step-by-step mechanism for the oxidation of 2-Phenylethanol to Phenylacetaldehyde using PCC

step-by-step mechanism for the oxidation of 2-Phenylethanol to Phenylacetaldehyde using PCC

The correct answer and explanation is:

Correct Answer:

The oxidation of 2-phenylethanol (C₆H₅CH₂CH₂OH) to phenylacetaldehyde (C₆H₅CH₂CHO) using Pyridinium chlorochromate (PCC) is a mild and selective oxidation that converts a primary alcohol to an aldehyde without overoxidizing it to a carboxylic acid.

Step-by-Step Mechanism:

1. Activation of the Alcohol:
   • PCC reacts with the hydroxyl group (-OH) of 2-phenylethanol.
   • The lone pair on the oxygen of the alcohol attacks the chromium atom of PCC, forming a chromate ester intermediate.
   C6H5CH2CH2OH+PCC→C6H5CH2CH2O−CrO2ClC_6H_5CH_2CH_2OH + PCC \rightarrow C_6H_5CH_2CH_2O−CrO_2Cl
2. Formation of Chromate Ester:
   • A proton (H⁺) is lost from the alcohol oxygen, stabilizing the chromate ester.
   • This step involves proton transfer with assistance from the pyridinium ion.
3. Elimination (E2-like mechanism):
   • A base (often Cl⁻ or pyridine) abstracts the β-hydrogen (the hydrogen on the carbon adjacent to the one bearing the O−Cr group).
   • This leads to the concerted elimination of the chromium group and formation of the C=O double bond (carbonyl group).
   • Result: phenylacetaldehyde and a reduced Cr(IV) species.
   C6H5CH2CH2O−CrO2Cl→C6H5CH2CHO+Cr(IV)speciesC_6H_5CH_2CH_2O−CrO_2Cl \rightarrow C_6H_5CH_2CHO + Cr(IV) species

300-Word Explanation:

The oxidation of 2-phenylethanol to phenylacetaldehyde using pyridinium chlorochromate (PCC) is a common reaction in organic synthesis, especially for converting primary alcohols into aldehydes without further oxidation to carboxylic acids. PCC is a mild, selective oxidizing agent composed of chromium(VI), pyridine, and HCl.

The reaction begins with the nucleophilic attack of the alcohol’s oxygen on the chromium atom in PCC, forming a chromate ester. This intermediate is crucial because it sets up the alcohol carbon for oxidation. A proton transfer facilitated by the acidic conditions or pyridinium ion allows the oxygen to become bonded to chromium in a stable ester form.

Next, an E2-like elimination occurs. A base (often pyridine or Cl⁻) removes a β-hydrogen from the carbon next to the one bearing the O−Cr group. This elimination is concerted: the electrons from the C−H bond move to form a double bond with the adjacent carbon, while the electrons in the C−O bond push onto the Cr, breaking the Cr−O bond and forming the aldehyde.

The result is phenylacetaldehyde, a valuable compound used in perfumery and pharmaceuticals. Importantly, PCC does not hydrate the aldehyde to a geminal diol under anhydrous conditions, which prevents further oxidation to phenylacetic acid.

This mechanism demonstrates how PCC selectively oxidizes alcohols to aldehydes under controlled conditions, avoiding overoxidation, making it an essential tool in synthetic organic chemistry.