Choose which of the following statements A, B, C or D best describe the reduction of a carboxylic acid to give an aldehyde by a reducing reagent.[Fill in the blank]

Question

BlackTom AI · Accepted Answer

Reduction of a carboxylic acid to an aldehyde involves net addition of hydrogen (and electrons) to the carbonyl carbon, so the carbon gains a C–H bond.

1) Relevant concepts:
- "Reduction" means gain of electrons and typically addition of hydrogen (or loss of oxygen).
- Compare bonding at the carboxyl carbon before and after: carboxylic acid is R–C(=O)–OH, aldehyde is R–C(=O)–H.
- Count bonds to that carbon to determine changes in C–H and C–O bonds.

2) Step-by-step reasoning:
- Start: carboxylic acid R–C(=O)–OH. The carbonyl carbon has: one C=O double bond, one C–O single bond (to OH), one C–R bond, and zero C–H bonds.
- End: aldehyde R–C(=O)–H. The carbonyl carbon has: one C=O double bond, one C–H bond, one C–R bond, and zero C–O single bonds.
- Compare: C–H bonds: 0 → 1 (net increase of 1). C–O single bonds: 1 → 0 (net decrease of 1). The C=O remains present (still one C=O bond).

3) Intermediate bond-count steps shown:
- Carboxylic acid: C–H = 0; C–O (single) = 1.
- Aldehyde: C–H = 1; C–O (single) = 0.
- Net change: Δ(C–H) = +1; Δ(C–O single) = −1.

4) Why the correct answer is correct:
- Option B states a net increase in the number of C–H bonds in the product aldehyde. This matches the bond-count result: the carbon gains one hydrogen (C–H bond) when reduced from R–C(=O)–OH to R–C(=O)–H. Reduction supplies the hydrogen/electrons needed to form that new C–H bond.

5) Why the other choices are incorrect:
- A ("net decrease in the number of electrons") contradicts the definition of reduction (which is electron gain), so it is wrong.
- C ("net increase in the number of C–O bonds") is wrong because the single C–O bond to OH is lost (1 → 0), not increased.
- D ("No net change in C–O and C–H bonds") is false because we do see a clear change: one C–H forms and one C–O single bond is lost.

6) Conclusion:
The correct statement is B: a net increase in the number of C–H bonds in the product aldehyde, which matches the provided answer.

Choose which of the following statements A, B, C or D best describe the reduction of a carboxylic acid to give an aldehyde by a reducing reagent.[Fill in the blank]Single choice

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