Which of the following dihalides can be used to prepare the alkyne $C_3{H}_6$ by double dehydrohalogenation?

• A. $\mathrm{Cl}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Cl}$
• B. $\mathrm{Br}-{\mathrm{CH}}_3-{\mathrm{CH}}_3-\mathrm{Br}$
• C. $\mathrm{Br}-{\mathrm{CH}}_2-{\mathrm{CH}}_3-\mathrm{Br}$
• D. $\mathrm{Br}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Br}$

Answer: (C)

Verified step by step guidance

1. Identify the target alkyne: C\_3H\_6 corresponds to propyne, which has a three-carbon chain with a triple bond between two carbons.
2. Recall that alkynes can be prepared by double dehydrohalogenation of vicinal or geminal dihalides, where two equivalents of a strong base remove two molecules of HX (where X is a halogen) to form a triple bond.
3. Examine the given dihalides and determine which have two halogen atoms on adjacent carbons (vicinal dihalides), as these are suitable for double elimination to form an alkyne.
4. Note that the dihalide Br-CH\_2-CH\_3-Br is not vicinal (halogens are on carbons 1 and 3, which is not possible for a three-carbon chain), so it cannot form the alkyne by double elimination.
5. Focus on the dihalide Br-CH\_2-CH\_2-Br, which has bromines on adjacent carbons (carbon 1 and carbon 2), making it a vicinal dibromide suitable for double dehydrohalogenation to yield propyne.