OCR A LEVEL CHEMISTRY MODULE 4

What is meant by a homologous series?

A series of organic compounds with the same functional group and similar chemical properties, where each successive member differs by a CH₂ group.

General formula for a carboxylic acid

CₙH₂ₙO₂ or CₙH₂ₙ₊₁COOH — both valid depending on how R is counted.

Definition of structural isomers

Compounds with the same molecular formula but different structural formulae (different atom arrangements).

Definition of stereoisomers

Compounds with the same structural formula but different spatial arrangements of atoms.

Why do branched alkanes have lower boiling points than straight-chain isomers?

Branching reduces surface area contact between molecules → weaker London forces → lower boiling points.

What are London forces (induced dipole-dipole interactions)?

Weak intermolecular forces caused by fluctuating dipoles; stronger with larger molecules or greater surface area.

Conditions and reagent for hydration of an alkene

Steam (H₂O(g)) and an acid catalyst (e.g. H₃PO₄ or H₂SO₄).

Why are alcohols more soluble in water than alkenes?

Alcohols form hydrogen bonds with water due to the OH group. Alkenes cannot form hydrogen bonds.

What does a curly arrow represent in mechanisms?

Movement of an electron pair (from a lone pair or π-bond to an electrophile or atom).

Definition of an electrophile

A species that accepts an electron pair — typically has a positive or δ⁺ charge (e.g. H⁺, NO₂⁺, I⁺).

Definition of a nucleophile

A species that donates an electron pair to form a covalent bond (e.g. OH⁻, CN⁻, NH₃).

Mechanism by which alkanes react with halogens under UV light

Free radical substitution.

General formula of an alkyl group

CₙH₂ₙ₊₁ — derived by removing one H atom from an alkane.

General formula for cycloalkanes

CₙH₂ₙ — same as alkenes, but only single bonds in a ring.

Why do larger cycloalkanes have higher boiling points?

More contact area between molecules → stronger London forces → more energy needed to separate them.

Reaction of bromine water with alkenes

Bromine water decolourises (orange to colourless) when it reacts with a C=C double bond.

What is the role of IBr in reactions?

IBr undergoes heterolytic fission to form I⁺ (electrophile) and Br⁻.

How many structural isomers of C₅H₁₂ exist?

3 — pentane, 2-methylbutane, and 2,2-dimethylpropane.

Why do more branched isomers have lower boiling points?

More branching = less surface contact = weaker London forces = lower boiling points.

Equation for full substitution of 3 Cl atoms in C₅H₁₂

C₅H₁₂ + 3Cl₂ → C₅H₉Cl₃ + 3HCl

Systematic name for CH₃CH(CH₃)CH₂OH

2-methylpropan-1-ol

Systematic name for allyl bromide

3-bromoprop-1-ene or 1-bromoprop-2-ene (both acceptable).

General formula of homologous series for allyl bromide

CₙH₂ₙ₋₁Br — characteristic of substituted alkenes.

Skeletal formula of pentane

Zigzag chain of five carbon atoms with no atom labels — shows backbone only.

Why do some isomers exhibit stereoisomerism?

C=C double bond prevents rotation and different groups attached → E/Z (cis-trans) isomerism possible.

Test for alkene presence

Add bromine water — decolourisation indicates presence of a double bond (electrophilic addition).

What is an alicyclic compound?

An aliphatic (non-aromatic) compound with carbon atoms joined in a ring.

Difference between aliphatic, alicyclic, and aromatic

Aliphatic = open chain; Alicyclic = ring, non-aromatic; Aromatic = contains benzene ring.

Percent yield formula

% Yield = (actual moles ÷ theoretical moles) × 100

Equation for elimination reaction: alcohol → alkene

C₆H₁₂O → C₆H₁₀ + H₂O (removal of water from alcohol).

Why must carbon not have five bonds in structural formulas?

Carbon forms 4 covalent bonds. Drawing 5 is chemically invalid — check structures for correctness.

Why are lowest locant numbers used in naming?

IUPAC rules favour lowest possible position numbers for functional groups and substituents.

How to identify aliphatic, alicyclic, and aromatic compounds

Aliphatic: straight/branched chain; Alicyclic: non-aromatic ring; Aromatic: benzene or delocalised ring.

Define radical substitution

A reaction involving free radicals where one atom (usually H) is replaced by another atom (usually halogen), initiated by UV light.

Initiation step of radical substitution

Cl₂ → 2Cl· (requires UV light to break Cl-Cl bond homolytically).

Propagation step 1 for butane and Cl·

C₄H₁₀ + Cl· → C₄H₉· + HCl

Propagation step 2 for butyl radical and Cl₂

C₄H₉· + Cl₂ → C₄H₉Cl + Cl·

Equation for full substitution of all H atoms in butane

C₄H₁₀ + 10Cl₂ → C₄Cl₁₀ + 10HCl

How to determine molecular formula from gas data

Use: moles = volume / molar volume → molar mass = mass / moles → use Mr to find empirical/molecular formula.

Definition of structural isomers

Compounds with the same molecular formula but different structural formulae.

Why branched alkanes have lower boiling points

Branching reduces surface contact between molecules → weaker London forces → lower boiling point.

Trend in boiling points across isomers

More branching = weaker London forces = lower boiling point.

Definition of London forces

Weak intermolecular forces caused by temporary dipoles between molecules.

Why less energy is needed to separate branched molecules

Fewer/weaker London forces → less energy required to overcome IMFs.

Mechanism name for alkane + Cl₂ under UV

Free radical substitution.

Why radical substitution forms many isomers

Any H atom can be substituted → multiple mono- and polysubstituted products form.

Equation for formation of trichloro derivative of C₅H₁₂

C₅H₁₂ + 3Cl₂ → C₅H₉Cl₃ + 3HCl

Condition for radical substitution

UV light (or sunlight) — initiates homolytic fission.

Definition of homolytic fission

Covalent bond breaks evenly so each atom takes one electron → forms two radicals.

Propagation step example: CH₃CH₂COOH and Cl·

Cl· + CH₃CH₂COOH → CH₃CHCOOH· + HCl

Displayed formula of CH₃CHCOOH·

Radical has unpaired electron on central carbon (C₂).

Further substitution product example from CH₃CHClCOOH

Structure with additional Cl on carbon chain, e.g. CH₃CCl₂COOH.

Why alcohols have higher boiling points than alkanes

Alcohols have hydrogen bonds (strong) vs alkanes' London forces (weak).

Equation for complete combustion of octane

C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O

How to check if combustion was incomplete

Compare moles of CO₂ actually produced with moles expected from complete combustion — if less, it's incomplete.

Definition of homologous series

Series of compounds with the same functional group, similar reactions, each member differing by CH₂.

General formula for cycloalkanes

CₙH₂ₙ

Why boiling points increase across cycloalkanes

More carbons = more contact = stronger London forces = higher boiling point.

Shape and bonding around C in cycloalkanes

Tetrahedral shape; four bonding pairs repel equally.

Initiation step with bromine in cyclohexane

Br₂ → 2Br·

Propagation steps with bromine and cyclohexane

C₆H₁₂ + Br· → C₆H₁₁· + HBr; C₆H₁₁· + Br₂ → C₆H₁₁Br + Br·

Termination step examples in free radical reactions

Br· + Br· → Br₂; C₆H₁₁· + Br· → C₆H₁₁Br; C₆H₁₁· + C₆H₁₁· → C₁₂H₂₂

Name a structural isomer of C₆H₁₀Br₂

e.g. 1,2-dibromocyclohexane or 1,4-dibromocyclohexane

Skeletal formula of product from hexane + Cl₂

Line structure of hexane with a Cl replacing one H.

Why radical substitution gives a mixture of products

Multiple H atoms can be substituted randomly → mono- and polysubstitutions at different positions.

Why CO is dangerous in incomplete combustion

CO is toxic — binds to haemoglobin and prevents oxygen transport.

How to assess safety in combustion based on oxygen

Compare oxygen consumed with oxygen required for complete combustion — if less, it's unsafe/incomplete.

How a sigma (σ) bond forms

Direct overlap of orbitals between bonding atoms.

Bond angle and shape around C=C in alkenes

120°, trigonal planar.

Definition of termination step in radical substitution

Two radicals combine to form a stable molecule, ending the chain reaction.

Definition of an electrophile

A species that accepts a pair of electrons — typically positively charged or δ+ (e.g. H⁺, Br⁺, I⁺).

What happens when Br₂ reacts with an alkene?

Bromine adds across the double bond; bromine water decolourises (orange → colourless).

What is the mechanism for alkene + Br₂?

Electrophilic addition: π electrons attack Br, Br-Br bond breaks heterolytically, Br⁻ adds to carbocation.

Conditions for hydrogenation of an alkene

H₂ gas and a nickel (Ni) catalyst; Pt, Pd or Rh also accepted.

Conditions for hydration of an alkene

Steam (H₂O(g)) and an acid catalyst (e.g. H₃PO₄ or H₂SO₄).

Why are alkenes more reactive than alkanes?

They contain a C=C double bond which is electron-rich and can be attacked by electrophiles.

What type of reaction occurs between alkenes and halogens?

Electrophilic addition.

Why does poly(propene) not have E/Z isomers?

Poly(propene) is saturated — it contains only single (σ) bonds, so there's free rotation.

Why do alkenes show E/Z stereoisomerism?

C=C bond has restricted rotation; each carbon in the double bond must have 2 different groups.

Definition of electrophilic addition

Reaction where an electrophile is attracted to a region of high electron density and adds to it.

What is the bond angle around each carbon in an alkene?

120°, trigonal planar.

Why can't pent-1-ene exhibit E/Z isomerism?

One carbon in the C=C bond has two identical groups (hydrogens), so stereoisomerism isn't possible.

What is Markovnikov's rule?

In HX addition, the H attaches to the carbon with more Hs already (major product comes from more stable carbocation).

How does carbocation stability affect product formation?

Tertiary carbocations are more stable than secondary/primary → major product forms via more stable intermediate.

Name the mechanism: alkene + HCl

Electrophilic addition.

Why is π bond weaker than σ bond?

π bond forms from sideways p-orbital overlap — electron density is above and below nuclei → more exposed and weaker.

How is a π bond formed?

By sideways overlap of two adjacent p-orbitals.

Why does the C=C bond have restricted rotation?

π bond prevents rotation — rotation would break overlap of p-orbitals.

Balanced equation for polymerisation of chloroethene

nCH₂=CHCl → -[CH₂-CHCl]-ₙ (use displayed formula with correct side links).

Problem with combustion of chloroalkene polymers

Releases toxic gases e.g. HCl which contributes to acid rain.

How is HCl removed during incineration?

Sodium hydrogencarbonate (NaHCO₃) neutralises HCl, forming NaCl, CO₂ and H₂O.

Give one advantage and disadvantage of incinerating polymers

Advantage: Energy production; Disadvantage: Toxic gases (HCl, CO, CO₂).

Define structural isomers

Compounds with the same molecular formula but different structural formulae (i.e. different connectivity of atoms).

How to test for C=C double bond

Add bromine water → decolourisation confirms presence of alkene.

How do alkenes react with H₂O (steam)?

Hydration reaction → electrophilic addition → forms alcohols.

Draw the intermediate in an alkene + HBr mechanism

A carbocation (C+) at the more stable site; curly arrow from Br⁻ to the C⁺.

Repeat unit of an addition polymer

Two carbon atoms from the original C=C, now single bonded, each with side links to next units.

Why do polymers not biodegrade easily?

Contain strong C-C and C-H bonds which are unreactive and not broken down by microbes.

What does a curly arrow show in a mechanism?

Movement of a pair of electrons.

Monomer used to make PTFE

Tetrafluoroethene: CF₂=CF₂