Complete the following reactions with appropriate structures...

Complete the following reactions with appropriate structures of products/reagents. $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2} \stackrel{\mathrm{Br}_{2}}{\longrightarrow}[\mathrm{A}]$ $$ \frac{\text { (i) } \mathrm{NaNH}_{2}(3.0 \text { equiv.) }}{\text { (ii) } \mathrm{CH}_{3} \mathrm{I}}[\mathrm{B}] $$

Complete the following reactions with appropriate structures of products/reagents. $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2} \stackrel{\mathrm{Br}_{2}}{\longrightarrow}[\mathrm{A}]$
$$
\frac{\text { (i) } \mathrm{NaNH}_{2}(3.0 \text { equiv.) }}{\text { (ii) } \mathrm{CH}_{3} \mathrm{I}}[\mathrm{B}]
$$

Key Concepts

Electrophilic Addition of Halogens to Alkenes

This reaction involves the attack of an alkene's ? electrons by a halogen molecule (such as Br2), leading to the formation of a cyclic halonium ion intermediate. Subsequent backside attack by a halide ion opens the ring, resulting in a vicinal dihalide. This is a classical transformation in organic chemistry used to functionalize alkenes with halogen atoms.

Dehydrohalogenation and Alkyne Formation

Vicinal dihalides can be converted into alkynes through the sequential elimination of two molecules of hydrogen halide. A strong base, like sodium amide (NaNH2), abstracts acidic protons adjacent to the halogen atoms, resulting in the elimination of bromide ions and formation of a triple bond. This method is fundamental in the synthesis of alkynes from halogenated precursors.

Acidity of Terminal Alkynes and Formation of Alkynyl Anions

Terminal alkynes have acidic hydrogen atoms due to the high s-character of the sp-hybridized carbon, which stabilizes the resulting negative charge upon deprotonation. When treated with strong bases such as NaNH2, these terminal alkynes are deprotonated to form alkynyl anions, which are powerful nucleophiles in organic synthesis.

Nucleophilic Substitution (SN2) for Alkylation of Alkynes

The alkynyl anion, formed from deprotonation of a terminal alkyne, can serve as a nucleophile and attack electrophilic carbons in alkyl halides, typically via an SN2 mechanism. This reaction leads to the alkylation of the alkyne, introducing new carbon-carbon bonds. It is an important strategy for the construction of more complex carbon skeletons from simple alkyne precursors.

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