Alcohols: combustion, halogenation and dehydration (Core Practical 6)

Naming and classes

Suffix -ol with a number: CH3CH(OH)CH3 is propan-2-ol. Primary (RCH2OH), secondary (R2CHOH), tertiary (R3COH), classified like halogenoalkanes.

Reactions

• Combustion: C2H5OH + 3O2 -> 2CO2 + 3H2O.
• Halogenation:

- PCl5 at room temperature: ROH + PCl5 -> RCl + POCl3 + HCl (steamy fumes; also the test for -OH)

- KBr and 50% concentrated H2SO4, warmed: makes HBr in situ, giving the bromoalkane

- red phosphorus and iodine, warmed: makes PI3 in situ, giving the iodoalkane

• Dehydration: heat with concentrated phosphoric acid: C2H5OH -> CH2=CH2 + H2O (elimination).

Core Practical 6

Shake 2-methylpropan-2-ol with concentrated hydrochloric acid in a separating funnel at room temperature (tertiary alcohols react readily), releasing pressure often:

(CH3)3COH + HCl -> (CH3)3CCl + H2O

Run off the lower aqueous layer. Wash the organic layer with sodium hydrogencarbonate solution to remove acid (vent the CO2), then with water. Dry with an anhydrous salt such as anhydrous sodium sulfate, then distil, collecting the fraction near the product's boiling point (around 50 °C).

Worked example

Give the reagent, conditions and organic product for converting propan-1-ol into 1-bromopropane.

1. HBr is made in the flask from KBr and concentrated sulfuric acid.
2. Warm the mixture and distil off the product.

Answer: KBr and 50% concentrated H2SO4, warm; CH3CH2CH2Br

Check yourself

1. Which products form when ethanol reacts with PCl5?
   • CH3CH2Cl, POCl3 and HCl
   • CH3CH2Cl and H2O only
   • CH2=CH2 and HCl
   • CH3CHO and HCl
   Show answer

   CH3CH2Cl, POCl3 and HCl. PCl5 replaces -OH with Cl, giving steamy HCl fumes.

2. Which reagent dehydrates an alcohol to an alkene?
   • Aqueous sodium hydroxide
   • Acidified potassium dichromate(VI)
   • Concentrated phosphoric acid, heated
   • PCl5
   Show answer

   Concentrated phosphoric acid, heated. Hot concentrated H3PO4 removes water.

3. Which reagents convert an alcohol into an iodoalkane?
   • Red phosphorus and iodine
   • Potassium iodide and water
   • Iodine solution at room temperature
   • Silver iodide
   Show answer

   Red phosphorus and iodine. PI3 is made in situ and reacts with the alcohol.

4. In Core Practical 6, why is the organic layer washed with sodium hydrogencarbonate solution?
   • To dry the product
   • To oxidise the alcohol
   • To make the product more volatile
   • To neutralise and remove remaining acid
   Show answer

   To neutralise and remove remaining acid. NaHCO3 reacts with HCl, giving CO2, so the funnel must be vented.

5. What is anhydrous sodium sulfate used for in Core Practical 6?
   • Neutralising acid
   • Removing water from the organic product
   • Catalysing the reaction
   • Increasing the yield
   Show answer

   Removing water from the organic product. It is a drying agent; the liquid turns from cloudy to clear.

6. How many moles of O2 are needed to burn 1 mol of propan-1-ol, C3H7OH, completely?
   Show answer

   4.5 mol. C3H7OH + 4.5O2 -> 3CO2 + 4H2O. O needed: 6 + 4 = 10 atoms, minus 1 in the alcohol = 9 atoms = 4.5 mol O2.

7. Why can 2-methylpropan-2-ol be chlorinated with concentrated HCl at room temperature?
   • It is a primary alcohol
   • It is a tertiary alcohol, which reacts readily
   • HCl is a catalyst
   • It contains a C=C bond
   Show answer

   It is a tertiary alcohol, which reacts readily. Tertiary alcohols substitute easily via a stable carbocation.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.