Organic chemistry/Mechanisms

Terminology

Mechanisms are a set of figures that uses arrows to indicate electron flows inside a molecule or between molecules. The arrow starts from the nucleophile and points towards the electrophile. In acid-base reactions, for instance, they start from the base and point towards acidic hydrogen or Lewis acid.

Examples

Unimolecular Elimination (E1) Mechanism:

       X*                H
       |                 |  (+)       base           
 H3C — C — CH3 --->  H — C — C — CH3 ------> H — C == C — CH3 + H2O
       |                 |   |                        |    |
       CH3               H   CH3                      H    H

Bimolecular Elimination (E2) Mechanism:

     X   H
     |   |       strong base
 H — C — C — H  -------------> H — C == C — H
     |   |                         |    | 
     H   H                         H    H

Unimolecular Substitution (S_N1) Mechanism:

Ex: Solvolysis                                    H
                                                  |
       X                       H              H — O(+)H                    OH
       |                  (+)  |      H2O         |   |                    |
 H3C — C — CH3 --->  H3C — C — C — H -----> H3C — C — C — H  ------> H3C — C — CH3
       |                   |   |                  |   |       -(H+)        |
       CH3                 CH3 H                  CH3 H                    CH3

Note: SN1 reactions produce racemates due to the achiral carbocation intermediate.

Bimolecular Substitution (S_N2) Mechanism:

    X                                                  H
    |        good nucleophile (I, BR, Cl, OH...)       |
H — C — CH3 -------------------------------------> H — C — CH3
    |                                                  |
    H                                                  I
Note: SN2 reactions undergo inversion of configuration.

• represents any good leaving group (i.e: Cl, Br, I, TsO, etc...)