This is because sodium acetylide will react with primary alkyl halides to give the corresponding alkylated product. If a secodary alkyl halide is used, elimination will be a competing reaction. And, if a tertiary alkyl halide is used, the elimination product with be the major product.
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CH3-CH2-CH2-CH2-CH2-CΞCH3 is the major product...
This is because sodium acetylide will react with primary alkyl halides to give the corresponding alkylated product. If a secodary alkyl halide is used, elimination will be a competing reaction. And, if a tertiary alkyl halide is used, the elimination product with be the major product.
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