Club Chemistry: Naming Linear Alkanes

Organic Nomenclature of Single-Bonded Carbon-Hydrogen Compounds

Alkanes are compounds consisting of chains of carbon atoms connected by single bonds. A few simple rules apply to the proper organic nomenclature of these molecules.

One of the first things a student of organic chemistry has to get a handle on is terminology. There are many more organic compounds than inorganic ones, and they exist in much greater complexity, so it's a challenge.

Millions of organic compounds are known, and many more are synthesized or discovered each year. By following some simple rules, one can properly name quite a few of them, and at least understand something about many others, just by knowing their names.

Organic Chemistry Defined

Organic chemistry is carbon chemistry. Life on Earth is often described as "carbon-based" because most of the important molecules in living systems are built on a skeleton or "backbone" of carbon atoms. Some carbon compounds, such as cyanide, carbonic acid and others may not be considered organic because there are no carbon-to-carbon bonds, but one will usually find those molecules discussed in organic chemistry texts.

The Hydrocarbons

The simplest organic molecules are composed of only carbon and hydrogen — the hydrocarbons. The simplest hydrocarbon of them all is methane, consisting of a single carbon atom bonded with four hydrogen atoms. The kinds of "bonds" that hold carbon atoms together are covalent bonds, which result when a pair of electrons is shared between the atoms.

Despite being only being made up of two elements, a huge number of hydrocarbons exist since there is a very high limit on the number of carbon atoms that can be linked together in a molecule. Hydrocarbons (and related organic molecules) are classified into one of three groups.

Alkanes

The alkanes are molecules with only single carbon-to-carbon bonds; e.g., C-C-C. The alkenes posses at least one instance of a carbon double bond; e.g., C-C=C-C. The third category is the alkynes, which contain a triple bonded carbon; e.g., C≡C.

Alkenes and Alkynes

Carbon atoms need to be connected to four other atoms to be stable, and in the case of hydrocarbons the other atoms will be either another carbon, or a hydrogen atom. For example, the complete structure of the simplest alkyne, acetylene, is H-C≡C-H (or simply HC≡CH). To save time in drawing them and for simplification, the hydrogen atoms are not shown and assumed to be there.

Organic Nomenclature of Alkanes

Alkanes consist of carbon atoms linked together by single bonds, and the first step in naming them is to count the number of carbons. The name of an alkane is based on using standard prefixes corresponding to the number of carbons, with an "-ane" stuck on the end. The most common prefixes are (with the number indicating the number of carbons):

meth
eth
but
prop
pent
hex
hept
oct
non
dec

So, C-C-C-C-C is pentane, C-C-C-C-C-C-C is heptane, etc.

Some additional analysis is needed when the carbons aren't all in a row (straight chain) but are branched at one or perhaps many places along the backbone. Take for example the six-carbon molecule in Figure 1 (click it to get the full view).

It appears to be a hexane, but it can't have the same name as the linear version C-C-C-C-C-C. In this case do as follows:

1) Count the number of carbons in the longest carbon chain that can be found. That will be the principle name.

In the Figure 1 example there are five, so it's a pentane.

2) Number the carbons in the main chain and identify the number(s) of the carbons on which the branches appear. Use a numbering that puts the side chain on the smallest number.

In the Figure 1 example the side chain is on carbon 2 (NOT carbon 4).

3) Determine the name of the functional group on the side chains. The name of a side chain is derived from the standard prefix, with an "-yl" added to the end (in this case it is a methyl group).

The name of this example molecule is 2-methylpentane. It takes practice to get good at naming alkanes. It's easy to find naming practice problems in textbooks and on the web.