Nomenclature of Organic Chemistry

Organic Nomenclature

Oxford Dictionaries (n.d.) defines nomenclature as “the devising or choosing of names for things, especially in a science or other discipline”{Dictionaries, #1@@author-year}.

I believe the easiest way to understand the rules associated with the IUPAC nomenclature of organic chemistry is with examples and so the majority of this essay will be dealing with examples and their explanations.

Simek (1999) introduces the systematic naming of an organic compound with a fundamental rule, that to begin naming, one must first identify the parent structure, “based on naming a molecule’s longest chain of carbons connected by single bonds, whether in a continuous chain or in a ring”. After which, “all deviations, either multiple bonds or atoms other than carbon and hydrogen, are indicated by prefixes or suffixes according to a specific set of priorities”.

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Simek (1999) also describes how alkanes are saturated hydrocarbons, which are molecules only containing carbon and hydrogen bonded by single bonds only. Alkanes can be subdivided into two main groups, linear and cyclic; linear describes molecules that can be bonded in continuous chains and cyclic describes molecules that are bonded in a ring like structure.

The simplest of all to name are straight chain alkanes:

CH4Methane

C2H6Ethane

C3H8Propane

C4H10Butane

C5H12Pentane

C6H14Hexane

C7H16Heptane

C8H18Octane

C9H20Nonane

C10H22Decane

C11H24Undecane

C12H26Dodecane

In order to name cycloalkanes, the prefix “cyclo” is used:

C3H6CyclopropaneC4H8Cyclobutane

C5H10Cycloheptane

Nomenclature of Branched Chain Alkanes

University of California, Davis (n.d.) describes how “An alkyl group is formed by removing one hydrogen from the alkane chain, and is described by the formula CnH2n+1. The removal of this hydrogen results in a stem change from-aneto-yl.” E.g. Propane to propyl.

1. In order to systematically name a molecule, first identify the parent structure. In this case the longest carbon chain is 6-Carbons long, as a result the parent structure is Hexane.

2. The carbons in the chain are numbered from the end giving the substituents (The group substituted in place of hydrogen, in this case the substituent is CH3-Methyl) the lowest possible number

3. The substituents or functional groups that are attached to the parent chain are then named. There are two, one-carbon long alkyl groups and as a result take methane, drop the -ane and replace it with –yl­, giving methyl.

N.B. If the alkyl group is two-carbons long (CH3CH2), the name would be ethyl, CH3CH2CH2- propyl, CH3CH2CH2CH2- butyl.

4. Number the substituents to identify their positions relative to the parent structure.

Here, substituent positions are 2 and 4.

Hardinger (2008) emphasises that a number must be assigned to each substituent, along with its prefix (di-, tri-, terta-, penta-, etc.), even if the same substituents are present in the molecule – In this example 2,4-dimethyl.

5. Position numbers for substituents are ordered numerically, substituent names are ordered alphabetically (prefixes such as di-, tri-, tetra-, etc., are excluded from alphabetical ordering, but cyclo, iso and neo are included) and are then written before the parent name.

If these rules are adhered to, the molecule is named as: 2,4-dimethylhexane.

Nomenclature of Alkenes

Alkenes are unsaturated hydrocarbons and differ from alkanes, as they have at least one C=C double bond. Alkenes have the general formula C_nH_2n, which is the same general formula for cycloalkanes.

1. Following the same basic rules as before, identify the parent structure. Here, the longest carbon chain is 7-Carbons long, thus the parent structure is heptane.

2. The molecule is numbered so that the substituents have the lowest possible position numbers.

3. The substituents are then named. As the molecule has a double bond, it is identified as an alkene and as the parent structure is heptane, it is named heptene. However, take into account there is also a methyl group.

4. Numbering the positions of the substituents gives, 2-methyl and 1,3-diene, since the molecule contains one methyl group and two double bonds.
5. Position numbers are ordered numerically, the substituents ordered alphabetically and both written before the parent name. Due to the fact the double bonds use a suffix (-ene is at the end of the name), 1,3-diene is not ordered before 2-methyl.

Systematically naming the molecule gives it an IUPAC name of: 2-methylhepta-1,3-diene.

Nomenclature of Haloalkanes

Haloalkanes are organic compounds, where an alkane contains at least one halogen. Haloalkanes have a general formula of C_nH_2n+1X (X=Halogen e.g. Cl)

In order to name haloalkanes, the –ine of the halogen name is removed, leaving the prefix (e.g. fluorine becomes floro-, chlorine becomes chloro-, etc.). The same rules are then applied to systematically name the haloalkane.

Nomenclature of Alkynes

Alkynes are unsaturated hydrocarbons, as they contain at least one C≡C bond. Alkynes have the general formula C_nH_2n-2.

1. Again identify the parent structure, the longest carbon chain is 7-Carbons long and therefore the parent structure is identified as heptane.

2. The molecule is numbered so substituent positions have the lowest possible numbers.

3. Here the substituents are: two methyl groups, one chloro group and one C≡C triple bond. The longest chain is 7-Carbons long and contains a C≡C triple bond; therefore, it is identified as heptyne.

4. The substituent positions are numbered giving: 6,6-dimethyl, 4-chloro and hept-2-yne.
5. Finally position numbers are ordered numerically, substituent names are ordered alphabetically and are written in front of the parent name.

The molecule is given an IUPAC name: 4-chloro-6,6-dimethylhept-2-yne.

Nomenclature of Alcohols

Alcohols are organic compounds containing at least one –OH group bonded to it. The hydroxyl group replaces a hydrogen on a carbon and because of this, alcohols have the general formula C_nH_2n+1OH.

1. The parent structure for this molecule is identified as octane, since the longest carbon chain is 8-Carbons long.

2. The molecule is numbered, giving the lowest possible numbers to substituents.

3. Substituents in this molecule are: one hydroxyl group and one chloro group. The longest carbon chain is 8-Carbons long and since it contains a hydroxyl group, it is identified as an alcohol.

4. The position numbers for substituents are: 4-chloro and octan-2-ol.
5. Position numbers are ordered numerically, substituent names are ordered alphabetically and are placed before the parent name (the hydroxyl group identifies the molecule as an alcohol, as such, it uses the suffix –ol instead of the prefix hydroxy-).

The molecule has an IUPAC name: 4-chlorooctan-2-ol.

Nomenclature of Amines

Amines are derivatives of ammonia (NH₃), the replacing of one or more hydrogens in ammonia with organic compound(s) creates an amine. Replacing one hydrogen, will create a primary amine, two hydrogens – secondary amine, three hydrogens – tertiary amine.

Methylamine (primary) Dimethylamine (secondary) Trimethylamine (tertiary)

When naming amines, the longest carbon chain including the amine group is determined and numbered so to give the amine group the lowest possible position number.

If the molecule is a secondary amine, the longest carbon chain is used as the parent structure and the other chain is denoted with N-alkyl (if both chains are of equal length, the molecule can be named dialkylamine).

If the molecule is a tertiary amine, like secondary amines the longest carbon chain is used as the parent structure and the other chains are denoted with N-alkyl (if all chains are of equal length, the molecule can be named trialkylamine).

Nomenclature of Ethers

University of California, Davis (n.d.) describes ethers as, organic compounds that contain two alkyl groups bonded to an oxygen atom (e.g. CH₃CH₂OCH₃).

Ethers only use the prefix alkoxy-, where the –ane of the alkane is removed. According to University of California, Davis (n.d.) the prefix alkoxy- is always treated as a substituent, because there is no suffix for ethers.

When naming the molecule the shorter carbon chain becomes the alkoxy- substituent (e.g. methoxy) and the longer carbon chain is identified as the parent structure.

Nomenclature of Aldehydes and Ketones

Both aldehydes and ketones are organic compounds that contain the carbonyl group C=O. Aldehydes feature at the end of a carbon chain (e.g. CH3CH2CH2CHO), whereas, ketones are part of the carbon chain (e.g. CH3CH2COCH3)

When naming aldehydes it is important to note that they exist only on the ends of carbon chains and therefore do not need a position number included in the name, the aldehyde is presumed to be position 1. Aldehyde’s use the suffix –al in naming and replace the –e at the end of alkanes (e.g. Butane becomes butanal).

Ketones use the suffix –one in naming and replace the –e at the end of alkanes (e.g. pentane becomes pentanone); however, unlike aldehydes ketones need position numbers, as there are multiple positions for the C=O bond (with the exception of simple ketones like propanone, as there is only one position for the carbonyl group).

Nomenclature of Carboxylic Acids

Carboxylic acids are organic compounds that contain the carboxyl group COOH. Like aldehydes carboxylic acids are only present at the end of carbon chains and therefore, do not have positions numbers. Carboxylic acids use the suffix –oic acid and replace the –e at the end of alkanes (e.g. Ethane becomes ethanoic acid).

N.B. One must be aware, that although there are systematic IUPAC names for all molecules, some molecules have common names e.g. ethanoic acid used to be known as acetic acid and is most commonly known as vinegar.

Nomenclature of Esters

Esters are formed from reacting a carboxylic acids with alcohols.

University of California, Davis (n.d.) briefly explains, that esters are named by treating the alkyl chain from the alcohol as a substituent and the carboxylic acid forms the parent structure, where the –oic acid part is replaced with –oate (e.g. Ethanoic acid becomes ethanoate).

Nomenclature of Aromatic Compounds

Simek (1999) describes, how aromatic compounds are derived from benzene rings (C₆H₆), by replacing one hydrogen with a substituent group. The removal of one hydrogen from the benzene results in the phenyl group.

When naming aromatic compounds the parent name benzene is used and appropriate prefixes.

Priority Rules of Nomenclature

When the molecule being named has multiple functional groups and substituent groups, the group with highest priority is numbered so it has the lowest possible position number.

This list based on Simek’s (1999) priority table, shows priorities from highest to lowest:

1. Carboxylic Acids
2. Esters
3. Aldehydes
4. Ketones
5. Alcohols
6. Amines
7. Alkenes
8. Alkynes
9. Alkyls
10. Ethers
11. Halo Compounds
12. Aromatic Compounds

References:

Simek, J., 1999.IUPAC_Handout. [e-book] San Luis Obispo: California Polytechnic State University. Available at: http://www.angelo.edu/faculty/kboudrea/organic/IUPAC_Handout.pdf [Accessed 10 Feb. 14]

Lam, D., n.d. [online] Nomenclature of Benzenes – Chemwiki. Available at: http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Aromatics/Nomenclature_of_Benzenes [Accessed 13 Feb. 14]

Oxford Dictionaries. N.d. [online]nomenclature: definition of nomenclature in Oxford dictionary (British & World English). Available at: http://www.oxforddictionaries.com/definition/english/nomenclature [Accessed 10 Feb. 14]

Hardinger, S., 2008. Nomenclature_02. [e-book] Los Angeles: University of California. Available at: http://www.chem.ucla.edu/harding/notes/nomenclature_02.pdf [Accessed 10 Feb. 14]

Clark, J. 2000. [online] Naming aromatic compounds. Available at: http://www.chemguide.co.uk/basicorg/conventions/names3.html [Accessed 12 Feb. 14]