Advices For Studying Organic Chemistry

Zn + Rδ+ Xδ− R−Zn2+ X− R H ++ 2

Reducing agent Alkylzinc halide Alkane

HX Zn2+ X−

4.18C ALKYLATION OF TERMINAL ALKYNES

1. Terminal alkyne: an alkyne with a hydrogen attached to a triply bonded carbon.
   
   
   1. The acetylenic hydrogen is weakly acidic (pKa ~ 25) and can be removed with a
      strong base (e.g. NaNH 2 ) to give an anion (called an alkynide anion or
      acetylide ion).
   
   
   
   


2. Alkylation: the formation of a new C—C bond by replacing a leaving group on
   an electrophile with a nucleophile.
   General Reaction

R CCH (NaNH−NH^2

3 )

R CC−Na+ (R'−NaXX) R CCR'

An alkyne Sodium amide An alkynide anion R' must be methyl or 1° and
unbranched at the second carbon

Specific Examples

H CCH (NaNH−NH^2

3 )

H CC−Na+ H(−^3 NaXC X) H CCCH 3

Ethyne Ethynide anion Propyne
(acetylene) (acetylide anion) 84%

3. The alkyl halide used with the alkynide anion must be methyl or primary and
   also unbranched at its second (beta) carbon.

1. Alkyl halides that are 2° or 3°, or are 1° with branching at the beta carbon,
   undergo elimination reaction predominantly.

4. After alkylation, the alkyne triple bond can be used in other reactions:
   
   
   1. It would not work to use propyne and 2-bromopropane for the alkylation step of
      this synthesis.