Question

Which of the following can be used to convert : (A) (i) \(\mathrm{N}_{2} \mathrm{H}_{4}\), (ii) \(\mathrm{OH}, \Delta\) (B) \(\mathrm{Zn}-\mathrm{Hg} / \mathrm{HCl}\) (C) LiAlH \(_{4}\) (D) Red P/HI

Short answer

The best option for converting an alcohol to an alkane among the given choices is \( (D) \) Red P/HI, followed by a suitable reducing agent like Li or Mg.

Step-by-step solution

Identify the target compound

Since the original question seems to be cut off and lacks specific information regarding the target compound, we will assume that we are converting an alcohol (R-OH) to an alkane (R-H). In this case, the reduction of an alcohol to an alkane will be the focus.

Analyze option (A)

(A) (i) N2H4 and (ii) OH−, Δ: The combination of N2H4 and OH− with heat (Δ) is known as the Wolff-Kishner reduction. This reaction mechanism is usually used to reduce carbonyl groups (C=O) to methylene groups (CH2). As the focus is on reducing an alcohol to an alkane, this option is not suitable.

Analyze option (B)

(B) Zn-Hg / HCl: The amalgamation of zinc and mercury in the presence of hydrochloric acid (HCl) is called the Clemmensen reduction. Similar to the Wolff-Kishner reduction, this reaction also converts carbonyl groups to methylene groups. Thus, this option is also not suitable for reducing alcohols to alkanes.

Analyze option (C)

(C) LiAlH4: Lithium aluminium hydride (LiAlH4) is a powerful reducing agent that can reduce a variety of functional groups. Although LiAlH4 is mainly used to reduce carbonyl groups, it is also capable of reducing carboxylic acids, esters, and amides. However, it is not capable of reducing alcohols to alkanes, so this option is not suitable as well.

Analyze option (D)

(D) Red P/HI: Red phosphorus (Red P) and hydroiodic acid (HI) are used together in a reaction called the Appel reaction, which is used to convert alcohols to alkyl iodides. However, this reaction does not reduce the alcohol to an alkane and instead provides an alkyl halide intermediate. Since none of the given options directly convert an alcohol to an alkane, it would be reasonable to choose the Appel reaction (D) and then further convert the alkyl iodide to an alkane using a reducing agent such as a metal like lithium (Li) or magnesium (Mg). Thus, the best option for converting an alcohol to an alkane among the given choices is (D) Red P/HI, followed by a suitable reducing agent like Li or Mg.

Key concepts

Wolff-Kishner Reduction

The Wolff-Kishner Reduction is a classic reaction in organic chemistry, primarily used to transform carbonyl groups (aldehydes and ketones) into methylene groups (\( \text{CH}_2 \)). This process is particularly useful in synthetic pathways where carbonyl functionalities need to be removed without affecting other sensitive groups.

• It involves the use of hydrazine (\(\text{N}_2\text{H}_4\)) and a strong base such as potassium or sodium hydroxide under heat.
• The reaction progresses through the formation of a hydrazone intermediate.
• Further heating dismantles this intermediate, expelling nitrogen gas and yielding the reduced methylene group.

It's important to note this reduction is not effective on alcohols, and requires a carbonyl structure to proceed. Therefore, in an exercise focusing on converting alcohols to alkanes, this reaction would not be appropriate.

Clemmensen Reduction

The Clemmensen Reduction is another method used to reduce carbonyl groups (ketones and aldehydes), but not functional groups like alcohols. In this reaction,

• Zinc amalgam (\( \text{Zn-Hg} \)) and hydrochloric acid (\(\text{HCl}\)) create a strongly acidic environment.
• This acidic medium facilitates the conversion of carbonyls into methylene groups (\(\text{CH}_2\)), similar to the Wolff-Kishner reduction but under acidic conditions.

Though effective for carbonyl groups, Clemmensen Reduction doesn't reduce alcohols. It’s particularly favored when substrates are stable under acidic conditions, unlike the basic conditions required for Wolff-Kishner Reduction. For students, understanding the nature of the substrate is key when choosing between these reductions.

LiAlH4 as a Reducing Agent

Lithium aluminum hydride (\(\text{LiAlH}_4\)) is revered as a potent reducing agent in organic chemistry. It's capable of reducing a wide variety of polar functional groups, yet certain limitations exist.

• \(\text{LiAlH}_4\) can efficiently reduce carbonyl groups, carboxylic acids, esters, and amides to their corresponding alcohols or amines.
• However, it does not possess the ability to reduce alcohols into alkanes; its action terminates at the alcohol stage because alcohols do not proceed to further reduce into nonpolar products.

While highly reactive, \(\text{LiAlH}_4\) demands careful handling and anhydrous conditions because it reacts violently with water. This makes it unsuitable for direct conversions from alcohols to alkanes, necessitating alternative steps for achieving such transformations.

Appel Reaction

The Appel Reaction represents a versatile approach to transform alcohols into alkyl halides, specifically alkyl iodides in the context of the given exercise. This is done through:

• Using triphenylphosphine (\(\text{PPh}_3\)) and carbon tetrachloride (\(\text{CCl}_4\)), or a similar halogen source like iodine and red phosphorus.
• This reaction allows alcohols to become reactive intermediates—alkyl halides—that can be further manipulated.

The transformation via Appel allows the subsequent reduction to alkanes using different methods, such as zinc-copper couple or other metal reductions. While Appel does not directly convert alcohols to alkanes, it sets the stage for such conversions to proceed more easily.