Nomenclature of organic compounds: Naming Alkanes

These are saturated hydrocarbons i.e., contain all single C - C bonds and only C and H atoms.

1. Naming straight chain hydrocarbons:

Prefix: indicates the number of carbons

Suffix: ‘-ane’

2. Branched chain alkanes:

• Identify the longest carbon chain
• Number the carbons in the parent carbon chain such that the substituents get minimum numbers
• If two substituents are found in equivalent positions, the lower number is given to the one coming first in the alphabetical listing
• If the same substituent occurs more than once, the location of each point on which the substituent occurs is given. In addition, the number of times the substituent group occurs is indicated by a prefix
• Write the name of the substituent along with its position before the name of the parent chain in alphabetical order

Nomenclature of Alkenes: These have the general formula C_nH_2n. These unsaturated hydrocarbons are isomeric with the saturated cycloalkanes. The IUPAC rules for naming alkenes are similar in many respects to those for naming alkanes.

1. Determine the root word by selecting the longest chain that contains the double bond and change the ending of the name of the alkane of identical length from ane to ene.

Number the chain so to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Designate the location of the double bond by using the number of the first atom of the double bond as prefix: Indicate the locations of this substituent groups by the numbers of the carbon atoms to which they are attached.

2. Two frequently encountered alkenyl groups are the vinyl group and the allyl group.

CH₂ = CH —                   CH₂ = CH CH₂ —

vinyl group                      allyl group (are not included in IUPAC system)

The following examples illustrate how these names are employed

CH₂ = CH — Br                  CH₂ = CH — CH₂ Cl

vinyl bromide                      allyl chloride (are not IUPAC name)

3. The geometry of the double bond of a disubstituted alkene is designated with the prefixes, cis and trans. If two identical group are on the same side of the double bond, it is cis, if they are on opposite sides; it is trans.

Nomenclature of Alkyne: In the IUPAC system the compounds are named as alkynes in which the final – ane of the parent alkane is replaced by the suffix – yne. The position of the triple bond is indicated by a number when necessary.

When both a double and triple bond are present, the hydrocarbon is named an alkenyne with numbers as low as possible given to the multiple bonds. In case of a choice, the double bond gets the lower number.
In complex structures the alkynyl group is used as a modifying prefix.