Alkoxymercuration Mechanism 

1.0Introduction 

• Alkoxymercuration is a key reaction in organic chemistry where an alkene reacts with an alcohol in the presence of mercuric salts to form an ether.
• It is closely related to oxymercuration, but instead of water as the nucleophile, an alcohol is used.
• This reaction is highly regioselective and follows Markovnikov’s rule without carbocation rearrangements.

2.0Introduction

Alkoxymercuration is the addition of an alcohol (ROH) and Hg(OAc)₂ across a double bond of an alkene, forming an alkoxymercurial intermediate, which, upon reduction, gives an ether.

General Reaction:

$RCH=C{H}_2+Hg(OAc{)}_2+ROH\to RCH(OR)-C{H}_{2}HgOAc$

Followed by reduction:

$RCH(OR)-C{H}_{2}HgOAc\stackrel{NaB{H}_4}{\to }RCH(OR)-C{H}_3$

3.0General Reaction of Alkoxymercuration

Alkene + Alcohol + Hg(OAc)₂ → Alkoxymercurial intermediate → Ether (on reduction with NaBH₄).

Example: Propene + Methanol in the presence of Hg(OAc)₂ → Isopropyl methyl ether.

4.0Stepwise Mechanism of Alkoxymercuration

The mechanism occurs in two main steps:

Step 1: Formation of Mercurinium Ion

• The alkene reacts with Hg(OAc)₂.
• A cyclic mercurinium ion intermediate is formed.
• This avoids free carbocation formation → prevents rearrangement.

Step 2: Nucleophilic Attack by Alcohol

• The alcohol molecule (ROH) attacks the more substituted carbon of the mercurinium ion.
• This follows Markovnikov’s rule, where the –OR group attaches to the more substituted carbon.

Step 3: Formation of Alkoxymercurial Intermediate

• The product is R–CH(OR)–CH₂HgOAc.

Step 4: Reduction with NaBH₄

• On treatment with sodium borohydride (NaBH₄), the –HgOAc group is replaced by –H.
• Final product: Ether (R–CH(OR)–CH₃).

5.0Regioselectivity in Alkoxymercuration (Markovnikov’s Rule)

1. The reaction follows Markovnikov’s orientation:

• The –OR group attaches to the more substituted carbon atom.
• The –H (from reduction) attaches to the less substituted carbon atom.

2. No carbocation rearrangement occurs due to the cyclic mercurinium ion intermediate.

6.0Importance of Alkoxymercuration in Organic Chemistry

• Provides a reliable method for ether synthesis without rearrangements.
• Useful in preparation of alkoxy alkanes from alkenes.
• Highlights the concept of electrophilic addition reactions for JEE aspirants.
• Avoids issues of direct acid-catalyzed etherification (which often causes rearrangements and poor selectivity).