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Bromine exists as two isotopes, $^{79}$Br and $^{81}$Br - VCE - SSCE Chemistry - Question 2 - 2011 - Paper 1
Question 2
Bromine exists as two isotopes, $^{79}$Br and $^{81}$Br. The mass spectrum of bromomethane, C$_2$H$_3$Br, with two molecular ion peaks at m/z 108 and 110, is shown b... show full transcript
Worked Solution & Example Answer:Bromine exists as two isotopes, $^{79}$Br and $^{81}$Br - VCE - SSCE Chemistry - Question 2 - 2011 - Paper 1
Step 1
Step 2
What do the two molecular ion peaks indicate about the relative abundance of $^{79}$Br and $^{81}$Br? Give a reason for your answer.
Answer
The presence of two peaks at m/z = 108 and m/z = 110 indicates that both isotopes of bromine are present in the compound CHBr. The peak at m/z = 110, which corresponds to CH_3$$^{81}Br, is slightly higher than the peak at m/z = 108, which corresponds to CH_3$$^{79}Br. This suggests that the relative abundance of Br is slightly more than that of Br, likely due to the natural abundance ratios of the isotopes.
Step 3
In the boxes below, draw the structural formula of each of the two compounds that have the molecular formula C$_2$H$_3$Br.
Answer
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Structure 1: The structure of bromoethane, which is CHBr, has a straight-chain arrangement.
- Structure 2: The structure of vinyl bromide, CHBr, has a double bond between the two carbon atoms.
Step 4
Circle the structure above that corresponds to the 'H NMR spectrum provided. Justify your selection by referring to both the 'H NMR spectrum and to the structure of the compound.
Answer
The structure corresponding to the provided 'H NMR spectrum is Structure 2 (vinyl bromide, CHBr). This is justified as the NMR spectrum shows a peak corresponding to the vinyl protons, which would be absent in the corresponding alkane (bromoethane). The region around 5-6 ppm typically indicates the presence of hydrogen atoms mentioned in vinyl groups.