Question

13 multiple answer 2 points which of the following are commonly used to synthesize alkenes? (select all that apply) dehydrohalogenation of alkyl halides. addition of hydrogen halides to alkynes. hydrogenation of alkenes. dehydration of alcohols.

Explanation:

• Dehydrohalogenation of alkyl halides: Alkyl halides undergo elimination reactions (dehydrohalogenation) in the presence of a base (e.g., alcoholic KOH) to form alkenes. The general reaction is \(R - CH_2 - CH_2 - X+KOH(alc)

ightarrow R - CH = CH_2+KX + H_2O\) (where \(X\) is a halogen).

• Addition of hydrogen halides to alkynes: This reaction leads to the formation of alkenyl halides (in the case of partial addition) or alkyl di - halides (in the case of complete addition), not alkenes. For example, \(CH\equiv CH+HX

ightarrow CH_2 = CHX\) (partial addition) and \(CH\equiv CH + 2HX
ightarrow CH_3 - CHX_2\) (complete addition).

• Hydrogenation of alkenes: This is a reduction reaction where alkenes react with hydrogen in the presence of a catalyst (e.g., \(Pd, Pt, Ni\)) to form alkanes. The reaction is \(R - CH = CH_2+H_2\xrightarrow{cat}R - CH_2 - CH_3\).
• Dehydration of alcohols: Alcohols undergo elimination reactions (dehydration) in the presence of concentrated \(H_2SO_4\) or \(H_3PO_4\) at an appropriate temperature to form alkenes. For example, \(CH_3 - CH_2 - OH\xrightarrow{conc.H_2SO_4, 170^{\circ}C}CH_2 = CH_2+H_2O\).

Answer:

A. Dehydrohalogenation of alkyl halides, D. Dehydration of alcohols