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4.2.3Organic Chemistry

Chlorination of 2-Methylpropan-2-ol — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 39 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Section 1 of 6The Reaction

A tertiary alcohol is converted into a tertiary halogenoalkane by concentrated hydrochloric acid.

Equation
  • Equation (CH₃)₃COH + HCl → (CH₃)₃CCl + H₂O, replacing the OH group with a chlorine atom.
Tertiary alcohol
  • Tertiary alcohol The OH sits on a carbon bonded to three other carbons, which makes substitution happen readily at room temperature.
Concentrated HCl
  • Concentrated HCl A high concentration of chloride ions and hydrogen ions drives the substitution forward.
Two layers
  • Two layers The organic product is insoluble in water and forms a separate layer above the aqueous acid.

Section 2 of 6Preparation Steps

The product is made and then purified in a fixed sequence.

1
In a fume cupboard add concentrated hydrochloric acid in small portions to 2-methylpropan-2-ol in a separating funnel.
2
Release the pressure regularly by opening the stopper, then leave the funnel to stand, shaking gently at intervals.
3
Allow the layers to separate and run off the lower aqueous layer through the tap.
4
Wash the organic layer with sodium hydrogencarbonate solution to remove acid, venting frequently because carbon dioxide forms.
5
Dry the organic liquid with anhydrous magnesium sulfate, decant it, and distil, collecting the fraction around 47–53 °C.

Section 3 of 6Purification Reasoning

Purification Reasoning
Separating funnel
  • Separating funnel Allows immiscible liquids to be separated cleanly by running off the lower layer.
Sodium hydrogencarbonate
  • Sodium hydrogencarbonate Neutralises leftover hydrochloric acid, producing CO₂ gas, so the funnel must be vented.
Anhydrous magnesium sulfate
  • Anhydrous magnesium sulfate Absorbs water from the organic layer, which turns from cloudy to clear when the drying is complete.
Distillation range
  • Distillation range The product boils near 51 °C, so collecting a narrow range removes impurities with different boiling points.

Section 4 of 6Safety and Yield

Fume cupboard
  • Fume cupboard Concentrated hydrochloric acid gives off corrosive hydrogen chloride gas, so the experiment must be done in a fume cupboard.
Pressure build-up
  • Pressure build-up Gases formed in a closed funnel can force the stopper out, so open the tap and vent regularly.
Flammability
  • Flammability The product is flammable, so keep it away from naked flames during distillation.
Yield
  • Yield Losses at every transfer, incomplete reaction and the product's volatility all reduce the percentage yield.

Section 5 of 6Worked Example

Question

  • 8.0 cm³ of 2-methylpropan-2-ol (density 0.78 g cm⁻³, M = 74.12 g mol⁻¹) gives 4.20 g of 2-chloro-2-methylpropane (M = 92.57 g mol⁻¹).
  • Calculate the percentage yield.

  1. Mass of alcohol = 8.0 × 0.78 = 6.24 g.
  2. Moles of alcohol = 6.24 ÷ 74.12 = 0.0842 mol.
  3. The reaction is 1 : 1, so the theoretical moles of product = 0.0842 mol.
  4. Actual moles obtained = 4.20 ÷ 92.57 = 0.04537 mol.
  5. Percentage yield = 0.04537 ÷ 0.0842 × 100 = 53.9%.

Answer Percentage yield = 53.9%

Section 6 of 6Exam Checklist & Tips

4.2.3 – Key Points for the Exam

  • Separating funnel Apparatus used to separate two immiscible liquids.
  • Drying agent Anhydrous salt, such as magnesium sulfate, used to remove traces of water from an organic liquid.
  • Tertiary alcohol An alcohol with the OH group on a carbon attached to three other carbon atoms.
  • Immiscible Describes liquids that do not mix and form separate layers.

Common Mistakes

  • Forgetting to release the pressure in the separating funnel after adding sodium hydrogencarbonate.
  • Using hydrated magnesium sulfate, which cannot remove water.
  • Collecting the whole distillate rather than only the fraction near the product's boiling point.
  • Running off the wrong layer; the organic layer here is the upper layer.

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