In Chapter 8 we shall look in detail at acids and bases, but at this point we need to tell you about one of their important roles in chemistry: they act as catalysts for a number of carbonyl addition reactions, among them hemiacetal and hydrate formation. To see why, we need to look back at the mechanisms of hemiacetal formation on p. 138 and hydrate formation on p. 134. Both involve proton-transfer steps, which we can choose to draw like this:

In the fi rst proton-transfer step, ethanol acts as a base, removing a proton; in the second it acts as an acid, donating a proton. You saw in Chapter 5 how water can also act as an acid or a base. Strong acids or strong bases (for example HCl or NaOH) increase the rate of hemiacetal or hydrate formation because they allow these proton-transfer steps to occur before the addition to the carbonyl group. In acid (dilute HCl, say), the mechanism is different in detail. The fi rst step is now protonation of the carbonyl group’s lone pair: the positive charge makes it much more electrophilic so the addition reaction is faster. Notice how the proton added at the beginning is lost again at the end—it is really a catalyst.

The mechanism in basic solution is slightly different again. The fi rst step is now deprotonation of the ethanol by hydroxide, which makes the addition reaction faster by making the ethanol more nucleophilic. Again, base (hydroxide) is regenerated in the last step, making the overall reaction catalytic in base.

The final step could equally well involve deprotonation of ethanol to give alkoxide—and alkoxide could equally well do the job of catalysing the reaction. In fact, you will often come across mechanisms with the base represented just as ‘B−’ because it doesn’t matter what the base is.

●For nucleophilic additions to carbonyl groups:

• acid catalysts work by making the carbonyl group more electrophilic

• base catalysts work by making the nucleophile more nucleophilic

• both types of catalysts are regenerated at the end of the reaction.

مواضيع ذات صلة

The molecular orbitals of butadiene

Conjugation

Multiple double bonds not in a ring

Molecules with more than one C=C double bond Benzene has three strongly interacting double bonds

The structure of ethene (ethylene, CH2=CH2)

Bisulfite addition compounds

Ketones also form hemiacetals

Hemiacetals from reaction of alcohols with aldehydes and ketones

Addition of water to aldehydes and ketones

Addition of organometallic reagents to aldehydes and ketones

Nucleophilic attack by ‘hydride’ on aldehydes and ketones

The angle of nucleophilic attack on aldehydes and ketones