Before You Begin

Question #44816

Difficulty: 8/10 | 🟢 Free

The Question

Trichloroacetaldehyde (chloral, denoted as $S$ ) undergoes nucleophilic addition with $\ce{CN^-}$ in an aqueous medium at 298 K to form a cyanohydrin. Chloral strongly interacts with water to form a highly stable chloral hydrate according to the equilibrium: $S + \ce{H2O} \rightleftharpoons S_{\text{hydrate}}$ with an equilibrium constant $K_h \gg 1$ . The cyanohydrin is formed exclusively via the slow, rate-determining attack of $\ce{CN^-}$ on the free (unhydrated) carbonyl $S$ with a rate constant $k$ . If $[S]_0$ represents the total analytical concentration of chloral (hydrated + unhydrated), which of the following represents the correct rate law for the formation of the cyanohydrin?

Options

A.
$\text{Rate} = \frac{k [S]_0 [\ce{CN^-}]}{K_h [\ce{H2O}]}$
B.
$\text{Rate} = k [S]_0 [\ce{CN^-}]$
C.
$\text{Rate} = k K_h [S]_0 [\ce{CN^-}] [\ce{H2O}]$
D.
$\text{Rate} = \frac{k [S]_0 [\ce{CN^-}]}{1 + K_h}$

Check Your Answer

Correct Answer

Step-by-Step Solution

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

Rate Law Derivation from Mechanism|Nucleophilic Addition Sterics|Hydrate Stability vs. Carbonyl

Why This Question Is Tricky

Common Mistakes

The 'Hidden Variable' being tested is the sequestration of the reactive substrate into an unreactive resting state (the hydrate).

Students typically overlook the mass balance $[S]_0 = [S] + [S_{\text{hydrate}}]$ and incorrectly assume $[S] \approx [S]_0$ , leading to a standard second-order rate law (Option B).

Because chloral has highly electron-withdrawing chlorine atoms, its hydrate formation equilibrium constant $K_h \gg 1$ .

This pre-equilibrium drastically reduces the effective concentration of the free carbonyl, resulting in an apparent rate law that exhibits an inverse dependence on the solvent (water) concentration.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Kinetics - Class 12
Concept	Rate Law Derivation from Mechanism
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free