Alkanes Nomenclature and Numbering

The simplest organic compounds are hydrocarbons. Hydrocarbons contain only two elements, hydrogen and carbon. A saturated hydrocarbon or alkane is a hydrocarbon in which all of the carbon-carbon bonds are single bonds. Each carbon atom forms four bonds and each hydrogen forms a single bond to a carbon. The bonding around each carbon atom is tetrahedral, so all bond angles are 109.5 degrees. As a result, the carbon atoms in higher alkanes are arranged in zig-zag rather than linear patterns.

Straight-Chain Alkanes

The general formula for an alkane is C_nH_2n+2 where n is the number of carbon atoms in the molecule. There are two ways of writing a condensed structural formula. For example, butane may be written as CH₃CH₂CH₂CH₃ or CH₃(CH₂)₂CH₃.

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Name the First 10 Alkanes

By Anne Marie Helmenstine, Ph.D.

Rules for Naming Alkanes

• The parent name of the molecule is determined by the number of carbons in the longest chain.
• In the case where two chains have the same number of carbons, the parent is the chain with the most substituents.
• The carbons in the chain are numbered starting from the end nearest the first substituent.
• In the case where there are substituents having the same number of carbons from both ends, numbering starts from the end nearest the next substituent.
• When more than one of a given substituent is present, a prefix is applied to indicate the number of substituents. Use di- for two, tri- for three, tetra- for four, etc. and use the number assigned to the carbon to indicate the position of each substituent.

Branched Alkanes

• Branched substituents are numbered starting from the carbon of the substituent attached to the parent chain. From this carbon, count the number of carbons in the longest chain of the substituent. The substituent is named as an alkyl group based on the number of carbons in this chain.
• The numbering of the substituent chain starts from the carbon attached to the parent chain.
• The entire name of the branched substituent is placed in parentheses, preceded by a number indicating which parent-chain carbon it joins.
• Substituents are listed in alphabetical order. To alphabetize, ignore numerical (di-, tri-, tetra-) prefixes (e.g., ethyl would come before dimethyl), but don't ignore don't ignore positional prefixes such as iso and tert (e.g., triethyl comes before tertbutyl).

Cyclic Alkanes

• The parent name is determined by the number of carbons in the largest ring (e.g., a cycloalkane such as cyclohexane).
• In the case where the ring is attached to a chain containing additional carbons, the ring is considered to be a substituent on the chain. A substituted ring that is a substituent on something else is named using the rules for branched alkanes.
• When two rings are attached to each other, the larger ring is the parent and the smaller is a cycloalkyl substituent.
• The carbons of the ring are numbered such that the substituents are given the lowest possible numbers.

Straight Chain Alkanes