11.4 : אתרים מאלקנים: תוספת כוהל ו-Alkoxymercuration-Demercuration

In addition to the alcohol dehydration and the Williamson ether synthesis pathways, ethers can also be prepared from alkenes through the acid-catalyzed addition of alcohols and the alkoxymercuration–demercuration methods.

The acid-catalyzed addition of alcohols to alkenes involves treating an alkene with an excess of alcohol in the presence of an acid catalyst to form an ether under suitable conditions.

For example, 2-methylpropene and methanol, when passed over an acid catalyst, give 2-methoxy-2-methylpropane.

The reaction begins with a proton transfer from the acid catalyst to the alkene's π bond, forming a carbocation intermediate.

Next, the carbocation acts as an electrophile and reacts with the nucleophilic oxygen of a methanol molecule, forming an oxonium ion.

Finally, proton transfer from the oxonium ion to the methanol solvent completes the reaction, forming the ether.

Another method for the preparation of ethers is the alkoxymercuration–demercuration of alkenes.

Alkoxymercuration is a two-step process to produce ethers by reacting an alcohol with an alkene in the presence of a mercury salt, such as mercuric acetate, followed by demercuration with sodium borohydride.

The reaction is similar to the oxymercuration reaction, but differs in the use of alcohol instead of water.

Mechanistically, alkoxymercuration–demercuration is an electrophilic addition reaction that proceeds in Markovnikov's manner and is anti-addition.

Consider the synthesis of 2-methoxypropane from propene.

The reaction begins with the electrophilic attack of mercuric acetate on the π bond of propene, resulting in the formation of a bridged mercurinium ion intermediate.

Next, the methanol molecule acts as a nucleophile and attacks the more-substituted carbon of the cyclic mercurinium ion and opens it, followed by deprotonation to form an organomercury intermediate.

Lastly, sodium borohydride acts as a reducing agent and substitutes the mercury acetate with a hydride to produce ether.

Overall, the net result is the Markovnikov addition of alcohol across the double bond of an alkene.