Alkylation of Terminal Alkynes (Chain Lengthening)

Rarely Tested

$$\text{R--C}\equiv\text{C}^-\text{Na}^+ + \text{R'--X} \xrightarrow{S_N2} \text{R--C}\equiv\text{C--R'} + \text{NaX}$$

Concept: Sodium alkynide acts as a powerful carbon nucleophile that attacks an alkyl halide via an $$S_N2$$ pathway to form higher internal alkynes.

Constraint: Successful strictly with primary ($$1^\circ$$) alkyl halides or methyl halides. With secondary ($$2^\circ$$) or tertiary ($$3^\circ$$) halides, the strongly basic alkynide causes $$E2$$ elimination, yielding alkenes instead of substitution products.

Question 1

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

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