R3.4Reactivity 3 — What are the mechanisms of chemical change?

Electron-pair sharing reactions — IB Chemistry HL Revision Notes & Topical Questions

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R3.4 Electron-pair sharing reactions — IB CHEMISTRY HL notes: Describe nucleophiles, electrophiles, heterolytic fission and the main electron-pair sharing mechanisms: nucleophilic substitution, electrophilic addition, Lewis acid-base reactions and electrophilic substitution in be…

Curly arrow
Polar bond
Meaning of the name
Transition state
Rate equation
What the symbols mean

Part 1 of 11Nucleophiles, electrophiles and heterolytic fission

1Nucleophile

Nucleophile an electron-rich species that donates a lone pair of electrons to form a covalent bond.
Neutral nucleophiles include H₂O, NH₃, ROH and RNH₂.
Charged nucleophiles include OH⁻, Cl⁻, CN⁻ and carbanions (R⁻).
A nucleophile is attracted to positively charged or partially positive centres.

2Electrophile

Electrophile an electron-deficient species that accepts a pair of electrons to form a covalent bond.
Neutral electrophiles include HX, X₂ and RX.
Cationic electrophiles include H⁺, NO₂⁺, NO⁺ and carbocations (R⁺).

3Nucleophile

Lewis base; electrophile = Lewis acid.

Charge and electronegativity set nucleophile strength.

A negatively charged species is a stronger nucleophile than a neutral one, so OH⁻ is stronger than H₂O.
A less electronegative donor holds its lone pair less tightly, so NH₃ is stronger than H₂O.
Order of strengthCN⁻ > OH⁻ > NH₃ > H₂O.

5Heterolytic fission

Heterolytic fission breaking of a covalent bond so that both bonding electrons go to the more electronegative atom.

It forms a negative ion (electron-rich, a nucleophile) and a positive ion (electron-deficient, an electrophile).
H–Cl breaks to give H⁺ and Cl⁻, with both electrons moving to chlorine.

6Curly arrow

Curly arrow a double-headed arrow shows the movement of one pair of electrons.
The arrow starts at a lone pair or a bond (a region of high electron density).
The arrow points to an electron-deficient atom or a positive charge.

Bond formation by a nucleophile is the reverse of heterolytic fission,

because the nucleophile donates a pair of electrons to make a coordinate covalent bond.

Homolytic fission differs because each atom takes one electron, giving two radicals shown with single-headed arrows.

Part 2 of 11Nucleophilic substitution in halogenoalkanes

Nucleophilic substitution in halogenoalkanes

Nucleophilic substitution

Nucleophilic substitution a nucleophile attacks a carbon atom carrying a partial positive charge and replaces the atom or group that carries a partial negative charge.
- (the leaving group)

Polar bond

Polar bond the C–X bond is polar because halogens are much more electronegative than carbon, so the carbon is open to attack.

Hydrolysis of halogenoalkanes uses aqueous NaOH or KOH, warmed because the reaction is very slow at room temperature.

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
The product is an alcohol and Br⁻ is the leaving group.

A neutral nucleophile such as water first gives a positively charged product, which then loses H⁺.

CH₃CH₂Cl + H₂O → CH₃CH₂OH + HCl

The bond that forms and the bond that breaks both involve the carbon bonded to the leaving group.

Part 3 of 11The SN2 mechanism (primary halogenoalkanes)

1Meaning of the name

S = substitution, N = nucleophilic, 2 = the rate depends on two reagents (bimolecular).

2Mechanism

Mechanism a single concerted step with no intermediate.
The nucleophile attacks the carbon from the side opposite the leaving group (backside attack).
The C–Nu bond forms while the C–X bond breaks heterolytically, and the halogen leaves as X⁻ taking both electrons.

3Transition state

Transition state a high-energy arrangement with partial bonds to both the nucleophile and the leaving group.
It appears as one peak on the energy profile and it is not an intermediate.

4Rate equation

Rate equation rate = k[halogenoalkane][nucleophile].
What the symbols mean: rate (mol dm⁻³ s⁻¹) · k = rate constant · [ ] = concentration (mol dm⁻³) · each power is the order with respect to that reactant
- (units depend on the overall order)

5Inversion of configuration

Inversion of configuration the three other groups on carbon flip, like an umbrella turning inside out.
Backside attack is needed because the large halogen blocks a frontal attack.

6Solvent

Solvent polar aprotic solvents favour SN2.

Part 4 of 11The SN1 mechanism (tertiary halogenoalkanes)

Meaning of the name

Meaning of the name the 1 means the rate depends on one reagent only (unimolecular).

Mechanism

Mechanism two steps.
Step 1 (slow, rate-determining) is heterolytic fission of the C–X bond, giving a tertiary carbocation and X⁻.
Step 2 (fast) is attack of the nucleophile on the positively charged carbon of the carbocation.

Rate equation

Rate equation rate = k[halogenoalkane], so the rate is independent of the nucleophile concentration.

Energy profile

Energy profile two peaks with a dip between them for the carbocation intermediate.

Stereochemistry

Stereochemistry the planar carbocation can be attacked from either face, so a chiral halogenoalkane gives a racemic mixture.

Solvent

Solvent polar protic solvents such as water and ethanol stabilise the carbocation and favour SN1.

Product

2-bromo-2-methylpropane with OH⁻ gives 2-methylpropan-2-ol.

Part 5 of 11Rates of nucleophilic substitution

Rates of nucleophilic substitution

Three factors set the rate

Three factors set the rate the nucleophile, the leaving group (halogen) and the class of halogenoalkane.

Halogen

Halogen the weaker the C–X bond, the faster the substitution.
Bond enthalpiesC–F 492, C–Cl 324, C–Br 285, C–I 228 kJ mol⁻¹.
Reactivity orderR–I > R–Br > R–Cl > R–F, and fluoroalkanes are almost unreactive.

Silver nitrate test

Silver nitrate test aqueous AgNO₃ detects the halide released.
Chloride gives a white precipitate, bromide a cream precipitate and iodide a yellow precipitate.
Fluoride gives no precipitate because AgF is soluble.
Yellow AgI forms fastest, which shows iodoalkanes are the most reactive.

Class of halogenoalkane

Class of halogenoalkane decides the mechanism.
Primary halogenoalkanes react by SN2 and form no carbocation.
Tertiary halogenoalkanes react by SN1 because they form stable carbocations.
Secondary halogenoalkanes use either route, depending on solvent, temperature and nucleophile.

Carbocation stability

Carbocation stability tertiary > secondary > primary.
Alkyl groups push electron density towards the positive carbon (positive inductive effect), which spreads the charge.

Part 6 of 11Electrophilic addition to alkenes

Electrophilic addition to alkenes

Why alkenes react

Why alkenes react the C=C bond is an area of high electron density that attracts electrophiles.
- (one σ-bond and one weaker π-bond)
The π-bond breaks and is replaced by two new σ-bonds, so the alkene becomes saturated.
The atoms around C=C are planar with bond angles of 120°.

Addition of hydrogen halides

HX adds to give a halogenoalkane, fastest in the order HI > HBr > HCl because weaker H–X bonds break more easily.

Addition of halogens

Br₂ adds readily at room temperature to give a 1,2-dibromoalkane.

Hydration

Hydration alkene + steam gives an alcohol using H₃PO₄ or H₂SO₄ catalyst at about 300 °C and 60 atm.

Bromine water test

Bromine water test an alkene decolourises orange-yellow bromine water, while a saturated compound leaves it unchanged.

Part 7 of 11Mechanisms of electrophilic addition and Markovnikov rule (HL)

Addition of HBr, two steps:

Step 1 — the C=C electrons attack the δ+ hydrogen of HBr, and heterolytic fission gives a carbocation and Br⁻.
Step 2 — Br⁻ donates a lone pair to the carbocation to give the halogenoalkane.

Addition of Br₂

Addition of Br₂ the same two steps, after Br₂ becomes polarised by the electron-rich C=C.
Br₂ is non-polar and relies on an induced dipole, whereas hydrogen halides are permanently polar.

Addition of steam

Addition of steam water is a weak electrophile, so H₃O⁺ acts as the electrophile.
Step 1: the C=C attacks H⁺ from H₃O⁺ to form a carbocation.
Step 2: water donates a lone pair to the carbocation to give a protonated alcohol, which loses H⁺ to regenerate the catalyst.

Hydrogenation is assumed prior knowledge, and no mechanism is required.

Markovnikov's rule

Markovnikov's rule when HX adds to an unsymmetrical alkene, the halogen bonds to the more substituted carbon atom.
For molecules with two different halogens (such as ICl), the more electronegative halogen bonds to the more substituted carbon.

Reason

Reason the electrophile can add to either carbon, giving two possible carbocations.
The more stable carbocation forms through a lower activation energy and gives the major product.
The less stable carbocation gives the minor product.

Propene + HBr

Propene + HBr the secondary carbocation gives 2-bromopropane (major) and the primary carbocation gives 1-bromopropane (minor).

Part 8 of 11Lewis acids and bases (HL)

Lewis acid

Lewis acid a lone-pair acceptor; Lewis base = a lone-pair donor.

Lewis theory is broader than Brønsted–Lowry theory because it includes reactions with no proton transfer.

Reaction: A⁺ + :B⁻ → A:B, with a coordinate covalent bond formed when the base donates a lone pair.

Species such as OH⁻, CN⁻, NH₃ and CH₃NH₂ act as both Lewis bases and Brønsted–Lowry bases.

Both kinds of base need a lone pair, which is donated to a Lewis acid or to H⁺.

BF₃ + NH₃ → F₃B←NH₃ is a Lewis acid-base reaction.

Boron is sp² hybridised with a vacant 2p_z orbital that accepts the lone pair from nitrogen.
Only an electron pair moves, so neither species acts as a Brønsted–Lowry acid or base.

An electrophile is a Lewis acid and a nucleophile is a Lewis base.

Water is amphoteric, so it can be a Lewis acid or a Lewis base depending on the reaction.

Part 9 of 11Complex ions and ligands (HL)

Complex ion

Complex ion a central metal ion bonded to ligands by coordinate bonds, written in square brackets.
The metal ion is the Lewis acid and each ligand is a Lewis base and a nucleophile.
Transition metals form complexes because they have partially filled d subshells.
Cu²⁺(aq) + 6H₂O(l) → [Cu(H₂O)₆]²⁺(aq)

Ligand types by number of coordinate bonds:

Type	Bonds per ligand	Examples
Monodentate	1	H₂O, NH₃, Cl⁻, CN−, OH⁻
Bidentate	2	1,2-diaminoethane (en), ethanedioate (ox)
Multidentate	more than 2	EDTA4− (hexadentate, 6 bonds)

Coordination number

Coordination number the number of coordinate bonds to the central ion.
For monodentate ligands it equals the number of ligands.
Coordination number 2 is linear, 4 is tetrahedral or square planar, and 6 is octahedral.

Charge of a complex

Charge of a complex charge on the metal ion + sum of the ligand charges.

In [CuCl₄]²⁻ the charge is (2+) + 4 × (1−) = 2−.
Neutral ligands such as H₂O add no charge, so [Fe(H₂O)₆]³⁺ has the charge of Fe³⁺.

Part 10 of 11Electrophilic substitution in benzene (HL)

Why substitution

Why substitution benzene has a stable delocalised π-system, so it keeps its ring and substitutes a hydrogen rather than adding across a double bond.

Nitration replaces H on the ring by a nitro group (–NO₂) and uses the electrophile NO₂⁺.

Steps of the mechanism:

Electrophilic substitution in benzene (HL)

Generation of the electrophile — concentrated HNO₃ and concentrated H₂SO₄ at 25–60 °C produce the nitronium ion NO₂⁺ in situ.

Electrophilic attack — a pair of π electrons from the ring forms a bond to NO₂⁺, disrupting aromaticity and giving a positively charged intermediate.

Regenerating aromaticity — the C–H bond breaks heterolytically and its electrons return to the ring,

giving nitrobenzene and regenerating the acid catalyst.

Part 11 of 11More from the specification

Hydroxide
- A compound containing the OH⁻ ion, formed by Group I metals with water.

Key terms

Term	Meaning
Nucleophile	An electron-rich species that donates a lone pair to form a covalent bond (a Lewis base).
Electrophile	An electron-deficient species that accepts a pair of electrons (a Lewis acid).
Heterolytic fission	Bond breaking in which both bonding electrons go to the more electronegative atom, forming ions.
SN2	One-step bimolecular nucleophilic substitution with backside attack and inversion of configuration.
SN1	Two-step unimolecular nucleophilic substitution through a carbocation intermediate.
Markovnikov's rule	In HX addition to an unsymmetrical alkene, X bonds to the more substituted carbon.
Ligand	A molecule or ion that donates a lone pair to a metal ion to form a coordinate bond.
Coordination number	The number of coordinate bonds to the central metal ion in a complex.

Avoid these mistakes

Calling the C–X bond break in SN1 homolytic — it is heterolytic and both electrons go to the halogen.

Saying tertiary halogenoalkanes react by SN2 — tertiary use SN1 because they form stable carbocations, and primary use SN2.

Treating the SN2 transition state as an intermediate — it is a temporary high-energy arrangement with partial bonds, not a species that can be isolated.

Drawing curly arrows with single heads or pointing them the wrong way — they are double-headed and run from a lone pair or bond to an electron-deficient atom.

Placing the halogen on the less substituted carbon in Markovnikov addition — it bonds to the more substituted carbon through the more stable carbocation.

Last updated: 18 October 2026