Chapter 25 Multiple Choice Questions

. Which of the following statements is wrong?

UV absorption is attributable to electronic transitions.

UV spectra provide information about valence electrons.

IR absorption is attributable to transitions between rotational energy levels of whole molecules.

NMR spectrometers use radiofrequency electromagnetic radiation.

. Which is the correct order of increasing wave number of the stretching vibrations of (1) C-H (alkane), (2) O-H (alcohol), (3) C=O (ketone), and (4) C≡C (alkyne)?

(4) < (3) < (2) < (1)

(3) < (4) < (2) < (1)

(3) < (4) < (1) < (2)

(4) < (3) < (1) < (2)

. How many signals does the aldehyde (CH₃)₃CCH₂CHO have in ¹H NMR and ¹³C NMR spectra?

five ¹H signals and six ¹³C signals

three ¹H signals and four ¹³C signals

five ¹H signals and four ¹³C signals

three ¹H signals and six ¹³C signals

. Which of hydrogens a-d in the following molecule gives a triplet signal in a normal ¹H NMR spectrum?

hydrogen a

hydrogen b

hydrogen c

hydrogen d

. Which hydrogen of 1-chloropent-2-ene shows the largest chemical (downfield) shift in its NMR spectrum?

the H on C1

the H on either C2 or C3

the H on C4

the H on C5

. Which carbon of (a)-(d) of hex-3-en-2-one shows the largest (most downfield) chemical shift in the NMR spectrum?

C1

C2

C4

C6

. Which of the following statements regarding IR spectroscopy is wrong?

Infrared radiation is higher in energy than UV radiation.

Infrared spectra record the transmission of IR radiation.

Molecular vibrations are due to periodic motions of atoms in molecules, and include bond stretching, torsional changes, and bond angle changes.

Infrared spectra give information about bonding features and functional groups in molecules.

. Which of the following statements regarding NMR spectroscopy is wrong?

NMR signals towards the left of the spectral chart correspond to larger chemical shifts.

Chemical shifts are larger when the frequencies of the radiation which induces the nuclear transitions are higher.

Chemical shifts are larger when shielding effects are greater.

A hydrogen signal splits into n+1 peaks by spin-spin coupling when the number of equivalent hydrogen atoms on adjacent atom(s) is n, and no other neighbouring atoms are involved.

. Which of the following statements regarding mass spectrometry is wrong?

In a normal mass spectrometer, electron impact causes a molecule to lose an electron and become a molecular radical cation which decomposes into fragment cations and radicals.

Only cations can be detected by a normal mass spectrometer.

A compound whose molecules contain just one bromine atom shows two molecular ion peaks of similar intensity, one at +1 and one at -1 of the average m/z value.

Molecular ion peaks always have even-numbered values of m/z.

. Which of the following statements is wrong?

A conventional mass spectrometer employs high energy UV radiation.

A conventional mass spectrometer does not employ a spectrophotometric detector.

Conventional mass spectrometry does not always require samples of high purity.

A mass spectrum does not show signals due to uncharged radicals.

. Absorption of radiation in the UV range attributable to n→π* electronic transitions is characteristic of which of the following types of compounds?

Aromatic hydrocarbons.

Unsaturated carbonyl compounds.

Non-conjugated polyenes.

Conjugated polyenes.

. Which of the following statements is wrong.

The wavenumber of a band in an IR spectrum is proportional to the frequency of the associated molecular vibration.

Water is a good solvent for recording UV spectra of water-soluble compounds.

Water is a good solvent for recording IR spectra of water-soluble compounds.

Hydrogen bonding in hydroxy compounds leads to broadening of spectral bands attributable to O-H stretching vibrations.

. Which is the correct order of increasing wave number of the stretching vibrations of (1) C-H (alkane), (2) C-H (alkene), (3) C-H (alkyne), and (4) C-H (arene)?

(1) < (2) ≈ (3) < (4)

(4) < (3) ≈ (2) < (1)

(3) < (4) ≈ (2) < (1)

(1) < (4) ≈ (2) < (3)

. How many signals does the unsaturated ketone
(CH₃)₂CHCH₂C(O)CH=CH₂ have in ¹H NMR and ¹³C NMR spectra?

five ¹H signals and six ¹³C signals

six ¹H signals and six ¹³C signals

six ¹H signals and seven ¹³C signals

five ¹H signals and seven ¹³C signals

. Which of the following statements in the context of ¹H NMR spectroscopy is true?

Arene C-H chemical shift (δ) values are greater than simple alkenes C-H chemical shift values because of the aromatic ring current.

Arene C-H chemical shift (δ) values are smaller than simple alkenes C-H chemical shift values because of the aromatic ring current.

Arene C-H signals are always multiplets.

Arene C-H signals are always singlets.

. Which carbon of (a)-(d) of hex-3-en-2-one has the smallest (most upfield) chemical shift in the NMR spectrum?

C1

C2

C4

C6

. Which of (a)-(d) indicates the multiplicities for hydrogens on C1, C3, and C4 of butanone attributable to spin-spin coupling in its ¹H NMR spectrum.

Hs on C1, singlet; Hs on C3, doublet; Hs on C4, triplet.

Hs on C1, singlet; Hs on C3, triplet; Hs on C4, quartet.

Hs on C1, singlet; Hs on C3, quartet; Hs on C4, triplet.

Hs on C1, triplet; Hs on C3, doublet; Hs on C4, triplet.

. Which of (a)-(d) indicates the correct order of carbon chemical shifts of the four carbons of the following compound.

C_Me < C2 < C3 < C1

C_Me < C3 < C2 < C1

C_Me < C2 < C1 < C3

C_Me < C1 < C2 < C3

. Which of the following statements regarding electron-impact mass spectrometry is true?

Samples need isotopic labels.

The base peak is formed by loss of one electron from each vaporised molecule by an electron beam.

Compounds must have a functional group to show a mass spectrum.

A meaningful mass spectrum can sometimes be obtained on a very small sample of an impure compound.

. Which of the following statements regarding mass spectrometry is false?

The base peak of a simple ketone is usually attributable to an acylium ion.

The molecular ion of carbonyl compounds with a -C-H readily undergoes elimination of an alkene to give a relatively stable enol radical cation.

The molecular ion peak of some alcohols is very weak because it readily loses an alkyl radical to give a relatively stable oxonium (hydroxycarbenium) ion.

Structurally isomeric alkanes cannot be distinguished by low resolution mass spectrometry.

Quiz Content