Q: A molecule absorbs infrared radiation and transitions from energy level $$E_1 to$$ $$E_2. $$Which equation correctly relates the energy difference to the frequency of absorbed light?

$$\Delta E = E_2$ - E_1$ = h\nu$$

Question 1

Which instrument is shown in the first image, and what is its primary use in organic chemistry?

Accepted Answer

It is an FT-IR spectrometer, used for measuring the absorption of infrared light by molecules to identify functional groups.

Question 2

Which of the following statements best describes the difference between IR spectroscopy and mass spectrometry?

Accepted Answer

IR spectroscopy involves the absorption of light, while mass spectrometry does not involve light but measures mass-to-charge ratios.

Question 3

Given the energy equation $$E = h
u$$, what happens to the energy of a photon as its wavelength decreases?

Accepted Answer

The energy increases.

Question 4

A molecule absorbs infrared radiation and transitions from energy level $$E_1 to$$ $$E_2. $$Which equation correctly relates the energy difference to the frequency of absorbed light?

Accepted Answer

$$\Delta E = E_2$ - E_1$ = h
u$$

Question 5

Which region of the IR spectrum is known as the 'fingerprint region' and why is it important?

Accepted Answer

1500–500 cm$$^{-1}$$; it is unique for each molecule and helps in identification.

Question 6

How many fundamental vibrational modes does a nonlinear molecule with 4 atoms have?

Accepted Answer

9

Question 7

Which of the following bonds will be IR-active?

Accepted Answer

A polar bond that changes its dipole moment during vibration.

Question 8

The vibrational frequency $$
u of a $$bond is given by $$
u = \frac{1}{2\pi} \sqrt{\frac{k}{\mu}}. $$What effect does increasing the reduced mass $$\mu$$ have on the vibrational frequency?

Accepted Answer

It decreases the vibrational frequency.

Question 9

Which functional group is most likely to show a strong, sharp IR absorption near 1700 cm$$^{-1}$$?

Accepted Answer

Ketone

Question 10

A compound shows a broad IR absorption around 3300 cm$$^{-1}$$ with a rounded tip. Which functional group is most likely present?

Accepted Answer

Alcohol

Question 11

Which of the following statements about mass spectrometry is correct?

Accepted Answer

It measures the mass-to-charge ratio of ions.

Question 12

In a mass spectrum, what is the base peak?

Accepted Answer

The most intense peak, assigned an abundance of 100.

Question 13

A mass spectrum of a compound containing chlorine shows two molecular ion peaks at m/z 112 and 114 in a 3:1 ratio. What does this indicate?

Accepted Answer

The compound contains one chlorine atom.

Question 14

Which isotope pattern would you expect for the molecular ion of p-dichlorobenzene (C$$_6H$_4Cl$_2$$)?

Accepted Answer

Three peaks in a 9:6:1 ratio.

Question 15

Which of the following best explains why the C–H stretch for an sp hybridized carbon appears at a higher wavenumber than for an sp$$^3$$ hybridized carbon?

Accepted Answer

sp hybridized C–H bonds have more s character, making them stronger and requiring more energy to stretch.

Question 16

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

Accepted Answer

Loss of 18 mass units due to water elimination.

Question 17

Which ionization technique is most commonly used in traditional mass spectrometry for organic molecules?

Accepted Answer

Electron Impact (EI) Ionization

Question 18

A molecule with a molecular ion at m/z = 86 shows a base peak at m/z = 43 in its mass spectrum. What does this suggest about the fragmentation?

Accepted Answer

The molecule fragments to form a stable tertiary carbocation at m/z = 43.

Question 19

Which of the following best describes the McLafferty rearrangement observed in mass spectrometry?

Accepted Answer

A rearrangement involving the transfer of a hydrogen atom to a carbonyl oxygen, resulting in cleavage and formation of an alkene.

Chapter 12: Infrared (IR) and Mass Spectroscopy – Study Notes

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