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MCQ on UV-Visible spectroscopy: Page-4

1. Given below is a cyclic six member enone. The base value for the chromophore is

(A) 202 nm

(B) 215 nm

(C) 253 nm

(D) 230 nm

Enone is a α, β-unsaturated carbonyl compound acts a chromophore just like a diene. In a cyclic form with 6 member ring, it shows base value at 215 nm. Even acyclic enone also has the same base value as the cyclic six member enone. base value of enone

2. The possible transitions in UV-visible region for the above compound will be
n to pi and pi to pi transitions

(A) P,Q

(B) P,R

(C) P,Q,R

(D) P,Q,R,S

Since it has both pi and n electrons, it can undergo pi to pi* and n to pi* transitions.

3. Which of the following absorbs at more wavelength

(A) Benzene

(B) Phenol

(C) Aniline

(D) Anilinium ion

As we know that auxochromes increase absorption, here we have to see which compound has strong auxochrome. Again the capacity of an auxochrome to increase absorption depends on the extension of conjugation by lone pair of electrons. Benzene has no auxochrome while anilinium ion has lost lone pair of electrons hence not acts as auxochrome. Now among phenol and aniline, the later has amine group which can show more auxochromic effect as nitrogen has low electronegativity than oxygen.

4. The increment for α alkyl substitution on enones is

(A) 18

(B) 12

(C) 10

(D) 5

5. The λmax in α,β-unsaturated acids can be determined by

(A) Wood ward fieser

(B) Nielsen rules

(C) Fieser-kuhn rules

(D) All of the above