Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 10.14 CORE PRACTICAL 6: Chlorination-Study Notes - New Syllabus

10.14 Core Practical 6: Chlorination of 2-methylpropan-2-ol with Concentrated Hydrochloric Acid

This practical investigates the reaction of a tertiary alcohol, 2-methylpropan-2-ol, with concentrated hydrochloric acid to form a tertiary halogenoalkane.

The reaction proceeds rapidly via a nucleophilic substitution mechanism and demonstrates the high reactivity of tertiary alcohols.

Principle of the Reaction

2-methylpropan-2-ol reacts with concentrated \( \mathrm{HCl} \) to form 2-chloro-2-methylpropane and water.

\( \mathbf{(CH_3)_3COH + HCl \rightarrow (CH_3)_3CCl + H_2O} \)

The reaction proceeds via an \( \mathrm{S_N1} \) mechanism due to the formation of a stable tertiary carbocation.

Mechanism (S_N1)

• The \( \mathrm{C-OH} \) bond is first protonated by \( \mathrm{H^+} \), making it a better leaving group.
• Water leaves, forming a stable tertiary carbocation.
• The \( \mathrm{Cl^-} \) ion acts as a nucleophile and attacks the carbocation.
• A tertiary halogenoalkane is formed.

Observations

• The mixture becomes cloudy or forms two layers.
• The halogenoalkane formed is insoluble in water, producing a cloudy appearance.
• The reaction occurs rapidly at room temperature.

Explanation of Observations

• The tertiary carbocation forms easily due to stabilisation by three alkyl groups.
• This leads to a fast \( \mathrm{S_N1} \) reaction.
• The product (2-chloro-2-methylpropane) is non-polar and immiscible with the aqueous layer.

Variables

• Independent variable: type of alcohol (primary, secondary, tertiary).
• Dependent variable: rate of formation of cloudiness or product.
• Controlled variables: temperature, volume of reagents, concentration of acid.

Key Points

• Tertiary alcohols react rapidly with concentrated \( \mathrm{HCl} \).
• The reaction proceeds via a carbocation intermediate.
• This practical demonstrates the effect of structure on reaction rate.