Mass-spectrometry

The figure shows the mass-spectrum of an unknown substance. Determine its molecular weight. Identify a substance if it is composed of two atoms of the same element.

The figure contains the mass-spectrum of the three most intense peaks with masses of 158, 160 and 162. The intensities of the peaks with masses 158 and 162 are equal and the intensity of the peak with the mass of 160 is twice as high. In addition, in the mass-spectrum there are peaks with masses 79 and 81. Knowing that an unknown substance consists of two identical atoms, we can talk about the presence of two isotopes with masses 79 and 81. From these data it follows that three of the most intense peaks are compounds of two isotopes, and therefore the average value of the unknown substance is equal to 160. The weight average value of the element is equal to mass 80. This is bromine.

IR-spectrrometry

The infra-red spectrum of citronellol is shown below. Mark on this spectrum, by the letter X, the absorption that confirms the presence of the functional group that is most easily identified from this spectrum.

X as shown below.

IR-spectrrometry

Justify which of the three spectra shown below is most likely to be that of ethanal.

Spectrum C only shows absorption at 1700 cm-1 for the \(C=O\). The other two spectra contain the OH-group absorption at approx 3000 cm-1

 

 

IR-spectrrometry

Lavandulol can be oxidized to produce either compound Y or compound Z.

An infra-red spectrum of either compound Y or compound Z was obtained and was found to contain an absorption between \( 1680-1750 cm^{–1}\). However, there was no broad absorption between \(2500-3300 cm^{–1}\).  Use this information to deduce whether the infra-red spectrum was of compound Y or of compound Z.

.

The infra-red spectrum was of compound Y, because the absorption between \(1680-1750 cm^{-1}\) indicates a \(C=O \), and the absence of a peak between \(2500-3300 cm^{-1}\) shows the absence of the OH hydrogen bonded in a carboxylic acid .

Mass-spectrometry

Predict two fragment ions that you would expect to see in the mass spectrum of butan-1-ol and state the m/z value of each ion.

Predict two fragment ions that you would expect to see in the mass spectrum of butan-1-ol and state the m/z value of each ion.

These fragments are \(CH_3^+: 15, C_4H_9^+: 57\)

Mass-spectrometry

Compound X is an atmospheric pollutant emitted from fuel combustion of petrol and diesel vehicles. Compound X is a potent human carcinogen.

•      Analysis of compound X showed the following percentage composition by mass:
C, 88.89%; H, 11.1%.

•      Mass spectrometry showed a molecular ion peak at m/z = 54.

•      Compound X reacts with H2 in the presence of a nickel catalyst in a 1 : 2 molar ratio.

Analyse and interpret this information to determine a possible structure for compound X.

mole ratio = 88.89/12 : 11.1/1 = 7.41 : 11.1
empirical formula = \(C_2H_3\) 
relative mass of \(C_2H_3 = 27\).
Mr = 2 * 29 so molecular formula = \(C_4H_6\)
X reacts with 2 mol \(H_2\) so there are 2 double bonds

Possible structure = 1,3-butadiene 

Mass-spectrometry

Is it possible to use mass spectrometry outside of the laboratory and for what purposes?

Outside the laboratory mass-spectrometry can be used for monitoring of air pollution and MOT emission testing.

IR-spectrrometry

What does the absorption in the region \(1680–1750 cm^{–1} \)indicate?

The absorption in the region \(1680–1750 cm^{–1} \)indicates containing of a \(C=O\)/ aldehyde, ketone, carboxylic acid and ester/carbonyl/carbonyl in an aldehyde.

IR-spectrrometry

What does the absence of a broad absorption in the region \(2500–3300 cm^{–1}\) indicate?

The absence of a broad absorption in the region \(2500–3300 cm^{–1}\) indicates the lack of \(O-H\) (hydrogen bonded) in a carboxylic acid.

Mass-spectrometry

Determine the empirical formula of a hydrocarbon that contains 83,3% of carbon by mass.

The mole ratio = 83,3/12 : 16,7/1 = 6,94 : 16,7

The empirical formula = \(C_2H_5\)