Question

A student completes the above experiment and gets a product, but gets a negative result when he/she tests his/her product for the presence of an alkyl halide. In which step is it mostly likely that he/she made a mistake, and what might his/her product be after the second distillation?

(The experiment is preparing 1-bromobutane from 1-butanol, tests are sodium iodide test and silver nitrate test)

Answer 1

Negative results for the presence of alkyl halide clearly indicates that the bromination of 1-butanol has not taken place or it is hydrolysed back to the starting alcohol.

The first assumption is very unlikely as the reaction of 1-butanol with the brominating mixture takes place very easily and would have certainly taken place.

Now, the second one is more likely where, after first distillation, the obtained crude 1-bromobutane is washed with the sodium hydroxide more than the required time or with higher strength of NaOH, before the second distillation.

This will hydrolyse the crude 1-bromobutane back to the 1-butanol and after second distillation, what he got is the starting 1-butanol only.

R-Br + NaOH --> R-OH + NaBr

That is the most possible reason that he is not getting the positive results for the alkyl halides. The sodium iodide and the silver nitrate tests are very sensitive and selective for the alkyl halides and  there is no chance that the presence of alkyl halide will not be detected by these tests.