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8.4. Organic chemistry reaction mechanism — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry8.4. Organic chemistry reaction mechanism1 Mark
MCQ
Its basic strength is $10^{10}$ more than $I$ -dimethyl amino naphthalene. Reason for high basic strength is
  • A
    resonance
  • ✓
    steric inhibitation of resonance
  • C
    ortho effect
  • D
    hyperconjugation
✓

Answer

Correct option: B.
steric inhibitation of resonance
b
$(b)$ Steric effects can indirectly affect acidity or basicity by affecting the  resonance. Steric effects can also be caused by other types of strain. $1,8$ - bis(diethylamino)- $2, 7$ -dimethoxynaphthalene $(1) $ is an extremely strong base  for a tertiary amine ($pK_a$ of the conjugate acid $= 16.3$; compare $N, N$ - dimethylaniline , $pK_a = 5.1$), but proton transfers to and from the nitrogen are  exceptionaly slow ; enough to be followed by a $uv$ -spectrophotometer. $1$ is  severely strained because the two nitrogen lorie pairs are forced to be near each  other. Protonation relieves the strain : one lone pair is now connected to a hydrogen,  which forms a hydrogen bond to the other lone pair (shown in $2$). The  same effects are found in $4, 5$ -bis (dimethylamino) fluorene $(3)$ and $4$, $5$ - bis (dimethylamino).

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