Consider the oxidation of isoborneol (1) to camphor (2) using hypochlorous acid to answer the following questions.
a. What is the oxidation number of the carbon atom bearing the alcohol functional group in 1?
b. What is the oxidation number of this carbon atom in 2?
c. What is reduced in this reaction, and what is it reduced to?
d. Suggest a reagent other than bleach that would oxidize 1 into 2.
The correct answer and explanation is :
To address the oxidation of isoborneol (1) to camphor (2) using hypochlorous acid (HOCl), let’s walk through each question with clarity and provide the chemical reasoning behind it.
a. What is the oxidation number of the carbon atom bearing the alcohol functional group in 1 (isoborneol)?
In isoborneol, the carbon bearing the hydroxyl (-OH) group is secondary, meaning it is bonded to two other carbon atoms and one hydrogen. To assign its oxidation number:
- Bond to hydrogen: carbon gets -1
- Bonds to two carbons: carbon gets 0 (electrons are shared equally)
- Bond to oxygen (in OH): carbon loses the electron → +1
Total: 0 (two C-C) + 1 (C-O) – 1 (C-H) = 0
🔹 Answer: 0
b. What is the oxidation number of this carbon atom in 2 (camphor)?
In camphor, the hydroxyl group has been oxidized to a ketone, meaning the carbon is now double bonded to oxygen:
- Bonds to two carbons: 0
- Double bond to oxygen: carbon loses both bonding electrons → +2
- No H bonded
Total: 0 (C-C) + 2 (C=O) = +2
🔹 Answer: +2
c. What is reduced in this reaction, and what is it reduced to?
The oxidizing agent is hypochlorous acid (HOCl). In this redox reaction:
- Isoborneol is oxidized (alcohol to ketone)
- HOCl is reduced
HOCl is reduced as follows:
- Cl in HOCl is initially in +1 oxidation state
- It is reduced to Cl⁻, where Cl is in –1 oxidation state
🔹 Answer: HOCl is reduced to Cl⁻
d. Suggest a reagent other than bleach that would oxidize 1 into 2.
Alternative oxidizing agents for secondary alcohols to ketones include:
- Chromic acid (H₂CrO₄)
- PCC (Pyridinium chlorochromate)
- Dess-Martin periodinane (DMP)
- Jones reagent (CrO₃ in H₂SO₄)
Among these, PCC is often preferred for milder, selective oxidation.
🔹 Answer: Pyridinium chlorochromate (PCC)
Explanation (300 words)
The conversion of isoborneol to camphor is a classic example of an oxidation reaction, where a secondary alcohol is transformed into a ketone. Oxidation, in organic chemistry, typically refers to an increase in the oxidation number of a carbon atom, often due to the replacement of a C-H bond with a C-O bond.
In isoborneol, the carbon bonded to the hydroxyl group is secondary and its oxidation number is 0. This is determined by assigning electrons based on electronegativity differences: the C-H bond gives carbon -1, C-C bonds are 0, and C-O gives carbon +1. The sum is 0.
In camphor, the same carbon forms a double bond with oxygen as part of a ketone group, leading to an oxidation number of +2. This shift from 0 to +2 clearly indicates oxidation has occurred.
The oxidizing agent used here is hypochlorous acid (HOCl), commonly generated in situ from sodium hypochlorite (bleach). HOCl contains chlorine in the +1 oxidation state, and as it oxidizes isoborneol, chlorine is reduced to chloride ion (Cl⁻), going to a –1 oxidation state. This redox pairing underpins the transformation of isoborneol to camphor.
For alternative reagents, several oxidants can achieve the same transformation. PCC (pyridinium chlorochromate) is particularly efficient and selective for converting secondary alcohols to ketones without overoxidation. Other options include Jones reagent and Dess-Martin periodinane, both reliable for secondary alcohol oxidation.
In conclusion, the oxidation of isoborneol to camphor is a redox reaction involving carbon oxidation (0 to +2) and chlorine reduction (+1 to –1), and various reagents can facilitate this transformation beyond hypochlorous acid.