Question

When succinaldehyde is treated with lithium diisopropylamide \((\mathrm{LDA}),\) it: becomes more nucleophilic. becomes less nucleophilic. generates a carbanion. A. I only B. II only C. I and III only D. II and III only

Short answer

C: I and III only.

Step-by-step solution

- Identify the reagents

The reagent used in the reaction is lithium diisopropylamide (LDA), which is a strong, non-nucleophilic base.

- Determine the effect of LDA on succinaldehyde

LDA, being a strong base, will deprotonate an acidic hydrogen in succinaldehyde. Succinaldehyde has two aldehyde groups. Deprotonation usually occurs at the alpha hydrogen (hydrogen adjacent to the carbonyl group), leading to the formation of a carbanion.

- Analyze nucleophilicity change

After the formation of the carbanion, the molecule becomes more nucleophilic because carbanions are generally good nucleophiles due to the presence of a negatively charged carbon atom.

- Conclusion with options

Therefore, when succinaldehyde is treated with LDA, it becomes more nucleophilic (I) and generates a carbanion (III). Based on the given options, the correct choice is C: I and III only.

Key concepts

lithium diisopropylamide (LDA)

Lithium diisopropylamide, commonly referred to as LDA, is a strong, non-nucleophilic base. It is often used in organic chemistry for deprotonating molecules, especially at positions adjacent to carbonyl groups. LDA is formed by reacting n-butyllithium with diisopropylamine. This reagent is particularly useful because it is highly selective and strong enough to deprotonate many organic compounds.
LDA is commonly employed in enolate chemistry, where it helps generate enolate ions by deprotonating carbonyl compounds. These enolate ions are key intermediates in many organic reactions, making LDA a staple in the synthesis toolkit.

• Selective deprotonation: Targets specific hydrogen atoms.
• Non-nucleophilic: Won't add to carbonyl groups.
• Strong base: Deprotonates weak acids like ketones and aldehydes.

carbanion formation

A carbanion is an anion in which carbon carries a negative charge. Carbanions are formed when a base deprotonates a carbon atom, removing a proton (H+) and leaving behind a lone pair of electrons. This results in a negatively charged carbon, which is termed a carbanion.
Carbanions are important intermediates in many organic reactions due to their high reactivity. They are typically formed in the presence of strong bases, like LDA, which can effectively remove protons from carbon atoms adjacent to electron-withdrawing groups like carbonyls.
Carbanions can participate in a variety of reactions, including nucleophilic substitutions and additions, making their formation a crucial step in organic synthesis.

• Definition: Carbon with a negative charge and lone pair.
• Formation: Requires a strong base like LDA.
• Reactivity: Highly reactive intermediates in organic reactions.

nucleophilicity

Nucleophilicity refers to the ability of a molecule or ion to donate a pair of electrons and form a new covalent bond. Nucleophiles are 'nucleus-loving' species that are attracted to positively charged or electron-deficient areas within other molecules. They are essential players in many organic reactions.
Carbanions are excellent nucleophiles because their negatively charged carbon atoms readily donate electrons to electron-poor sites. When succinaldehyde is treated with LDA, a carbanion is formed, making it more nucleophilic.
Increased nucleophilicity means the molecule can participate more readily in reactions that involve forming new bonds, such as addition and substitution reactions.

• Definition: Ability to donate electrons and form a bond.
• Carbanions: Highly nucleophilic due to negative charge.
• Reaction participation: Key for addition and substitution reactions.

alpha hydrogen deprotonation

Alpha hydrogen deprotonation is a common process in organic chemistry where a hydrogen atom adjacent to a carbonyl group (the alpha position) is removed. This process is typically facilitated by a strong base like LDA.
The carbonyl group stabilizes the negative charge left on the carbon atom after deprotonation, forming an enolate ion. This stability is due to resonance structures that can delocalize the negative charge between the oxygen and the alpha carbon.
The enolate ion formed through deprotonation is a versatile intermediate that can participate in various organic transformations, including alkylation and condensation reactions.

• Alpha position: Hydrogen adjacent to the carbonyl group.
• Deprotonation: Removal facilitated by strong bases like LDA.
• Stabilization: Negative charge stabilized by resonance.