A student obtained a GC chromatogram of his distillation from an early fraction

Question

A student obtained a GC chromatogram of his distillation from an early fraction of
fraction distillation. His original mixture contained 50:50 (by volume) cyclohexane:
toluene. The GC showed the early fraction to contain only a trace amount of cyclohexane
and mostly toluene. Is this result in accord with distillation theory? If no, explain how
this result could have come about if the student followed identical procedure to the ones
we did in lab. -

The answer would be that this result is not in accord with distillation theory. According to theory, the substance with the lower boiling point should distill first and therefore the sample should contain mostly cyclohexane. However, in this case the student's sample contains mostly the compound with the higher boiling point - toluene, contradicting theory. Where I am lost is - how could this result come about from conducting a normal fractional distillation of cyclohexane and toluene??

Explanation / Answer

The boiling point of cyclohexane is 81C and that of toluene is 110C. Therefore, in accordance with the distillation theory one would expect cyclohexane to distill out first and the early fractio to show more of cyclohexane and less of toluene (although both will be present, the intial distillate will only be enriched in cyclohexane). If all goes well, the GC of this sample should show a higher concentration of cyclohexane than toluene. However, this is not observed. The possible reason could be improper injection of sample into the GC. If more volume of sample is injected than the injection port volume then some of it might fllashback. Also, if the temperature of the oven is not properly set, then there could be a potential loss of sample.

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