Exercise 12: - Nucleophilic substitution reactions of alkyl halides

  1.  
  1. Select the member of each pair of compounds that will react faster by an SN2 mechanism:

 

                      i.        CH3CH2CH2CH2F or CH3CH2CH2CH2OCH3

 

CH3CH2CH2CH2F

 

                    ii.        CH3CH2CH2CH2Cl or CH3CH2CH2CH2F

 

CH3CH2CH2CH2Cl

 

  1. Which SN2 reaction of each pair would you expect to take place more readily in a protic solvent?

 

                      i.        CH3CH2CH2Cl + (C6H5)3N ® CH3CH2CH2N(C6H5)3+ + Cl-

or

CH3CH2CH2Cl + (C6H5)3P  ® CH3CH2CH2P(C6H5)3+  + Cl-

                                CH3CH2CH2Cl + (C6H5)3P  ® CH3CH2CH2P(C6H5)3+  + Cl-

                    ii.        CH3CH2CH2Cl + CH3CH2S- ® CH3CH2CH2SCH2CH3 + Cl-

or

CH3CH2CH2Cl + CH3CH2SH ® CH3CH2CH2SCH2CH3 + HCl

                        CH3CH2CH2Cl + CH3CH2S- ® CH3CH2CH2SCH2CH3 + Cl-

  1. Which SN1 reaction of each pair would you expect to take place more rapidly?

 

                      i.        (CH3)3CBr + H2O ® (CH3)3COH + HBr

or

CH3CHBrCH3 + H2O ® CH3CHOHCH3 + HBr

                        (CH3)3CBr + H2O ® (CH3)3COH + HBr

                    ii.        (CH3)3CBr + H2O ® (CH3)3COH + HBr

or

(CH3)3CCl + H2O ® (CH3)3COH + HCl

(CH3)3CBr + H2O ® (CH3)3COH + HBr

  1. 1-Chlorobutane reacts with 0.01 M NaCN in ethanol to give predominantly CH3CH2CH2CH2CN whereas under the same conditions (CH3)­3­CCl  reacts to give (CH3)3COCH2CH3. Propose mechanisms to explain these reactions.

 

  1. When 2-propanol is heated in the presence of H2SO4, a possible product is diisopropyl ether, (CH3)2CH-O-CH(CH3)2.

 

a.    To what type of reaction does this belong?

Nucleophilic substitution

b.    Propose a possible mechanism(s).

  1. The SN2 reaction can occur intramolecularly.  What product would you expect from treatment of 4-bromo-1-butanol with sodium methoxide?

 

 

  1. (S)-2-butanol slowly racemizes in dilute acid solution.  Propose a mechanism.

 

  1. SN2 reactions take place with inversion of configuration whereas racemization occurs in SN1 reactions.  Explain why retention of configuration occurs in the following reaction:

 

 

  1. Choline, a constituent of phospholipids, has the formula C5H15O2N.  It dissolves in water to form a strongly basic solution.  It can be prepared by the reaction of oxacyclopropane with trimethylamine, (CH3)3N, in the presence of water.  What is its structure?