Backward and Forward Groups

Figure 14.3 Standing in Group 2A(2), looking backward to 1A(1) and forward to3A(13).

Figure 14.11 Standing in Group 4A(14), iooking backward to 3A(13) and forward to 5A(15).

Figure 14.16 Standing in Group 6A(16), looking backward to Group 5A(15) and forward to Group 7A(17).

We have now discussed Fischer esterification (formation of an ester in an acidic solution of an alcohol) and fhe hydrolysis of an ester in acidic water. When we discussed Fischer esterification, we pointed out that it is an equilibrium reaction. Ester hydrolysis in aqueous acid is also an equilibrium reaction. The two reactions proceed via the same nucleophilic addition/elimination mechanism, except that they are the reverse of each other. As first introduced in Section 10.6, the prmdple of microscopic reversibility states that for any reversible reaction, the sequence of intermediates and transition states must be the same but in reverse order for the backward versus forward reaction. In general, the reverse of protonation (Add a proton) is deprotonation (Take a proton away). The reverse of nucleophilic affack (Make a bond between a nucleophile and an electrophile) is leaving group departure (Break a bond to give stable molecules or ions). 

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