IUPAC Nomenclature of Alkanes, Alkenes and Alkynes

What is IUPAC Nomenclature?

A single chemical compound can have several acceptable systematic names and the systematic method of naming organic chemical compounds is called IUPAC Nomenclature but no two compounds can have the same name.

Table of Contents

• IUPAC Nomenclature
• IUPAC Nomenclature of Alkanes, Alkynes, and Alkenes
• Unsaturated Carbon Chains – Alkene Formula
• IUPAC System of Alkenes
• Alkene Formula
• Recommended Videos
• Frequently Asked Questions – FAQs

IUPAC Nomenclature

Earlier, most of the compounds with the same structural formula were known by different names depending on the regions where they were synthesized. This naming system was very trivial since it raised a lot of confusion. Finally, a common naming system enlisting the standard rules was set up by IUPAC (International Union for Pure and Applied Chemistry) for the naming of compounds. This method of naming is known as IUPAC naming or IUPAC nomenclature.

Nomenclature the way that names are given to things is one main point of difference between the language of chemistry as of other science and natural languages. The other is the importance of the written language compared with the spoken one. In chemistry there are several nomenclatures not only elements and compounds must be named but also reactions methods, pieces of apparatus and theoretical concepts.

IUPAC Nomenclature of Alkanes, Alkynes, and Alkenes

This method of naming is known as IUPAC naming or IUPAC nomenclature. IUPAC nomenclature of alkanes, alkynes, and alkenes are explained below:

Alkanes are the simplest hydrocarbons known to us. They have a general formula of C_nH_2n+2. Alkanes belong to the family of saturated hydrocarbons that is; they contain only sigma bond linkages between carbon and hydrogen. The organic compounds form a series, known as homologues series in which the successive compounds contain the same functional group and differ from one another by a ‘–CH₂’ group.

Alkenes and alkynes, on the other hand, are unsaturated hydrocarbons. In the case of alkenes, double bond linkages are seen and in alkynes, triple bond linkages are present. Rules underlying IUPAC nomenclature of alkanes, alkenes, and alkynes are discussed below:

• The longest hydrocarbon chain is selected and is termed as parent chain in case of alkanes. In the case of alkenes and alkynes, hydrocarbon chain with the double and triple bond is chosen as parent chain. The parent chain is named with the help of Greek alphabets such as hepta, octa etc.
• For alkanes suffix ‘-ane’ is used, for alkenes, the suffix ‘-ene’ is used and the suffix ‘yne’ is used for alkynes. For example, C₂H₆ is known as ethane, C₂H₄ is known as ethene and C₂H₂ is known as ethyne.
• The parent chain is numbered such that we reach to the double-bonded or triple bonded carbon atom earliest. The position of the carbon atom with the double bond is mentioned in numerals. For example, CH₃CH=CHCH₂CH₃ is named as Pent-2-ene.
• In the case of multiple double bonds in the carbon chain, Greek numerical prefixes such as di, tri are used to denote their number.

Unsaturated Carbon Chains – Alkene Formula

Hydrocarbons in which one or more of the carbon atoms in the chain are bonded to another by a double or a triple bond are said to be unsaturated. The two families of unsaturated hydrocarbons are alkenes and alkynes.

Alkenes of hydrocarbons has one double bond between carbon atoms. Like the alkenes, their molecular formula increases by a fixed amount with each carbon atom added to the chain. A family of compounds that grows in such a way is called a homologous series. Alkenes bear the general formula CnH2n. Note that this is the same as the alkane formula, except that we subtract two hydrogen atoms to allow for the double bond.

IUPAC System of Alkenes

• The longest carbon chain containing the carbon-carbon double bond is selected as the parent alkene.
• The suffix ‘ane’ of the alkane is replaced by ‘ene’. If a double bond occurs twice or thrice in the parent chain the alkene is called diene or triene respectively.
• The position of double bonds or side chains indicated by numbers 1, 2, 3 etc.
• The longest chain is numbered from that end, which gives the lowest number to the carbon atom of the double bond and written just before the suffix ‘ene’. If while numbering the chain the double bond gets the same number from either side the carbon chain is numbered in such a manner that the substituent gets the lowest number.
• In case there are two or more double bonds, the lowest sum rule should be followed.
• The name and position of other groups (substituents) is indicated by prefixes.

Alkene Formula

Named alkenes are unsaturated molecules. This means they contain at least one double bond of carbon-carbon which displaces two atoms of hydrogen and, thus, alkenes do not have the maximum number of atoms of hydrogen per atom.

For the homologous series of alkenes, the general formula is CnH2n where n is the number of carbon atoms. Because alkenes are hydrocarbons, the alkene homologous series starts at ethene C2H4, with at least one carbon-carbon double bond.

The compounds are called alkanes, but with the end, “ene” rather than “ane.” Each successive molecule in the homologous series of alkenes is formed by adding to the preceding molecule one carbon and two hydrogen atoms or one CH2 (methylene group). Accordingly, the incremental change in relative molecular mass is 14.

Recommended Videos

Detail discussion on Alkanes Physical Property