Before You Begin

Question #44687

Difficulty: 8/10 | 🟢 Free

The Question

An unknown, symmetrically substituted N,N-dimethylaniline derivative, $\mathbf{X}$ , is subjected to precise combustion analysis. Combustion of exactly $2.3340$ g of $\mathbf{X}$ produces $7.0414$ g of \ce{CO2} and $2.4320$ g of \ce{H2O}. (Use exact atomic weights: C = 12.011, H = 1.008, N = 14.007, O = 15.999). Aqueous basicity measurements reveal that $\mathbf{X}$ is a significantly stronger base than standard N,N-dimethylaniline ( $\Delta pK_b > 2$ ). Which of the following correctly identifies $\mathbf{X}$ and the primary electronic/steric reason for its anomalous basicity?

Options

A.
$\mathbf{X}$ is N,N-dimethyl-2,6-di-tert-butylaniline; the bulky ortho-groups cause Steric Inhibition of Resonance (SIR), localizing the nitrogen lone pair and overcoming steric hindrance to solvation.
B.
$\mathbf{X}$ is N,N-dimethyl-3,5-di-tert-butylaniline; the strong +I (inductive) and hyperconjugative effects of the meta-tert-butyl groups drastically increase the electron density on nitrogen.
C.
$\mathbf{X}$ is N,N-dimethyl-2,6-di-isopropylaniline; the ortho-groups prevent the solvation of the conjugate acid, which paradoxically increases the thermodynamic stability of the free base.
D.
$\mathbf{X}$ is N,N-dimethyl-2,6-di-tert-butylaniline; the ortho-effect creates a hydrophobic pocket that enhances the hydrogen bonding of water to the neutral amine, increasing its basicity.

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

Precision in Atomic Mass Units|Ortho-Effect in Anilines|Basicity-Solvation Competition

Why This Question Is Tricky

Common Mistakes

This problem tests the 'Precision in Atomic Mass Units' trap combined with the 'Ortho-Effect Exception'.

Students who use rounded atomic masses (C=12, H=1) will calculate a C/N ratio of 15.62, failing to identify the \ce{C16H27N} empirical formula.

Furthermore, it tests the exception to the standard ortho-effect: while ortho-substituents typically decrease basicity in primary anilines due to steric hindrance to solvation, in N,N-dialkylanilines, bulky ortho-groups cause Steric Inhibition of Resonance (SIR).

This twists the \ce{-NMe2} group out of plane, localizing the lone pair on nitrogen and drastically increasing basicity.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Some Basic Concepts of Chemistry - Class 11 Part 1
Concept	Precision in Atomic Mass Units
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free