Question

Reaction not involving a carbanion is: (a) Aldol condensation (b) Claisen condensation (c) Pinacol pinacolone rearrangement (d) Michael reaction

Short answer

The correct answer is (c) Pinacol pinacolone rearrangement.

Step-by-step solution

Understand Carbanion Involvement

A carbanion is a type of anion where a carbon atom exhibits a negative charge. It's important to identify reactions where a carbanion acts as an intermediate. Aldol condensation, Claisen condensation, and Michael addition all proceed through carbanion intermediates, which initiate nucleophilic attacks.

Analyze Each Reaction

Let's briefly analyze each option: (a) **Aldol Condensation** involves the creation of a carbanion by deprotonation of an aldehyde or ketone, which then attacks another carbonyl compound. (b) **Claisen Condensation** also involves the formation of a carbanion from an ester, which then attacks another ester. (c) **Pinacol Pinacolone Rearrangement** involves the rearrangement of a diol (pinacol) to a ketone (pinacolone), which typically proceeds through a carbocation intermediate, not a carbanion. (d) **Michael Reaction** is a nucleophilic addition that often involves a carbanion attacking an α,β-unsaturated carbonyl compound.

Identify the Reaction Without Carbanion

By examining the reaction mechanisms, we notice that the Pinacol Pinacolone rearrangement proceeds through a carbocation transition state rather than using a carbanion. This distinguishes it from the other reactions, making it not reliant on a carbanion for its process.

Key concepts

Aldol Condensation

Aldol condensation is a critical reaction in organic chemistry that involves the formation of carbon-carbon bonds. This reaction typically occurs between aldehyde or ketone molecules that possess alpha-hydrogen atoms. The key mechanism begins with the deprotonation of the alpha carbon, creating a carbanion intermediate.
This carbanion, being electron-rich, can act as a nucleophile. It attacks the carbonyl carbon of another molecule, forming a β-hydroxy aldehyde or ketone, known as the "aldol" product.
Further reaction stages can lead to dehydration, resulting in an α,β-unsaturated carbonyl compound.

• The reaction requires a basic or acidic medium, although bases are more common for facilitating the initial deprotonation.
• Temperature conditions may vary depending on whether the focus is on creating the aldol product or pushing towards dehydration.
• This reaction is widely used in synthetic chemistry for building complex molecules with ease.

The beauty of aldol condensation lies in its ability to combine simple starting materials into more complex structures, pivotal in the synthesis of various natural products and pharmaceuticals.

Claisen Condensation

Claisen condensation is a vital reaction within organic chemistry focused on esterification and enolization processes. In this reaction, two ester molecules react in the presence of a strong base, leading to the formation of a β-keto ester.
The process begins with the deprotonation of an α-hydrogen of the ester, generating a carbanion intermediate. This carbanion is highly reactive and moves on to perform a nucleophilic attack on another ester’s carbonyl carbon.

• A very specific base, usually ethoxide ion, is employed because it matches the alkyl group of the ester involved.
• After the nucleophilic attack, the molecule rearranges, resulting in a tetrahedral intermediate that collapses to expel an alkoxide ion.
• This reaction is favoured when the esters used have no alpha hydrogen, aiding in avoiding side reactions.

The Claisen condensation is integral in constructing complex carbon frameworks, often serving as a stepping-stone in multi-step organic syntheses.

Pinacol Pinacolone Rearrangement

The Pinacol pinacolone rearrangement is a fascinating transformation involving alcohols, specifically converting a diol into a ketone. Unlike other reactions discussed, it proceeds through a carbocation intermediate, not involving a carbanion.
The reaction kicks off when the diol compound is treated with an acid. A protonation step makes one of the alcohol groups leave, creating a carbocation.

• The carbocation rearranges through migration of an alkyl or aryl group to form a more stable carbocation intermediate.
• Eventually, this rearranged carbocation loses a proton to form a carbonyl group, culminating in the formation of a ketone.
• This rearrangement is useful in organic synthesis to strategically alter the backbone of a molecule, offering access to challenging chemical spaces.

The Pinacol rearrangement showcases the diverse reactivity of alcohols under acidic conditions and exemplifies rearrangement reactions in organic chemistry.

Michael Reaction

The Michael reaction, or Michael addition, is a fundamental tool for forming carbon-carbon bonds, set apart by its conjugate addition mechanism. This reaction involves the addition of a nucleophile, often a carbanion, to an α,β-unsaturated carbonyl compound.
The carbanion intermediate, created by deprotonating a compound like malonate or cyanoacetate, acts as a Michael donor. This donor forms a bond with the beta-carbon of an α,β-unsaturated carbonyl compound, which acts as a Michael acceptor.

• The process requires a base to generate the nucleophilic carbanion from the donor compound.
• It's a highly regioselective reaction, favouring the 1,4-addition over the 1,2-addition.
• The Michael reaction is widely used in synthetic organic chemistry for its ability to introduce functional groups into molecules efficiently.

Often used in complex molecule synthesis, the Michael reaction plays a pivotal role in the formation of pharmaceuticals and biologically active compounds, demonstrating its relevance in both industrial and academic settings.