Carbanions and Aldol Condensation

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Questions and Answers

What characterizes a carbanion?

It has no relation to carbon acids.

It has a formal positive charge.

It can only form linear geometries.

It is a negatively charged ion with trivalence. (correct)

In what type of reaction does aldol condensation occur?

In the presence of an alkali hydroxide. (correct)

With a saturated hydrocarbon.

When mixed with a strong acid.

Only in the presence of heat.

What is formed as a direct result of aldol condensation?

A β-hydroxy ketone or β-hydroxy aldehyde. (correct)

An alkaloid.

A saturated hydrocarbon.

A conjugated acid.

What does the term 'crossed aldol condensation' refer to?

Condensation between two different carbonyl compounds. Signup and view all the answers

Which step in the mechanism of aldol condensation produces the enolate ion?

The removal of a proton from the α-carbon. Signup and view all the answers

What is the role of the alkali hydroxide ion in aldol condensation?

To generate an enolate ion. Signup and view all the answers

What is the final product following dehydration in the aldol condensation mechanism?

A conjugated enone. Signup and view all the answers

Which of the following correctly describes the geometry of carbanions?

Carbanions can exhibit bent, linear, or trigonal pyramidal geometries. Signup and view all the answers

What is the first step in the preparation of Wittig reagents?

Preparation of phosphonium salt from triphenylphosphine and an alkyl halide Signup and view all the answers

Which property enhances the stability of a carbanion when the carbon is sp hybridized?

Increased s character Signup and view all the answers

Which factor does NOT affect the stability of carbanions?

Temperature Signup and view all the answers

In what type of reactions do carbanions serve as intermediates?

Nucleophilic substitutions Signup and view all the answers

What role do carbanions typically play in organic synthesis?

Strong bases Signup and view all the answers

During the preparation of Wittig reagents, what base is commonly used for deprotonation?

n-Butyllithium Signup and view all the answers

Which Wittig rearrangement converts ethers into secondary or tertiary alcohols?

[1,2]-Wittig rearrangement Signup and view all the answers

How does resonance contribute to the stability of carbanions?

It delocalizes the negative charge. Signup and view all the answers

What is produced as an intermediate in the nucleophilic addition reaction described?

Alkoxide Signup and view all the answers

What product is formed when the alkoxide ion accepts a proton from water?

β-hydroxy aldehyde Signup and view all the answers

What occurs during the dehydration of aldol products?

Formation of α,β-unsaturated aldehydes or ketones Signup and view all the answers

What type of reaction is the Favorskii rearrangement?

Base-catalyzed Signup and view all the answers

Which of the following compounds can be obtained from the Favorskii rearrangement?

Carboxylic acid derivatives Signup and view all the answers

What is the main product of the Wittig reaction when an aldehyde is reacted with a Wittig reagent?

Alkene Signup and view all the answers

What is the driving force behind the Wittig reaction?

Formation of triphenylphosphine oxide Signup and view all the answers

What can influence the outcome of products in the Wittig reaction when R groups are similar?

Selectivity for E or Z configurations Signup and view all the answers

Study Notes

Carbanion

A carbanion is a negatively charged ion with a carbon atom that has three bonds and a formal negative charge of at least -1.
Carbanions are the conjugate bases of carbon acids.
Carbanions can have bent, linear, or trigonal pyramidal molecular geometries.

Condensation Reaction of Carbanions

Aldol Condensation: An aldol condensation occurs when an aldehyde with an α-hydrogen reacts with a dilute base to form a β-hydroxy aldehyde, also known as an aldol.
Crossed Aldol Condensation: A crossed aldol condensation occurs when two different carbonyl compounds react.
Aldol Condensation Mechanism:
- Step I: An enolate ion is formed by removing a proton from the α-carbon of one molecule of the aldehyde using a base.
- Step II: The enolate ion attacks the carbonyl carbon of the second molecule of the aldehyde in a nucleophilic addition reaction, forming an alkoxide intermediate.
- Step III: The alkoxide ion accepts a proton from water to form a β-hydroxy aldehyde (aldol).
- Step IV: The aldol product can undergo dehydration to produce α, β-unsaturated aldehydes or ketones.

Favorskii Rearrangement

The Favorskii rearrangement involves the rearrangement of cyclopropanones or α-halo ketones to form carboxylic acid derivatives.
This reaction is base-catalyzed and can form acids, esters, or amides depending on the base used.
Mechanism:
- Step I: The base removes a proton from the carbon adjacent to the carbonyl group.
- Step II: The enolate ion attacks the carbonyl carbon, forming a cyclic intermediate.
- Step III: The cyclic intermediate opens up to form a carboxylic acid derivative.

Wittig Reaction

The Wittig reaction involves the reaction of an aldehyde or ketone with a triphenylphosphonium ylide (Wittig reagent) to form an alkene and triphenylphosphine oxide.
This reaction is widely used in organic synthesis to prepare alkenes.
Preparation of Wittig Reagents:
- Step I: Triphenylphosphine reacts with an alkyl halide via an SN2 reaction to form phosphonium salt.
- Step II: The phosphonium salt is deprotonated with a strong base such as n-butyllithium to form the ylide.

Stability of Carbanions

Induction: Highly electronegative substituents stabilize carbanions by drawing electrons towards themselves. Electropositive substituents destabilize carbanions.
- Hybridization: Carbanions with more s character are more stable.
- Resonance: Resonance delocalizes the negative charge, increasing stability. Aromatic systems are particularly stable.

Significance of Carbanions

Reaction Intermediates: Carbanions are intermediates in many important organic reactions.
Base Catalysts: Carbanions are strong bases that catalyze many reactions, especially in the formation of carbon-carbon bonds.
Synthetic Applications: Carbanions are used in the synthesis of complex organic compounds.

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Description

Explore the fascinating world of carbanions and their role in aldol condensation reactions. This quiz covers the definition of carbanions, their properties, and the step-by-step mechanism of aldol condensation. Test your knowledge on how these key concepts in organic chemistry interconnect.