Q: Which IR absorption would you expect to be absent in a tertiary amine (R$$_3N$$)?

Broad absorption at $$3300\ \text{cm}^{-1}$$

Q: Which IR absorption is characteristic of a nitrile group?

Sharp, intense absorption at $$2200$$–$$2300\ \text{cm}^{-1}$$

Q: Which IR absorption is most likely to be observed for an alkyne with a terminal hydrogen?

Sharp absorption at $$3300\ \text{cm}^{-1}$$

Q: Which IR absorption is most likely to be observed for a carboxylic ester?

C=O stretch at $$1735\ \text{cm}^{-1}$$ and C–O stretch between $$1300$$–$$1000\ \text{cm}^{-1}$$

Question 1

Which functional group is indicated by a broad absorption around $$3300\ 	ext{cm}^{-1} in an IR $$spectrum, with a rounded tip, and is often associated with hydrogen bonding?

Accepted Answer

Alcohol

Question 2

A compound shows a strong, sharp IR absorption at $$1710\ 	ext{cm}^{-1}$$ and two additional peaks at $$2720$$ and $$2820\ 	ext{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Aldehyde

Question 3

Which IR absorption would you expect to be absent in a tertiary amine (R$$_3N$$)?

Accepted Answer

Broad absorption at $$3300\ 	ext{cm}^{-1}$$

Question 4

A mass spectrum of a compound shows a molecular ion peak (M$$^+) at$$ $$86$$ and a strong base peak at $$57. $$Which fragmentation process is most likely responsible for the base peak?

Accepted Answer

Loss of a propyl radical ($$\ce{C3H7})$$

Question 5

Which IR absorption is characteristic of a nitrile group?

Accepted Answer

Sharp, intense absorption at $$2200$$–$$2300\ 	ext{cm}^{-1}$$

Question 6

Which of the following statements about mass spectrometry is TRUE?

Accepted Answer

It measures the mass-to-charge ratio ($$m/z) of $$ions.

Question 7

A compound’s IR spectrum shows a very broad absorption from $$2500 to$$ $$3500\ 	ext{cm}^{-1}$$ and a strong absorption at $$1710\ 	ext{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Carboxylic acid

Question 8

Which IR absorption is most likely to be observed for an alkyne with a terminal hydrogen?

Accepted Answer

Sharp absorption at $$3300\ 	ext{cm}^{-1}$$

Question 9

Which of the following best explains why the IR spectrum of an alkane is important for structure determination?

Accepted Answer

The absence of other functional group absorptions confirms the presence of only C–H and C–C bonds.

Question 10

A mass spectrum of a compound containing bromine shows two molecular ion peaks (M$$^+$$ and M$$^{+2}) of $$equal height. What does this indicate?

Accepted Answer

The compound contains one bromine atom.

Question 11

Which IR absorption is characteristic of a conjugated C=C bond?

Accepted Answer

1620–1640\ 	ext{cm}^{-1}

Question 12

A mass spectrum of an amine shows a molecular ion with an odd mass number. What does this suggest about the structure?

Accepted Answer

The molecule contains an odd number of nitrogen atoms.

Question 13

Which IR absorption is most likely to be observed for a carboxylic ester?

Accepted Answer

C=O stretch at $$1735\ 	ext{cm}^{-1}$$ and C–O stretch between $$1300$$–$$1000\ 	ext{cm}^{-1}$$

Question 14

Which of the following is a limitation of IR spectroscopy?

Accepted Answer

The absence of a signal is definite proof that a functional group is absent.

Question 15

A compound’s IR spectrum shows a sharp absorption at $$1640\ 	ext{cm}^{-1}$$ and a broad absorption at $$3300\ 	ext{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Amide

Question 16

Which mass spectrometry fragmentation is characteristic of ketones and aldehydes with a hydrogen three atoms away from the carbonyl group?

Accepted Answer

McLafferty rearrangement

Question 17

A mass spectrum of a compound containing chlorine shows a molecular ion peak (M$$^+) $$three times higher than the M$$^{+2}$$ peak. What does this indicate?

Accepted Answer

The compound contains one chlorine atom.

Question 18

Which IR absorption is characteristic of an aromatic ring?

Accepted Answer

C=C stretch at about $$1600\ 	ext{cm}^{-1}$$

Question 19

Which of the following best describes the principle behind mass spectrometry fragmentation patterns?

Accepted Answer

The most stable cation forms in greater amounts.

Infrared Spectroscopy and Mass Spectrometry: Structure Determination in Organic Chemistry

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