Question 1

Which of the following statements best describes the information provided by mass spectrometry (MS) in organic chemistry?

Accepted Answer

It provides the molecular mass and information about the structure based on the fragmentation pattern.

Question 2

A mass spectrum of an unknown organic compound shows a molecular ion peak at $$m/z = 86$$ and a base peak at $$m/z = 43. $$What does the base peak represent?

Accepted Answer

The most abundant fragment ion detected in the spectrum.

Question 3

Which of the following best explains why some fragments are more abundant than others in a mass spectrum?

Accepted Answer

Fragments that form more stable carbocations are more likely to be detected in greater abundance.

Question 4

In a mass spectrum, a small peak at $$M+1 is $$observed next to the molecular ion peak. What is the primary cause of this $$M+1$$ peak?

Accepted Answer

The presence of $$^{13}\mathrm{C}$$ isotopes in the molecule.

Question 5

A compound's mass spectrum shows two peaks at $$m/z = 78$$ and $$m/z = 80$$ with nearly equal intensity. What does this suggest about the compound?

Accepted Answer

It contains a bromine atom.

Question 6

Which of the following functional groups is most likely to show a strong, broad absorption band around $$3200-3600\ \mathrm{cm}^{-1} in an IR $$spectrum?

Accepted Answer

Alcohol ($$-\mathrm{OH})$$

Question 7

Which region of the IR spectrum is known as the 'functional group region'?

Accepted Answer

$$4000-1500\ \mathrm{cm}^{-1}$$

Question 8

Which of the following statements about IR absorption bands is correct?

Accepted Answer

More polar bonds show more intense absorption bands.

Question 9

A compound shows a strong IR absorption at $$1710\ \mathrm{cm}^{-1}$$ and a weak absorption just above $$2700\ \mathrm{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Aldehyde

Question 10

Which of the following best describes the effect of hydrogen bonding on an IR absorption band?

Accepted Answer

It broadens the absorption band.

Question 11

Which of the following IR absorption bands would you expect to find just above $$3000\ \mathrm{cm}^{-1}$$?

Accepted Answer

sp$$^2 C-H $$stretch

Question 12

A molecule contains a symmetrical alkyne ($$\mathrm{C}\equiv\mathrm{C}) $$group. What is most likely observed in its IR spectrum?

Accepted Answer

No significant absorption for the $$\mathrm{C}\equiv\mathrm{C}$$ stretch due to symmetry.

Question 13

Which of the following statements about the absence of an IR absorption band is correct?

Accepted Answer

The absence of a prominent absorption band (e.g., O-H or C=O) indicates the absence of that functional group.

Question 14

Which of the following best explains why the C-H stretch for sp$$^3$$ hybridized carbons appears just below $$3000\ \mathrm{cm}^{-1}$$, while the C-H stretch for sp$$^2$$ hybridized carbons appears just above $$3000\ \mathrm{cm}^{-1}$$?

Accepted Answer

sp$$^2 C-H $$bonds are shorter and stronger, requiring more energy to stretch.

Question 15

A mass spectrum of an unknown compound shows a molecular ion peak at $$m/z = 92$$ and a significant $$M+2$$ peak with about one-third the intensity of the molecular ion. What does this suggest?

Accepted Answer

The compound contains a chlorine atom.

Question 16

Which of the following is a common fragmentation pattern for alcohols in mass spectrometry?

Accepted Answer

Alpha cleavage next to the carbon bearing the hydroxyl group.

Question 17

Which of the following best describes the relationship between frequency ($$
u$$), wavelength ($$\lambda$$), and the speed of light ($$c) $$for electromagnetic radiation?

Accepted Answer

$$c = \lambda 
u$$

Question 18

Which of the following IR absorption bands would be most useful for distinguishing between an aldehyde and a ketone?

Accepted Answer

A weak absorption just above $$2700\ \mathrm{cm}^{-1} ($$aldehyde C-H stretch)

Question 19

Which of the following statements about the fingerprint region of an IR spectrum is correct?

Accepted Answer

It is unique for each molecule and is found below $$1500\ \mathrm{cm}^{-1}.$$

Question 20

Which of the following is true regarding the effect of electron delocalization on the position of an IR absorption band?

Accepted Answer

Electron delocalization lowers the frequency (wavenumber) of the absorption band.

Spectroscopic Methods in Organic Chemistry: Mass Spectrometry and Infrared Spectroscopy

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