Q.Convert: 1-Bromobutane to Hex-1-yne.

Step 1. Recognise the carbon-count problem. Hex-1-yne (HC(triple bond)C-CH2CH2CH2CH3) has 6 carbons, but 1-bromobutane (CH3CH2CH2CH2Br) supplies only 4. The remaining 2 carbons, plus the terminal triple bond itself, must come from a second building block.

Step 2. Use the terminal-alkyne alkylation method (section 15.3.2). That method reacts a metal acetylide (from a terminal alkyne) with a PRIMARY alkyl halide to extend the chain at the metal-bearing carbon, while the other end of the alkyne keeps its terminal hydrogen. Starting from ethyne itself (the smallest possible terminal alkyne, 2 carbons) supplies exactly the missing 2 carbons and the terminal C-triple-bond-C-H unit that the final product needs.

Step 3. Deprotonate ethyne. Ethyne reacts with lithium amide: H-C(triple bond)C-H + LiNH2 -> H-C(triple bond)C-Li (lithium ethynide) + NH3. …

Counting carbons to see 1-bromobutane alone cannot supply all 6, then using it as the primary-alkyl-halide alkylating agent on lithium ethynide ( …

• Trying to build hex-1-yne FROM 1-bromobutane alone by elimination -- 1-bromobutane has no second halogen and cannot be turned into a 6-carbon alkyne without an added 2-carbon unit. …