Nomenclature and Isomerism

Naming Alkynes: Two Systems

In the older common system, alkynes are named as derivatives of acetylene ($\mathrm{HC \equiv CH}$). For example, $\mathrm{CH_3C \equiv CH}$ is called methylacetylene. This system is simple but becomes unwieldy for larger molecules.

The modern IUPAC system is systematic and unambiguous. Alkynes are named as derivatives of the corresponding alkanes. The suffix '-ane' of the alkane is replaced by '-yne'. The position of the triple bond is indicated by the number of the first triply bonded carbon atom. The numbering of the parent chain is done so that the triple bond gets the lowest possible number.

The table below gives the common and IUPAC names for the first few members of the alkyne series.

Isomerism in Alkynes

The first two members of the alkyne series, ethyne ($\mathrm{C_2H_2}$) and propyne ($\mathrm{C_3H_4}$), have only one possible structure each. However, for the next homologue, butyne ($\mathrm{C_4H_6}$), two distinct structures are possible.

Position Isomerism: The two structures of butyne differ in the location of the triple bond along the carbon chain.

• But-1-yne: $\mathrm{HC \equiv C - CH_2 - CH_3}$ (triple bond between C1 and C2)
• But-2-yne: $\mathrm{H_3C - C \equiv C - CH_3}$ (triple bond between C2 and C3)

Because these compounds have the same molecular formula but differ in the position of the functional group (the triple bond), they are called position isomers.

Isomers of Pentyne ($\mathrm{C_5H_8}$)

For the next homologue, pentyne ($\mathrm{C_5H_8}$), we can arrange the five carbon atoms in different ways, leading to both position and chain isomers. The three possible structures are:

I. $\mathrm{HC \equiv C - CH_2 - CH_2 - CH_3}$ — Pent-1-yne

II. $\mathrm{H_3C - C \equiv C - CH_2 - CH_3}$ — Pent-2-yne

III. $\mathrm{H_3C - CH - C \equiv CH}$ — 3-Methylbut-1-yne

$\hspace{2.5cm} |$

$\hspace{2.5cm} \mathrm{CH_3}$

• Structures I and II are position isomers because they have the same carbon skeleton (a straight chain of five carbons) but the triple bond is in a different position.
• Structures I and III (or II and III) are chain isomers because they have different carbon skeletons. Structure III has a branched chain (a methyl group attached to the third carbon), while Structures I and II have a continuous, unbranched chain.

Problem 9.13: Isomers of the 5th Member of the Alkyne Series

The 5th member of the alkyne series has the molecular formula $\mathrm{C_6H_{10}}$. The possible structural isomers are:

(a) $\mathrm{HC \equiv C - CH_2 - CH_2 - CH_2 - CH_3}$ — Hex-1-yne

(b) $\mathrm{CH_3 - C \equiv C - CH_2 - CH_2 - CH_3}$ — Hex-2-yne

(c) $\mathrm{CH_3 - CH_2 - C \equiv C - CH_2 - CH_3}$ — Hex-3-yne

(d) $\mathrm{HC \equiv C - CH - CH_2 - CH_3}$ — 3-Methylpent-1-yne

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$\hspace{2.5cm} \mathrm{CH_3}$

(e) $\mathrm{HC \equiv C - CH_2 - CH - CH_3}$ — 4-Methylpent-1-yne

$\hspace{2.5cm} |$

$\hspace{2.5cm} \mathrm{CH_3}$

(f) $\mathrm{H_3C - C \equiv C - CH - CH_3}$ — 4-Methylpent-2-yne

$\hspace{2.5cm} |$

$\hspace{2.5cm} \mathrm{CH_3}$

(g) $\mathrm{HC \equiv C - C - CH_3}$ — 3,3-Dimethylbut-1-yne

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$\hspace{2.5cm} \mathrm{CH_3}$

Type of Isomerism: …