Q: Which IR absorption is characteristic of a nitrile group (C≡N)?

$$2200\text{--}2300\ \mathrm{cm}^{-1}$$, sharp and intense

Question 1

Which of the following statements best explains why conjugation lowers the IR stretching frequency of a C=O bond?

Accepted Answer

Conjugation allows for resonance delocalization, which reduces the double bond character of the C=O bond, making it easier to stretch and thus lowering the frequency.

Question 2

A compound shows a strong, sharp IR absorption at $$1718\ \mathrm{cm}^{-1}$$ and two weak absorptions at $$2720\ \mathrm{cm}^{-1}$$ and $$2820\ \mathrm{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Aldehyde

Question 3

Which of the following IR absorptions would you expect to be present in the spectrum of a primary amine (RNH$$_2$$)?

Accepted Answer

A broad absorption with two sharp spikes near $$3300\ \mathrm{cm}^{-1}$$

Question 4

A mass spectrum of an unknown compound shows a molecular ion peak (M$$^+) at$$ $$m/z = 122$$ and an M$$^+$$ and M$$^{+2}$$ peak of equal intensity. Which element is most likely present in the molecule?

Accepted Answer

Bromine

Question 5

Which of the following best describes the 'base peak' in a mass spectrum?

Accepted Answer

The tallest peak, assigned an abundance of 100%, representing the most stable fragment ion

Question 6

Which IR absorption is characteristic of a nitrile group (C≡N)?

Accepted Answer

$$2200	ext{--}2300\ \mathrm{cm}^{-1}$$, sharp and intense

Question 7

A compound's mass spectrum shows a molecular ion peak at $$m/z = 78$$ and an M$$^+ to M$^{+2}$$ ratio of approximately 3:1. Which element is likely present?

Accepted Answer

Chlorine

Question 8

Which of the following statements about the fingerprint region in IR spectroscopy is correct?

Accepted Answer

It is unique for every molecule (except enantiomers) and is found between $$600$$ and $$1400\ \mathrm{cm}^{-1}.$$

Question 9

Which of the following best explains why the IR absorption for an O–H stretch in a carboxylic acid is broader than that in an alcohol?

Accepted Answer

Carboxylic acids have stronger hydrogen bonding, leading to a broader O–H absorption between $$2500$$ and $$3500\ \mathrm{cm}^{-1}.$$

Question 10

A mass spectrum of an unknown organic compound shows a prominent fragment at $$m/z = 43. $$This is most likely due to which of the following ions?

Accepted Answer

Isopropyl cation ($$\mathrm{(CH_3$)_2CH^+})$$

Question 11

Which of the following is a limitation of IR spectroscopy when determining the structure of an unknown compound?

Accepted Answer

IR spectroscopy cannot always unambiguously identify a functional group, but the absence of a signal is strong evidence that a group is not present.

Question 12

Which of the following best describes the effect of increasing atomic mass on the IR stretching frequency of a bond?

Accepted Answer

The stretching frequency decreases with increasing atomic mass.

Question 13

A compound with a strong IR absorption at $$1735\ \mathrm{cm}^{-1} is $$most likely which of the following?

Accepted Answer

An ester

Question 14

Which of the following is true regarding the molecular ion (M$$^+) in $$mass spectrometry?

Accepted Answer

It corresponds to the unfragmented molecule with a single positive charge.

Question 15

Which of the following IR absorptions would you expect to be absent in the spectrum of a tertiary amine (R$$_3N$$)?

Accepted Answer

A broad N–H stretch near $$3300\ \mathrm{cm}^{-1}$$

Question 16

A mass spectrum of a compound shows a molecular ion peak at an odd m/z value. What does this suggest about the compound's composition?

Accepted Answer

The compound contains at least one nitrogen atom.

Question 17

Which of the following best explains why the C–H stretch of an sp-hybridized carbon appears at a higher frequency than that of an sp$$^2 or sp$^3$$ carbon?

Accepted Answer

sp-hybridized carbons have greater s-character, making the C–H bond stronger and requiring more energy to stretch.

Question 18

Which of the following is a typical fragmentation pattern observed in the mass spectrum of an alcohol?

Accepted Answer

Loss of water to form an alkene ion

Question 19

A compound's IR spectrum shows a strong absorption at $$1640\ \mathrm{cm}^{-1}$$ and a broad absorption at $$3300\ \mathrm{cm}^{-1}$$ with two sharp spikes. Which functional group is most likely present?

Accepted Answer

Primary amide

Infrared Spectroscopy and Mass Spectrometry in Organic Chemistry

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