9701 · 15.1
Halogenoalkanes flashcards
Revision flashcards for Cambridge 9701 Halogenoalkanes (syllabus 15.1). Flip, recall, then mark a real past-paper question.
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What is a nucleophile?
An electron-pair donor. It is a species (an ion or a molecule) that is attracted to an electron-deficient centre, where it donates a pair of electrons to form a new covalent bond. Examples: OH⁻, CN⁻, NH₃.
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What is a halogenoalkane?
An alkane in which one or more hydrogen atoms have been replaced by a halogen atom (F, Cl, Br, I). General formula CₙH₂ₙ₊₁X.
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Why is the carbon-halogen (C-X) bond polar?
Because halogen atoms are more electronegative than carbon atoms. This pulls the electron density in the bond towards the halogen, creating a partial negative charge (δ−) on the halogen and a partial positive charge (δ+) on the carbon.
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What are the key features of an SN2 mechanism?
Stands for Substitution, Nucleophilic, Bimolecular. It is a one-step mechanism with a single transition state. It involves a 'backside' attack and results in inversion of configuration. Favoured by primary halogenoalkanes.
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What are the key features of an SN1 mechanism?
Stands for Substitution, Nucleophilic, Unimolecular. It is a two-step mechanism involving a carbocation intermediate. The first step (formation of the carbocation) is rate-determining. It can result in a racemic mixture. Favoured by tertiary halogenoalkanes.
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What conditions favour nucleophilic substitution over elimination?
Using a warm, aqueous solution of the nucleophile (e.g., NaOH(aq)). Water is a polar solvent that stabilises the ions involved.
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What conditions favour elimination over nucleophilic substitution?
Using a hot, concentrated solution of a strong base in an ethanol solvent (e.g., KOH in ethanol). The hydroxide ion acts as a base rather than a nucleophile.
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Why do iodoalkanes react faster than chloroalkanes in substitution reactions?
The C-I bond is weaker and has a lower bond enthalpy than the C-Cl bond. Therefore, less energy is required to break the C-I bond, leading to a faster rate of reaction.
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Why do tertiary halogenoalkanes favour the SN1 mechanism?
They form a stable tertiary carbocation intermediate. The stability is due to the positive inductive effect of the three alkyl groups, which donate electron density to the positive carbon, spreading out the charge.
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What is meant by 'inversion of configuration' in an SN2 reaction?
The stereochemistry at the carbon centre is inverted, similar to an umbrella flipping inside out. The nucleophile attacks from the opposite side of the leaving group, causing the other three groups to 'flip' over.
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What is a racemic mixture and why does an SN1 reaction produce one?
A racemic mixture is a 50:50 mixture of two enantiomers (chiral molecules that are mirror images). The SN1 mechanism proceeds via a planar carbocation, which the nucleophile can attack from either side with equal probability, forming both enantiomers in equal amounts.