Alcohols can be obtained from all methods except:

Options

A. Hydroboration-oxidation
B. Oxymercuration-demercuration
C. Reduction of aldehyde`//`ketones with Zn-Hg`//`HCl
D. By fermentation of starch

Answer

To determine which methods do not yield alcohols, we need to analyze the processes mentioned in the video transcript. Here’s a step-by-step solution:

### Step 1: Understand Hydroboration-Oxidation
- Hydroboration-oxidation is a method where alkenes are converted into alcohols.
- The reagents involved are BH3 (boron hydride) followed by H2O2 (hydrogen peroxide) in the presence of NaOH (sodium hydroxide).
- This method successfully produces alcohols from alkenes.

**Hint:** Hydroboration-oxidation is a two-step reaction that converts alkenes to alcohols.

### Step 2: Analyze Oxymercuration-Demercuration
- Oxymercuration is another method that converts alkenes into alcohols.
- The reagents used include mercuric acetate (Hg(OAc)2) and water, followed by reduction with NaBH4 (sodium borohydride).
- This method also results in the formation of alcohols from alkenes.

**Hint:** Oxymercuration involves the addition of mercury and water to alkenes, leading to alcohol formation.

### Step 3: Consider the Reduction of Aldehydes and Ketones
- The reduction of aldehydes and ketones can produce alcohols.
- However, the specific reduction process mentioned is the Clemmensen reduction, which uses zinc amalgam (Zn/Hg) and HCl.
- This reaction converts aldehydes and ketones into hydrocarbons, not alcohols.

**Hint:** Clemmensen reduction is known for converting carbonyl compounds to alkanes, not alcohols.

### Step 4: Evaluate Fermentation
- Fermentation is a biological process where sugars (like starch) are converted into alcohol (ethanol) by yeast.
- This method is a well-known way to produce alcohols.

**Hint:** Fermentation is a natural process that converts sugars into alcohols, primarily ethanol.

### Conclusion
From the analysis, the method that does not yield alcohols is the **reduction of aldehydes or ketones using Clemmensen reduction**. Therefore, the correct answer is:

**Final Answer:** Alcohols can be obtained from all methods except the reduction of aldehydes or ketones with zinc amalgam and HCl (Clemmensen reduction).