Q: Which equation 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 and how is it formed?

Accepted Answer

The molecular ion is a radical cation formed when a molecule loses an electron after being hit by an electron beam: $$\mathrm{M} + 	ext{electron beam} ightarrow \mathrm{M}^{+\cdot} + e$$^{-}$$.

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 base peak at $$m/z = 43$$ indicates the most stable fragment, which is the propyl cation ($$\mathrm{CH_3CH_2CH_2}^+).$$

Question 3

Which fragmentation pathway in pentane leads to the formation of a fragment with $$m/z = 29$$?

Accepted Answer

C-2—C-3 fragmentation

Question 4

In mass spectrometry, why is the peak at $$m/z = 57$$ more abundant for isopentane than for pentane?

Accepted Answer

Because isopentane forms a secondary carbocation at $$m/z = 57$$, which is more stable than a primary carbocation.

Question 5

How can you differentiate an alkyl bromide from an alkyl chloride using mass spectrometry?

Accepted Answer

By the ratio of M and M+2 peaks: bromides show two peaks 2 mass units apart with almost equal intensities, while chlorides show M+2 peak at 1/3 the height of M.

Question 6

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

Accepted Answer

Heterolytic cleavage of the carbon-halogen bond

Question 7

What is the effect of resonance electron donation on the IR absorption frequency of a carbonyl group?

Accepted Answer

Resonance electron donation decreases the frequency.

Question 8

Which bond absorbs IR radiation most intensely, and why?

Accepted Answer

O—H bond, because it is the most polar.

Question 9

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

Accepted Answer

Carbonyl (C=O)

Question 10

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

Accepted Answer

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

Question 11

Why do hydrogen-bonded OH groups stretch at a lower frequency in IR spectroscopy?

Accepted Answer

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

Question 12

Which region of the IR spectrum is most useful for identifying functional groups?

Accepted Answer

Functional group region (4000–1400 cm$$^{-1})$$

Question 13

What is the Beer–Lambert Law and what does each variable represent?

Accepted Answer

$$A = ecl$$, where $$A is $$absorbance, $$e is $$molar absorptivity, $$c is $$concentration, and $$l is $$path length.

Question 14

Which type of electronic transition is responsible for UV/Vis absorption in compounds with conjugated double bonds?

Accepted Answer

$$\pi \rightarrow \pi^*$$ transition

Question 15

What effect does increasing conjugation have on the wavelength of maximum absorption ($$\lambda_{max}) in UV/$$Vis spectroscopy?

Accepted Answer

Increasing conjugation increases $$\lambda_{max}.$$

Question 16

Which of the following bonds would show the highest wavenumber in IR absorption?

Accepted Answer

C≡N triple bond

Question 17

A mass spectrum of 2-hexanol shows a peak at $$m/z = 84. $$What fragmentation process is responsible for this peak?

Accepted Answer

Loss of water ($$\mathrm{H_2O}) $$from the molecular ion

Question 18

Which IR absorption is characteristic of a hydrogen-bonded carboxylic acid dimer?

Accepted Answer

$$3300–2500\ 	ext{cm}^{-1}$$

Question 19

Which of the following statements about IR activity is correct?

Accepted Answer

A bond absorbs IR radiation only if its dipole moment changes during vibration.

Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet/Visible Spectroscopy: Principles and Applications

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