Ch 1 · Halogenoalkanes · AS & A Level Chemistry 9701

Topic 1 of 1 in Halogen compounds — AS & A Level Chemistry 9701 — 68 lessons.

Introduction

Halogenoalkanes contain a polar carbon–halogen bond that makes them far more reactive than alkanes towards nucleophiles. Cambridge examiners expect the two nucleophilic substitution mechanisms, S_N1 and S_N2, to be linked precisely to the class of halogenoalkane (primary, secondary or tertiary) that favours each one, and expect the trend in reactivity down the halogens to be explained using bond enthalpy, not electronegativity.

Key ideas — Producing halogenoalkanes

Halogenoalkanes can be produced by several routes already met in this module: the free-radical substitution of an alkane with a halogen in ultraviolet light; the electrophilic addition of a hydrogen halide or a halogen to an alkene; and by reacting an alcohol with a suitable halogenating agent, such as HBr, PCl_5, or NaBr with concentrated H_2SO_4, which substitutes the -OH group for a halogen.

🧑‍🏫 Teacher's explanation

Think of a halogenoalkane as a molecule where a carbon atom is 'holding hands' with a halogen atom. Because halogens are more electronegative than carbon, they pull the electron density toward themselves, making the carbon atom slightly positive (partial positive charge, delta+). This makes the carbon an 'electrophile magnet'—it's looking for something rich in electrons to attack it.

📖 Definitions

🔍 Worked example — Easy

Calculate the percentage yield of bromoethane if 0.50 moles of bromoethane are produced from 0.60 moles of ethene in the reaction: C_2H_4 + HBr arrow C_2H_5Br.

🔍 Worked example — Standard

A sample of 2-chlorobutane is reacted with aqueous KOH. If the reaction follows an S_N2 mechanism, describe the change in the stereochemistry of the chiral center.

🔍 Worked example — Exam-style

Compare the reactivity of CH_3CH_2Cl and (CH_3)_3CCl towards reaction with NaOH(aq). Which is faster and why?

⚠️ Common errors

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🎯 Exam question

Key ideas — Classifying halogenoalkanes

A halogenoalkane is classified by the number of carbon (alkyl) groups directly attached to the carbon bearing the halogen: a primary (1^°) halogenoalkane has one alkyl group attached to that carbon (e.g. 1-bromobutane), a secondary (2^°) halogenoalkane has two (e.g. 2-bromobutane), and a tertiary (3^°) halogenoalkane has three (e.g. 2-bromo-2-methylpropane).

🧑‍🏫 Teacher's explanation

Imagine a carbon atom as a central hub in a busy airport. The halogen atom (like Cl or Br) is a passenger waiting to leave. The 'classification' of the halogenoalkane simply tells us how many other 'passengers' (carbon groups) are sitting in the seats right next to that central hub.

📖 Definitions

🔍 Worked example — Easy

Classify the following compound: CH_3CH_2CH(Cl)CH_3.

🔍 Worked example — Standard

Calculate the molar mass (M_r) of 2-bromo-2-methylpropane, given C=12, H=1, Br=80.

🔍 Worked example — Exam-style

A halogenoalkane reacts via the S_N1 mechanism. If the rate-determining step involves the formation of a carbocation from CH_3CH_2CH(Br)CH_3, classify the starting material.

⚠️ Common errors

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🎯 Exam question

(a) Classify the halogenoalkane as primary, secondary, or tertiary. [1] (b) State the mechanism that is most likely to occur. [1] (c) Draw the structure of the organic product formed. [1] [3 marks]

Key ideas — Nucleophilic substitution reactions

The C-X bond in a halogenoalkane is polarised C^delta+-X^delta-, making the carbon atom susceptible to attack by nucleophiles. Warming a halogenoalkane with:

🧑‍🏫 Teacher's explanation

Imagine a halogenoalkane as a molecule with a 'weak spot'. Because halogens like Chlorine or Bromine are more electronegative than Carbon, they pull the electron density toward themselves. This leaves the Carbon atom feeling slightly positive (electron-deficient), like a magnet with a weak positive pole. A nucleophile is a 'nucleus-loving' species—it has extra electrons and is looking for that positive spot to crash into.

📖 Definitions

🔍 Worked example — Easy

Calculate the mass of 1-bromobutane ($ ext{C}_4 ext{H}_9 ext{Br}$) required to react completely with 0.500 mol of NaOH in a nucleophilic substitution reaction to form 1-butanol. (A_r: C=12, H=1, Br=80).

🔍 Worked example — Standard

A sample of 2-bromo-2-methylpropane reacts via an S_N1 mechanism. If the rate of reaction is proportional to the concentration of the halogenoalkane, [R-Br] = 0.020 mol dm^-3, and the rate constant k = 0.005\ ext s^-1, calculate the initial rate of reaction.

🔍 Worked example — Exam-style

Explain why 1-chlorobutane reacts faster via S_N2 than 2-chloro-2-methylpropane, and identify the mechanism for the latter.

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🎯 Exam question

Key ideas — Elimination with ethanolic hydroxide

Heating a halogenoalkane with sodium or potassium hydroxide dissolved in ethanol favours elimination instead of substitution: hydroxide acts as a base, removing a hydrogen atom from the carbon adjacent to the one bearing the halogen, while the C-X bond breaks heterolytically, forming an alkene, water and a halide ion, e.g. CH_3CH_2Br + OH^- xrightarrowethanol, heat CH_2=CH_2 + H_2O + Br^-. Using aqueous hydroxide favours substitution, while ethanolic hydroxide favours elimination; this contrast in solvent and mechanism is a common examination point.

🧑‍🏫 Teacher's explanation

Think of a halogenoalkane as a molecule with a 'bulky' leaving group (the halogen) that is waiting to escape. When you use aqueous hydroxide, the hydroxide acts like a surgical scalpel (a nucleophile), attacking the carbon atom to swap the halogen for an -OH group. However, when you use hydroxide dissolved in ethanol and heat it, the environment changes. The hydroxide becomes more 'aggressive' and acts like a sledgehammer (a base).

📖 Definitions

🔍 Worked example — Easy

Predict the products of the reaction: ceCH3CH2Br + ceKOH xrightarrowethanol, heat

🔍 Worked example — Standard

A student reacts 2-bromobutane with ethanolic KOH. Draw the major organic product.

🔍 Worked example — Exam-style

Calculate the mass of ethene produced from the elimination of 10.9g of bromoethane (Ar: Br=80, C=12, H=1, O=16) using excess ethanolic KOH.

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🎯 Exam question

Key ideas — SN2 and SN1 mechanisms

Nucleophilic substitution can occur by two distinct mechanisms:

🧑‍🏫 Teacher's explanation

Imagine you are trying to enter a crowded room. In an S_N2 mechanism, it is like a person pushing through a revolving door; you enter at the exact same moment someone else is exiting. It is a single, coordinated movement where the 'new person' (nucleophile) pushes the 'old person' (leaving group) out from the opposite side. This 'backside attack' is why the molecular shape flips, like an umbrella blowing inside out in a strong wind.

📖 Definitions

🔍 Worked example — Easy

Identify whether the following reaction is likely to proceed via S_N1 or S_N2 mechanism: CH_3CH_2CH_2Br + OH^- arrow CH_3CH_2CH_2OH + Br^-.

🔍 Worked example — Standard

A reaction between 2-bromo-2-methylpropane and water follows first-order kinetics: Rate = k[C_4H_9Br]. Determine the rate law if the concentration of the halogenoalkane is doubled and the concentration of water remains constant.

🔍 Worked example — Exam-style

Explain why (S)-2 ext-chlorobutane reacts with OH^- to form a racemic mixture of 2 ext-butanol via the S_N1 mechanism.

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🎯 Exam question

Key ideas — Why mechanism depends on class, and why reactivity depends on bond strength

S_N2 is disfavoured at more substituted carbons because the bulky alkyl groups around a tertiary (or even secondary) carbon sterically hinder the nucleophile's backside attack. S_N1 is favoured at more substituted carbons because each additional alkyl group releases electron density towards the developing positive charge through the positive inductive effect (+I), stabilising the carbocation intermediate; a tertiary carbocation, with three alkyl groups, is markedly more stable than a primary carbocation, with only one, which almost never forms.

🧑‍🏫 Teacher's explanation

Imagine you are trying to walk through a doorway to reach a prize. In an S_N2 reaction, the 'prize' is the carbon atom, and the 'doorway' is the backside of the molecule. If the room is crowded with large, bulky furniture (alkyl groups), you can't get through the door easily—this is 'steric hindrance'. In an S_N1 reaction, the door is already open because a piece of the furniture has fallen off (the halogen has left), leaving a 'carbocation' intermediate. The more furniture you have, the more the 'weight' of that furniture helps support the structure while the door is open—this is the 'inductive effect'.

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Halogenoalkanes