Question

Treatment of buta-\(1,3\)-diyne with \(\mathrm{NaNH_2}\) (2 equivalents), followed by reaction with excess of trans-\(\mathrm{CH_3-CH=CH-CH_2-Br}\) gives \(X\) as the major product. The maximum number of carbon atoms that are collinear (in a straight line) in \(X\) is ____.

Hint:

Remember: \[ sp \text{-hybridized carbons} \] have bond angle: \[ 180^\circ \] Hence long chains containing consecutive: \[ \mathrm{C{\equiv}C} \] units tend to become linear. While counting collinear atoms:
• identify all \(sp\)-hybridized carbons
• extend the straight line as far as geometry permits

Answer 1

Correct Answer: 8

Concept: Carbon atoms connected through triple bonds possess: \[ sp \text{-hybridization} \] An \(sp\)-hybridized carbon has bond angle: \[ 180^\circ \] Thus atoms connected through continuous \(sp\)-hybridized systems become collinear. The problem asks for the maximum number of carbon atoms lying in a straight line in the final product.

Step 1: Understanding the starting compound. Buta-\(1,3\)-diyne is: \[ \mathrm{HC{\equiv}C-C{\equiv}CH} \] It contains terminal acidic hydrogens on both ends. Treatment with \(2\) equivalents of \(\mathrm{NaNH_2}\) removes both acidic hydrogens: \[ \mathrm{^-C{\equiv}C-C{\equiv}C^-} \] Thus a dianion is formed.

Step 2: Reaction with allylic bromide. The reagent is: \[ \mathrm{trans-CH_3-CH=CH-CH_2Br} \] This undergoes nucleophilic substitution at: \[ \mathrm{CH_2Br} \] Both terminal acetylide ions react with bromide molecules. Hence final product becomes: \[ \mathrm{CH_3-CH=CH-CH_2-C{\equiv}C-C{\equiv}C-CH_2-CH=CH-CH_3} \]

Step 3: Finding maximum collinear carbon atoms. The central portion: \[ \mathrm{-CH_2-C{\equiv}C-C{\equiv}C-CH_2-} \] contains consecutive \(sp\)-hybridized carbons. All carbons connected through the diyne system remain linear. Now count the carbons lying along the same straight line: \[ \mathrm{CH_2-C{\equiv}C-C{\equiv}C-CH_2} \] This contributes: \[ 6 \text{ carbons} \] Additionally, because the alkene substituents are trans arranged, one carbon from each side can align linearly with the central chain. Thus total maximum collinear carbons become: \[ 8 \] Final Answer: \[ \boxed{8} \]