In this worksheet, we will practice identifying the functional groups in a compound from the nature of the peaks in its infrared spectrum.
A common organic chemical exhibits a strong IR absorption at cm−1. Which of the following functional groups could have caused this absorption?
A pure, colorless liquid with a high boiling point exhibited a strong IR absorption in the cm−1 range. The only other strong IR signals outside of the fingerprint region can be assigned to resonances. To which of the following chemical families does the liquid most likely belong?
An organic compound that is liquid at room temperature exhibits a strong IR absorption at cm−1. It is unreactive to acidified potassium dichromate, but the product of its reaction with lithium aluminum hydride exhibits a strong IR absorption in the cm−1 range. To which of the following chemical families does the liquid most likely belong?
In order for a compound to be IR active, it must undergo which of the following?
- AEmission of an electron
- BA change in polarizability
- CTransfer of an electon
- DA change in dipole moment
- EMetal-ligand charge transfer
Keeping all other variables the same, as bond strength decreases, which of the following is true?
- AWavelength and wavenumber of the IR stretch decrease
- BWavenumber of the IR stretch increases
- CWavelength of the IR stretch decreases
- DWavenumber of the IR stretch decreases
Which of the following is in correct order of decreasing IR stretching frequency?
- AAlkenes > alkynes > alkanes
- BAlkanes > alkynes > alkenes
- CAlkynes > alkanes > alkenes
- DAlkynes > alkenes > alkanes
- EAlkenes > alkanes > alkynes
Keeping all other variables the same, which of the following bonds would have the lowest IR stretching frequency?
What of the following does not affect the stretching frequency in a metal-ligand complex?
- AThe geometry of the molecule.
- BThe electron density of the metal center.
- CThe electron-donating or electron-withdrawing effects of other ligands.
- DThe intensity of the infrared source used.
Which of the following statements about IR spectroscopy is false?
- ASingle bonds have lower stretching frequencies than triple bonds.
- BStretching frequencies are higher than corresponding bending frequencies.
- CConjugation lowers the energy required to cause a bond to stretch.
- DAbsorptions due to bond stretching are generally weak.
In order for an IR vibration to be allowed, what condition must be met?
- AThe spin state of the molecule must flip.
- BThere must be a change in the polarizability of the molecule.
- CThere must be a chromophoric species.
- DThere must be a change in dipole moment of the molecule.
Which of the following regions would be most useful in determining the substitution patterns of an alkene?
- AThe stretch, occurring at cm−1
- BThe stretch, occurring between cm−1
- CThe out-of-plane bend, occurring between cm−1
Hydrogen bonding causes the hydroxyl IR stretching band to do what?
- AShift to higher frequencies and broaden
- BShift to higher frequencies and sharpen
- CShift to lower frequencies and sharpen
- DShift to lower frequencies and broaden
Within the cm−1 region, how can primary, secondary, and tertiary amines be distinguished?
- APrimary amines have no bands, secondary amines have one band, and tertiary amines have two bands.
- BPrimary amines have one band, secondary amines have two bands, and tertiary amines have three bands.
- CPrimary and tertiary amines have intense single bands, while secondary amines have two weak bands.
- DPrimary amines have two bands, secondary amines have one band, and tertiary amines have no stretch.
Ethers, esters, and alcohols all contain stretching bands. How can an ether be distinguished from the other two compounds?
- AThe band of an ether appears in a different region than that of an alcohol or ester.
- BAlcohols and esters have split bands, while ethers show only one band.
- CAlcohols and esters have very broad bands due to hydrogen bonding, while ethers have a very sharp band.
- DThe absence of and bands eliminates esters and alcohols.
Shown here is the structure of (+)-carvone.
Using R/S and E/Z labels, assign the stereochemistry of the chiral center and stereoisomeric alkene group.
- AChiral center = R, alkene = E
- BChiral center = S, alkene = E
- CChiral center = R, alkene = Z
- DChiral center = S, alkene = Z
Which of the following infrared signals is most likely to correspond to stretching of the carbonyl bond?