Question

From the following website: find the spectra for: I) benzyl bromide II) benzyl chloride III) p-nitrotoluene a) Describe how you would use relative Isotopic abundances and the nitrogen rule to discern the presence of Br, Cl, and N in benzyl bromide, benzyl chloride and p-nitrotoluene, respectively by simply looking at their mass spectra. benzyl bromide _____ benzyl chloride _____ p-nitrotoluene _____ b) What is the structure of the molecular ion of each? Include the correct charge and mass to charge ratio, m/z. benzyl bromide benzyl chloride p-nitrotoluene c)) What Is the mass: charge (m/z) value of the base peak of each?? benzyl bromide benzyl chloride p-nitrotoluene d) Give the fragment (with its correct charge and corresponding m/z value) responsible for the base peak of benzyl bromide and benzyl chloride? What common name is given to this fragment? e) Propose a synthesis for benzyl bromide from toluene (PhCH_3).

Answer 1

a) benzyl bromide shows m/z values at M and M+2 in 1:1 ratio due to 50:50 isotopic abundance of Br79 and Br81

benzyl chloride shows m/z values at M and M+2 in 3:1 ratio due to 75:25 isotopic abundance of Cl35 and Cl37

Nitrogen rule: Compound having odd no. of nitrogen contain m/z peaks at even numbers. p-nitrotoulene contain one N and shows m/z at even number

b) Molecular ion peaks

c) Base peak of benzyl bromide and benzylchloride is 77(C6H5+) and base peak of p-nitrotoulene is m/z=91 (benzyl cation)

d) Fragmentation

e) Benzyl bromide can be synthesied from toulene by using N-bromosuccinimide or Br2 in presence of light

Ph-CH3 + Br2 +hv -----> Ph-CH2-Br