Halogenoalkanes

Halogenoalkanes and their classification

A halogenoalkane (alkyl halide) is an alkane in which one hydrogen atom has been replaced by a halogen atom (F, Cl, Br or I). They are also called halogen derivatives of alkanes. The general formula is $\mathrm{R}-\mathrm{X}$, where R is an alkyl group and X is the halogen. With one halogen atom on an open chain the molecular formula is $\mathrm{C}{}_n\mathrm{H}{}_{2n+1}\mathrm{X}$.

When the halogen is bonded directly to a benzene ring the compound is a halogenoarene (aryl halide), general formula $\mathrm{Ar}-\mathrm{X}$, where Ar is an aryl (arene) group. Chlorobenzene $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{Cl}$ is the simplest example. Halogenoarenes behave very differently from halogenoalkanes (section 9.2).

Primary, secondary and tertiary halogenoalkanes

Halogenoalkanes are classified by the carbon atom that carries the halogen. Count how many other carbon atoms (alkyl groups) are bonded to that carbon.

Physical properties

Halogenoalkanes have higher melting and boiling points than alkanes of similar size. The $\mathrm{C}-\mathrm{X}$ bond is polar, so the molecules attract each other by dipole–dipole forces as well as London forces. A halogen atom also adds many electrons, which makes the London forces stronger. Boiling points rise from fluoro to iodo compounds and with chain length. Halogenoalkanes do not form hydrogen bonds with water, so they are almost insoluble in water but dissolve in organic solvents.

9.1Preparation of halogenoarenes and halogenoalkanesOften examined

Benzene does not react with chlorine or bromine on its own, because its delocalised π electrons are very stable. A halogen carrier (a Lewis acid catalyst) such as $\mathrm{FeCl}{}_3$, $\mathrm{FeBr}{}_3$ or iron filings is needed. The reaction is electrophilic substitution: one hydrogen atom of the ring is replaced by a halogen atom and the ring stays aromatic.

benzene gives bromobenzene

benzene gives chlorobenzene

Conditions: room temperature, dry reagents, in the dark. Iron filings work because iron reacts with bromine to form the real catalyst: $2\mathrm{Fe}+3\mathrm{Br}{}_2\to 2\mathrm{FeBr}{}_3$. $\mathrm{AlCl}{}_3$ is also a Lewis acid and can act as a halogen carrier.

Preparation of halogenoalkanes (background)

Halogenoalkanes themselves are made by reactions from the Grade 11 units on alkanes, alkenes and alcohols. They are worth knowing for retrosynthesis (section 9.6).

9.2Reactivity of halogenoalkanes and halogenoarenesOften examined

The strength and polarity of the $\mathrm{C}-\mathrm{X}$ bond decide how a halogen compound reacts. Halogens are more electronegative than carbon, so carbon carries a partial positive charge ($\delta +$) and the halogen a partial negative charge ($\delta -$).

In chloroethane the $\delta +$ carbon is attacked by a nucleophile (an electron-pair donor, $\mathrm{Nu}{}^{-}$). The $\mathrm{C}-\mathrm{Cl}$ bond breaks and chlorine leaves as a chloride ion. This is nucleophilic substitution:

In chlorobenzene one lone pair on the chlorine atom overlaps with the π electrons of the ring and is delocalised over the ring. The $\mathrm{C}-\mathrm{Cl}$ bond gains partial double-bond character, so it is shorter and stronger than in chloroethane and difficult to break. The chlorine atom therefore cannot be replaced by a nucleophile under normal conditions. The resonance structures show this:

In structures II to IV the C–Cl bond is drawn as a double bond, and negative charge appears at the ortho (2, 6) and para (4) carbons. The real molecule is a hybrid of I to IV.

Why chlorobenzene resists nucleophilic substitution

• Partial double bond. The $\mathrm{C}-\mathrm{Cl}$ bond is shorter ($169\mathrm{pm}$, against $177\mathrm{pm}$ in chloroethane) and stronger.
• **$s{p}^2$ carbon.** An $s{p}^2$ carbon holds its electrons more tightly than an $s{p}^3$ carbon, so the bond is less polar and the carbon less $\delta +$.
• Electron-rich ring. The π cloud repels an approaching nucleophile, and the ring blocks attack from the back of the carbon (no ${\mathrm{S}}_{\mathrm{N}}2$).
• Unstable cation. A phenyl cation $\mathrm{C}{}_6\mathrm{H}{}_5{}^{+}$ would be very unstable, so the ${\mathrm{S}}_{\mathrm{N}}1$ route is not possible either.

Electrophilic substitution of chlorobenzene

Chlorobenzene still has an aromatic ring, so it reacts with electrophiles. The chlorine atom directs the incoming group to the ortho (2-) and para (4-) positions, where its lone pair raises the electron density (structures II to IV). For example, nitration gives a mixture of 2-nitrochlorobenzene and 4-nitrochlorobenzene:

the two products are the ortho and para isomers

9.3Polarity of the C–X bond and the inductive effect

Halogens are more electronegative than carbon (Pauling values: F 4.0, Cl 3.0, Br 2.8, I 2.5, C 2.5). The halogen pulls the bonding pair towards itself, so it gains a partial negative charge and the carbon a partial positive charge. The halogen atoms induce a dipole in the carbon–halogen bond:

Inductive effect

The permanent shift of σ-bond electrons towards the more electronegative atom, which makes the bond polar. Groups that pull electrons (halogens, \ce{-NO2}) show a -I effect; groups that push electrons (alkyl groups such as \ce{-CH3}) show a +I effect.

The polar \ce{C-X} bond explains the two types of reactions of halogenoalkanes:

1. Nucleophilic substitution (SN reactions): a nucleophile attacks the \delta+ carbon and replaces X.
2. Elimination: a base removes \ce{H} and \ce{X} from neighbouring carbons, forming an alkene.

9.3.1Nucleophilic substitution: substrate, nucleophile and leaving groupOften examined

The reaction in which the halogen atom of an alkyl halide is replaced by a stronger nucleophile is a nucleophilic substitution (SN) reaction. The general equation is:

\ce{Nu-}: attacking nucleophile; \ce{R-X}: substrate; \ce{X-}: leaving group

Substrate

The alkyl halide molecule that the nucleophile attacks.

Nucleophile

A species with a lone pair of electrons that it donates to the electrophilic (\delta+) carbon of the alkyl halide to form a new bond. It may be negatively charged (\ce{OH-}, \ce{CN-}) or neutral (\ce{H2O}, \ce{NH3}). The word means 'nucleus-loving'.

Leaving group

The group that departs with the bonding pair of electrons. In an alkyl halide it is the halide ion \ce{X-}.

Leaving groups. A good leaving group is a weak base that is stable on its own: \ce{I-}, \ce{Br-}, \ce{Cl-} and \ce{HSO4-}. Strong bases such as \ce{OH-}, \ce{RO-} and \ce{NH2-} are poor leaving groups. The incoming nucleophile must be stronger than the departing one. The iodide ion is both a good nucleophile and a good leaving group.

Every nucleophilic substitution involves two events: (1) breaking of the σ bond between carbon and halogen, and (2) formation of a new σ bond between carbon and the attacking nucleophile. In primary (and many secondary) alkyl halides both events happen together in one step: this is the **\mathrm{S_N2} mechanism. In bulky tertiary alkyl halides the C–X bond breaks first and the nucleophile bonds afterwards, in two steps: this is the \mathrm{S_N1}** mechanism.

Products of nucleophilic substitution

9.3.1Carbocations and their stabilityOften examined

Carbocation

A carbon atom that is attached to three atoms or groups of atoms and carries a unit positive charge. The positive carbon has only six electrons in its outer shell. The book also uses the older name carbonium ion.

The positive carbon is sp^2 hybridised. Its three bonds lie in one plane at 120^\circ (trigonal planar), and the empty 2p orbital lies above and below that plane. The empty orbital is what a nucleophile attacks, from either side.

Carbocations are classified like halogenoalkanes, by the number of alkyl groups on the positive carbon: methyl \ce{CH3+}, primary \ce{RCH2+}, secondary \ce{R2CH+}, tertiary \ce{R3C+}.

most stable on the left, least stable on the right (book Fig. 9.1)

Why alkyl groups stabilise a carbocation

1. **Inductive effect (+I).** An alkyl group is electron-releasing. It pushes σ-bond electron density towards the positive carbon, which partly neutralises the charge. Spreading (dispersing) a charge over more atoms lowers the energy and makes the ion more stable. A tertiary cation has three alkyl groups pushing; a methyl cation has none.
2. Hyperconjugation. A C–H σ bond on a carbon next to \ce{C+} can overlap with the empty 2p orbital and share its electron pair a little. The more such C–H bonds, the greater the stabilisation: \ce{(CH3)3C+} has 9, \ce{(CH3)2CH+} has 6, \ce{CH3CH2+} has 3 and \ce{CH3+} has none.

9.3.1SN1 mechanismOften examined

\mathrm{S_N1}

Substitution, nucleophilic, unimolecular. A two-step mechanism in which the substrate R–X first ionises into a carbocation (\ce{R+}) and \ce{X-}; the carbocation is then attacked by the nucleophile. Only one species, the substrate, takes part in the slow (rate-determining) step.

Rate equation

Only the substrate takes part in the slow step, so the rate depends only on its concentration. The nucleophile reacts after the slow step, so changing [\ce{OH-}] does not change the rate.

The reaction is first order overall: first order in R–X and zero order in the nucleophile. 'Unimolecular' means one molecule takes part in the rate-determining step. k is the rate constant (unit \mathrm{s^{-1}}).

Stereochemistry: racemisation

If the carbon carrying the halogen is a chiral centre, \mathrm{S_N1} gives a 50 : 50 mixture of the two enantiomers (a racemic mixture), which is optically inactive.

Conditions that favour \mathrm{S_N1}

• a tertiary (highly branched) substrate: it gives a stable carbocation and is too crowded for attack from the back
• a weak or neutral nucleophile such as \ce{H2O} or an alcohol
• a polar protic solvent such as water, which stabilises the ions formed in step 1
• a good leaving group (\ce{I-} > \ce{Br-} > \ce{Cl-})

SN2 mechanismOften examined

\mathrm{S_N2}

Substitution, nucleophilic, bimolecular. A one-step (concerted) mechanism: the nucleophile attacks the carbon from the side opposite the halogen while the C–X bond breaks. Two species take part in the single step.

Rate equation

Both species are in the single step, so doubling either concentration doubles the rate. The reaction is first order in each reactant and second order overall; k has the unit \mathrm{dm^3\,mol^{-1}\,s^{-1}}.

Conditions that favour \mathrm{S_N2}

• a methyl or primary substrate: the back of the carbon is not crowded
• a strong, negatively charged nucleophile (\ce{OH-}, \ce{CN-}, \ce{RO-}) at good concentration
• a polar aprotic solvent (for example propanone), which does not surround the nucleophile with hydrogen bonds
• a good leaving group

Effect of alkyl groups: which mechanismOften examined

The structure of the halogenoalkane decides the mechanism. Alkyl groups on the carbon carrying X have two effects that point in opposite directions.

Secondary halogenoalkanes are in between. Their carbocation is fairly stable and the back of the carbon is partly crowded, so they react by a mixture of both mechanisms. A strong nucleophile in a polar aprotic solvent pushes them towards \mathrm{S_N2}; a weak nucleophile in water pushes them towards \mathrm{S_N1}.

Elimination reactionsOften examined

A halogenoalkane can also lose a hydrogen halide (\ce{HX}) to form an alkene. Here the \ce{OH-} ion acts as a base: it removes a proton (\ce{H+}) from the carbon next to the C–X carbon instead of attacking the carbon itself. This is elimination (dehydrohalogenation).

α-carbon

The carbon atom bonded to the halogen.

β-carbon

A carbon atom next to the α-carbon; the hydrogen removed in elimination comes from here.

β-elimination

Loss of X from the α-carbon and H from the β-carbon, which forms a C=C double bond between them.

bromoethane gives ethene

2-bromopropane gives propene

Tertiary halogenoalkanes can also eliminate by an E1 route: the same slow ionisation as \mathrm{S_N1}, then loss of \ce{H+} from a β-carbon of the carbocation; \text{rate}=k[\ce{R-X}].

Which alkene forms: Saytzeff (Zaitsev) rule

If the halogen carbon has two different β-carbons, two alkenes can form. The major product is the more substituted alkene (fewer hydrogen atoms on the C=C carbons), because it is more stable.

but-2-ene is the major product (about 80%), but-1-ene the minor one

Substitution or elimination

Predicting products: KCN, ammonia and hydroxideOften examined

Many questions give a halogenoalkane and a reagent and ask for the product. Decide first whether the reagent acts as a nucleophile or a base, then use the conditions and the structure of the halogenoalkane.

With KCN: nitriles

The cyanide ion attacks the \delta+ carbon through its carbon atom, so a new C–C bond forms and the chain grows by one carbon. Chloroethane (2 C) gives propanenitrile (3 C), not ethanenitrile. Ethanol is used as the solvent because in water some \ce{OH-} would form and give the alcohol as well.

Nitriles are useful because the \ce{-C#N} group can be changed further: acid hydrolysis gives a carboxylic acid and reduction gives an amine.

With ammonia: amines

The lone pair on nitrogen attacks the carbon, giving an ethylammonium salt. A second ammonia molecule removes a proton and frees the amine. Overall: \ce{CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br}. Ammonia is used in excess, because the amine formed also has a lone pair and would attack more halogenoalkane to give secondary and tertiary amines and finally a quaternary ammonium salt. The sealed tube keeps the volatile ammonia in the hot solution.

With hydroxide: aqueous or alcoholic

substitution, the main reaction at lower temperature in water

elimination, the main reaction in hot ethanol

Reactivity and C–X bond strength: the silver nitrate testOften examined

Two properties of the C–X bond could control how fast it breaks: its polarity (largest for C–F) and its strength (also largest for C–F). Experiments show that reactivity increases from fluoro to iodo compounds, so bond strength is the deciding factor: the weaker the bond, the faster it breaks.

Reaction with aqueous silver nitrate

A few drops of the halogenoalkane are added to silver nitrate solution in ethanol and water (ethanol lets the two liquids mix), and the tube is warmed in a water bath. Water slowly hydrolyses the C–X bond. Each halide ion released meets an \ce{Ag+} ion and forms a silver halide precipitate, so the time taken for the precipitate to appear shows how fast the C–X bond breaks.

The book writes the overall change as \ce{R-X + AgNO3 -> R-NO3 + AgX}, where X is Cl, Br or I. Alkyl fluorides do not react, because the C–F bond is too strong and no fluoride ion is released.

9.6Alkyl halides and retrosynthesisOften examined

Alkyl halides are good starting materials for many organic compounds, because the halogen can be replaced by many different groups. Retrosynthesis means working backwards from the product (the target) to find the starting materials. The open arrow \Longrightarrow means 'can be made from'.

In \ce{R-X + Nu- -> R-Nu + X-} the new bond is C–Nu. So we mentally disconnect the target at that bond: the carbon part R came from the halogenoalkane, and the group Nu came from the nucleophile.

Uses and environmental effects

CFCs and the ozone layer

CFCs are unreactive in the lower atmosphere, because their C–F and C–Cl bonds are strong, so they slowly rise into the stratosphere. There, ultraviolet light breaks the weaker C–Cl bond and releases chlorine atoms (free radicals), which destroy ozone in a chain reaction:

The chlorine atom is regenerated in the last step, so one chlorine atom can destroy many thousands of ozone molecules; the net change is \ce{O3 + O -> 2O2}. A thinner ozone layer lets more harmful ultraviolet radiation reach the Earth, which increases skin cancer and eye cataracts and damages crops and plankton.

• Ozone depletion: CFCs, halons and \ce{CCl4}.
• Global warming: CFCs and HFCs are strong greenhouse gases.
• Persistence and bioaccumulation: DDT and other organochlorine pesticides build up in food chains and harm birds and fish.
• Toxicity: many chlorinated solvents damage the liver and the nervous system.

Formula sheet

Key terms

Halogenoalkane (alkyl halide)

A compound in which a hydrogen atom of an alkane is replaced by a halogen atom; general formula \ce{R-X}.

Halogenoarene (aryl halide)

A compound with a halogen atom bonded directly to a benzene ring; general formula \ce{Ar-X}.

Primary, secondary, tertiary halogenoalkane

The carbon carrying the halogen is bonded to one, two or three other carbon atoms.

Halogen carrier

A Lewis acid such as \ce{FeCl3}, \ce{FeBr3} or \ce{AlCl3} that polarises a halogen molecule to give the electrophile \ce{X+}.

Arenium ion

The positively charged intermediate formed when an electrophile bonds to a benzene ring (σ-complex).

Inductive effect

The shift of σ-bond electrons towards the more electronegative atom; -I groups pull electrons, +I groups (alkyl groups) push them.

Nucleophile

A species with a lone pair of electrons that it donates to an electron-poor carbon to form a new bond.

Electrophile

A species that accepts a pair of electrons; it is attracted to electron-rich sites.

Substrate

The molecule (here the halogenoalkane) that is attacked by the reagent.

Leaving group

The atom or group that departs with the bonding pair of electrons, such as \ce{Cl-}, \ce{Br-} or \ce{I-}.

Nucleophilic substitution

A reaction in which a nucleophile replaces an atom or group (here the halogen) in a molecule.

\mathrm{S_N1}

Unimolecular nucleophilic substitution: two steps, via a carbocation; \text{rate}=k[\ce{R-X}].

\mathrm{S_N2}

Bimolecular nucleophilic substitution: one step, backside attack through a transition state; \text{rate}=k[\ce{R-X}][\ce{Nu-}].

Rate-determining step

The slowest step of a mechanism; it controls the overall rate.

Carbocation (carbonium ion)

A species with a positively charged carbon that is bonded to three atoms or groups.

Hyperconjugation

Stabilisation of a carbocation by overlap of neighbouring C–H σ bonds with its empty p orbital.

Steric hindrance

Slowing of a reaction because bulky groups block the approach of the attacking species.

Transition state

The highest-energy arrangement of atoms during a reaction step; it cannot be isolated.

Intermediate

A species formed in one step and used up in a later step, such as the carbocation in \mathrm{S_N1}.

Walden inversion

The umbrella-like inversion of configuration at a carbon atom during an \mathrm{S_N2} reaction.

Racemic mixture

An equal mixture of two enantiomers; it is optically inactive. \mathrm{S_N1} at a chiral carbon gives one.

Elimination (dehydrohalogenation)

Removal of H and X from neighbouring carbon atoms by a base, forming an alkene.

Saytzeff (Zaitsev) rule

In elimination the more substituted alkene is the major product.

Retrosynthesis

Working backwards from a target molecule to find suitable starting materials; shown with the open arrow \Longrightarrow.

Disconnection

An imaginary breaking of a bond in the target molecule during retrosynthesis.

CFC

Chlorofluorocarbon, such as \ce{CCl2F2}; releases chlorine atoms in the stratosphere that destroy ozone.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. \mathrm{S_N1} mechanism of tert-butyl bromide with water: carbocation, inductive effect, rate equation. (long)
   Asked as a 7-mark Section C question in 2026 (Set B Q.3 OR).
   Go to answer
2. Why tertiary halides favour \mathrm{S_N1} and primary halides favour \mathrm{S_N2}. (short)
   Old-syllabus 2026 Q.2 (vii); needs steric hindrance and carbocation stability for each.
   Go to answer
3. \mathrm{S_N2} mechanism with transition state, inversion and rate equation. (long)
   The partner of the \mathrm{S_N1} question; \mathrm{S_N2} appears in the book's key points and in MCQs every year.
   Go to answer
4. Order of stability of carbocations and the reasons. (short)
   2026 Set D Q.2 (ix), 1 + 1 + 1 marks.
   Go to answer
5. Retrosynthesis: the halogenoalkane that gives propan-2-ol or butan-2-ol. (short)
   Both 2026 new-syllabus sets asked this (Set B Q.2 (vii), Set D Q.2 (viii) OR).
   Go to answer
6. Products with KCN, \ce{NH3}, aqueous \ce{OH-} and alcoholic \ce{OH-}. (mcq)
   Three 2026 MCQs came from this table (Set B MCQ 8 and 11, Set D MCQ 9).
   Go to answer
7. Why chlorobenzene is less reactive than chloroethane towards nucleophiles. (short)
   SLO 2 of the unit and book exercise MCQ viii; draw the resonance structures.
   Go to answer
8. Halogenation of benzene with a halogen carrier, with mechanism. (short)
   SLO 1 and three book exercise MCQs; write the catalyst over the arrow.
   Go to answer
9. C–X bond strength and the silver nitrate test (colours of AgX). (short)
   SLO 7; a standard short question on reactivity order R–I > R–Br > R–Cl.
   Go to answer
10. Substitution versus elimination and the Saytzeff rule. (short)
    Predicting the major product from conditions is an SLO and a frequent MCQ.
    Go to answer