Organic Chemistry Nomenclature

A Translator’s Guide to Organic Chemical Nomenclature

Part XV

by Chester E. Claff, Jr., Ph. D.

Download the zipped version of the first installments of this series, originally published in the Sci-Tech Translation Journal, of the American Translators Association. The file is approximately 87 KB.

Previous installments of this series appeared in the July 1997, October 1997, January 1998, April 1998, July 1998, October 1998, and January 1999 issues of the Translation Journal.

A note on searching:

If you have saved the HTML files of this series on your hard drive, they can be quickly and easily searched for any term from the Windows 95 desktop by:

START - FIND - FILES - ADVANCED - INTERNET DOCUMENT.

It's fast!

The orderly diversity of organic chemical nomenclature

Reference has been made repeatedly to different systems of nomenclature (radicofunctional, substitutive, trivial, etc.) and to the fact that multiple names are often acceptable for the same compound. This situation is pointedly recognized in the following advertisement from a recent issue of Chemical and Engineering News:

VII. Alicyclic Compounds (continued)

Chirality (continued)

The concept of chirality is of such current importance in organic chemistry and medicine that it will be worthwhile to explore the subject a little further before returning to specific chemical nomenclature.
    J.R. Biot in 1815 discovered that many naturally occurring organic compounds were optically active, i.e. they rotated the plane of polarized light. The extent of this rotation was expressed in terms of specific rotation [a]; for example the expression [a]_D²⁵ = +n° means that a (hypothetical) solution of 1 gram of a substance in 1 ml of solution in a tube 10 cm long would rotate polarized light from incandescent sodium (its so-called D line) by n° to the right at 25°C.
   In 1848 Louis Pasteur noticed that certain salts of racemic tartaric acid crystallized from water in two different mirror-image forms. When meticulously separated with tweezers, one form proved to be dextrorotatory and the other levorotatory, with the same but opposite values of [a]. He called the two isomers d-tartaric acid and l-tartaric acid, and their racemic mixture dl-tartaric acid.
   Some confusion began to reign when the configuration of chiral centers was sometimes designated by a small capital D or L, depending on how the configuration of the compound was related to d- or l-glyceraldehyde. These descriptors no longer always correlated with the direction of rotation of polarized light, so that D-(+)-, D-(-)-, L-(+)-, and L-(-)- descriptors were added to indicate this property.
    Cahn, Ingold, and Prelog in 1956 brought some order out of this chaos by proposing the R,S descriptors discussed in the preceding part of this series.
    We have already shown graphically the three configurations of 1,2-dichlorocyclopentane. Another, more up-to-date depiction of these three isomers would be:


(R,S)-cis-	(S,S)-trans-	(R,R)-trans-
1,2-dichlorocyclopentane

In these illustrations, the solid wedges indicate bonds that rise above the plane of the page, and the shaded ones dip below it. This convention is now universal, and is applied also to fused ring systems and achiral compounds:


cis-decalin	trans-decalin

With this (admittedly tedious) coverage of stereoisomerism behind us, let's now return to elements other than C, H, O, S, and halides commonly found in organic compounds.

VIII. Organic Nitrogen Compounds

Amines

While the halides are monovalent, oxygen and sulfur are divalent, and carbon is tetravalent, nitrogen is trivalent; the saturated compounds of these elements with hydrogen reflect this:

HCl	Hydrogen chloride
H₂O	Water
H₂S	Hydrogen sulfide
NH₃	Ammonia
CH₄	Methane

The reason why hydrogen precedes the other element in some of these formulas and follows it in others is neither known nor significant, but this order is nevertheless conventional.
The nitrogen atom of ammonia has a free, unbonded electron pair and therefore has great affinity for protons or other positively charged entities. Such compounds are said to be nucleophilic, while their willing positive partners are electrophilic. Ammonia readily abstracts a proton from water or hydrogen chloride to give ammonium hydroxide or ammonium chloride:

NH₃ + H₂O --> NH₄⁺OH^-

NH₃ + HCl --> NH₄⁺Cl^-

A dramatic demonstration of the latter reaction consists of placing a beaker of ammonia solution next to a beaker of hydrochloric acid solution and blowing across the top. When the invisible NH₃ and HCl gases unite, a dense cloud of solid NH₄Cl particles is formed. Nucleophilicity in action!
The hydrogen atoms of ammonia can readily be replaced by alkyl groups to give primary, secondary, or tertiary amines:

CH₃NH₂	Methylamine (a primary amine)
(CH₃)₂NH	Dimethylamine (a secondary amine)
(CH₃)₃N	Trimethylamine (a tertiary amine)

When the alkyl groups of a secondary or tertiary amine differ, all of the alkyl groups are named in sequence, with no spaces between them:

CH₃NHC₂H₅	Methylethylamine (or N-methylethylamine)
(CH₃)₂NC₃H₇	Dimethylpropylamine (or N,N-dimethylpropylamine)
(CH₃)(C₂H₅)NC₃H₇	Methylethylpropylamine

It may be confusing (but again prescribed by convention) that the terms primary, secondary, and tertiary have different meanings when applied to amines than when applied to alcohols or halides. A primary alcohol is a compound with a hydroxyl group bonded to a carbon atom that carries two hydrogen atoms (RCH₂OH). A primary amine, on the other hand, can be bonded to any carbon atom, but there must be two hydrogen atoms on the nitrogen atom (RNH₂). A secondary alcohol is R₂CHOH, while a secondary amine is R₂NH, and the tertiary compounds are R₃COH and R₃N. Therefore, tert-butylamine is a primary amine (CH₃)₃CNH₂. Strange but true!

Amines, like ammonia, are nucleophilic and form salts with acids:

C₄H₉NH₃⁺Cl^- or C₄H₉NH₂^.HCl	Butylammonium chloride (or butylamine hydrochloride)
[(CH₃)₃NH⁺]₂SO₄^2-	Trimethylammonium sulfate

Quaternary ammonium compounds

Tertiary amines also react with alkylating agents including alkyl halides or alkyl sulfates (such as methyl iodide or dimethyl sulfate) to give quaternary ammonium salts:

(C₂H₅)₃N + C₂H₅Br --> (C₂H₅)₄N⁺Br^-	Tetraethylammonium bromide
(CH₃)₃N + CH₃OSO₂OCH₃ --> (CH₃)₄N⁺CH₃OSO₂O^-	Tetramethylammonium methosulfate

Long-chain quaternary ammonium chlorides are marketed in very large volume as fabric softeners.
Quaternary ammonium hydroxides also exist. They are very strong alkalies.

Part XVI will address amino acids and proteins.

Readers are urged to e-mail questions, comments, or suggestions for further topics in the field of organic nomenclature to the author at: 74654.1335@compuserve.com.