organic chem question?

Question

I have a 4 carbon chain and there is a Br on the 3rd + 4th carbon and I need to know steps will produce a 6 carbon chain with a double bond from the 4th to the 5th carbon.

Answer 1

Starting material:
CH3CH2CHBrCH2Br ?

Product wanted:
CH3CH2CH2CH=CHCH3 ? Cis or Trans?

If the starting material is 1,2 Dibromobutane, then treating it with base (eg. NaNH2) will give:

CH3CH2CHBr–CH2Br + base ——> CH3CH2CH=CHBr + base ——> CH3CH2C≡CH

If this is done with excess NaNH2 in liquid ammonia or dry diethyl ether, then the sodium salt will be produced:

(CH3CH2C≡C)- Na+

(CH3CH2C≡C)- Na+ + BrCH2CH3 ——> CH3CH2C≡CCH2CH3

CH3CH2C≡CCH2CH3 when heated gives the more stable compound CH3CH2CH2C≡CCH3 (this is 2-hexyne). The path will be different from here depending on if you want a cis or trans product. A controlled catalytic addition (Lindlar catalyst) of H2 will give the Cis form. Na in liquid ammonia will give the Trans.

There might be an easier way, but this path is a classic.