Spectrometry Questions

Name: ……………………………………………Unit 4.11

Spectrometry questions

DATA

Table of infra-red absorption data

Bond / Wavenumber/cm–1
C–H / 2850–3300
C–C / 750–1100
C=C / 1620–1680
C=O / 1680–1750
C–O / 1000–1300
O–H (alcohols) / 3230–3550
O–H (acids) / 2500–3000

1.Each of the parts (a) to (f) below concerns a different pair of isomers. Deduce one possible structural formula for each of the species A to L, using, where appropriate, the table of infra-red absorption data provided.

(a)A and B have the molecular formula C2H4O2. An aqueous solution of A reacts with calcium carbonate to liberate carbon dioxide, but B does not.

(2)

(b)C and D have the molecular formula C3H8O. C has a broad absorption band at 3300 cm-1in its infra-red spectrum, but D does not.

(2)

Kinetics, Equilibria and Organic Chemistry 4.11Question sheet

(c)E and F have the molecular formula C3H6C12. E has only one peak in its low resolution proton n.m.r. spectrum but F has two.

(2)

(d)G and H have the molecular formula C3H6O and both have strong absorption bands at about 1700 cm-1in their infra-red spectra. G turns acidified potassium dichromate(VI) solution green but H does not.

(2)

(e)I and J have the molecular formula C4H9Br. I shows optical activity, but J does not.

(2)

(f)K and L have the molecular formula C5H10. K has a weak absorption band at 1650 cm-1in its infra-red spectrum, but L does not.

(2)

(Total 12 marks)

2.(a)The fragmentation of a molecular ion (M–R)+, formed in the ionisation chamber of a mass spectrometer, can be represented by the equation

(M–R)+ M+ + R

Identify the three types of species shown in the equation and explain what takes place in this conversion.

………………………………………………………………………………………………………………………………………………………………………………..

(4)

(b)The mass spectrum of chloroethane shows two molecular ion peaks at m/z values of 64 and 66. The peak at m/z = 64 is approximately three times as intense as that at m/z = 66. Explain this observation and show, by means of an equation, how the molecular ion of chloroethane fragments to give rise to a peak at an m/z value of 29.

(4)

(c)Suggest why the mass spectrum of 1,2-dichloroethane shows peaks at m/z values of 98, 100 and 102.

(4)

3.(a)Explain what is meant by the fingerprint region of an infra-red spectrum and describe how it is used to confirm the identity of organic molecules.

……………………………………………………………………………………………………………

(4)

(b)Each of three isomeric esters, Q, R and S, of molecular formula C5H10O2 has a branched structure.

Q has only two peaks in its low resolution proton n.m.r. spectrum, but R and S each have three peaks.

Hydrolysis of R in acid conditions forms compounds T and U. The low-resolution proton n.m.r. spectrum of T has three peaks and that of U has two peaks. Infra-red spectra of T and U are shown below.

Draw structures of the compounds Q, R,S, T and U and explain how the n.m.r. and i.r. data support your conclusions,using the data sheet provided.

Q / R
S / T
U

(11)

(Total 15 marks)

4.The proton n.m.r. spectrum of an organic compound F, C4H8O, is shown below.

The measured integration trace gives the ratio 0.5 to 0.75 to 0.75 for the peaks at  2.47, 2.15 and 1.05, respectively. The infra-red spectrum of compound F has an intense band at 1715 cm-1. In the mass spectrum of F, dominant fragmentation peaks appear at m/z = 43 and m/z = 57.

(a)Give the structural formula of the compound responsible for the signal at in the n.m.r. spectrum.

(1)

(b)How many different types of proton are present in compound F?

(1)

(c)What is the ratio of the numbers of each type of proton?

(1)

(d)Which alkyl group is responsible for the signal at 2.15 in the n.m.r. spectrum?

(1)

(e)The peaks at 2.47 and 1.05 arise from one group in compound F. Identify this group and explain the appearance of the splitting pattern shown in the n.m.r. spectrum.

Group......

Explanation ......

(3)

(f)Which group is responsible for the band at 1715 cm–1 in the infra-red spectrum of compound F?

(1)

(g)Give the structures of the species responsible for the peaks at m/z values of 43 and 57 in the mass spectrum.

Peak at m/z = 43......

Peak at m/z = 57......

(2)

(h)Give the structure of compound F.

(1)

(Total 11 marks)

Kinetics, Equilibria and Organic Chemistry 4.11Question sheet