Meet Vipul Singh: AIEEE (AIR-1,2010), IITJEE (AIR-5, 2010)

Meet Vipul Singh: AIEEE (AIR-1,2010), IITJEE (AIR-5, 2010)

Crossed Claisen Condensation

Crossed Claisen condensation is a variation of Claisen condensation.

Claisen Condensation:

Crossed Claisen Condensation:

eg:

mechanism:
Step 1:  The alkoxide ion deprotanates the enolizable ester reversibly.

Step 2 and 3:  Enolate ion 1 undergoes a nucleophilic acyl substitution preferentially with the non-enolizable ester, which has the sterically less hindered and, therefore, more accessible carbonyl carbon, giving a  β-ketoester.

Step 4:  The alkoxide ion deprotonates the β-ketoester irreversibly.

Step 5:  The acid protonates enolate ion 2

Wittig Reaction

The Wittig reaction is the reaction of a Wittig reagent with an aldehyde or a ketone to afford an alkene as the organic product.

eg:

Mechanism:

Williamson Ether Synthesis

Williamson ether synthesis is a method of preparation of ethers.  It is a nucleophilic aliphatic substitution at saturated carbon in which thenucleophile is either an alkoxide ion or a phenoxide ion.

R¹ = alkyl, allylic, benzylic

R² = alkyl, aryl

eg:

Some symmetrical ethers can not be prepared using Williamson either synthesis because, in addition to nucleophilic aliphatic substitution,1,2-elimination could occur between a substrate bearing beta hydrogens and an alkoxide ion, leading to an alkene as the organic product.

eg:

To prepare ether 1 using Williamson ether synthesis, a tert-butyl substrate (2) should be reacted with the tert-butoxide ion (3).

Since alkoxide ions are strongly basic and since the substrate is a tertiary alkyl substrate, the major reaction between 2 and 3 would be 1,2-elimination, giving alkene 4 as the organic product.

Preparation of unsymmetrical ethers using Williamson ether synthesis requires planning because, again, in addition to nucleophilic aliphatic substitution, 1,2-elimination could occur between some substrates and alkoxide ion and could be the dominant process.

eg:

Wagner-Meerwein Rearrangement

A Wagner-Meerwein rearrangement is any reaction in which the carbon skeleton of a reactant changes due to one or more rearrangementsinvolving carbocations.

eg:

mechanism:

Sandmeyer Reaction

Each of the following three reactions of aromatic diazonium ions is called a Sandmeyer reaction.

Each reaction, overall, is a nucleophilic substitution.

None of the reactions, however, occurs in the absence of copper (I) ion, which is a reducing agent.  The mechanism of Sandmeyer reactions is not fully understood.

Ruff Degradation

The Ruff degradation is a synthetic protocol used to remove one carbon atom from the molecule of an aldose.

eg: