Q.Observe the following and answer the questions given below.
$CH_2=CH-\overset{..}{X}: \longleftrightarrow\ ^{\ominus}CH_2-CH=\overset{..}{X}{}^{\oplus}$
(the printed resonance pair of a vinylic halide, with a curved arrow on the left structure)
a. Name the type of halogen derivative
b. Comment on the bond length of C-X bond in it
c. Can react by SN1 mechanism? Justify your answer.

Step 1, sequence (a): 2-bromobutane to A. 2-Bromobutane heated with alcoholic KOH undergoes dehydrohalogenation (section 10.6.5); by Saytzeff's rule the major product is the more substituted alkene, but-2-ene: A = CH3-CH=CH-CH3.

Step 2, sequence (a): A to B. But-2-ene treated with Br2 undergoes simple electrophilic addition across the double bond (section 10.3.2), giving the vicinal dihalide: B = CH3-CHBr-CHBr-CH3 (2,3-dibromobutane).

Step 3, sequence (a): B to C. 2,3-Dibromobutane treated with NaNH2 (a strong, bulky base) undergoes double dehydrohalogenation (two successive eliminations), converting the vicinal dihalide all the way to an alkyne: C = CH3-C(triple bond)C-CH3 (but-2-yne). …

Applying, in sequence, the specific named reactions the reagents indicate: alc. KOH elimination (Saytzeff), Br2 addition, NaNH2 double elimination …

• Missing that NaNH2 performs TWO eliminations on the vicinal dibromide (not just one), which is what is needed to reach a full alkyne (C) rather than stopping at a monobromoalkene. …