Nomenclature of Alkane, Alkene and Alkyne

What is IUPAC Nomenclature?

The proliferation of organic chemistry gave rise to a number of organic compounds with broadly the same structural formula. To ensure that the nomenclature of such compounds was homogenised and was not at odds with one another, a set of rules were established by the International Union for Pure and Applied Chemistry. Thus, the nomenclature of alkanes, for instance, would differ based upon the IUPAC system. Let us look at the IUPAC names of alkane, alkene and alkyne along with the rules for naming such compounds.

IUPAC Nomenclature

The basic structure of the nomenclature system formulated by IUPAC rests on two factors. 

  1. That the name should give an indication of the number of carbon atoms that the compound in question has, thus being indicative of the structural features of the compound whether in the form of a lattice or a ring.

  2. Subsequently, the name should also indicate the presence of any functional groups that the compound consists of.

A special exception made in this regard is hydrogen. Since tetra-valency of carbon is consolidated by default with hydrogen, and hence, the amount and location of hydrogen do not need to be specified.

IUPAC Name of Alkanes

Alkanes are the most basic type of hydrocarbons which have no double or triple bond functional groups in the structures. Although they do not contain any functional groups, they form the basic framework upon which subsequent organic structure is built upon and are thereby also important when it comes to nomenclature rules. The fact that the given structure or formula is an alkane can easily be determined from the general structure CnH2n+2 and the suffix -ane in the name of the compound. Alkanes also have the highest hydrogen : carbon ratio thus making them stable and saturated hydrocarbons.

IUPAC Nomenclature of Alkanes 

  1. The longest and most continuous carbon chain is taken into consideration.

  2. The functional groups attached to this chain must be identified and named.

  3. The longest carbon chain identified now needs to be numbered, and the numbering should begin from the end, which is the nearest to a substituent group.

  4. Each of the substituent groups thus identified must be assigned the proper name. The number of the substituent group would depend upon the position of the carbon to which it has been attached.

  5. Construct the name of the compound, naming the groups in alphabetical order. In the case of multiple groups, the prefixes di, tri, etc. are also used though not necessarily in alphabetical order.

Examples of alkanes include Methane (CH4), Ethane (C2H6) and Propane (C3H8).

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IUPAC Nomenclature of Alkenes

Alkanes and alkynes can be classified as unsaturated hydrocarbons having double bond carbon-carbon and triple bond carbon-carbon functional groups respectively. Alkenes are identified primarily from the general structure CnH2n.

IUPAC Rules for Naming Alkenes

  1. The compound name should always end with the suffix -ene.

  2. The longest carbon chain that should be identified to name the compound should include the double bond in the compound along with both the carbons that share that bond.

  3. While numbering the carbons on the root chain, the numbering should begin from the carbon nearest to the double bond in the structure. If the double bond is at the middle of the compound, the nearest substituent rule must be employed.

  4. Between the two numbered carbon atoms sharing the double bond, the number which is lower must be considered while naming the bond. If there are two or more double bonds, then prefixes like diene, trine, etc. should be used, and the double bonds should be assigned a number.

Examples of alkenes include: Ethene (C2H4), Propene (C3H6)

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IUPAC Name of Alkynes

As already noted above, alkynes are a type of unsaturated hydrocarbons. However, unlike alkenes, they follow the broader structure of CnH2n-2.

IUPAC Nomenclature of Alkynes

  1. The compound name should always end with the suffix -yne.

  2. The longest carbon chain that should be identified to name the compound should include the triple bond in the compound along with both the carbons that share that bond.

Note: The other conventions and rules for the nomenclature of alkynes are exactly the same as with the alkenes, instead of a double bond, it is the triple bond which must be taken into consideration and used as the fulcrum for naming the compounds.

Examples of alkynes include: Ethyne (C2H2) and Octyne (C8H14)

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FAQ (Frequently Asked Questions)

1. How do you Identify an Alkene or an Alkyne?

Ans: While it is very easy to identify an alkane due to its relatively simple structure, the confusion usually is between identifying alkenes and alkynes. The fundamental difference between alkanes and alkenes are the presence of double bonds and triple bonds between carbon-carbon atoms, respectively. Alkene compounds end with the suffix -ene, and alkyne compounds end with the suffix -yne. 

When it comes to chemical tests, bromine water can also be used to differentiate between alkanes and alkenes. Since alkene contains double bonds between carbon-carbon atoms, the bromine water which is brown in colour turns colourless. This result can be observed on any unsaturated compounds with double bonds in them.

2. What are Some of the Rules you Must Follow while Writing the Names of the Compounds?

Ans: IUPAC observes certain naming conventions involving the nomenclature of alkenes, alkanes and alkynes. The basename of the compound is derived from the number of carbons present in the longest carbon chain. Among special characters, only dashes and commas are used. Commas are used while writing numbers and dashes are used for both letters and numbers. The name, however, should be written without any spaces.