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Question #44740

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The Question

Consider the two-step synthesis involving the diazotization of aniline (Reaction I) to form intermediate $P$ , followed by its electrophilic coupling with 3-methylphenol in a mildly alkaline medium (pH 9-10) to yield the major product $Q$ . Reaction I is strictly carried out at $273-278\text{ K}$ . Which of the following statements correctly identifies the structure of $Q$ and the fundamental physicochemical rationale for the strict thermal constraints of Reaction I?

Options

A.
$Q$ is 3-methyl-4-(phenyldiazenyl)phenol. $P$ is thermodynamically unstable ( $\Delta G_{\text{decomp}} < 0$ ) but kinetically stable at $273\text{ K}$ ; the low temperature prevents the thermal activation required for its entropy-driven decomposition into phenol and \ce{N2}.
B.
$Q$ is 3-methyl-6-(phenyldiazenyl)phenol. $P$ is thermodynamically stable only at $T < 278\text{ K}$ ( $\Delta G_{\text{form}} < 0$ ); at higher temperatures, the negative entropy of diazotization ( $\Delta S_{\text{form}} < 0$ ) makes its formation non-spontaneous.
C.
$Q$ is 3-methyl-4-(phenyldiazenyl)phenol. $P$ is thermodynamically stable at all temperatures, but at $T > 278\text{ K}$ , the electrophilic coupling shifts from kinetic control to thermodynamic control, yielding a sterically hindered regioisomer.
D.
$Q$ is 3-methyl-2-(phenyldiazenyl)phenol. $P$ is kinetically unstable at all temperatures, but the high concentration of \ce{HCl} at $273\text{ K}$ suppresses the decomposition equilibrium via the Common Ion Effect and Le Chatelier's principle.

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Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Gibbs Energy and Spontaneity|Electrophilic Substitution Control|Inductive Effect Trends|Diazotization Temperature Sensitivity

Why This Question Is Tricky

Common Mistakes

This question targets a pervasive misconception regarding the temperature constraints of diazotization.

Students frequently misapply the Gibbs free energy equation ( $\Delta G = \Delta H - T\Delta S$ ) to the formation of the diazonium salt, falsely concluding that the forward reaction becomes non-spontaneous at higher temperatures due to a negative entropy of formation.

The trap exposes the hidden variable: the diazonium ion is always thermodynamically unstable with respect to decomposition ( $\Delta G_{\text{decomp}} < 0$ ) due to the massive entropy gain of releasing \ce{N2} gas.

The $273\text{ K}$ constraint is purely a kinetic requirement to prevent the system from overcoming the activation barrier for this entropy-driven collapse.

Furthermore, it tests advanced regioselectivity by forcing students to weigh competing $+M$ and $+I$ directing groups alongside steric constraints in the coupling phase.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Electrochemistry - Class 12
Concept	Gibbs Energy and Spontaneity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free