Question

Competing Nucleophiles

NUCLEOPHILIC SUBSTITUTION REACTIONS:

COMPETING NUCLEOPHILES

1.      Give a full balanced equation for the reaction of 1-Butanol and 2-methyl-2-propanol (t-butyl alcohol) as per this experiment. Why are ammonium salts used here instead of using the related acids HBr and HCl?

2.    Using the two GC Chromatograms provided showing the analysis the product mixture of each of the reactions, determine the % relative amounts of alkyl chloride and alkyl bromide in each of the two products. Show your calculations in full.

Answer 1

The ammonium salts are used in this reaction to not compete as a nucleophile in this reaction, as ammonium is a poor nucleophile. Still, if HBr and HCl are used, then Cl- or Br- can compete as a nucleophile and affect the yield and mechanism of the reaction.

Answer 2

2. Dehydration of 2-methylcyclohexanol: Gas Chromatographic Analysis of the resulting product mixture.

In Gas Chromatography ("GC," or sometimes "VPC" for Vapor Phase Chromatography), we inject a liquid solution which is rapidly vaporized and then passed as a vapor over a solid material which usually has a very high boiling liquid (such as Carbowax) adsorbed on it to act as the "adsorbent." The separation occurs as the vaporized mixture is adsorbed, vaporized, reabsorbed, vaporized, etc., as it passes along the column. Since different materials will be adsorbed and vaporized at different rates, separation will occur if the column is long enough. The various components will come off the column at different times ("retention times," or 'Rt'). As each component exits the column, it will be detected and registered as a "peak" on the recorder.

Helium is commonly used as a carrier gas. The gas must be supplied at a controlled flow rate. Liquid samples are vaporized in a heated chamber. A thermal conductivity cell is commonly used as the detector. A chart recorder makes a record of the elution of components of the mixture.

The column is the critical component of a vapor phase chromatograph. For separation to occur, conditions must be achieved that result in components of the mixture having different retention times on the column.

Four experimental factors affect the retention time of a given compound:

•the length of the column

•the temperature at which the column is maintained

•the rate of flow of the carrier gas

•the nature of the stationary liquid phase

Apiezon grease (SE-30) (nonpolar) < Silicone oil < Carbowax (polar)

A vapor phase chromatogram may be used in the qualitative and quantitative analysis of a sample of volatile components.

For a given set of conditions, retention time is a reproducible property of a compound. It is useful in the identification of a compound. The area under the peak is related to the amount of compound that is characterized by the peak. The relative area of the two peaks of a binary mixture chromatogram essentially describes the weight percentage ratio in the mixture.