Q.For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated :
Chain isomers arise from different carbon skeleton arrangements while keeping the same functional group. For (one double bond), we get three isomers; for (one triple bond), we get three isomers.
Understanding Chain and Position Isomerism
When a molecular formula can accommodate different structural arrangements, we encounter isomerism. For unsaturated hydrocarbons, two main types appear: chain isomers (different carbon skeletons) and position isomers (same skeleton, different functional group location). The double or triple bond can slide along the chain or the chain itself can branch, giving us distinct compounds with identical molecular formulas but different properties.
The degree of unsaturation helps us verify our formula. For , the degree is . Both our cases have degree 1, confirming one -bond.
(a) with one double bond
We need all alkenes with four carbons. Start with the longest chain, then systematically shorten it.
1. Four-carbon straight chain
The double bond can occupy two distinct positions. Placing it between gives but-1-ene; between gives but-2-ene. (A double bond at is identical to by renumbering.)
IUPAC name: But-1-ene
IUPAC name: But-2-ene
But-2-ene actually exists as cis and trans geometric isomers, but the question asks only for chain/position isomers, so we count it once.
2. Three-carbon chain with one methyl branch
With a three-carbon backbone, we can attach a methyl group. The only way to maintain with one double bond is to place the double bond at the first carbon and the methyl at the second carbon.
IUPAC name: 2-Methylprop-1-ene (or Isobutylene)
Any other arrangement either duplicates one of the above or violates the molecular formula.
A common mistake is writing , but this is identical to 2-methylprop-1-ene by renumbering from the other end.
(b) with one triple bond
Now we need all alkynes with five carbons.
1. Five-carbon straight chain
The triple bond can be positioned at two distinct locations.
IUPAC name: Pent-1-yne
IUPAC name: Pent-2-yne
(Pent-3-yne would be identical to pent-2-yne by renumbering.)
2. Four-carbon chain with one methyl branch
We can create a branched skeleton. The triple bond must be terminal (at ) because an internal triple bond on a four-carbon chain with a methyl substituent would require more than five carbons total or violate valency.
IUPAC name: 3-Methylbut-1-yne
For alkynes, internal triple bonds are more stable than terminal ones, but both are perfectly valid isomers. The question asks for all possibilities, not just the most stable.
Any attempt to place a methyl group elsewhere or use a three-carbon main chain either duplicates the above or creates a structure that doesn't satisfy .
Summary Table
| Molecular Formula | Isomer | IUPAC Name | Structure |
|---|---|---|---|
| 1 | But-1-ene | ||
| 2 | But-2-ene | ||
| 3 | 2-Methylprop-1-ene | ||
| 1 | Pent-1-yne | ||
| 2 | Pent-2-yne | ||
| 3 | 3-Methylbut-1-yne |
For (one double bond), the three isomers are but-1-ene, but-2-ene, and 2-methylprop-1-ene. For (one triple bond), the three isomers are pent-1-yne, pent-2-yne, and 3-methylbut-1-yne.
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