Organic Reactions

The reaction mechanisms

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HYDROXIDE-ION-PROMOTED ESTER HYDROLYSIS

The rate of hydrolysis of an ester can be increased by hydroxide ion. Like an acid catalyst, hydroxide ion increases the rates of the two slow steps of the reaction—namely, formation of the tetrahedral intermediate and collapse of the tetrahedral intermediate.

MECHANISM FOR THE HYDROXIDE-ION-PROMOTED HYDROLYSIS OF AN ESTER

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ACID-CATALYZED ESTER HYDROLYSIS

Hydrolysis of An Ester with a Tertiary Alkyl Group

The hydrolysis of an ester with a tertiary alkyl group is an SN1 reaction rather than a nucleophilic addition–elimination reaction, because the carboxylic acid leaves behind a relatively stable tertiary carbocation.

MECHANISM FOR THE HYDROLYSIS OF AN ESTER WITH A TERTIARY ALKYL GROUP

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ACID-CATALYZED ESTER HYDROLYSIS

Hydrolysis of An Ester with a Primary or Secondary Alkyl Group

When an acid is added to a reaction, the first thing that happens is the acid protonates the atom in the reactant that has the greatest electron density. Therefore, when an acid is added to an ester, the acid protonates the carbonyl oxygen.

MECHANISM FOR ACID-CATALYZED ESTER HYDROLYSIS

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THE REACTIONS OF ACYL CHLORIDES

Acyl chlorides react with alcohols to form esters, with water to form carboxylic acids, and with amines to form amides because, in each case, the incoming nucleophile is a stronger base than the departing halide ion.

MECHANISM FOR THE REACTION OF AN ACYL CHLORIDE WITH AN ALCOHOL

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Epoxidation of an Alkene

An alkene can be converted to an epoxide by a peroxyacid.An epoxide is an ether in which the oxygen is incorporated into a three-membered ring, and a peroxyacid is a carboxylic acid with an extra oxygen.

MECHANISM FOR THE EPOXIDATION OF AN ALKENE

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Hydroboration

Another way to convert an alkene to an alcohol is by two successive reactions known as hydroboration–oxidation.

As the following reactions show, the alcohol formed from hydroboration–oxidation of an alkene has the H and OH groups switched compared to the alcohol formed from the acid-catalyzed addition of water.

MECHANISM FOR HYDROBORATION WITH BH3

As boron (an electrophile) accepts the

π electrons from the alkene and forms a bond with one sp2 carbon, it gives a hydride ion (a nucleophile) to the other sp2 carbon.

Hydroboration is an example of a concerted reaction.

MECHANISM FOR HYDROBORATION WITH R2BH

When the hydroboration reaction is over, an aqueous solution of sodium hydroxide and hydrogen peroxide (HOOH) is added to the reaction mixture in order to replace BR2 with an OH group.

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Ozonolysis

When an alkene is treated with ozone (O3) at a low temperature, both the s and p bonds of the double bond break and the carbons that were doubly bonded to each other are now doubly bonded to oxygens instead. This is an oxidation reaction—called ozonolysis — because the number of C— O bonds increases.

Reaction Mechanism

The molozonide is unstable because it has two O—O bonds; it immediately rearranges to a more stable ozonide.

Because ozonides are explosive, they are not isolated. Instead, they are immediately conver ted to ketones and/or aldehydes by dimethyl sulfide (CH3-S-CH3 ) or zinc in acetic acid (CH3CO2H).