be used for naming organic compounds in which skeletal atoms of a parent structure are replaced by other skeletal atoms, or in which oxygen atom and/or hydroxy groups of characteristic groups are replaced by other atoms or groups.

It is very important to recognize that, in general, the rules of organic nomenclature are written in terms of classical valence bonding and do not imply electronic configurations of any kind.

Full details of the way in which parent names may be combined with appropriate prefixes and suffixes are given in ; rules for selection of a unique systematic name, if required, will be described in a separate document. Methods for the specification of stereochemistry and those for denoting isotopic modification .

Alkanes

Hydrocarbons having no double or triple bond functional groups are classified as alkanes or cycloalkanes, depending on whether the carbon atoms of the molecule are arranged only in chains or also in rings. Although these hydrocarbons have no functional groups, they constitute the framework on which functional groups are located in other classes of compounds, and provide an ideal starting point for studying and naming organic compounds. The alkanes and cycloalkanes are also members of a larger class of compounds referred to as aliphatic. Simply put, aliphatic compounds are compounds that do not incorporate any aromatic rings in their molecular structure.
The following table lists the IUPAC names assigned to simple continuous-chain alkanes from C-1 to C-10. A common "ane" suffix identifies these compounds as alkanes. Longer chain alkanes are well known, and their names may be found in many reference and text books. The names methane through decane should be memorized, since they constitute the root of many IUPAC names. Fortunately, common numerical prefixes are used in naming chains of five or more carbon atoms.

Name	Molecular Formula	Structural Formula	Isomers	Name	Molecular Formula	Structural Formula	Isomers
methane	CH₄	CH₄	1	hexane	C₆H₁₄	CH₃(CH₂)₄CH₃	5
ethane	C₂H₆	CH₃CH₃	1	heptane	C₇H₁₆	CH₃(CH₂)₅CH₃	9
propane	C₃H₈	CH₃CH₂CH₃	1	octane	C₈H₁₈	CH₃(CH₂)₆CH₃	18
butane	C₄H₁₀	CH₃CH₂CH₂CH₃	2	nonane	C₉H₂₀	CH₃(CH₂)₇CH₃	35
pentane	C₅H₁₂	CH₃(CH₂)₃CH₃	3	decane	C₁₀H₂₂	CH₃(CH₂)₈CH₃	75

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Some important behavior trends and terminologies:

(i)   The formulas and structures of these alkanes increase uniformally by a CH₂ increment.
(ii)   A uniform variation of this kind in a series of compounds is called homologous.
(iii)   These formulas all fit the C_nH_2n+2 rule. This is also the highest possible H/C ratio for a stable hydrocarbon.
(iv)   Since the H/C ratio in these compounds is at a maximum, we call them saturated (with hydrogen).

Beginning with butane (C₄H₁₀), and becoming more numerous with larger alkanes, we note the existence of alkane isomers. For example, there are five C₆H₁₄ isomers, shown below as abbreviated line formulas (A through E):

Although these distinct compounds all have the same molecular formula, only one (A) can be called hexane. How then are we to name the others?

The IUPAC system requires first that we have names for simple unbranched chains, as noted above, and second that we have names for simple alkyl groups that may be attached to the chains. Examples of some common alkyl groups are given in the following table. Note that the "ane" suffix is replaced by "yl" in naming groups. The symbol R is used to designate a generic (unspecified) alkyl group.

following standard IUPAC numerical multiplier rules. The first few are:

Group	CH₃–	C₂H₅–	CH₃CH₂CH₂–	(CH₃)₂CH–	CH₃CH₂CH₂CH₂–	(CH₃)₂CHCH₂–	CH₃CH₂CH(CH₃)–	(CH₃)₃C–	R–
Name	Methyl	Ethyl	Propyl	Isopropyl	Butyl	Isobutyl	sec-Butyl	tert-Butyl	Alkyl

Number of carbons	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	20	30
Prefix	Meth	Eth	Prop	But	Pent	Hex	Hept	Oct	Non	Dec	Undec	Dodec	Tridec	Tetradec	Pentadec	Eicos	Triacont

For example, the simplest alkane is CH₄ methane, and the nine-carbon alkane CH₃(CH₂)₇CH₃ is named nonane. If one was to name the 157-carbon alkane

CH₃(CH₂)₁₅₅CH₃ the name would be heptapentahectane.Parentheses are used to indicate the repetition of the enclosed molecule (ie. (CH₂)₁₅₅ indicates that a molecule contains a chain of 155 CH₂ groups.) Cyclic alkanes are simply prefixed with "cyclo-", for example C₄H₈ is cyclobutane and C₆H₁₂ is cyclohexane.

IUPAC Rules for Alkane Nomenclature

1.   Find and name the longest continuous carbon chain.
2.   Identify and name groups attached to this chain.
3.   Number the chain consecutively, starting at the end nearest a substituent group.
4.   Designate the location of each substituent group by an appropriate number and name.
5.   Assemble the name, listing groups in alphabetical order.
       The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when            alphabetizing.

For the above isomers of hexane the IUPAC names are: B 2-methylpentane C 3-methylpentane D 2,2-dimethylbutane E 2,3-dimethylbutane

Halogen substituents are easily accomodated, using the names: fluoro (F-), chloro (Cl-), bromo (Br-) and iodo (I-). For example, (CH₃)₂CHCH₂CH₂Br would be named 1-bromo-3-methylbutane. If the halogen is bonded to a simple alkyl group an alternative "alkyl halide" name may be used. Thus, C₂H₅Cl may be named chloroethane (no locator number is needed for a two carbon chain) or ethyl chloride.

For additional examples of how these rules are used in naming branched alkanes, and for some sub-rules of nomenclature showing in the following illustrations.

Illustration 1.

CH₃(CH₂)₂CH(CH₃)CH₂CH₃

When viewing a condensed formula of this kind, one must recognize that parentheses are used both to identify repeating units, such as the two methylene groups on the left side, and substituents, such as the methyl group on the right side. This formula is elaborated and named as follows:

The condensed formula is expanded on the left. By inspection, the longest chain is seen to consist of six carbons, so the root name of this compound will be hexane. A single methyl substituent (colored red) is present, so this compound is a methylhexane. The location of the methyl group must be specified, since there are two possible isomers of this kind. Note that if the methyl group were located at the end of the chain, the longest chain would have seven carbons and the root name would be heptane not hexane. To locate the substituent the hexane chain must be numbered consecutively, starting from the end nearest a substituent. In this case it is the right end of the chain, and the methyl group is located on carbon #3. The IUPAC name is thus: 3-methylhexane

Illustration 2.

(CH₃)₂C(C₂H₅)₂

Again, the condensed formula is expanded on the left, the longest chain is identified (five carbons) and substituents are located and named. Because of the symmetrical substitution pattern, it does not matter at which end of the chain the numbering begins.

When two or more identical substituents are present in a molecule, a numerical prefix (di, tri, tetra etc.) is used to designate their number. However, each substituent must be given an identifying location number. Thus, the above compound is correctly named: 3,3-dimethylpentane.
Note that the isomer (CH₃)₂CHCH₂CH(CH₃)₂ would be named 2,4-dimethylpentane.

Illustration 3.

(CH₃)₂CHCH₂CH(C₂H₅)C(CH₃)₃

This example illustrates some sub-rules of the IUPAC system that must be used in complex cases. The expanded and line formulas are shown below.

Sub-Rules for IUPAC Nomenclature

1. If there are two or more longest chains of equal length, the one having the largest number of   substituents is chosen.
2. If both ends of the root chain have equidistant substituents:
          (i)   begin numbering at the end nearest a third substituent, if one is present.
        (ii)   begin numbering at the end nearest the first cited group (alphabetical order).

In this case several six-carbon chains can be identified. Some (colored blue) are identical in that they have the same number, kind and location of substituents. The IUPAC name derived from these chains will not change. Some (colored magenta) differ in the number, kind and location of substituents, and will result in a different name. From rule 1 above the blue chain is chosen, and it will be numbered from the right hand end by application of rule (i). Remembering the alphabetical priority, we assign the following IUPAC name: 3-ethyl-2,2,5-trimethylhexane.

Illustration 4.

Write a structural formula for the compound 3,4-dichloro-4-ethyl-5-methylheptane.
First, we draw a chain of seven carbon atoms to represent the root name "heptane". This chain can be numbered from either end, since no substituents are yet attached. From the IUPAC name we know there are two chlorine, one ethyl and one methyl substituents. The numbers tell us where the substituents are located on the chain, so they can be attached, as shown in the middle structure below. Finally, hydrogen atoms are introduced to satisfy the tetravalency of carbon. The structural formula on the right can then be written in condensed form as: CH₃CH₂CHClCCl(C₂H₅)CH(CH₃)CH₂CH₃ or C₂H₅CHClCCl(C₂H₅)CH(CH₃)C₂H₅

In naming this compound it should be noted that the seven carbon chain is numbered from the end nearest the chlorine by applying rule (ii) above.

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Cycloalkanes

Cycloalkanes have one or more rings of carbon atoms. The simplest examples of this class consist of a single, unsubstituted carbon ring, and these form a homologous series similar to the unbranched alkanes. The IUPAC names of the first five members of this series are given in the following table. The last (yellow shaded) column gives the general formula for a cycloalkane of any size. If a simple unbranched alkane is converted to a cycloalkane two hydrogen atoms, one from each end of the chain, must be lost. Hence the general formula for a cycloalkane composed of n carbons is C_nH_2n.

Examples of Simple Cycloalkanes
Name	Cyclopropane	Cyclobutane	Cyclopentane	Cyclohexane	Cycloheptane	Cycloalkane
Molecular Formula	C₃H₆	C₄H₈	C₅H₁₀	C₆H₁₂	C₇H₁₄	C_nH_2n
Structural Formula						(CH₂)_n

Substituted cycloalkanes are named in a fashion very similar to that used for naming branched alkanes. The chief difference in the rules and procedures occurs in the numbering system. Since all the carbons of a ring are equivalent (a ring has no ends like a chain does), the numbering starts at a substituted ring atom.

IUPAC Rules for Cycloalkane Nomenclature

1.   For a monosubstituted cycloalkane the ring supplies the root name (table above) and the substituent group is named as usual. A location number is unnecessary.
2.   If the alkyl sustituent is large and/or complex, the ring may be named as a substituent group on an alkane.
3.   If two different substituents are present on the ring, they are listed in alphabetical order, and the first cited substituent is assigned to carbon #1. The numbering of ring carbons then continues in a direction (clockwise or counter-clockwise) that affords the second substituent the lower possible location number.
4.   If several substituents are present on the ring, they are listed in alphabetical order. Location numbers are assigned to the substituents so that one of them is at carbon #1 and the other locations have the lowest possible numbers, counting in either a clockwise or counter-clockwise direction.
5.   The name is assembled, listing groups in alphabetical order and giving each group (if there are two or more) a location number. The prefixes di, tri, tetra etc., used to designate several groups of the same kind, are not considered when alphabetizing.

For examples of how these rules are used in naming substituted cycloalkanes .

The following two cases provide examples of monosubstituted cycloalkanes.

In the first case, on the left, we see a seven-carbon ring bearing a C₄H₉ substituent group. Earlier this substituent was identified as the tert-butyl group, so a name based on the cycloheptane root is easily written. In the second case, on the right, a four-carbon ring is attached to a branched six-carbon alkyl group. This C₆H₁₃ group could be named "isohexyl", but a better approach is to name this compound as a disubstituted pentane. The four-membered ring substituent is called a cyclobutyl group.

More highly substituted cycloalkanes are named in a similar fashion, but care must be taken in numbering the ring.

In the example on the left, there are three substituents on the six-membered ring and two are on the same carbon. The disubstituted carbon becomes #1 because the total locator numbers are thereby kept to a minimum. The ethyl substituent is then located on carbon #3 (counter-clockwise numbering), not #5 (clockwise numbering). Alphabetical listing of the substituents then leads to the name "3-ethyl-1,1-dimethylcyclohexane", being careful to assign a locator number to each substituent. Note that if only one methyl substituent was present, the alphabetical citation rule would assign the ethyl group to carbon #1 and the methyl to #3. The second example, on the left, has five substituents, and the numbering is assigned so that the first, second and third arbitrarily chosen substituents have the lowest possible numbers (1,1 & 2 in this case).

Small rings, such as three and four membered rings, have significant angle strain resulting from the distortion of the sp³ carbon bond angles from the ideal 109.5º to 60º and 90º respectively. This angle strain often enhances the chemical reactivity of such compounds, leading to ring cleavage products. It is also important to recognize that, with the exception of cyclopropane, cycloalkyl rings are not planar (flat). The three dimensional shapes assumed by the common rings (especially cyclohexane and larger rings) are described and discussed in the Conformational Analysis Section.

Hydrocarbons having more than one ring are common, and are referred to as bicyclic (two rings), tricyclic (three rings) and in general, polycyclic compounds. The molecular formulas of such compounds have H/C ratios that decrease with the number of rings. In general, for a hydrocarbon composed of n carbon atoms associated with m rings the formula is: C_nH_{(2n + 2 - 2m)}. The structural relationship of rings in a polycyclic compound can vary. They may be separate and independent, or they may share one or two common atoms. Some examples of these possible arrangements are shown in the following table.

Examples of Isomeric C₈H₁₄ Bicycloalkanes

Isolated Rings	Spiro Rings	Fused Rings	Bridged Rings
No common atoms	One common atom	One common bond	Two common atoms

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Alkenes and Alkynes

Alkenes and alkynes are hydrocarbons which respectively have carbon-carbon double bond and carbon-carbon triple bond functional groups. The molecular formulas of these unsaturated hydrocarbons reflect the multiple bonding of the functional groups:

Alkane	R–CH₂–CH₂–R	C_nH_2n+2	This is the maximum H/C ratio for a given number of carbon atoms.
Alkene	R–CH=CH–R	C_nH_2n	Each double bond reduces the number of hydrogen atoms by 2.
Alkyne	R–C≡C–R	C_nH_2n-2	Each triple bond reduces the number of hydrogen atoms by 4.

As noted earlier in the Analysis of Molecular Formulas section, the molecular formula of a hydrocarbon provides information about the possible structural types it may represent. For example, consider compounds having the formula C₅H₈. The formula of the five-carbon alkane pentane is C₅H₁₂ so the difference in hydrogen content is 4. This difference suggests such compounds may have a triple bond, two double bonds, a ring plus a double bond, or two rings. Some examples are shown here, and there are at least fourteen others!

IUPAC Rules for Alkene and Cycloalkene Nomenclature

1.   The ene suffix (ending) indicates an alkene or cycloalkene.
2.   The longest chain chosen for the root name must include both carbon atoms of the double bond.
3.   The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
4.   The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator. If more than one double bond is present the compound is named as a diene, triene or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
5.   In cycloalkenes the double bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
6.   Substituent groups containing double bonds are:
            H₂C=CH–   Vinyl group
            H₂C=CH–CH₂–   Allyl group

IUPAC Rules for Alkyne Nomenclature

1. The yne suffix (ending) indicates an alkyne or cycloalkyne.
2.   The longest chain chosen for the root name must include both carbon atoms of the triple bond.
3.   The root chain must be numbered from the end nearest a triple bond carbon atom. If the triple bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
4.   The smaller of the two numbers designating the carbon atoms of the triple bond is used as the triple bond locator.
5.   If several multiple bonds are present, each must be assigned a locator number. Double bonds precede triple bonds in the IUPAC name, but the chain is numbered from the end nearest a multiple bond, regardless of its nature.
6.   Because the triple bond is linear, it can only be accommodated in rings larger than ten carbons. In simple cycloalkynes the triple bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
7.   Substituent groups containing triple bonds are:
            HC≡C–   Ethynyl group
            HC≡CH–CH₂–   Propargyl group

For examples of how these rules are used in naming alkenes, alkynes and cyclic analogs are showing below .

Illustration 1 (CH₃)₂C=CHCH₂C(CH₃)₃		Illustration 2 (CH₃CH₂CH₂)₂C=CH₂

Expanding these formulas we have:

Both these compounds have double bonds, making them alkenes. In example (1) the longest chain consists of six carbons, so the root name of this compound will be hexene. Three methyl substituents (colored red) are present. Numbering the six-carbon chain begins at the end nearest the double bond (the left end), so the methyl groups are located on carbons 2 & 5. The IUPAC name is therefore: 2,5,5-trimethyl-2-hexene.
In example (2) the longest chain incorporating both carbon atoms of the double bond has a length of five. There is a seven-carbon chain, but it contains only one of the double bond carbon atoms. Consequently, the root name of this compound will be pentene. There is a propyl substituent on the inside double bond carbon atom (#2), so the IUPAC name is: 2-propyl-1-pentene.

Illustration 3 (C₂H₅)₂C=CHCH(CH₃)₂		Illustration 4 CH₂=C(CH₃)CH(CH₃)C(C₂H₅)=CH₂

The next two examples illustrate additional features of chain numbering. As customary, the root chain is colored blue and substituents are red.

The double bond in example (3) is located in the center of a six-carbon chain. The double bond would therefore have a locator number of 3 regardless of the end chosen to begin numbering. The right hand end is selected because it gives the lowest first-substituent number (2 for the methyl as compared with 3 for the ethyl if numbering were started from the left). The IUPAC name is assigned as shown.
Example (4) is a diene (two double bonds). Both double bonds must be contained in the longest chain, which is therefore five- rather than six-carbons in length. The second and fourth carbons of this 1,4-pentadiene are both substituted, so the numbering begins at the end nearest the alphabetically first-cited substituent (the ethyl group).

Illustrations 5, 6, 7 & 8

These examples include rings of carbon atoms as well as some carbon-carbon triple bonds. Example (6) is best named as an alkyne bearing a cyclobutyl substituent. Example (7) is simply a ten-membered ring containing both a double and a triple bond. The double bond is cited first in the IUPAC name, so numbering begins with those two carbons in the direction that gives the triple bond carbons the lowest locator numbers. Because of the linear geometry of a triple bond, a-ten membered ring is the smallest ring in which this functional group is easily accomodated. Example (8) is a cyclooctatriene (three double bonds in an eight-membered ring). The numbering must begin with one of the end carbons of the conjugated diene moiety (adjacent double bonds), because in this way the double bond carbon atoms are assigned the smallest possible locator numbers (1, 2, 3, 4, 6 & 7). Of the two ways in which this can be done, we choose the one that gives the vinyl substituent the lower number.

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The last three examples, (9), (10) & (11), illustrate some fine points of the nomenclature rules.

Sub-Rules for IUPAC Nomenclature

The root chain is that which contains the maximum number of multiple bonds.
• If more than one such chain is found, the longest is chosen as the root.
• If the chains have equal length the one with the most double bonds is chosen.

The first two acyclic cases are branched chains containing several multiple bonds. Example (9) has two possible seven-carbon chains, each having three multiple bonds. The one selected has three double bonds and the triple bond becomes a substituent group. In example (10) we find a six-carbon chain containing two double bonds, and a seven-carbon chain with a double and a triple bond. The latter becomes the root chain and the second double bond is a vinyl substituent on that chain. The last example (11) shows that in numbering a cycloalkene one must first consider substituents on the double bond in assigning sites #1 and #2. Here the double bond carbon atom to which the ethyl group is attached becomes #1 and the other carbon of the double bond is necessarily carbon #2. Sometimes this results in other substituents having high locator numbers, as does bromine in this case.

Alcohols

Alcohols (R-OH) drop the terminal "e" from the name of the parent alkane, and take the suffix "-ol" with an infix numerical bonding position: CH₃CH₂CH₂OH is propan-1-ol. (Methanol and ethanol are unambiguous and do not require position numbers). The suffixes -diol, -triol, -tetraol, etc., are used for multiple -OH groups: Ethylene glycol CH₂OHCH₂OH is ethane-1,2-diol.

If higher precedence functional groups are present (see order of precedence, below), the prefix "hydroxy" is used with the bonding position: CH₃CHOHCOOH is 2-hydroxypropanoic acid.

Halogens (Alkyl Halides)

Halogen functional groups are prefixed with the bonding position and take the form fluoro-, chloro-, bromo-, iodo-, etc., depending on the halogen. Multiple groups are dichloro-, trichloro-, etc, and dissimilar groups are orded alphabetically as before. For example, CHCl₃ (chloroform) is trichloromethane. The anesthetic Halothane (CF₃CHBrCl) is 2-bromo-2-chloro-1,1,1-trifluoroethane.

Ketones

In general ketones (R-CO-R) take the suffix "-one" (pronounced own, not won) with an infix position number: CH₃CH₂CH₂COCH₃ is pentan-2-one. For common ketones some traditional names such as acetone and benzophenone predominate, and these are acceptable IUPAC names, although some introductory chemistry texts use alternative names for acetone such as propan-2-one or propanone (see diagram). Additionally, in such unambiguous cases as propanone, the infixed number can be dropped. If a higher precedence suffix is in use, the prefix "oxo-" is used: CH₃CH₂CH₂COCH₂CHO is 3-oxohexanal.

Aldehydes

Aldehydes (R-CHO) take the suffix "-al". Since they are always at the end of an alkane chain, they do not need a position number: HCHO (formaldehyde) is methanal, CH₃CHO (acetaldehyde) is ethanal. If other functional groups are present, the chain is numbered such that the aldehyde carbon is in the "1" position.

If a prefix form is required, "oxo-" is used (as for ketones), with the position number indicating the end of a chain: CHOCH₂COOH is 3-oxopropanoic acid. If the carbon in the carbonyl group cannot be included in the attached chain (for instance in the case of cyclic aldehydes), the prefix "formyl-" or the suffix "-carbaldehyde" is used: C₆H₁₁CHO is cyclohexanecarbaldehyde.

Carboxylic acids

In general carboxylic acids are named with the suffix -oic acid (etymologically a back-formation from benzoic acid). As for aldehydes, they take the "1" position on the parent chain, but do not have their position number indicated. For example, CH₃CH₂CH₂CH₂COOH (valeric acid) is named pentanoic acid. For common carboxylic acids some traditional names such as acetic acid are in such widespread use they are considered retained IUPAC names, although "systematic" names such as ethanoic acid are also acceptable. For carboxylic acids attached to a benzene ring such as Ph-COOH, these are named as benzoic acid or its derivatives.

If there are multiple carboxyl groups on the same parent chain, the suffix "-carboxylic acid" can be used (as -dicarboxylic acid, -tricarboxylic acid, etc.). In these cases, the carbon in the carboxyl group does not count as being part of the main alkane chain. The same is true for the prefix form, "carboxyl-". Citric acid is one example; it is named 2-hydroxypropane- 1,2,3-tricarboxylic acid, rather than 2-carboxy, 2-hydroxypentanedioic acid.

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Ethers

Ethers (R-O-R) consist of an oxygen atom between the two attached carbon chains. The shorter of the two chains becomes the first part of the name with the -ane suffix changed to -oxy, and the longer alkane chain become the suffix of the name of the ether. Thus CH₃OCH₃ is methoxymethane, and CH₃OCH₂CH₃ is methoxyethane (not ethoxymethane). If the oxygen is not attached to the end of the main alkane chain, then the whole shorter alkyl-plus-ether group is treated as a side-chain and prefixed with its bonding position on the main chain. Thus CH₃OCH(CH₃)₂ is 2-methoxypropane.

Esters (R-CO-O-R') are named as alkyl derivatives of carboxylic acids. The alkyl (R') group is named first. The R-CO-O part is then named as a separate word based on the carboxylic acid name, with the ending changed from -oic acid to -oate. For example, CH₃CH₂CH₂CH₂COOCH₃ is methyl pentanoate, and (CH₃)₂CHCH₂CH₂COOCH₂CH₃ is ethyl 4-methylpentanoate. For esters such as ethyl acetate (CH₃COOCH₂CH₃), ethyl formate (HCOOCH₂CH₃) or dimethyl phthalate that are based on common acids, IUPAC recommends use of these established names, called retained names. The -oate changes to -ate. Some simple examples, named both ways, are shown in the figure above.

If the alkyl group is not attached at the end of the chain, the bond position to the ester group is infixed before "-yl": CH₃CH₂CH(CH₃)OOCCH₂CH₃ may be called but-2-yl propanoate or but-2-yl propionate.

Amines and Amides

Amines (R-NH₂) are named for the attached alkane chain with the suffix "-amine" (e.g. CH₃NH₂ methanamine). If necessary, the bonding position is infixed: CH₃CH₂CH₂NH₂ propan-1-amine, CH₃CHNH₂CH₃ propan-2-amine. The prefix form is "amino-".

For secondary amines (of the form R-NH-R), the longest carbon chain attached to the nitrogen atom becomes the primary name of the amine; the other chain is prefixed as an alkyl group with location prefix given as an italic N: CH₃NHCH₂CH₃ is N-methylethanamine. Tertiary amines (R-NR-R) are treated similarly: CH₃CH₂N(CH₃)CH₂CH₂CH₃ is N-ethyl-N-methylpropanamine. Again, the substituent groups are ordered alphabetically.

Amides (R-CO-NH₂) take the suffix "-amide". There is no prefix form, and no location number is required since they always terminate a carbon chain, e.g. CH₃CONH₂ (acetamide) is named ethanamide.

Cyclic compounds

Cycloalkanes and aromatic compounds can be treated as the main parent chain of the compound, in which case the position of substituents are numbered around the ring structure. For example, the three isomers of xylene CH₃C₆H₄CH₃, commonly the ortho-, meta-, and para- forms, are 1,2-dimethylbenzene, 1,3-dimethylbenzene, and 1,4-dimethylbenzene. The cyclic structures can also be treated as functional groups themselves, in which case they take the prefix "cycloalkyl-" (e.g. "cyclohexyl-") or for benzene, "phenyl-".

The IUPAC nomenclature scheme becomes rapidly more elaborate for more complex cyclic structures, with notation for compounds containing conjoined rings, and many common names such as phenol, furan, indole, etc. being accepted as base names for compounds derived from them.

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Order of precedence of groups

When compounds contain more than one functional group, the order of precedence determines which groups are named with prefix or suffix forms. The highest precedence group takes the suffix, with all others taking the prefix form. However, double and triple bonds only take suffix form (-en and -yn) and are used with other suffixes.Prefixed substituents are ordered alphabetically (excluding any modifiers such as di-, tri-, etc.), e.g. chlorofluoromethane, not fluorochloromethane. If there are multiple functional groups of the same type, either prefixed or suffixed, the position numbers are ordered numerically (thus ethane-1,2-diol, not ethane-2,1-diol.) The N position indicator for amines and amides comes before "1", e.g. CH₃CH(CH₃)CH₂NH(CH₃) is N,2-dimethylpropanamine.

Priority	Functional group	Formula	Prefix	Suffix
1	Cations e.g. Ammonium	–NH₄⁺	-onio- ammonio-	-onium -ammonium
2	Carboxylic acids Thiocarboxylic acids Selenocarboxylic acids Sulfonic acids Sulfinic acids Sulfenic acids	–COOH –COSH –COSeH –SO₃H –SO₂H –SOH	carboxy- thiocarboxy- selenocarboxy- sulfo- sulfino- sulfeno-	-oic acid* -thioic acid* -selenoic acid* -sulfonic acid -sulfinic acid -sulfenic acid
3	Carboxylic acid derivatives Esters Acyl chlorides Amides Imides Amidines	–COOR –COCl –CONH₂ –CON=C< –C(=NH)NH₂	R-oxycarbonyl- chloroformyl- carbamoyl- -imido- amidino-	-oyl chloride* -amide* -imide* -amidine*
4	Nitriles Isocyanides	–CN –NC	cyano- isocyano-	-nitrile* isocyanide
5	Aldehydes Thioaldehydes	–CHO –CHS	formyl- thioformyl-	-al* -thial*
6	Ketones Thioketones	>CO >CS	oxo- thiono-	-one -thione
7	Alcohols Thiols Selenols Tellurols	–OH –SH –SeH –TeH	hydroxy- sulfanyl- selanyl- tellanyl-	-ol -thiol -selenol -tellurol
8	Hydroperoxides	–OOH	hydroperoxy-	-hydroperoxide
9	Amines Imines Hydrazines	–NH₂ =NH –NHNH₂	amino- imino- hydrazino-	-amine -imine -hydrazine
10	Ethers Thioethers Selenoethers	–O– –S– –Se–	-oxy- -thio- -seleno-
11	Peroxides Disulfides	–OO– –SS–	-peroxy- -disulfanyl-

*Note: These suffixes, in which the carbon atom is counted as part of the preceding chain, are the most commonly used. See individual functional group articles for more details.

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Common nomenclature - trivial names

Common nomenclature is an older system of naming organic compounds. Instead of using the prefixes for the carbon skeleton above, another system is used. The pattern can be se

Number of carbons	Prefix as in new system	Common name for alcohol	Common name for aldehyde	Common name for acid
1	Meth	Methyl alcohol (wood alcohol)	Formaldehyde	Formic acid
2	Eth	Ethyl alcohol (grain alcohol)	Acetaldehyde	Acetic acid
3	Prop	Propyl alcohol	Propionaldehyde	Propionic acid
4	But	Butyl alcohol	Butyraldehyde	Butyric acid
5	Pent	Amyl alcohol	Valeraldehyde	Valeric acid
6	Hex	-	Caproaldehyde	Caproic acid
7	Hept	Enanthyl alcohol	Enanthaldehyde	Enanthoic acid
8	Oct	Capryl alcohol	Caprylaldehyde	Caprylic acid
9	Non	-	Pelargonaldehyde	Pelargonic acid
10	Dec	Capric alcohol	Capraldehyde	Capric acid
11	Undec	-	-	-
12	Dodec	Lauryl alcohol	Lauraldehyde	Lauric acid
13	Tridec	-	-	-
14	Tetradec	-	Myristaldehyde	Myristic acid
15	Pentadec	-	-	-
16	Hexadec	Cetyl alcohol	Palmitaldehyde	Palmitic acid
17	Heptadec	-	-	Margaric acid
18	Octadec	Stearyl alcohol	Stearaldehyde	Stearic acid
19	Nonadec	-	-	-
20	Eicos	Arachidyl alcohol	-	Arachidic acid
21	Heneicos	-	-	-
22	Docos	Behenyl alcohol	-	Behenic acid
24	Tetracos	Lignoceryl alcohol	-	Lignoceric acid
26	Hexacos	Cerotinyl alcohol	-	Cerotinic acid
28	Octacos	Montanyl alcohol	-	Montanic acid
30	Triacont	Melissyl alcohol	-	Melissic acid

Ketones

Common names for ketones can be derived by naming the two alkyl or aryl groups bonded to the carbonyl group as separate words followed by the word ketone.

Acetone

Acetophenone

Benzophenone

Ethyl isopropyl ketone

Diethyl ketone

The first three of the names shown above are still considered to be acceptable IUPAC names.

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Aldehydes

The common name for an aldehyde is derived from the common name of the corresponding carboxylic acid by dropping the word acid and changing the suffix from -ic or -oic to -aldehyde.

Formaldehyde

Acetaldehyde

Ions

The IUPAC nomenclature also provides rules for naming ions.

Hydron

Hydron is a generic term for hydrogen cation; protons, deuterons and tritons are all hydrons.

Parent hydride cations

Simple cations formed by adding a hydron to a hydride of a halogen, chalcogen or nitrogen-family element are named by adding the suffix "-onium" to the element's root: H₄N⁺ is ammonium, H₃O⁺ is oxonium, and H₂F⁺ is fluoronium. Ammonium was adopted instead of nitronium, which commonly refers to NO₂⁺.If the cationic center of the hydride is not a halogen, chalcogen or nitrogen-family element then the suffix "-ium" is added to the name of the neutral hydride after dropping any final 'e'. H₅C⁺ is methanium, HO-O⁺H₂ is dioxidanium (HO-OH is dioxidane), and H₂N-N⁺H₃ is diazanium (H₂N-NH₂ is diazane).

Cations and substitution

The above cations except for methanium are not, strictly speaking, organic, since they do not contain carbon. However, many organic cations are obtained by substituting another element or some functional group for a hydrogen.The name of each substitution is prepended to the hydride cation name. If many substitutions by the same functional group occur, then the number is indicated by prepending "di-", "tri-" as with halogenation. (CH₃)₃O⁺ is trimethyloxonium. CH₃F₃N⁺ is trifluoromethylammonium.

Types of hydrocarbons

The classifications for hydrocarbons defined by IUPAC nomenclature of organic chemistry are as follows:

1. Saturated hydrocarbons (alkanes) are the most simple of the hydrocarbon species and are composed entirely of single bonds and are saturated with hydrogen; they are the basis of petroleum fuels and are either found as linear or branched species of unlimited number. The general formula for saturated hydrocarbons is C_nH_2n+2 (assuming non-cyclic structures).

2. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with one double bond are called alkenes, with the formula C_nH_2n (assuming non-cyclic structures). Those containing triple bonds are called alkynes, with general formula C_nH_2n-2.

3. Cycloalkanes are hydrocarbons containing one or more carbon rings to which hydrogen atoms are attached. The general formula for a saturated hydrocarbon containing one ring is C_nH_2n

4. Aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring.

Hydrocarbons can be gases (e.g. methane and propane), liquids (e.g. hexane and benzene), waxes or low melting solids (e.g. paraffin wax and naphthalene) or polymers (e.g. polyethylene, polypropylene and polystyrene).

General properties

Because of differences in molecular structure, the empirical formula remains different between hydrocarbons; in linear, or "straight-run" alkanes, alkenes and alkynes, the amount of bonded hydrogen lessens in alkenes and alkynes due to the "self-bonding" or catenation of carbon preventing entire saturation of the hydrocarbon by the formation of double or triple bonds.This inherent ability of hydrocarbons to bond to themselves is referred to as catenation, and allows hydrocarbon to form more complex molecules, such as cyclohexane, and in rarer cases, arenes such as benzene. This ability comes from the fact that bond character between carbon atoms is entirely non-polar, in that the distribution of electrons between the two elements is somewhat even due to the same electronegativity values of the elements (~0.30), and does not result in the formation of an electrophile.Generally, with catenation comes the loss of the total amount of bonded hydrocarbons and an increase in the amount of energy required for bond cleavage due to strain exerted upon the molecule; in molecules such as cyclohexane, this is referred to as ring strain, and occurs due to the "destabilized" spatial electron configuration of the atom.In simple chemistry, as per valence bond theory, the carbon atom must follow the "4-hydrogen rule", which states that the maximum number of atoms available to bond with carbon is equal to the number of electrons that are attracted into the outer shell of carbon. In terms of shells, carbon consists of an incomplete outer shell, which comprises 4 electrons, and thus has 4 electrons available for covalent or dative bonding.According thermodynamics studies hydrocarbons are stable in great depths within the earth. Hydrocarbons also have great abundance in the universe. In Titan (a Saturn moon) there are lakes and seas of liquid methane and ethane confirmed by Cassini-Huygens Mission.

Simple hydrocarbons and their variations

Number of carbon atoms	Alkane	Alkene	Alkyne	Cycloalkane	Alkadiene
1	Methane	—	—	—	—
2	Ethane	Ethene	Ethyne	—	—
3	Propane	Propene	Propyne	Cyclopropane	Allene
4	Butane Isobutane	Butene	Butyne	Cyclobutane Methylcyclopropane	Butadiene
5	Pentane Isopentane Neopentane	Pentene	Pentyne	Cyclopentane	Pentadiene Isoprene
6	Hexane	Hexene	Hexyne	Cyclohexane	Hexadiene
7	Heptane	Heptene	Heptyne	Cycloheptane Methylcyclohexane	Heptadiene
8	Octane	Octene	Octyne	Cyclooctane	Octadiene
9	Nonane	Nonene	Nonyne	Cyclononane	Nonadiene
10	Decane	Decene	Decyne	Cyclodecane	Decadiene

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Benzene Derivatives

The nomenclature of substituted benzene ring compounds is less systematic than that of the alkanes, alkenes and alkynes. A few mono-substituted compounds are named by using a group name as a prefix to "benzene", as shown by the combined names listed below. A majority of these compounds, however, are referred to by singular names that are unique. There is no simple alternative to memorization in mastering these names.

Two commonly encountered substituent groups that incorporate a benzene ring are phenyl, abbreviated Ph-, and benzyl, abbreviated Bn-. These are shown here with examples of their use. Be careful not to confuse a phenyl (pronounced fenyl) group with the compound phenol (pronounced feenol). A general and useful generic notation that complements the use of R- for an alkyl group is Ar- for an aryl group (any aromatic ring).

When more than one substituent is present on a benzene ring, the relative locations of the substituents must be designated by numbering the ring carbons or by some other notation. In the case of disubstituted benzenes, the prefixes ortho, meta & para are commonly used to indicate a 1,2- or 1,3- or 1,4- relationship respectively. In the following examples, the first row of compounds show this usage in red. Some disubstituted toluenes have singular names (e.g. xylene, cresol & toluidine) and their isomers are normally designated by the ortho, meta or para prefix. A few disubstituted benzenes have singular names given to specific isomers (e.g. salicylic acid & resorcinol). Finally, if there are three or more substituent groups, the ring is numbered in such a way as to assign the substituents the lowest possible numbers, as illustrated by the last row of examples. The substituents are listed alphabetically in the final name. If the substitution is symmetrical (third example from the left) the numbering corresponds to the alphabetical order.

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IUPAC Rules for Alkane Nomenclature

Illustration 1.

Illustration 2.

Illustration 3.

Sub-Rules for IUPAC Nomenclature

Illustration 4.

Examples of Simple Cycloalkanes

IUPAC Rules for Cycloalkane Nomenclature

Examples of Isomeric C8H14 Bicycloalkanes

IUPAC Rules for Alkene and Cycloalkene Nomenclature

IUPAC Rules for Alkyne Nomenclature

Sub-Rules for IUPAC Nomenclature

Examples of Isomeric C₈H₁₄ Bicycloalkanes