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Loading home…Halogenoalkanes are compounds in which a halogen atom replaces hydrogen in an alkane.
The general representation is R–X, where X = F, Cl, Br, or I.
They are classified as primary, secondary, or tertiary by the carbon bearing X.
Chloroform (CHCl₃) was historically used as an anaesthetic.
Many halogenoalkanes are useful synthetic intermediates.
Bond strength and polarity of C–X control their reactivity.
Aqueous KOH favours nucleophilic substitution to give alcohols.
R–X + KOH(aq) → R–OH + KX.
Alcoholic KOH favours elimination to give alkenes.
Heat and ethanolic conditions promote dehydrohalogenation.
The same substrate can give different products by changing the medium.
This contrast is a classic exam point for halogenoalkane chemistry.
SN2 rate depends on the leaving group and the C–X bond strength.
C–I is the weakest carbon–halogen bond in the series.
Iodide is an excellent leaving group.
Bond strength order: C–F > C–Cl > C–Br > C–I.
Therefore iodoalkanes usually react fastest in SN2 among R–X for the same R.
Fluoroalkanes are often least reactive toward nucleophilic substitution.
NaCN (often in ethanol/water) substitutes X by CN to give a nitrile.
R–X + NaCN → R–CN + NaX.
The product contains one extra carbon atom (chain extension).
Nitriles can be hydrolysed to carboxylic acids or reduced to amines.
This is a key carbon–carbon bond forming / chain-lengthening step.
It is widely used in organic synthesis planning.
Freons are chlorofluorocarbon (CFC) refrigerants and related compounds.
They were used in refrigerators, air conditioners, and aerosols.
In the stratosphere they release chlorine that destroys ozone.
International agreements phased out many ozone-depleting Freons.
Safer alternatives with lower ozone-depletion potential are now used.
They illustrate how useful chemicals can have global environmental costs.
Alkyl halide in ethanol with AgNO₃ can give a silver halide precipitate.
The organic halide ionises or reacts so X⁻ meets Ag⁺.
AgCl is white, AgBr is cream, and AgI is yellow.
Reactivity order often follows leaving-group ability (I > Br > Cl).
This test helps classify and compare halogenoalkanes.
Aqueous AgNO₃ alone is less suitable for covalent R–X without assistance.
Nucleophilic substitution replaces X in R–X by a nucleophile Nu⁻ (or Nu).
SN2 is bimolecular: rate = k[R–X][Nu].
The nucleophile attacks from the back as X leaves in one step.
The transition state has pentacoordinate carbon character.
SN2 is favoured for methyl and primary substrates.
Steric hindrance slows SN2 at tertiary centres.
SN1 is unimolecular: rate = k[R–X] in the slow step.
The halide ionises to a carbocation intermediate, then Nu attacks.
SN1 is favoured for tertiary substrates that form stable carbocations.
Polar protic solvents stabilise ions and help SN1.
Polar aprotic solvents often accelerate SN2 by freeing nucleophiles.
SN2 inverts configuration at a chiral carbon (Walden inversion).
SN1 often gives racemisation if a planar carbocation is free.
Leaving group ability: I⁻ > Br⁻ > Cl⁻ > F⁻ generally.
Strong nucleophiles favour SN2; weak nucleophiles/solvolysis favour SN1.
Competing elimination can occur, especially with strong bulky bases.
Exam answers should give rate laws, steps, and substrate preferences.
Draw mechanisms with curly arrows when required.
State that the same R–X may follow different paths under different conditions.
Link mechanism choice to product stereochemistry questions.
Practice with CH₃X (SN2) versus t-butyl chloride (SN1) examples.
Reactivity depends on C–X bond strength and leaving-group ability.
Bond strength decreases down the group: C–F strongest, C–I weakest.
Iodoalkanes are usually most reactive toward nucleophiles; fluoroalkanes least.
Primary, secondary, and tertiary structure changes mechanism preference.
Tertiary halides favour SN1/E1; primary favour SN2/E2 under many conditions.
Aryl and vinyl halides are much less reactive in ordinary SN reactions.
Uses include solvents (e.g. dichloromethane historically), refrigerants, and intermediates.
Chloroform was used as an anaesthetic; modern practice uses safer agents.
Freons (CFCs) were refrigerants until ozone concerns limited them.
Alkyl halides are starting materials for alcohols, ethers, amines, and nitriles.
In synthesis, R–X + Nu gives substitution products of wide variety.
Polymers such as PVC come from halogen-containing monomers.
Environmental and toxicity issues require careful handling and disposal.
Some halogen organics persist and bioaccumulate.
School answers should connect structure → mechanism → reactivity order.
Give at least three uses with a named example each.
Mention C–X bond polarity: carbon is δ⁺ and attracts nucleophiles.
Compare boiling points: larger halogen → higher BP generally.
Density of many alkyl bromides/iodides is greater than water.
Always balance usefulness against health and environmental risk.
Modern green chemistry seeks fewer persistent halogen pollutants.
Substitution with aqueous alkali gives alcohols.
Elimination with alcoholic KOH gives alkenes (Saytzeff orientation often).
Reaction with NaCN gives nitriles (chain extension).
Reaction with alcoholic ammonia gives amines (then polyalkylation possible).
With AgCN, isocyanides may form (linkage isomer nucleophile).
With KNO₂/AgNO₂, nitroalkanes or nitrites can form depending on conditions.
Finkelstein reaction: R–Cl/Br + NaI in acetone → R–I (precipitates NaCl/NaBr).
Wurtz coupling with sodium gives higher alkanes.
Grignard reagents form from R–X + Mg in dry ether.
Iodoform reaction: compounds with CH₃CH(OH)– or CH₃CO– give yellow CHI₃.
Ethanol and acetaldehyde give positive iodoform tests; methanol does not.
Iodoform historically used as an antiseptic.
Write conditions clearly: aq vs alc KOH changes the pathway.
Mechanism labels (SN1/SN2/E1/E2) earn method marks.
Competing reactions mean product mixtures need control of conditions.
Exam answers should include equations for substitution and elimination.
Mention alcoholic AgNO₃ precipitate test for halide identification.
Link NaCN and NH₃ reactions to synthesis of acids and amines.
State that tertiary substrates eliminate readily with strong bases.
Summarise: halogenoalkanes are versatile synthetic building blocks.
Always note the leaving group X⁻ in balanced equations.
From alkanes: free-radical halogenation with Cl₂/Br₂ and UV light.
This method often gives mixtures and is less selective.
From alkenes: addition of HX gives halogenoalkanes.
Markovnikov’s rule predicts the major product for unsymmetrical alkenes.
Addition of X₂ gives vicinal dihalides.
From alcohols: reaction with HX, PX₃, PCl₅, or SOCl₂ replaces –OH by X.
Primary alcohols often need catalysts such as ZnCl₂ with HCl (Lucas idea).
PCl₅ with alcohols gives alkyl chlorides and POCl₃ + HCl.
SOCl₂ is useful for chlorides with gaseous by-products.
From carboxylic acids: Hunsdiecker and related methods are more advanced.
Industrial processes may use oxychlorination and other catalytic routes.
Choice of method depends on desired isomer purity and halogen type.
Laboratory preparations emphasise alcohols + PCl₅/SOCl₂/HX.
Write equations and state anhydrous conditions where needed.
Avoid moisture when making Grignard precursors of high quality.
Safety: many halogenating agents are corrosive or toxic.
School exams often ask alcohol → chloroalkane with PCl₅.
Compare ease: 3° alcohols react fastest with HX in many cases.
Purify products by washing and distillation when required.
Confirm identity by boiling point and halide tests.
Preparation knowledge feeds directly into synthesis roadmap questions.
Elimination of HX from R–X with base gives alkenes.
Alcoholic KOH is the usual school reagent for dehydrohalogenation.
Saytzeff rule: preferential formation of the more substituted alkene.
E2 is concerted; E1 involves carbocation intermediates.
Bulky bases may favour less substituted alkenes (Hofmann product).
CFCs are halogenoalkanes/related compounds used historically as Freons.
They are inert in the troposphere but photolyse in the stratosphere.
Chlorine radicals catalyse ozone destruction.
This environmental harm led to global phase-outs.
NaCN substitution yields nitriles for chain extension.
Ammonia yields amines; excess NH₃ reduces polyalkylation.
Alcoholic AgNO₃ gives AgX precipitates used in halide characterisation.
Finkelstein: halide exchange with NaI/acetone drives R–I formation.
These reactions form a toolkit for converting R–X into many families.
Exam answers should connect each reagent to its product class.
Write at least one equation for each named transformation.
Comment on green alternatives where ozone-depleting substances are discussed.
Stereochemistry and competing substitution/elimination may be asked.
Summarise halogenoalkanes as environmental concern + synthetic opportunity.
Tie Saytzeff products back to alkene chemistry in the previous chapter.
Clear conditions (solvent, heat) separate similar-looking reactions.