Q: Which equation correctly relates the energy of electromagnetic radiation to its frequency and wavelength?

$$E = h\nu = \frac{hc}{\lambda}$$

Question 1

In mass spectrometry, what is the molecular ion (M⁺•) and how is it formed?

Accepted Answer

It is a radical cation formed when a molecule loses an electron due to an electron beam.

Question 2

A mass spectrum of pentane shows a base peak at $$m/z = 43. $$What does this indicate about the fragmentation pattern?

Accepted Answer

The most stable and abundant fragment is the propyl cation.

Question 3

Which of the following statements best explains why the $$m/z = 57$$ peak is more abundant in isopentane than in pentane?

Accepted Answer

Isopentane forms a more stable secondary carbocation fragment.

Question 4

In the mass spectrum of 1-bromopropane, two molecular ion peaks (M and M+2) of nearly equal intensity are observed. What does this indicate?

Accepted Answer

The presence of two bromine isotopes, $$^{79}Br $$and $$^{81}Br$$, in nearly equal abundance.

Question 5

Which type of bond cleavage is most likely to occur in an alkyl halide during mass spectrometry?

Accepted Answer

Heterolytic cleavage of the carbon-halogen bond.

Question 6

A compound shows a strong IR absorption at $$	ilde{
u} = 1700\ \mathrm{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Ketone (C=O)

Question 7

Which of the following bonds will show the most intense IR absorption?

Accepted Answer

O–H

Question 8

Why does the O–H stretching frequency appear at a lower wavenumber in concentrated solutions compared to dilute solutions?

Accepted Answer

Because hydrogen bonding weakens the O–H bond, making it easier to stretch.

Question 9

Which of the following statements about the IR spectrum of 2-cyclohexenone is correct?

Accepted Answer

The C=O stretch appears at a lower wavenumber than in 2-pentanone due to resonance.

Question 10

Which equation correctly relates the energy of electromagnetic radiation to its frequency and wavelength?

Accepted Answer

$$E = h
u = \frac{hc}{\lambda}$$

Question 11

A compound absorbs UV light at a longer wavelength after conjugation. What is the reason for this shift?

Accepted Answer

Conjugation decreases the energy gap between HOMO and LUMO, making the transition easier.

Question 12

Which of the following is true about the base peak in a mass spectrum?

Accepted Answer

It is the fragment with the greatest abundance.

Question 13

Which of the following IR stretching vibrations requires the most energy?

Accepted Answer

Asymmetric stretching

Question 14

A mass spectrum of 2-hexanol shows a prominent peak at $$m/z = 84. $$What is the most likely explanation for this peak?

Accepted Answer

Loss of water ($$M - 18) $$from the molecular ion.

Question 15

Which of the following best describes the fingerprint region in an IR spectrum?

Accepted Answer

1400–600 cm$$^{-1}$$, unique for each compound.

Question 16

Which of the following is NOT a requirement for a bond to be IR active?

Accepted Answer

The bond must be nonpolar.

Question 17

Which of the following statements about the Beer–Lambert Law is correct?

Accepted Answer

Absorbance is directly proportional to concentration and path length.

Question 18

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

Accepted Answer

Carboxylic acids form strong hydrogen-bonded dimers.

Question 19

Which of the following is a chromophore in UV/Vis spectroscopy?

Accepted Answer

A conjugated system of double bonds

Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy: Structural Elucidation in Organic Chemistry

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