Question

1. a) Predict the behavior of all five compounds (biphenyl, benzophenone, ortho-dichlorobenzene, 1-naphthol, and benzhydrol (also called "benzohydrol") on both TLC (Thin Layer Chromatography) and GC-MS (Gas Chromatography-Mass Spectroscopy). Which one do you expect to have the largest Rf on the TLC? Which one will have the smallest Rf? Which will have the longest and shortest retention times on the GC?

1. b) Ortho-dichlorobenzene has a molecular weight of 147.00 g/mol, but its MS has a very small relative abundance of the 147 peak. Instead, it has peaks at 146 (100% relative abundance), 148 (65%), and 150 (10.6%). The peak at 147 is about 6.6% as big as the 146 peak. What causes this distinctive molecular ion region for this compound? What peaks (note the plural) would you expect to see on the mass spectrum for the fragment (note the lack of plural) caused by loss of one chlorine atom (one of the C-Cl bonds breaking) during the ionization process?

1. c) For the other four compounds, note which peak on the mass spectrum you expect to corresponds to the molecular ion for the molecule, and give the approximate M+1 abundance (relative to the molecular ion) expected based on the isotopic abundance the elements present. In addition, predict at least one major fragment which you would expect for each compound.

1. d) Based on your fragmentation analysis for the previous question, describe how you would expect to be able to tell these five compounds apart by their GC-MS even if you were unable to detect the molecular ion region (that is, if all three compounds fragmented 100% of the time).

Answer 1

1. a) The Rf on TLC and the retention times on GC-MS depends on the polarity of the compound.

The least polar compound will have the largest Rf on the TLC and the most polar compound will has the smallest Rf value.

The compound with more polarity will have the longest retention time on the GC-MS and the least polar compound will have the shortest retention time.

The increasing order of the polarity of the five compounds is as follows:

Biphenyl < ortho-dichlorobenzene < Benzophenone < Benzohydrol < 1-naphthol

From the above polarity order of the five compounds, the compound with largest Rf value on TLC is Biphenyl. And the compound with smallest Rf value is 1-naphthol.

Similarly on GC-MS, the compound with longest retention time is 1-naphthol and the compound with shortest retention time is Biphenyl.

b) The loss of one Cl atom for the molecular ion result in the peaks with m/z 77 , m/z 75, m/z 52 and m/z 39 .

The peak with m/z 113 is due to the isotopic forms of Chlorine atoms.

c) For biphenyl, the molecular ion peak is with m/z 154. M+1 peak is not expected as the natural isotopic abundance is very low. One of the prominent peak is (M-28).

For benzophenone, the molecular ion peak is with m/z 182. +1 peak is not expected as the natural isotopic abundance is very low. One of the prominent peak is (M-77)

For 1-naphthol, the molecular ion peak is with m/z 145. +1 peak is not expected as the natural isotopic abundance is very low. One of the prominent peak is (M-CO).

For benzohydrol, the molecular ion peak is with m/z 183. +1 peak is not expected as the natural isotopic abundance is very low. One of the prominent peaks are (M-CO) and (M-COH).

d) The compound may be identified by the fragment ions produced.