Write structures of different isomers corresponding to the 5 th member of alkyne series. Also write IUPAC names of all the isomers. What type of isomerism is exhibited by different pairs of isomers?

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> [!TLDR]
> The 5th alkyne is $\ce{C6H10}$ (hexyne). It has **seven** alkyne isomers — hex-1-yne, hex-2-yne, hex-3-yne, 3-methylpent-1-yne, 4-methylpent-1-yne, 4-methylpent-2-yne and 3,3-dimethylbut-1-yne — showing position isomerism (same skeleton, different triple-bond position) and chain isomerism (different skeletons).

### Identifying the 5th member

The alkyne homologous series ($\ce{C_nH_{2n-2}}$) runs ethyne ($\ce{C2H2}$), propyne ($\ce{C3H4}$), butyne ($\ce{C4H6}$), pentyne ($\ce{C5H8}$) — so the **5th member is $\ce{C6H10}$**, hexyne.

### Generating every alkyne isomer systematically

**Six-carbon straight chain.** The triple bond can sit at three distinct positions:
1. **Hex-1-yne** — $\ce{HC#C-CH2-CH2-CH2-CH3}$
2. **Hex-2-yne** — $\ce{CH3-C#C-CH2-CH2-CH3}$
3. **Hex-3-yne** — $\ce{CH3-CH2-C#C-CH2-CH3}$ (numbering from either end gives 3 — the symmetric case)

**Five-carbon chain with one methyl branch.** A triple-bonded carbon can hold only one substituent, so the methyl must sit on a saturated carbon:
4. **3-Methylpent-1-yne** — $\ce{HC#C-CH(CH3)-CH2-CH3}$
5. **4-Methylpent-1-yne** — $\ce{HC#C-CH2-CH(CH3)-CH3}$
6. **4-Methylpent-2-yne** — $\ce{CH3-C#C-CH(CH3)-CH3}$

**Four-carbon chain with two methyl branches.** Both methyls must go on the one available saturated interior carbon:
7. **3,3-Dimethylbut-1-yne** — $\ce{HC#C-C(CH3)2-CH3}$

No further skeleton works: a triple bond cannot be accommodated on a more highly branched four-carbon frame without exceeding carbon's four bonds.

### The isomerism shown by different pairs …

Isomer Name	Common Name	Structure (Condensed)	Boiling Point (°C)
Pentane	n-Pentane	$CH_3(CH_2)_3CH_3$	36.1
2-Methylbutane	Isopentane	$(CH_3)_2CHCH_2CH_3$	27.7
2,2-Dimethylpropane	Neopentane	$C(CH_3)_4$	9.5

Q.Write structures of different isomers corresponding to the 5 th member of alkyne series. Also write IUPAC names of all the isomers. What type of isomerism is exhibited by different pairs of isomers?

Concept understanding — Chain Isomerism

Chain Isomerism: The Skeleton of a Molecule

The Intuition

The Precise Statement

A Concrete Example: Pentane ( $C_5H_{12}$ )

When Does Chain Isomerism Occur?

Identifying the 5th member

Generating every alkyne isomer systematically

The isomerism shown by different pairs …

Structure	IUPAC name
$\ce{HC#C-CH2-CH2-CH2-CH3}$	Hex-1-yne
$\ce{CH3-C#C-CH2-CH2-CH3}$	Hex-2-yne
$\ce{CH3-CH2-C#C-CH2-CH3}$	Hex-3-yne
$\ce{HC#C-CH(CH3)-CH2-CH3}$	3-Methylpent-1-yne
$\ce{HC#C-CH2-CH(CH3)-CH3}$	4-Methylpent-1-yne
$\ce{CH3-C#C-CH(CH3)-CH3}$	4-Methylpent-2-yne
$\ce{HC#C-C(CH3)2-CH3}$	3,3-Dimethylbut-1-yne

Q.Write structures of different isomers corresponding to the 5 th member of alkyne series. Also write IUPAC names of all the isomers. What type of isomerism is exhibited by different pairs of isomers?

Identifying the 5th member

Generating every alkyne isomer systematically

The isomerism shown by different pairs …

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