For the reactions of 2-methyl-2-propanol ( tert -butyl alcohol) on p. 1 with hyd

Question

For the reactions of 2-methyl-2-propanol (tert-butyl alcohol) on p. 1 with hydrobromic acid (HBr) and sulfuric acid (H2SO4), both reactions have the identical mechanism up to the formation of the tertiary carbocation.

Which two factors favor substitution with hydrobromic acid (HBr) to give 2-bromo-2-methylpropane (tert-butyl bromide), instead of elimination to 2-methylpropene (isobutene)?

Acidity of hydrobromic acid (HBr)

Basicity of bromide (Br-)

Nucleophilicity of bromide (Br-)

Temperature of 20 oC instead of 100 oC

Acidity of hydrobromic acid (HBr)

Basicity of bromide (Br-)

Nucleophilicity of bromide (Br-)

Temperature of 20 oC instead of 100 oC

Explanation / Answer

Two conditions that favour the substitution reaction over elimination reaction are as given below.

1) Nuclephilicity of bromide ion. The nucleophilicty of bromide ion is higher than its basicity. This favors substitution reaction.

2) Temperature of 20 oC instead of 100 oC. At low temperature, thermodynamically controlled substitution product is favored and at high temperature kinetically controlled elimination product is formed.

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