Reaction types, mechanisms, nucleophiles and electrophilesEdexcel International A Level Chemistry: Revision notes

Section 1

Types of organic reaction

Addition: two molecules join to form one, usually across a C=C bond, e.g. ethene + HBr.
Substitution: one atom or group is replaced by another, e.g. $C_2H_5Br \rightarrow C_2H_5OH$ .
Elimination: a small molecule (e.g. HBr or $H_2O$ ) is removed, forming a C=C bond.
Oxidation: gain of oxygen or loss of hydrogen, e.g. alcohol to aldehyde.
Reduction: loss of oxygen or gain of hydrogen.
Hydrolysis: a bond is broken by reaction with water or hydroxide ions, e.g. halogenoalkane to alcohol.
Polymerisation: many monomers join to form a long chain.

Key termsadditionsubstitutioneliminationhydrolysispolymerisation

Exam tip

A reaction can fit two labels. Bromoethane with aqueous NaOH is both substitution and hydrolysis.

Section 2

Reaction mechanisms and curly arrows

A reaction mechanism is a step-by-step description of how a reaction occurs, showing which bonds break and form. Curly arrows show the movement of a pair of electrons: the arrow starts at the lone pair or bond that moves, and ends where the pair goes.

Key termsreaction mechanismcurly arrow

Section 3

Heterolytic bond breaking

In heterolytic fission a covalent bond breaks with both bonding electrons going to one atom. This forms a positive ion and a negative ion, e.g. $H{-}Br \rightarrow H^+ + Br^-$ . Heterolytic breaking produces the electrophiles and nucleophiles that attack molecules in these mechanisms.

Key termsheterolytic fission

Section 4

Nucleophiles and electrophiles

A nucleophile is an electron-pair donor: it uses a lone pair to form a covalent bond. Examples: $OH^-$ , $H_2O$ . Nucleophiles are attracted to δ+ (electron-deficient) atoms.

An electrophile is an electron-pair acceptor. Examples: $H^+$ , the δ+ hydrogen in HBr, a δ+ bromine atom in a polarised $Br_2$ molecule. Electrophiles are attracted to electron-rich regions.

Key termsnucleophileelectrophile

Common mistake

A nucleophile donates a pair of electrons, an electrophile accepts a pair. Do not swap them, and do not say 'loves nuclei' without the electron-pair idea.

Section 5

Bond polarity and the type of mechanism

Bond polarity decides which type of attacker a molecule meets.

In a halogenoalkane the C–X bond is polar, because the halogen is more electronegative, so carbon is δ+. It is attacked by nucleophiles: nucleophilic substitution, e.g. $C_2H_5Br + OH^- \rightarrow C_2H_5OH + Br^-$ .
In an alkene the C=C bond is electron-rich, so it is attacked by electrophiles: electrophilic addition, e.g. $CH_2{=}CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$ . The double bond polarises the $Br_2$ molecule.

Key termsnucleophilic substitutionelectrophilic addition

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Reaction types, mechanisms, nucleophiles and electrophiles

A student summarises five reactions of ethene and related compounds. Reaction 1: ethene reacts with hydrogen bromide to form bromoethane. Reaction 2: bromoethane is warmed with aqueous sodium hydroxide to form ethanol and sodium bromide. Reaction 3: bromoethane is warmed with ethanolic sodium hydroxide to form ethene, sodium bromide and water. Reaction 4: ethanol is warmed with acidified potassium dichromate(VI) to form ethanal. Reaction 5: many ethene molecules join under high pressure to form poly(ethene).
Name the type of reaction in Reaction 3 and in Reaction 5, and state what happens to the organic molecules in each.2 marks
Bromoethane reacts with aqueous hydroxide ions to form ethanol: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. The reaction can be explained by a mechanism in which curly arrows show the movement of pairs of electrons.
Describe, in terms of the movement of electron pairs, how bromoethane forms ethanol in this reaction.2 marks
Ethene reacts with bromine at room temperature to form 1,2-dibromoethane: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br. The reaction occurs readily even though ethene and bromine are both non-polar molecules.
Explain why ethene is attacked by electrophiles, and why bromine behaves as an electrophile in this reaction.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).