Question 1

Which region of the IR spectrum is known as the 'fingerprint region,' and why is it important in identifying organic compounds?

Accepted Answer

The region between $$600$$ and $$1500 cm$^{-1}$$; it contains complex vibrations unique to each molecule.

Question 2

A compound shows a strong IR absorption at $$1718 cm$^{-1}$$ and no significant absorption near $$3300 cm$^{-1}. $$Which functional group is most likely present?

Accepted Answer

Ketone

Question 3

Which statement best explains why conjugation lowers the IR stretching frequency of a carbonyl group?

Accepted Answer

Conjugation delocalizes electrons, reducing the double bond character and lowering the stretching frequency.

Question 4

Which IR absorption would you expect for a terminal alkyne (C≡C-H)?

Accepted Answer

A sharp absorption near $$2200 cm$^{-1}$$ for C≡C and a strong absorption near $$3300 cm$^{-1}$$ for C-H.

Question 5

A mass spectrum of an unknown compound shows a molecular ion peak (M$$^+) at$$ $$122$$ and another peak of equal intensity at $$124. $$What element is most likely present?

Accepted Answer

Bromine

Question 6

Which of the following statements about mass spectrometry is TRUE?

Accepted Answer

The base peak in a mass spectrum corresponds to the most stable carbocation fragment.

Question 7

An IR spectrum shows a broad absorption centered around $$3300 cm$^{-1}$$ with two sharp spikes. Which functional group is most likely present?

Accepted Answer

Primary amine ($$RNH_2)$$

Question 8

Which of the following best describes the effect of atomic mass and bond energy on IR stretching frequency?

Accepted Answer

Stretching frequency decreases with increasing atomic mass and increases with increasing bond energy.

Question 9

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

Accepted Answer

It contains a nitrogen atom.

Question 10

Which IR absorption is characteristic of a carboxylic acid?

Accepted Answer

A sharp C=O stretch near $$1710 cm$^{-1}$$ and a broad O-H stretch from $$2500 to$$ $$3500 cm$^{-1}.$$

Question 11

Which fragmentation pattern is most commonly observed in the mass spectrum of an alcohol?

Accepted Answer

Loss of water to form an alkene.

Question 12

A compound's IR spectrum shows a strong absorption at $$2200 cm$^{-1}$$ and no absorption near $$3300 cm$^{-1}. $$Which functional group is most likely present?

Accepted Answer

Nitrile

Question 13

Which isotope pattern in a mass spectrum would indicate the presence of chlorine in a molecule?

Accepted Answer

M$$^+$$ and M$$^++2 $$peaks in a 3:1 ratio.

Question 14

Which of the following is TRUE regarding the IR spectrum of an alkane?

Accepted Answer

It shows a broad C-H stretch between $$2800$$ and $$3000 cm$^{-1}$$ and no other significant bands.

Question 15

Which type of cation is favored in the fragmentation of aromatic compounds in mass spectrometry?

Accepted Answer

Benzylic cation

Question 16

A compound shows a strong IR absorption at $$1735 cm$^{-1}. $$Which functional group is most likely responsible for this absorption?

Accepted Answer

Ester

Question 17

Which of the following statements about the use of IR spectroscopy in structure determination is CORRECT?

Accepted Answer

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

Question 18

Which of the following is a limitation of mass spectrometry?

Accepted Answer

It is a destructive technique and the sample cannot be recovered.

Question 19

Which IR absorption would you expect for an amide functional group?

Accepted Answer

Strong C=O absorption at $$1640$$-$$1680 cm$^{-1}$$ and N-H absorption near $$3300 cm$^{-1} if $$hydrogens are present.

Spectroscopy in Organic Chemistry: Infrared (IR) and Mass Spectrometry (MS)

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