Question

In which case, the intermediate involved is incorrect? (a) Pinacol - pinacolone rearrangement - carbocation (b) Hofmann's bromamide reaction \(-\) Nitrene (c) Aldol condensation - carbocation (d) Anti Markonikoff's addition of HBr to propene Free radical.

Short answer

(c) Aldol condensation - carbocation is incorrect; the intermediate is an enolate ion, not a carbocation.

Step-by-step solution

Identify Intermediates

The first task is to identify the intermediates involved in each of the reactions mentioned in the options. (a) In the Pinacol-pinacolone rearrangement, the intermediate is a carbocation. (b) In Hofmann's bromamide reaction, the intermediate is a nitrene. (c) In Aldol condensation, the reaction typically involves an enolate ion, not a carbocation. (d) Anti-Markovnikov's addition of HBr to propene involves a free radical intermediate.

Match Intermediates to Reactions

Next, we need to match each reaction with the listed intermediate to check for correctness: (a) Pinacol-pinacolone rearrangement - Correct intermediate (carbocation). (b) Hofmann's bromamide reaction - Correct intermediate (nitrene). (c) Aldol condensation - Incorrect, as it involves an enolate ion, not a carbocation. (d) Anti-Markovnikov's addition - Correct intermediate (free radical).

Identify the Incorrect Match

From Step 2, it is evident that the reaction and intermediate pair that is incorrectly matched is in option (c): 'Aldol condensation - carbocation'. Unlike the other options, which correctly associate the reactions with their respective intermediates, Aldol condensation actually involves an enolate ion.

Key concepts

Pinacol-pinacolone rearrangement

The pinacol-pinacolone rearrangement is a fascinating organic reaction where a 1,2-diol transforms into a ketone. It showcases the beauty of rearrangement reactions through its elegant mechanism.

In this reaction, the intermediate is a carbocation. The process begins with the protonation of the alcohol group of the diol. This creates a good leaving group, which departs to form the carbocation.

• The newly formed carbocation is highly reactive and will rearrange to form a more stable carbocation if possible. This rearrangement involves the migration of an alkyl or aryl group.
• Following this rearrangement, the molecule captures a water molecule to form the final ketone product.

The reaction often requires acidic conditions, and the driving force behind the rearrangement is the stability gained by forming the ketone structure. It is a perfect example of how molecular rearrangement is utilized to obtain a more stable configuration.

Hofmann's bromamide reaction

Hofmann's bromamide reaction is a classic method used to prepare primary amines from amides. It involves a series of interesting steps that lead to the formation of a nitrene intermediate.

The reaction is critically dependent on the formation of a nitrene, a highly reactive species characterized by a nitrogen atom with six electrons, lacking an octet.

• The process begins with the reaction of the amide with bromine and a strong base, typically sodium or potassium hydroxide.
• The bromide attacks the amide nitrogen, leading to the formation of the nitrene intermediate.
• This nitrene then undergoes rearrangement, eventually leading to the formation of a primary amine.

Overall, this reaction illustrates the power of rearrangements and the importance of understanding reactive intermediates such as nitrenes.

Aldol condensation

Aldol condensation is a fundamental reaction in organic chemistry, especially in the synthesis of carbon-carbon bonds. Contrary to the given exercise's claim, the actual intermediate here is not a carbocation.

Instead, aldol condensation involves the formation of an enolate ion, a powerful nucleophile.

• The reaction begins with the deprotonation of an aldehyde or ketone at the alpha position, forming the enolate ion.
• This enolate then attacks another carbonyl compound, leading to the formation of a β-hydroxy ketone or aldehyde.
• In many cases, this product can dehydrate to form a conjugated enone.

Enolate ions rather than carbocations highlight the significance of electron delocalization, providing stability and allowing nucleophilic attack, which is central to the aldol condensation process.

Anti-Markovnikov's addition

Anti-Markovnikov's addition is a chemical reaction that, as opposed to the typical addition reactions, results in the less-substituted product in the case of alkenes. It's best exemplified by the radical mechanism involving the addition of HBr to alkenes, such as propene.

Here, the key player is the free radical intermediate.

• The reaction typically uses a peroxide to initiate the formation of free radicals.
• This leads to the formation of the bromine radical, which adds to the least substituted carbon of the alkene, following an anti-Markovnikov rule.
• The hydrogen radical then completes the addition by pairing with the other carbon.

This process is an exciting demonstration of chemistry's adaptability, showing how different conditions can lead to alternative reaction pathways and products. Understanding free radicals is crucial here, as they direct the addition away from the typical, more stable carbocation pathway.