Question

A student makes a solution containing ammonium chloride and ammonium bromide salts dissolved in concentrated sulfuric acid. She adds a portion of tert-butyl alcohol and shakes the mixture vigorously. After allowing the layers to separate, she dried the alkyl halide layer that resulted and analyzed it by GCMS.

a. Predict the products and their approximate percentages.

b. Draw the complete mechanisms to explain the formation of the products.

c. Explain your percentage predictions

d. Would the change to an aprotic solvent such as DMSO have an effect on the product ratio? How?

Answer 1

a) The reaction betwen ammonium chloride and ammonium bromide with conc. H2SO4 results HCl & HBr respectively. HCl and HBr ionizes to produce Cl- and Br- ions which can acts as nucleophiles.

NH4Cl + H2SO4 ----> NH4(HSO4) + HCl

NH4Br + H2SO4 ----> NH4(HSO4) + HBr

=> When t-butyl alcohol is added to the mixture it undergo dehydration to form t-butyl carbocation as reaction intermediate. This carbocation will reacts with nucleophiles Cl- and Br- present in the reaction mixture and forms t-butyl chloride and t-butyl bromide respectively in equal amounts(50:50)

b) The reaction between t-butyl alcohol and halides takes place via SN1 mechanism i.e. in two steps

c) Since the carbocation formed have planar geometry and hence both the nucelophiles can attack from both planes equally and thus produces equal amounts of alkylchloride and alky bromide.

d) If solvent is changes to aprotic solvent then the mechanism of the reaction change form SN1 to SN2. Then the raio of products depnds on the relative nucleophilicity of Cl- and Br-.

SInce Br- is nuclephilic than Cl-, it can attack on t-butyl alcohol more effectiely than Cl- and produces more amount of alkylbromide.