Hydrocarbons · Question 1

Treatment of buta-1,3-diyne with NaNH2 (2 equivalents), followed by reaction with excess of trans- CH3-CH=CH-CH2-Br gives X as the major product. The maximum number of carbon atoms that are collinear (in a straight line) in X is .

Answer

6

Solution

[figure] [-2 NH3]2 NaNH2 [figure] (both terminal ≡C–H are acidic)

[figure] + 2 [figure] [-2 Br-]SN2 X

X = CH3-CH=CH-CH2-C C-C C-CH2-CH=CH-CH3, i.e. (2E,10E)-dodeca-2,10-diene-5,7-diyne.

The four sp carbons (C5–C8) and the two CH2 carbons (C4, C9) bonded to them lie on one straight line (180° at every sp carbon).

C3 and C10 are attached to the sp³ carbons C4 and C9 (109.5°), so they bend away from this line.

Maximum collinear carbons = 4 + 2 = 6

Answer: 6