UNIT 2 CHEMISTRY · PEARSON AAQ · E1.7.4
Substitution of Alcohols with PCl₅ and HBr
FREE CHEMISTRY REVISION · TOPIC 59
Substitution of Alcohols with PCl₅ and HBr
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · E1.7.4
What you will learn
Predict, draw and name the halogenoalkanes formed from alcohols using phosphorus(V) chloride, PCl₅, or hydrogen bromide, HBr. Write balanced equations including the different by-products, and state the conditions. Pearson E1.7.4 requires these two substitution routes; detailed mechanisms and practical techniques are not required.
1. Recognise the substitution
An alcohol contains an OH group bonded through oxygen to carbon. In the reactions studied here, that whole OH group is replaced by a halogen atom: R–OH becomes R–Cl or R–Br. It is not simply the hydrogen on oxygen that is replaced. For the simple examples below, retain the carbon framework and put the halogen on the carbon that originally carried OH.
| Reagent | Organic product | Other products | Standard conditions |
|---|---|---|---|
| Phosphorus(V) chloride, PCl₅ | Chloroalkane, R–Cl | POCl₃ and HCl | Dry PCl₅; reaction occurs at room temperature, without UV |
| Hydrogen bromide, HBr | Bromoalkane, R–Br | H₂O | For the primary-alcohol examples: concentrated aqueous HBr and heat; heating under reflux is a standard condition |
The two routes have different by-products and must not be combined into one memorised equation. The temperatures and speeds depend on the alcohol and method: more reactive alcohols can react with HX under milder conditions. There is no universal numerical temperature specified for every alcohol in Pearson’s AAQ point.
2. Alcohols with PCl₅
R–OH + PCl₅ → R–Cl + POCl₃ + HCl
PCl₅ is phosphorus(V) chloride, also called phosphorus pentachloride. POCl₃ is phosphoryl chloride, also called phosphorus oxychloride. PCl₅ supplies five Cl atoms overall: one appears in the chloroalkane, three in POCl₃ and one in HCl.
Use dry reagent/conditions: water reacts with PCl₅ and would consume it. For these simple alcohol examples the reaction occurs at room temperature; neither UV nor a metal catalyst is needed. Hydrogen chloride gas can form acidic mist in moist air. These observations are context for the equation, not a practical instruction or an invitation to perform the reaction.
Worked reaction 1 — ethanol with PCl₅
CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl
Product: chloroethane. Conditions: dry PCl₅, room temperature. Check C=2 and O=1 each side. H: 6 on the left, 5+1=6 on the right. Cl: 5=1+3+1. P: 1=1. Do not add H₂O to this equation.
Worked reaction 2 — propan-1-ol
CH₃CH₂CH₂OH + PCl₅ → CH₃CH₂CH₂Cl + POCl₃ + HCl
Product: 1-chloropropane. The OH was terminal, so Cl must be terminal. The organic formula changes C₃H₈O → C₃H₇Cl; the removed O and H are accounted for in the inorganic by-products. Use dry PCl₅ at room temperature.
Worked reaction 3 — propan-2-ol
CH₃CH(OH)CH₃ + PCl₅ → CH₃CHClCH₃ + POCl₃ + HCl
Product: 2-chloropropane. The middle carbon retains two C–C bonds and one C–H bond; its C–O bond is replaced by C–Cl. It still has four bond units. Do not move Cl to the end or draw carbon bonded to both the old OH and the new Cl.
3. Check the complete OH replacement
Worked reaction 4 — methanol
CH₃OH + PCl₅ → CH₃Cl + POCl₃ + HCl
The organic product is chloromethane, not CH₃OCl. CH₃Cl contains one carbon, three H and one Cl; oxygen is absent from the organic product. The displayed diagram shows every organic atom and bond, and each carbon has four single bonds.
4. Alcohols with HBr
R–OH + HBr → R–Br + H₂O
For ethanol and other simple primary alcohols, use concentrated aqueous hydrobromic acid with heat; heating under reflux is a common condition. HBr may be supplied directly or generated in the reaction mixture by an appropriate bromide/acid system. In situ means made in that mixture. Detailed reagent preparation and apparatus are not required here.
HBr is a reactant, not a catalyst: its Br enters the organic product and its H contributes to the water product in the overall equation. Do not give the UV conditions used for substituting an alkane, or substitute Br₂ for HBr. Br₂ does not describe this alcohol-to-bromoalkane route.
Worked reaction 5 — ethanol with HBr
CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O
Product: bromoethane. Use concentrated aqueous HBr and heat/reflux for this primary-alcohol example. Atom check: C=2, Br=1, O=1 on each side. Total H: 6+1=7 on the left, 5+2=7 on the right. Unlike the PCl₅ route, this route produces water and no phosphorus compound.
Worked reaction 6 — propan-1-ol with HBr
CH₃CH₂CH₂OH + HBr → CH₃CH₂CH₂Br + H₂O
Product: 1-bromopropane, C₃H₇Br. Use concentrated aqueous HBr with heat/reflux. The carbon chain retains three carbons, and the Br replaces OH on the terminal carbon. CH₃CHBrCH₃ would put Br in the wrong position for this straightforward substitution.
Worked reaction 7 — butan-1-ol with HBr
CH₃CH₂CH₂CH₂OH + HBr → CH₃CH₂CH₂CH₂Br + H₂O
Product: 1-bromobutane. The starting alcohol is C₄H₁₀O and the organic product is C₄H₉Br. Left H=10+1=11; right H=9+2=11. Use concentrated HBr and heating/reflux. A balanced molecular formula alone would not identify the terminal bromine position; retain the structural formula.
5. Compare routes and work backwards
| Alcohol | With PCl₅ | With HBr |
|---|---|---|
| Ethanol | Chloroethane + POCl₃ + HCl | Bromoethane + H₂O |
| Propan-1-ol | 1-Chloropropane + POCl₃ + HCl | 1-Bromopropane + H₂O |
| Propan-2-ol | 2-Chloropropane + POCl₃ + HCl | 2-Bromopropane + H₂O |
| Butan-1-ol | 1-Chlorobutane + POCl₃ + HCl | 1-Bromobutane + H₂O |
Worked example 8 — choose a starting alcohol and reagent
Target: 2-bromopropane, CH₃CHBrCH₃. Start from propan-2-ol, CH₃CH(OH)CH₃, and use HBr under appropriate acidic substitution conditions, commonly warmed/heated. The desired halogen is Br, so PCl₅ would give the wrong halogenoalkane. Propan-1-ol would place the substituent on the wrong carbon. State the reagent and conditions as well as the product.
Worked example 9 — compare theoretical amounts
Both equations use one mole of reagent per mole of a monohydric alcohol. For 0.0500 mol ethanol reacting by the stated substitution, the theoretical amount of organic halide is 0.0500 mol. With PCl₅, theoretical HCl is also 0.0500 mol; with HBr, theoretical H₂O is 0.0500 mol. These are stoichiometric limits; incomplete conversion, competing pathways and losses can reduce isolated yield.
Practice with answers
| Task | Answer |
|---|---|
| Name CH₃CHClCH₃ | 2-Chloropropane. |
| Write the other products for ethanol + PCl₅ | POCl₃ and HCl; no water in this overall equation. |
| Which reagent gives Br replacing OH? | HBr, not Br₂ or PCl₅. |
| Does the organic halide retain oxygen? | No. The whole OH is replaced in these routes. |
| Must every alcohol/HBr reaction use the same exact temperature? | No. Reactivity depends on the alcohol; give the stated standard conditions for the example. |
Common misconceptions
- Only O–H hydrogen is replaced, leaving C–O–X: the specified organic product is C–X.
- PCl₅ gives a bromoalkane: it gives a chloroalkane.
- The PCl₅ and HBr routes have the same by-products: PCl₅ gives POCl₃ + HCl; HBr gives H₂O.
- Both routes require UV: UV belongs to the alkane-halogen substitution studied earlier.
- HBr is a catalyst: it is consumed in the balanced reaction.
- All Cl atoms in PCl₅ enter one organic molecule: the other four are in POCl₃ and HCl.
- An overall balanced equation guarantees a pure quantitative yield: it gives atom ratios, not experimental selectivity or yield.
Examination tips
- Identify which carbon carries OH before replacing that group.
- Name the halogenoalkane with the correct chain length and locant.
- Learn the PCl₅ equation including both inorganic by-products.
- State dry room-temperature PCl₅ conditions versus concentrated aqueous HBr and heating for the primary examples.
- Check oxygen and all five chlorine atoms particularly carefully in PCl₅ equations.
- Give conditions without expanding into mechanisms or practical procedures.
Reference checks: Pearson, alcohol-to-halogenoalkane reagent overview; RSC teaching notes, HBr generated in situ and heating under reflux. The required AAQ scope is linked below.
Check your understanding
Choose one answer, select Check Answer, then read the explanation. Try Again resets the question. Use Tab between controls, arrow keys within a radio group, and Enter or Space on buttons. Answers are not saved.