Halogenoalkanes and Alcohols — A-Level Chemistry Study Guide

For: A-Level Chemistry candidates sitting A-Level Chemistry.

Covers: Nucleophilic substitution (SN1/SN2) of halogenoalkanes, elimination reactions, key alcohol reactions (oxidation, dehydration, esterification), and functional group distinguishing tests.

You should already know: IGCSE Chemistry, basic algebra.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the A-Level Chemistry style for educational use. They are not reproductions of past Cambridge International examination papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official Cambridge mark schemes for grading conventions.

1. What Is Halogenoalkanes and Alcohols?

Halogenoalkanes are alkane derivatives with one or more hydrogen atoms replaced by halogen atoms (F, Cl, Br, I), while alcohols are alkane derivatives with a hydroxyl (-OH) functional group bound to an sp³ hybridized carbon. These two homologous series are core to A-Level organic chemistry, as they are common synthetic intermediates for a wide range of organic products, and make up ~10-15% of the organic chemistry content in A-Level Chemistry Papers 1, 2 and 4. Synonyms for halogenoalkanes include haloalkanes and alkyl halides; alcohols may be referred to as alkanols in older syllabus documents.

2. Nucleophilic substitution — SN1 vs SN2

Nucleophilic substitution is a reaction where an electron-rich nucleophile (Nu⁻, e.g. OH⁻, CN⁻, NH₃) displaces a halide leaving group (X⁻) from the electrophilic carbon bonded to the halogen in a halogenoalkane. There are two distinct mechanisms, tested frequently in A-Level exams:

SN2 (Bimolecular Nucleophilic Substitution)

The reaction rate depends on the concentration of both the halogenoalkane and the nucleophile, so it is second order overall: $$\text{rate}=k[\text{R-X}][{\text{Nu}}^{-}]$$ Mechanism: The nucleophile attacks the electrophilic carbon from the back side (opposite the leaving group, as steric hindrance blocks front-side attack), forming a high-energy transition state with partial bonds to both the nucleophile and leaving group. The leaving group then dissociates, resulting in inversion of configuration (Walden inversion) at the reactive carbon. SN2 is favored by primary halogenoalkanes (least steric hindrance around the reactive carbon) and polar aprotic solvents (no O-H or N-H bonds, e.g. propanone, which do not solvate the nucleophile, increasing its reactivity).

SN1 (Unimolecular Nucleophilic Substitution)

The reaction rate only depends on the concentration of the halogenoalkane, so it is first order overall: $$\text{rate}=k[\text{R-X}]$$ Mechanism: The slow, rate-determining step is dissociation of the halogenoalkane to form a planar carbocation intermediate. The nucleophile then attacks the positively charged carbon from either side of the plane, forming a racemic mixture of products if the reactive carbon is chiral. SN1 is favored by tertiary halogenoalkanes (the tertiary carbocation is stabilized by hyperconjugation from adjacent alkyl groups) and polar protic solvents (e.g. water, ethanol, which solvate the carbocation and leaving group, speeding up dissociation).

Worked Example: State which mechanism 2-bromo-2-methylpropane (tertiary halogenoalkane) follows when reacted with aqueous sodium hydroxide, and give the rate equation.

Answer: It follows the SN1 mechanism, as it forms a stable tertiary carbocation intermediate that cannot be formed by primary or secondary halogenoalkanes. The rate equation is $\text{rate}=k[{\text{(CH}}_3{\text{)}}_3\text{CBr}]$. Exam tip: Always draw curly arrows starting from lone pairs or electron-dense bonds when drawing mechanisms, as examiners mark these explicitly.

3. Elimination reactions

Elimination is a reaction where a small molecule (e.g. HX, H₂O) is removed from an organic molecule to form a C=C double bond, producing an alkene. For halogenoalkanes, this reaction is called dehydrohalogenation: the halogen atom is removed from one carbon, and a hydrogen atom is removed from an adjacent carbon (called the β-carbon).

Key reaction conditions

Elimination is favored when the halogenoalkane is heated under reflux with concentrated ethanolic sodium or potassium hydroxide. The hydroxide ion acts as a base (not a nucleophile, as in substitution) to remove the β-hydrogen. To distinguish from substitution: aqueous hydroxide favors substitution (alcohol product), while ethanolic hydroxide favors elimination (alkene product).

Zaitsev's Rule

When multiple alkene products are possible, the major product is the most substituted alkene (the one with the most alkyl groups attached to the double bond carbons), as it is the most thermodynamically stable.

Worked Example: 2-bromobutane is heated under reflux with concentrated ethanolic KOH. Name the major organic product and explain your answer.

Answer: The major product is but-2-ene, per Zaitsev's rule. But-2-ene has two alkyl groups attached to its double bond carbons, so it is more stable than the minor product but-1-ene, which only has one alkyl group attached to its double bond.

4. Alcohols — oxidation, dehydration, esterification

Alcohols are classified by the number of alkyl groups attached to the carbon bonded to the -OH group: primary (1 alkyl group, RCH₂OH), secondary (2 alkyl groups, R₂CHOH), tertiary (3 alkyl groups, R₃COH). Their reactivity varies significantly by classification, a common exam question focus:

1. Oxidation

Uses the oxidizing agent acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄), which changes from orange (Cr₂O₇²⁻) to green (Cr³⁺) when reduced:

• Primary alcohols: Oxidized first to aldehydes if distilled off immediately, then further oxidized to carboxylic acids if heated under reflux with excess oxidizing agent.
• Secondary alcohols: Only oxidized to ketones, with no further oxidation possible.
• Tertiary alcohols: No oxidation under these conditions, as there is no hydrogen atom attached to the carbon bonded to the -OH group to remove, so no color change is observed.

2. Dehydration

Elimination of water to form an alkene, following Zaitsev's rule for major products. Conditions: Concentrated H₂SO₄ or H₃PO₄ catalyst heated to 170°C, or passing alcohol vapor over heated Al₂O₃ catalyst.

3. Esterification

Reversible condensation reaction between an alcohol and a carboxylic acid to form an ester and water, catalyzed by concentrated H₂SO₄ and heated under reflux: $$\text{RCOOH}+\text{R’OH}\underset{\Delta }{\overset{\text{conc }{H}_{2}S{O}_4}{\rightleftharpoons }}\text{RCOOR’}+H_{2}O$$ Esters are named as alkyl alkanoate, where the alkyl group comes from the alcohol and the alkanoate group comes from the carboxylic acid (e.g. ethanol + ethanoic acid → ethyl ethanoate).

Worked Example: State the product formed when propan-2-ol is heated under reflux with excess acidified K₂Cr₂O₇, and describe the observed color change.

Answer: Propan-2-ol is a secondary alcohol, so it is oxidized to propanone. The color of the reaction mixture changes from orange to green.

5. Distinguishing tests for functional groups

These tests are examined in both multiple choice and practical (Paper 3) questions, so you must memorize reagents, conditions, and observations clearly:

1. Halogenoalkane test: Add ethanolic silver nitrate (AgNO₃) and heat. The halide leaving group forms a silver halide precipitate: yellow (AgI, fastest to form, weakest C-I bond), cream (AgBr, medium rate), white (AgCl, slowest, strongest C-Cl bond). Tertiary halogenoalkanes form precipitates faster than secondary, which are faster than primary, as SN1 dissociation is faster for more stable carbocations.
2. Alcohol (-OH) test: Add phosphorus pentachloride (PCl₅) at room temperature. Steamy fumes of HCl are produced, which turn damp blue litmus paper red and form white fumes with ammonia gas. Note: Carboxylic acids also give a positive result, so first test with sodium carbonate: if no effervescence (no CO₂ produced), the positive PCl₅ test confirms an alcohol.
3. Primary/secondary vs tertiary alcohol test: Heat with acidified K₂Cr₂O₇. Orange to green color change confirms primary/secondary alcohol; no color change confirms tertiary alcohol.
4. Aldehyde vs ketone test (from alcohol oxidation products): Warm with Tollens' reagent (ammoniacal silver nitrate). A silver mirror on the test tube wall confirms an aldehyde; no change confirms a ketone. Alternatively, warm with Fehling's solution: blue to brick red precipitate of Cu₂O confirms an aldehyde.

Worked Example: You have three unlabelled test tubes containing 1-chlorobutane, 1-bromobutane, and 1-iodobutane. Describe a test to distinguish them, including observations.

Answer: Add equal volumes of ethanolic silver nitrate to each test tube, and heat in a 60°C water bath. The test tube with 1-iodobutane will form a yellow precipitate first, followed by a cream precipitate in the 1-bromobutane test tube, and finally a white precipitate in the 1-chlorobutane test tube. This is because C-X bond strength increases in the order C-I < C-Br < C-Cl, so weaker bonds dissociate faster to form the silver halide precipitate.

6. Common Pitfalls (and how to avoid them)

• Wrong move: Stating aqueous NaOH for halogenoalkane elimination reactions. Why students do it: Mix up substitution and elimination solvents. Correct move: Memorize "aqueous = substitution (alcohol product), ethanolic = elimination (alkene product)" — examiners award explicit marks for correct solvent.
• Wrong move: Claiming tertiary alcohols can be oxidized by acidified dichromate. Why students do it: Forget tertiary alcohols have no hydrogen attached to the -OH bound carbon. Correct move: Draw the alcohol structure to confirm its classification before answering oxidation questions.
• Wrong move: Drawing SN2 mechanisms with front-side nucleophile attack. Why students do it: Ignore steric hindrance from the leaving group. Correct move: Always draw the nucleophile attacking from the opposite side of the leaving group for SN2, and show the transition state with partial dative bonds.
• Wrong move: Using only PCl₅ to distinguish alcohols from carboxylic acids. Why students do it: Only associate PCl₅ with -OH groups. Correct move: First add sodium carbonate: if no effervescence (no CO₂), the PCl₅ test confirms an alcohol, as carboxylic acids react with carbonates to produce CO₂.
• Wrong move: Stating Zaitsev's rule only applies to halogenoalkane elimination. Why students do it: Learn it exclusively in the halogenoalkane topic. Correct move: It applies to all elimination reactions producing alkenes, including alcohol dehydration.

7. Practice Questions (A-Level Chemistry Style)

Question 1

2-chlorobutane reacts with aqueous sodium hydroxide via a mixture of SN1 and SN2 mechanisms. (a) (i) State the order of reaction with respect to NaOH for the SN1 mechanism. [1 mark] (ii) Explain why 2-chlorobutane can react via both mechanisms, while 1-chlorobutane only reacts via SN2. [3 marks] (b) State the displayed formula of the organic product of this reaction. [1 mark]

Worked Solution

(a) (i) 0. The SN1 rate only depends on halogenoalkane concentration, so NaOH does not appear in the rate equation. (ii) 2-chlorobutane is a secondary halogenoalkane: it has moderate steric hindrance, so backside attack for SN2 is possible, and it forms a relatively stable secondary carbocation, so SN1 can also occur. 1-chlorobutane is a primary halogenoalkane: it has very low steric hindrance, so SN2 is favored, but it forms an unstable primary carbocation, so SN1 cannot occur. (b) ${\text{CH}}_3{\text{CH(OH)CH}}_2{\text{CH}}_3$ (butan-2-ol)

Question 2

(a) Ethanol is heated under reflux with excess ethanoic acid and concentrated sulfuric acid. (i) Name the type of reaction occurring. [1 mark] (ii) Give the structural formula of the organic product. [1 mark] (b) State two observations you would make if propan-1-ol is warmed with acidified potassium dichromate(VI), the product is collected by distillation, then tested with Tollens' reagent. [2 marks]

Worked Solution

(a) (i) Esterification (or nucleophilic substitution/condensation) (ii) ${\text{CH}}_3{\text{COOCH}}_2{\text{CH}}_3$ (ethyl ethanoate) (b) First, the orange acidified dichromate solution turns green as propan-1-ol is oxidized to ethanal. Second, a silver mirror forms on the test tube wall when ethanal is warmed with Tollens' reagent.

Question 3

Three unlabelled organic liquids are known to be tertiary butanol, propanal, and propanoic acid. Describe a sequence of chemical tests to distinguish all three, including observations for each test. [4 marks]

Worked Solution

Step 1: Add a small amount of solid sodium carbonate to each test tube. The test tube that shows effervescence of a gas that turns limewater milky is propanoic acid (carboxylic acids react with carbonates to produce CO₂, while alcohols and aldehydes do not). Step 2: Add acidified potassium dichromate(VI) to the remaining two test tubes and heat. The test tube that shows an orange to green color change is propanal (aldehydes are oxidized to carboxylic acids), while the test tube with no color change is tertiary butanol (tertiary alcohols cannot be oxidized under these conditions). (Alternative valid sequences, e.g. using Tollens' reagent first to identify propanal, are also accepted.)

8. Quick Reference Cheatsheet

9. What's Next

This topic forms the foundation for multiple later units in the A-Level Chemistry syllabus, including carboxyl compounds (aldehydes, ketones, carboxylic acids, esters), organic synthesis, and polymer chemistry. You will use the reaction pathways you learned here to plan multi-step synthetic routes for complex organic molecules, a common high-mark question in Paper 4. The functional group tests are also required for Paper 3 practical assessments, so practicing these observations will help you score full marks in qualitative analysis tasks.

If you are struggling with any mechanism, reaction conditions, or test observations, you can ask Ollie, our AI tutor, for step-by-step explanations, extra practice questions, or custom quizzes tailored to your weak areas. You can also find more A-Level Chemistry study guides and past paper walkthroughs on the homepage to prepare for your upcoming exams.

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