Worksheet: Functional Groups in IR Spectra
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 1,645 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 1,710 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?
Keeping all other variables the same as bond strength decreases, which of the following is true?
- AThe wave number of the IR stretch decreases.
- BThe wavelength and wave number of the IR stretch decrease.
- CThe wavelength of the IR stretch decreases.
- DThe wave number of the IR stretch increases.
Which of the following is in correct order of decreasing IR stretching frequency?
- AAlkenes > alkynes > alkanes
- BAlkenes > alkanes > alkynes
- CAlkynes > alkanes > alkenes
- DAlkynes > alkenes > alkanes
- EAlkanes > alkynes > alkenes
Keeping all other variables the same, which of the following bonds would have the lowest IR stretching frequency?
Which of the following does not affect the stretching frequency in a metal–ligand complex?
- AThe electron-donating or electron-withdrawing effects of other ligands
- BThe electron density of the metal center
- CThe intensity of the infrared source used
- DThe geometry of the molecule
Which of the following statements about IR spectroscopy is false?
- AAbsorptions due to bond stretching are generally weak.
- BStretching frequencies are higher than corresponding bending frequencies.
- CSingle bonds have lower stretching frequencies than triple bonds.
- DConjugation lowers the energy required to cause a bond to stretch.
In order for an IR vibration to be allowed, what condition must be met?
- AThere must be a change in the polarizability of the molecule.
- BThere must be a change in dipole moment of the molecule.
- CThe spin state of the molecule must flip.
- DThere must be a chromophoric species.
Which of the following regions would be most useful in determining the substitution patterns of an alkene?
- AThe stretch, occurring between 1,660 and 1,600 cm−1
- BThe out-of-plane bend, occurring between 1,000 and 650 cm−1
- CThe stretch, occurring between 3,100 and 3,010 cm−1
- DNone of the choices would indicate the substitution patterns.
What does hydrogen bonding cause the hydroxyl IR stretching band to do?
- AShift to lower frequencies and broaden
- BShift to higher frequencies and sharpen
- CShift to lower frequencies and sharpen
- DShift to higher frequencies and broaden
Within the cm−1 region, how can primary, secondary, and tertiary amines be distinguished?
- APrimary amines have two bands, secondary amines have one band, and tertiary amines have no stretch.
- BPrimary amines have one band, secondary amines have two bands, and tertiary amines have three bands.
- CPrimary amines have no bands, secondary amines have one band, and tertiary amines have two bands.
- DPrimary and tertiary amines have intense single bands, while secondary amines have two weak bands.
Ethers, esters, and alcohols all contain stretching bands. How can an ether be distinguished from the other two compounds?
- AThe absence of and bands eliminates esters and alcohols.
- 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 band of an ether appears in a different region than that of an alcohol or ester.
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 = S, alkene = E
- BChiral center = S, alkene = Z
- CChiral center = R, alkene = Z
- DChiral center = R, alkene = E
Which of the following infrared signals is most likely to correspond to stretching of the carbonyl bond?