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Hydrocarbons Halogenoalkanes And Alcohols

Learn and apply Hydrocarbons Halogenoalkanes And Alcohols in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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H1 Hydrocarbons, Halogenoalkanes and Alcohols: Orientation

Organic reaction questions become manageable when you treat each arrow as a contract: identify the starting functional group, name the reagent, state the decisive condition and solvent, then predict the product.

H1 8873 scope
  • Learn the specified reactions of alkanes, alkenes, halogenoalkanes and alcohols, including reagents, essential conditions, products and observations.
  • Use reaction types and functional-group changes to plan short syntheses.
  • Curly-arrow mechanisms, SN1/SN2 labels, detailed radical-chain steps and H2 selectivity rules are not required.

Definitions (Must Know)

  • Combustion is reaction with oxygen; complete combustion of a hydrocarbon forms carbon dioxide and water.
  • Substitution replaces an atom or group by another atom or group.
  • Addition joins atoms across a multiple bond so that one product forms.
  • Elimination removes atoms or groups from adjacent atoms and forms a multiple bond.
  • Oxidation of an organic compound commonly increases its oxygen content or decreases its hydrogen content.
  • Dehydration eliminates water from an alcohol to form an alkene.
  • Reflux heats a mixture while condensing vapour back into the flask, allowing prolonged reaction without losing volatile material.

Detailed Explanations

A. Alkanes: combustion and photochemical substitution

Ethane undergoes complete combustion in excess oxygen. At room temperature, chlorine reacts in ultraviolet light by substitution, giving chloroethane and hydrogen chloride. H1 requires the overall change and conditions, not a radical-chain mechanism.

B. Alkenes: addition across C=C

The double bond provides the reaction site. Bromine in CCl₄ adds across C=C and is decolourised, while hydrogen adds with a nickel catalyst to give an alkane. These are addition reactions: one product forms as the π bond is replaced by two new σ bonds.

C. Halogenoalkanes: solvent controls the pathway

Hot aqueous hydroxide replaces bromine by OH, so bromoethane gives ethanol. Hot ethanolic hydroxide removes H and Br from adjacent carbons, so bromoethane gives ethene. The same reagent formula is not enough; the solvent must be stated.

D. Alcohols: oxidation and dehydration

Heating ethanol under reflux with acidified K₂Cr₂O₇ or acidified KMnO₄ gives ethanoic acid. Heating ethanol with concentrated H₃PO₄ catalyst eliminates water and forms ethene.

E. Build reaction maps rather than isolated flashcards

Build routes only from the reactions in this course. Ethane can be converted to chloroethane and then ethanol; ethanol can be oxidised to ethanoic acid or dehydrated to ethene; ethene can be hydrogenated to ethane or reacted with bromine in CCl₄.

Worked Examples

Modelled example 1

Select Substitution or Elimination

Core

Problem

State reagents and conditions to convert bromoethane separately into ethanol and ethene.
Study the worked solution

  1. Plan the ethanol route

    Method

    Heat bromoethane with aqueous sodium hydroxide.

    Reason

    Aqueous hydroxide favours substitution: OH replaces Br without changing the carbon skeleton.

    Working

    CH₃CH₂Br → [NaOH(aq), heat] CH₃CH₂OH.

  2. Plan the ethene route

    Method

    Heat bromoethane with sodium hydroxide in ethanol.

    Reason

    Ethanolic hydroxide with heat favours elimination of H and Br from adjacent carbons, forming C=C.

    Working

    CH₃CH₂Br → [NaOH, ethanol, heat] CH₂ = CH₂.

  3. Compare the condition cue

    Method

    Keep the reagent, solvent and temperature together in each answer.

    Reason

    The solvent changes the favoured pathway even though hydroxide appears in both routes.

    Working

    aqueous → substitution; ethanolic + heat → elimination.

Guided practice 2

Convert Ethene to Ethane

About 5 min

Problem

Choose the reagent and catalyst for converting ethene to ethane, then state the bond change.

Try this before viewing the solution

Reagent
Catalyst

Hints

Hint 1: reaction family
This is an addition reaction: two atoms are added to the alkene.
Hint 2: bond change
The C=C becomes C–C as one hydrogen bonds to each carbon.
View solution step by step

  1. Select the conditions

    Method

    Use hydrogen with a nickel catalyst.

    Reason

    These are the required hydrogenation conditions.

    Working

    H₂/Ni.

  2. Track the bond change

    Method

    Add one H to each carbon of C=C and form a single C–C bond.

    Reason

    Addition uses the double bond while preserving the two-carbon skeleton.

    Working

    CH₂ = CH₂ + H₂ → CH₃CH₃.

Guided practice 3

Dehydrate Ethanol to Ethene

About 5 min

Problem

State one complete route for converting ethanol to ethene and identify the small molecule removed.

Try this before viewing the solution

Complete route
Small molecule removed

Hints

Hint 1: name clue
De-hydration means removal of water.
Hint 2: catalyst
The required catalyst is concentrated H₃PO₄.
View solution step by step

  1. Give the required route

    Method

    Heat ethanol with concentrated phosphoric acid catalyst.

    Reason

    The acid catalyst and heat promote elimination from the alcohol.

    Working

    concentrated H₃PO₄, heat.

  2. Account for the products

    Method

    Form ethene and water.

    Reason

    Removing H and OH from adjacent positions creates C=C.

    Working

    CH₃CH₂OH → CH₂ = CH₂ + H₂O.

Common misconception 4

Use the Required Bromine Medium

Find and correct the mistake

Common mistake

A learner writes “add bromine water” for the required reaction of ethene with bromine. Correct the reagent and state the product and observation.

Try this before viewing the solution

Required reagent
Organic product

View solution step by step

  1. Correct the reagent

    Method

    Use Br₂ in CCl₄.

    Reason

    This is the bromine medium specified for the H1 alkene reaction.

    Working

    Br₂/CCl₄.

  2. Track the addition

    Method

    Form 1,2-dibromoethane and state that the orange solution is decolourised.

    Reason

    Bromine adds across C=C, placing one Br on each carbon.

    Working

    CH₂ = CH₂ + Br₂ → CH₂BrCH₂Br.

Examiner practice 5

Plan Ethane to Ethanol

4 marks

Examination question

Suggest a two-step route from ethane to ethanol. State the intermediate and the reagent and condition for each step. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Form the halogenoalkane

    2 marks

    Method

    React ethane with chlorine under UV light at room temperature to form chloroethane.

    Reason

    Alkane halogenation supplies a leaving group for the next conversion; detailed radical-chain steps are not required here.

    Working

    CH₃CH₃ → [Cl₂, UV, room temperature] CH₃CH₂Cl.

  2. Form the alcohol

    2 marks

    Method

    Heat chloroethane with aqueous sodium hydroxide to form ethanol.

    Reason

    Aqueous hydroxide replaces chlorine by OH through substitution.

    Working

    CH₃CH₂Cl → [NaOH(aq), heat] CH₃CH₂OH.

Challenge 6

Choose between Two Ethanol Routes

Minimal support

Synthesis transfer

Starting from ethanol, state complete reagents and conditions to prepare (a) ethanoic acid and (b) ethene. Classify each change.

Try this before viewing the solution

Hints

Hint 1: acid route
The carbon skeleton remains but the oxygen-to-hydrogen balance changes.
Hint 2: alkene route
Remove the elements of water using the required acid catalyst.
View solution step by step

  1. Prepare ethanoic acid

    Method

    Heat ethanol under reflux with acidified K₂Cr₂O₇ or acidified KMnO₄.

    Reason

    Continued oxidation of the primary alcohol forms the carboxylic acid.

    Working

    CH₃CH₂OH → [[O], reflux] CH₃CO₂H; oxidation.

  2. Prepare ethene

    Method

    Heat ethanol with concentrated H₃PO₄ catalyst.

    Reason

    Elimination removes the elements of water and creates C=C.

    Working

    CH₃CH₂OH → [conc. H₃PO₄, heat] CH₂ = CH₂ + H₂O; elimination.

Mind Stretchers

Attempt each task before opening its hint.

Mind stretcher 1: Diagnosing four unlabelled reaction arrowsExtension

Question. A map contains ethane → chloroethane → ethanol → ethene → ethane. Classify each arrow and give the decisive reagent or condition.

Show Hint

Use, in order, alkane halogenation, hydrolysis, dehydration and hydrogenation.

Show Answer

Ethane to chloroethane is substitution with chlorine under UV light at room temperature. Chloroethane to ethanol is substitution with hot aqueous hydroxide. Ethanol to ethene is elimination/dehydration with concentrated phosphoric acid catalyst and heat. Ethene to ethane is addition/hydrogenation with hydrogen and nickel catalyst.

Mind stretcher 2: Using conditions and observations as evidenceExtension

Question. Two colourless compounds are ethane and ethene. Bromine in CCl₄ is added in the absence of UV light. Predict the results and explain why the conditions make the test discriminating.

Show Hint

Alkene addition is rapid under test conditions; alkane substitution needs UV initiation.

Show Answer

Ethene rapidly decolourises the orange bromine solution because bromine adds across C=C. Ethane shows no rapid change because its substitution reaction requires UV light. Excluding UV therefore prevents the alkane reaction from confusing the test.