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Difficulty: HardTetrahedral Carbon, Bonding, and Hybridization

Consider the unsaturated hydrocarbon 2-methylbut-1-en-3-yne, which has the condensed structural formula CH2=C(CH3)C≡CHCH_2=C(CH_3)C\equiv CH. What is the total number of sigma (σ\sigma) bonds present in one molecule of this compound?

Answer: 10 / 10 sigma bonds / 10 bonds

Answer

The total number of sigma (σ\sigma) bonds present in one molecule of 2-methylbut-1-en-3-yne is 10.
In 2-methylbut-1-en-3-yne (CH2=C(CH3)C≡CHCH_2=C(CH_3)C\equiv CH), there are 6 carbon-hydrogen single bonds (2 from C1, 3 from the methyl group, and 1 from C4) and 4 carbon-carbon sigma bonds (1 from the C1=C2 double bond, 1 connecting C2 to the methyl carbon, 1 connecting C2 to C3, and 1 from the C3\equiv C4 triple bond). Summing these gives 10 sigma bonds in total.

Step-by-Step Solution

1
Expand the condensed structural formula to identify all individual carbon-hydrogen (C-H) bonds.
The =CH2=CH_2 group contains 2 C-H σ\sigma bonds, the methyl group (−CH3-CH_3) contains 3 C-H σ\sigma bonds, and the terminal alkynyl group (≡CH\equiv CH) contains 1 C-H σ\sigma bond, giving a total of 6 C-H σ\sigma bonds.
Every single bond between carbon and hydrogen is a single sigma bond.
2
Identify all carbon-carbon (C-C) sigma bonds in the backbone and side chain.
The C=CC=C double bond contributes 1 C-C σ\sigma bond, the single bond to the methyl branch contributes 1 C-C σ\sigma bond, the C2-C3 single bond contributes 1 C-C σ\sigma bond, and the C≡CC\equiv C triple bond contributes 1 C-C σ\sigma bond, giving a total of 4 C-C σ\sigma bonds.
Multiple bonds (double or triple) contain exactly one sigma bond each, with the remaining bonds being pi (π\pi) bonds.
3
Sum the total number of C-H and C-C sigma bonds.
6 (C-H σ bonds)+4 (C-C σ bonds)=10 total σ bonds6 \text{ (C-H } \sigma\text{ bonds)} + 4 \text{ (C-C } \sigma\text{ bonds)} = 10 \text{ total } \sigma \text{ bonds}.
Adding all localized σ\sigma bonds yields the total count for the molecule.

Key Concept

Determination of sigma (\sigma) and pi (\pi) bond counts in complex open-chain hydrocarbons
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Topics

Tetrahedral Carbon, Bonding, and Hybridization

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