Organic Chemistry Questions and Answers – Infrared Spectroscopy - 2

1 Why in the IR spectrum of Benzoyl chloride, a weak band near 1750 cm^-1 is formed?

A Inductive effect

B Fermi resonance between C = O band and first overtone

C Conjugation effect

D Hyperconjugation effect

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Answer: Fermi resonance between C = O band and first overtone

2 Which compound having molecular formula C₅H₁₀ shows absorption at 1380 cm^-1?

A 2-Methyl-1-butene

B Cyclopentane

C Pentyne

D Methylcyclobutane

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Answer: Cyclopentane

3 Why ketenes absorb in IR at a very high frequency (2150 cm^-1)?

A The inner C is sp hybridized

B The more s character in a bond, the stronger it is

C Inner C is sp² hybridized

D Inner C is sp³ hybridized

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Answer: Inner C is sp² hybridized

4 What is the effect of ring strain in lactone (cyclic ester) or a lactam (cyclic amide)?

A Increases carbonyl stretching frequency

B Decreases carbonyl stretching frequency

C Increases C = C frequency

D Decreases C = C frequency

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Answer: Increases carbonyl stretching frequency

5 A compound C₈ H₆ decolorizes Br₂ in CCl₄ and gives a white precipitate with Tollen’s reagent. It has sharp band at 3300 cm^-l and weak bands at 3085, 2110 cm^-l. What is this compound?

A Phenyl acetylene

B Phenyl propylene

C Phenyl ethylene

D Octene

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Answer: Phenyl acetylene

6 What is the relation between wave number of IR absorption and the reduced mass?

A Wave number is directly proportional to reduced mass

B Wave number is inversely proportional to reduced mass

C Wave number is independent of the reduced mass

D Wave number is directly proportional to square of reduced mass

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Answer: Wave number is inversely proportional to reduced mass

7 What is the number of vibrational degrees of freedom in C₆H₅CH₃?

A 39

B 15

C 18

D 40

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Answer: 39

8 Which of the following molecules will not show infrared spectrum?

A H₂

B HCI

C CH₄

D H₂0

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Answer: H₂

9 The phosphorescence spectrum of the excited species is due to which transition?

A Singlet to triplet transitions

B Triplet to singlet transitions

C Vibration modes

D Electron spin transitions

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Answer: Triplet to singlet transitions

10 Why Monomeric saturated aliphatic carboxylic acids show carbonyl stretching frequency near 1760 cm^-1, while saturated aliphatic ketones near 1720 cm^-1?

A Mesomeric (M) effect is dominant in acids over the inductive (I) effect

B I effect is dominant in carboxylic acids over the mesomeric effect

C I effect on ketones is dominant over the M effect

D M effect in ketones is dominant

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