Question

Give the correct sequences of TRUE (T) and FALSE (F) for the following statements. (a) \(\mathrm{SOCl}_{2}, \mathrm{PCl}_{3}\) and \(\mathrm{HCl}\) all converts alcohol into alkyl halide (b) Mechanism of all reagents are identical (c) Mechanism of \(\mathrm{SOCl}_{2}\) and \(\mathrm{PCl}_{3}\) is same but different mechanism in case of \(\mathrm{HCl}\) (d) They all are used commercially to convert acetic acid to acetyl chloride. (A)T FFF (B) \(\mathrm{T} \mathrm{TF} \mathrm{T}\) (C) \(\mathrm{TFTF}\) (D) \(\mathrm{TFF} \mathrm{T}\)

Short answer

(C) TFTF

Step-by-step solution

Statement (a): SOCl2, PCl3, and HCl all convert alcohol into alkyl halide

This statement is true. SOCl2, PCl3, and HCl are all known to convert alcohols into alkyl halides.

Statement (b): Mechanisms of all reagents are identical

This statement is false. The mechanisms for these reactions are not identical. While SOCl2 and PCl3 follow a similar mechanism involving the formation of a chlorosulfite or phosphorus intermediate, HCl follows a different mechanism where nucleophilic substitution occurs directly.

Statement (c): Mechanism of SOCl2 and PCl3 is the same but different mechanism in case of HCl

This statement is true. As mentioned above, SOCl2 and PCl3 follow a similar mechanism involving the formation of a chlorosulfite or phosphorus intermediate, whereas HCl follows a different mechanism.

Statement (d): They all are used commercially to convert acetic acid to acetyl chloride

This statement is false. Though SOCl2 and PCl3 are used for this conversion, HCl is not used commercially to convert acetic acid to acetyl chloride. According to our analysis, the correct sequence of true (T) and false (F) statements is TFTF, which corresponds to option (C). Therefore, the correct answer is (C) TFTF.

Key concepts

Alcohol to Alkyl Halide Conversion

Converting an alcohol to an alkyl halide is a fundamental reaction in organic chemistry. This transformation is often required in synthesis as alkyl halides are versatile intermediates.
There are different reagents for this conversion, such as

• Thionyl chloride (\(\text{SOCl}_2\))
• Phosphorus trichloride (\(\text{PCl}_3\))
• Hydrochloric acid (\(\text{HCl}\))

These reagents help in replacing the hydroxyl group (\(\text{-OH}\)) of alcohols with a halide (such as chloride) to form the alkyl halide.

Each reagent has unique properties and conditions under which they work best. \(\text{SOCl}_2\), for example, is often preferred because it generally produces gaseous byproducts, making the isolation of the alkyl halide easier.
\(\text{PCl}_3\), on the other hand, is used in milder conditions but requires careful handling, and \(\text{HCl}\) works well for converting tertiary alcohols.

Reaction Mechanisms

Reaction mechanisms describe the detailed steps or path a chemical reaction follows. While \(\text{SOCl}_2\) and \(\text{PCl}_3\) share a similar mechanism, \(\text{HCl}\) follows a different path.

Using \(\text{SOCl}_2\) or \(\text{PCl}_3\), the mechanism involves an initial reaction where these reagents form

• a chlorosulfite ester or
• a phosphite ester, respectively,

which are then displaced by the halide ion. This is a substitution reaction categorized as \(\text{S}_\text{N}2\), where the nucleophile attacks the electrophilic carbon from the opposite side, causing the leaving group to depart.

However, when \(\text{HCl}\) is involved, the mechanism largely depends on the structure of the alcohol. For primary and secondary alcohols, a \(\text{S}_\text{N}2\) mechanism is common, while tertiary alcohols favor a \(\text{S}_\text{N}1\) mechanism, involving the formation of a carbocation intermediate before the halide ion attaches. This difference is crucial in organic synthesis to predict the reaction outcome and to select the right conditions.

Commercial Applications of Reagents

These reagents are not just a staple in academic settings but have significant commercial utility as well.
\(\text{SOCl}_2\) and \(\text{PCl}_3\) are widely used in the chemical industry to convert carboxylic acids, like acetic acid, into acyl chlorides, like acetyl chloride.

While \(\text{SOCl}_2\) is favored due to its efficiency and the ease of removing byproducts, \(\text{PCl}_3\) is preferred for large-scale industrial applications because it is more cost-effective and offers versatility in different synthesis pathways.
However, \(\text{HCl}\) is not typically used for converting acetic acid to acetyl chloride in commercial settings due to the challenges in yield and selectivity.

Overall, these reagents exemplify the intersection of theoretical organic chemistry with practical industrial processes, showcasing the relevance of understanding reaction mechanisms for real-world chemical manufacturing.