IB Docs (2) Team MC test: Spectroscopic identification of organic compounds (AHL)
Multiple choice test on 21.1 Spectroscopic identification of organic compounds (AHL)
Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).
If you get an answer wrong, read through the explanation carefully to learn from your mistakes.
Which technique can be used to identify the bond lengths and bond angles in a crystalline solid?
X-ray crystallography involves the diffraction of a beam of X-rays by the atoms in a crystalline solid. From the resulting diffraction pattern the precise position of each atom and the distances between the atoms can be determined.
Which is the correct order of increasing wavelength (smallest first)
X-rays are in the order of 10−10 m, visible between 4 and 7 x 10−7 m, infrared between about 10−6 to 10−4 m and radio waves between about 101 and 108 m.
Which factors help to explain why tetramethylsilane is used as a reference material in 1H NMR spectroscopy?
I. It is soluble in water.
II. It absorbs upfield well away from most other protons.
III. All twelve protons are in the same chemical environment so it gives a strong single signal.
Si(CH3)4 is non-polar and is not soluble in water.
Which factors help to explain why tetramethylsilane is used as a reference material in 1H NMR spectroscopy?
I. It is very unreactive so does not interfere with the sample.
II. It is volatile so can easily be removed from the sample after the spectrum has been run.
III. It is safe to handle as it is not toxic.
All three of these reasons together with the fact that it absorbs upfield well away from most other protons and that it gives a strong single signal (because it contains 12 protons in the same chemical environment) make it an ideal reference.
Which compound will show a signal in its 1H NMR spectrum that is split into a doublet?
A doublet will be the splitting pattern shown by a proton (or protons) which is (are) adjacent to just one single proton.
Which will show a single signal with no splitting pattern (i.e. a singlet) in their high resolution 1H NMR spectrum?
I. benzene
II. 2,2-dimethylpropane
III. 2-methylpropane
In both benzene, C6H6 and 2,2-dimethylpropane, C(CH3)4 all the hydrogen atoms in the molecule are in an identical chemical environment. In 2-methylpropane the single proton will be split by the protons in the three methyl groups and the signal from the methyl protons will be split into a doublet by the single proton.
Which row describes correctly the 1H NMR spectra for the two isomers of dibromoethane?
| 1,1-dibromoethane | 1,2-dibromoethane | ||||
| Row | Number of signals | Splitting pattern(s) | Number of signals | Splitting pattern(s) | |
| 1 | 1 | triplet | 1 | triplet | |
| 2 | 2 | doublet & quartet | 1 | singlet | |
| 3 | 2 | doublet & quartet | 2 | triplet & triplet | |
| 4 | 2 | triplet & singlet | 2 | singlet |
In 1,2-dibromethane all the hydrogen atoms are in an identical chemical environment so there will be a single signal that is not split. In 1,1-dibromoethane the single hydrogen atom on the first carbon atom will split the signal from the three hydrogen atoms on the adjacent carbon atom into a doublet and its own signal will be split by the methyl group into a quartet.
Which splitting patterns will be shown by the signals due to the protons marked a, b and c in the 1H NMR spectrum of ibuprofen?
a is split by the single proton on the adjacent carbon atom into a doublet. b is split by the three protons on the adjacent carbon atom into a quartet. As there are no protons on the carbon atom adjacent to the acidic proton, c is not split.
Which information from their 1H NMR spectra could be used to distinguish between methyl propanoate and ethyl ethanoate?
Methyl propanoate, CH3CH2COOCH3 and ethyl ethanoate, CH3COOCH2CH3 will both give three signals as the hydrogen atoms are in three separate environments. Both will have the same ratio of 3:3:2 and both will show a singlet (from the isolated -methyl group), a triplet and a quartet (due to the ethyl group). The only difference will be in the chemical shift.
Which compound shows a singlet, a triplet and a quartet splitting pattern for its three signals in its 1H NMR spectrum?
The ony two to show a singlet are propanoic acid and 3-methylbutan-2-one as they possess hydrogen atoms in a chemical environment where there are no other hydrogen atoms on adjacent carbon atoms. 3-methylbutan-2-one has a doublet as the two methyl groups are split by a single hydrogen atom. The ethyl group in propanoic acid provides the triplet and quartet.
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