Test: IR Spectroscopy - Chemistry MCQ

C₅H₁₀ → 1380 cm^-1. 1380 cm^-1 represents a C–C bond band. All have C – C bond. Option Pentyne does not fulfill the H-atom requirement. So, correct option is Cyclopentane.

Carbonyl stretching order in both the compounds is 6 > 5 > 4 > 3. Ring strain increases, carbonyl frequency increases.

, frequency is directly proportional to wave number. So, wave number is inversely proportional to reduced mass as shown in the above relation.

Correct option is H₂ as HH₂ do not have dynamic dipole moment, so no spectrum will be observed.

Monomeric saturated aliphatic carboxylic acids show carbonyl stretching frequency near 1760 cm^-1, while saturated aliphatic ketones near 1720 cm^-1 because I effect is dominant in carboxylic acids over the mesomeric effect.

Benzoyl chloride

A weak band near 1750 cm^-1. Since, band is of weak intensity, it must be due to fermi Resonance. So correct option is Fermi resonance between C = O band and first overtone.

Ketenes absorb in IR at a very high frequency (2150 cm^-1) because inner C is sp² hybridized. Bonds with more s character absorb at a higher frequency.

C₈ H₆ → Sharp band at 3300 cm^-l
Weak band at 3085, 2110 cm^-l
Double bond equivalent = C + 1 – H⁄2 – X⁄2 + N⁄2,
(where C→ Carbon, H→ Hydrogen, X→ Halogen, N→ Nitrogen)
= 8 + 1 – 6⁄2 = 6.
One double bond equivalent represents either one bond or one ring.
Now, band at 3300 shows C–H bond, 2110 may be because of C≡C triple bond
Now, we check each option one by one
Option Octene → octene → does not match m.f. C₈ H₆.


C ≡ C → 2110
D.B.E. → 5πbond
M.f. → C₈ H₆. Hence, the correct option is Phenyl acetylene.

C₆H₅CH₃ is nonlinear.
No. of vibration degree of freedom = 3N – 6
= 3(15) – 6 = 39.

The phosphorescence spectrum of the excited species is due to triplet to singlet transitions. In an excited singlet state, the electron is promoted in the same spin orientation as it was in the ground state (paired). In a triplet excited stated, the electron that is promoted has the same spin orientation (parallel) to the other unpaired electron.