Halogenoalkanes

· Halogenoalkanes contain a halogen atom bonded to an alkyl group: R–X, where X = F, Cl, Br or I.
· The C–X bond is polar because halogens are more electronegative than carbon: Cδ+–Xδ−.
· The δ+ carbon is attacked by nucleophiles in nucleophilic substitution reactions.
· Key reaction types: nucleophilic substitution, elimination, hydrolysis, and formation of nitriles/amines.
· Reactivity depends strongly on C–X bond strength: C–I weakest, C–Cl strongest; therefore iodoalkanes react fastest and chloroalkanes slowest.

This diagram compares the energy changes in SN1 and SN2 reactions. SN1 has two steps with a carbocation intermediate, while SN2 has one step with one transition state. Use it to remember why tertiary and primary halogenoalkanes often react by different mechanisms. Source

Making Halogenoalkanes

· From alkanes: free-radical substitution using Cl₂ or Br₂ in ultraviolet light.
· Example: ethane + chlorine → chloroethane + HCl, with possible further substitution products.
· From alkenes: electrophilic addition with a halogen, X₂, or hydrogen halide, HX(g), at room temperature.
· Example: ethene + HBr → bromoethane.
· From alcohols: substitution of the –OH group by halogen.
· Reagents for alcohol → halogenoalkane include HX(g), KCl + concentrated H₂SO₄, KCl + concentrated H₃PO₄, PCl₃ + heat, PCl₅, or SOCl₂.

Classification of Halogenoalkanes

· Primary halogenoalkane: carbon bonded to the halogen is attached to one alkyl group.
· Example: CH₃CH₂Br.
· Secondary halogenoalkane: carbon bonded to the halogen is attached to two alkyl groups.
· Example: CH₃CHBrCH₃.
· Tertiary halogenoalkane: carbon bonded to the halogen is attached to three alkyl groups.
· Example: (CH₃)₃CBr.
· Classification affects mechanism: primary → mainly SN2, tertiary → mainly SN1, secondary → mixture of SN1 and SN2.

Nucleophilic Substitution Reactions

· Nucleophilic substitution = a nucleophile replaces the halogen atom.
· The halogen leaves as a halide ion, X⁻.
· General equation: R–X + Nu⁻ → R–Nu + X⁻.
· With NaOH(aq) + heat: halogenoalkane → alcohol.
· Example: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻.
· With KCN in ethanol + heat: halogenoalkane → nitrile.
· Example: CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻.
· This reaction increases the carbon chain length by one carbon, useful in organic synthesis.
· With NH₃ in ethanol, heated under pressure: halogenoalkane → amine.
· Example: CH₃CH₂Br + NH₃ → CH₃CH₂NH₂ + HBr.
· Exam focus: always state reagent + solvent + conditions + product.

This image shows OH⁻ attacking the δ+ carbon while the C–Br bond breaks. It illustrates the one-step SN2 mechanism with a transition state. It is useful for learning how to draw curly arrows correctly in exam answers. Source

SN2 Mechanism

· SN2 = substitution nucleophilic bimolecular.
· Occurs in one step: the nucleophile attacks as the C–X bond breaks.
· The rate-determining step involves both halogenoalkane and nucleophile.
· Typical for primary halogenoalkanes because there is less steric hindrance around the δ+ carbon.
· Curly arrow 1: from lone pair on nucleophile to the δ+ carbon.
· Curly arrow 2: from C–X bond to the halogen atom.
· There is a transition state, not a carbocation intermediate.
· Exam phrase: bond making and bond breaking occur simultaneously.

SN1 Mechanism

· SN1 = substitution nucleophilic unimolecular.
· Occurs in two steps.
· Step 1: C–X bond breaks heterolytically to form a carbocation and X⁻.
· Step 1 is the slow rate-determining step.
· Step 2: nucleophile attacks the carbocation to form the substituted product.
· Typical for tertiary halogenoalkanes because the carbocation is stabilised by alkyl groups.
· Alkyl groups donate electron density by the positive inductive effect, stabilising the positive carbocation.
· Exam phrase: tertiary carbocations are more stable than secondary, which are more stable than primary.

This diagram shows the key feature of SN1: the leaving group departs first to form a carbocation intermediate. The nucleophile then attacks the carbocation. This helps explain why tertiary halogenoalkanes favour SN1. Source

Elimination Reaction

· Halogenoalkanes can undergo elimination to form alkenes.
· Reagent and conditions: NaOH in ethanol + heat.
· Example: bromoethane → ethene.
· Equation: CH₃CH₂Br + OH⁻ → CH₂=CH₂ + H₂O + Br⁻.
· Elimination removes HX from the halogenoalkane.
· Conditions matter: aqueous NaOH favours substitution; ethanolic NaOH favours elimination.
· Exam tip: when asked for an alkene product, look for removal of H from an adjacent carbon and X from the halogen-bearing carbon.

This page helps compare the two competing reactions of halogenoalkanes with hydroxide ions. Aqueous hydroxide tends to give substitution, while ethanolic hydroxide promotes elimination. It is useful for choosing the correct product in exam questions. Source

Silver Nitrate Test for Halogen in Halogenoalkanes

· Reagent: aqueous silver nitrate in ethanol.
· Purpose: identifies the halogen present and compares relative reactivity of halogenoalkanes.
· Ethanol helps the halogenoalkane dissolve and water allows hydrolysis.
· The halogenoalkane hydrolyses to release X⁻ ions.
· Ag⁺ + X⁻ → AgX(s).
· AgCl = white precipitate.
· AgBr = cream precipitate.
· AgI = yellow precipitate.
· Faster precipitate formation means faster hydrolysis and greater reactivity.
· Reactivity trend: R–I > R–Br > R–Cl, because C–I is weakest and C–Cl is strongest.

This page explains how halogenoalkanes release halide ions that form silver halide precipitates. The colour of the precipitate identifies the halogen. The speed of precipitate formation shows the effect of C–X bond strength on reactivity. Source

Reactivity of Halogenoalkanes

· Reactivity depends mainly on the strength of the carbon–halogen bond.
· C–I bond is longest and weakest, so iodoalkanes hydrolyse fastest.
· C–Br bond is intermediate, so bromoalkanes hydrolyse faster than chloroalkanes.
· C–Cl bond is strongest, so chloroalkanes hydrolyse slowest.
· The C–F bond is very strong, so fluoroalkanes are much less reactive in substitution.
· Exam phrase: weaker C–X bond breaks more readily, so reaction is faster.
· Do not explain the trend mainly by bond polarity; for CIE, focus on C–X bond strength.

Common Exam Conversions

· Alkane → halogenoalkane: Cl₂ or Br₂, UV light, free-radical substitution.
· Alkene → halogenoalkane: HX(g), room temperature, electrophilic addition.
· Alkene → dihalogenoalkane: X₂, room temperature, electrophilic addition.
· Alcohol → halogenoalkane: use HX, PCl₃, PCl₅, SOCl₂, or KCl + concentrated acid.
· Halogenoalkane → alcohol: NaOH(aq), heat, nucleophilic substitution.
· Halogenoalkane → nitrile: KCN, ethanol, heat, nucleophilic substitution.
· Halogenoalkane → amine: NH₃, ethanol, heat under pressure, nucleophilic substitution.
· Halogenoalkane → alkene: NaOH in ethanol, heat, elimination.