Haloalkanes and Haloarenes: chapter on a page

A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).

About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.

This note's current text has no applied fact-check on record.

Mind map

Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.

The mind map as a list

Key ideas
- The C–X bond is polar (carbon is δ+…
- Bond dissociation enthalpy follows…
- In the SN2 mechanism the nucleophile…
- In the SN1 mechanism the C–X bond…
- Elimination reactions…
- Haloarenes are less reactive toward…
- In electrophilic aromatic…
Formulas and facts
- General formula of haloalkanes
- IUPAC naming
- Order of reactivity of alkyl halides…
- Order of reactivity toward SN1
- Grignard reagent formation
- Wurtz reaction
- Friedel–Crafts alkylation of benzene…
Worked examples
- IUPAC naming
- Predicting SN1 vs SN2
- Elimination (Zaitsev product)
Watch out for
- Confusing inversion with racemisation
- Thinking chlorobenzene reacts easily…
- Numbering the chain to give the…
- Forgetting that Grignard reagents…
- Believing a stronger C–X bond means…

What this chapter is about

This chapter introduces organic compounds in which one or more hydrogen atoms of a hydrocarbon are replaced by halogen atoms (F, Cl, Br or I). When the halogen is bonded to an sp³-hybridised carbon of an alkyl group, the compound is a haloalkane (alkyl halide). When it is bonded directly to a carbon of a benzene ring, it is a haloarene (aryl halide). These compounds are important intermediates in organic synthesis because the carbon–halogen bond can be broken and replaced by other functional groups.

Key ideas

The C–X bond is polar (carbon is δ+, halogen is δ−) because halogens are more electronegative than carbon; this polarity makes the carbon susceptible to attack by nucleophiles.
Bond dissociation enthalpy follows the order C–F > C–Cl > C–Br > C–I, so iodoalkanes react fastest and fluoroalkanes slowest in substitution and elimination.
In the SN2 mechanism the nucleophile attacks from the side opposite to the leaving group in one concerted step, giving inversion of configuration at a chiral centre (Walden inversion).
In the SN1 mechanism the C–X bond breaks first to form a carbocation; the nucleophile then attacks this planar intermediate, producing a racemic mixture if the carbon was chiral.
Elimination reactions (dehydrohalogenation) occur when a strong base abstracts a β-hydrogen; E2 is concerted, E1 proceeds via a carbocation, and Zaitsev's rule predicts the more substituted alkene as the major product.
Haloarenes are less reactive toward nucleophilic substitution because the C–X bond has partial double-bond character due to resonance between the lone pair on the halogen and the benzene ring.
In electrophilic aromatic substitution, halogens are deactivating (withdraw electrons by induction) yet ortho- and para-directing (donate electrons by resonance to those positions).

Formulas and facts to remember

General formula of haloalkanes: CₙH₂ₙ₊₁X (for mono-substituted, saturated, open-chain).
IUPAC naming: the halogen is named as a prefix (fluoro, chloro, bromo, iodo) with position numbers; for example, CH₃–CHBr–CH₃ is 2-bromopropane.
Order of reactivity of alkyl halides toward SN2: CH₃X > primary > secondary > tertiary (steric hindrance increases).
Order of reactivity toward SN1: tertiary > secondary > primary > CH₃X (carbocation stability increases).
Grignard reagent formation: R–X + Mg (dry ether) → R–Mg–X. This reagent reacts with water or acids to give alkanes, and with carbonyl compounds to give alcohols.
Wurtz reaction: 2 R–X + 2 Na (dry ether) → R–R + 2 NaX. Used to prepare symmetrical alkanes.
Friedel–Crafts alkylation of benzene with R–Cl in presence of anhydrous AlCl₃ gives alkylbenzene.
Chlorobenzene requires very harsh conditions (high temperature, pressure, NaOH) for nucleophilic substitution; the presence of strong electron-withdrawing groups (–NO₂) at ortho or para positions activates the ring toward substitution.

Common mistakes

Confusing inversion with racemisation → Inversion (one flip) occurs in SN2; racemisation (50-50 mixture) occurs in SN1 because the carbocation is planar.
Thinking chlorobenzene reacts easily with NaOH like chloroethane → Resonance strengthens the C–Cl bond in haloarenes; special conditions are required.
Numbering the chain to give the halogen the highest number → Always number to give the lowest locant to the first point of difference.
Forgetting that Grignard reagents react with even traces of water → Dry ether and anhydrous conditions are essential.
Believing a stronger C–X bond means faster reaction → A weaker bond (C–I) breaks more easily, so iodoalkanes react fastest.

Quick revision

Haloalkanes: halogen on sp³ C; haloarenes: halogen on benzene C.
SN2: one step, inversion, favoured by primary substrates and strong nucleophiles.
SN1: two steps, racemisation, favoured by tertiary substrates and polar protic solvents.
Elimination with alcoholic KOH gives the more substituted alkene (Zaitsev rule).
Haloarenes resist nucleophilic substitution but are ortho-para directing in electrophilic substitution.
Grignard reagent (R–Mg–X) is a versatile nucleophile for forming new C–C bonds.