Q15P - Organic Chemistry (Mcmurry)

Question

Which of the following compounds would you expect to show ultraviolet absorptions in the 200 to 400 nm range?

Step-by-Step Solution

Answer

Compounds (b),(c),(d),(e),(f) showing UV absorption in the 200 to 400 nm range are shown as follows:

 

Ultraviolet Radiation

When the molecule absorbs the UV visible radiation, the electrons excite from its “highest occupied to lowest unoccupied” molecular orbital, creating an energy difference $\mathbf{∆}\mathit{E}$ between the two levels.

The “energy difference” $\mathbf{∆}\mathit{E}$ value decides the maximum absorption of UV radiation.

UV absorption of compound (a)

Ultraviolet absorption in the range from 200-400 nm is shown by the compound having alternative single and multiple bonds.

Compound (a) cyclohexa-1,4-diene is the cyclic compound having two double bonds, but there is no complete conjugation present; this compound has no “complete resonance” thus, compound (a) is not show absorption in the range from 200-400 nm is it is not UV-active.

 

 

UV inactive compound cyclohexa-1,4-diene

UV absorption of compound (b)

Ultraviolet absorption in the range from 200-400 nm is shown by the compound having alternative single and multiple bonds.

Compound (b) cyclohexa-1,3-diene is the cyclic compound having two double bonds that are in conjugation with each other; both the double bonds show resonance with each other; thus, compound (b) shows absorption in the range from 200-400 nm is UV-active.

UV active compound cyclohexa-1,3-diene

UV absorption of compound (c)

Ultraviolet absorption in the range from 200-400 nm is shown by the compound having alternative single and multiple bonds.

Compound (c) is acrylonitrile having a double bond in conjugation with the triple bond of the cyano group  ; complete resonance is present in the compound; thus, compound (c) shows absorption in the range from 200-400 nm is UV-active.

UV active compound acrylonitrile

UV absorption of compound (d)

Ultraviolet absorption in the range from 200-400 nm is shown by the compound having alternative single and multiple bonds.

Compound (d) is aspirin with the aromatic ring attached to the carbonyl and ester group; thus, the aromatic 6-membered ring is in conjugation with both the carbonyl and ester group with complete resonance present inside as well as outside the ring.

 

Therefore compound (d) shows absorption from 200-400 nm and is UV-active.

UV active compound aspirin.

UV absorption of compound (e)

Ultraviolet absorption in the range from 200-400 nm is shown by the compound having alternative single and multiple bonds.

Compound (e) is 4a-methyl-5,6,7,8-tetrahydronapthalen-2(4aH)-one is a bicyclic compound having two double bonds in conjugation with each other, the ring, which is further is in conjugation with a carbonyl group attached to that ring.

 

Therefore compound (e) shows absorption from 200-400 nm and is UV-active.

UV active compound 4a-methyl-5,6,7,8-tetrahydronapthalen-2(4aH)-one

UV absorption of compound (f)

Ultraviolet absorption in the range from 200-400 nm is shown by the compound having alternative single and multiple bonds.

Compound (f) is indole is a heterocyclic compound having six and five-membered rings fused, and both rings are in conjugation with each other; complete resonance is present in the compound. 

 

Therefore compound (f) shows absorption from 200-400 nm and is UV-active.

UV active compound indole