Elimination reactions, comparison leaving groups

Comparison of SN2 and acyl addition-elimination reactions with methoxide as the leaving group. In the concerted SN2, methoxide leaves in a slightly endothermic step, and the bond to methoxide is largely broken in the transition state. In the acyl substitution, methoxide leaves in an exothermic second step with a reactant-like transition state The bond to methoxide has just begun to break in the transition state. 

The last two trends are particularly relevant to the elimination reactions of the 2-hexyl halides. As indicated by the data in Table 10.7, leaving group ability in methanol solution follows the trend I > Br > Cl > F. In comparison with the other 2-hexyl halides, therefore, the transition structure for 2-hexyl fluoride should have relatively more carbanion character. Therefore, the stabilities of the developing carbon-carbon double bonds are less important in determining values of AG for 2-hexyl fluoride than is the case for the other 2-hexyl halides. ° ... 

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