Halogenoalkanes (Alkyl Halides)
Senior 5 · Chemistry
Halogenoalkanes (Alkyl Halides)
Lesson 3 of 4
25 min
In the 1930s Edward Hughes and Christopher Ingold, at University College London, measured how fast halogenoalkanes react with hydroxide ions. For bromomethane, doubling the concentration of either reactant doubled the rate. For 2-bromo-2-methylpropane, the rate depended only on the halogenoalkane: adding more hydroxide changed nothing. The overall equations look the same, so the two reactions must take different paths. Hughes and Ingold named them SN2 and SN1, names used ever since.
Definition
Nucleophilic substitution: a reaction in which a nucleophile replaces an atom or group (the leaving group, here ) on an electron-poor carbon. Example: .
SN2: substitution, nucleophilic, bimolecular: one step in which two species (nucleophile and halogenoalkane) collide; rate .
SN1: substitution, nucleophilic, unimolecular: two steps, the slow one involving only the halogenoalkane; rate .
Rate-determining step: the slowest step of a mechanism, which controls the overall rate.
Figure 3.1: SN2: one step, with attack from the back. The arrangement of the other three groups is inverted.
The nucleophile approaches the carbon from the side opposite the halogen, where it is not blocked. As the C–O bond forms, the C–Br bond breaks. In a primary halogenoalkane the back of the carbon is open (only one alkyl group), so SN2 is fast; in a tertiary one three bulky alkyl groups block the way.
Figure 3.2: SN1: two steps. The slow first step involves only the halogenoalkane.
Figure 3.3: Energy profiles. In SN1 the first step has the larger activation energy, so it is rate-determining.
A tertiary halogenoalkane ionises because its carbocation is stabilised by three alkyl groups (Unit 3), while a primary carbocation would be too unstable to form. Secondary halogenoalkanes react by both routes.
Table 3.1: SN1 and SN2 compared
| SN2 | SN1 | |
|---|---|---|
| typical substrate | primary (and methyl) | tertiary |
| steps | one, through a transition state | two, through a carbocation intermediate |
| rate equation | (second order) | (first order) |
| chiral substrate gives | one enantiomer, with inverted arrangement | a racemic mixture (attack on both faces of the flat carbocation) |
| Nucleophile and conditions | Equation (bromoethane) | Product |
|---|---|---|
| : NaOH(aq), warm under reflux | alcohol | |
| : KCN in ethanol, reflux | nitrile (propanenitrile): the chain grows by one carbon | |
| : excess concentrated ammonia in ethanol, heated in a sealed tube | primary amine (ethylamine); with less ammonia, secondary and tertiary amines also form | |
| : sodium methoxide in methanol | ether (Williamson synthesis, Unit 5) | |
| : water, warm | alcohol, slowly (water is a weak nucleophile) |
The same hydroxide ion can also act as a base and remove HX (Unit 3, Lesson 2). Water as solvent and gentle warming favour substitution; ethanol as solvent, a high temperature and a tertiary halogenoalkane favour elimination.
Which halogen leaves fastest? The order of reactivity is R–I > R–Br > R–Cl ≫ R–F. If polarity decided, the C–F bond, the most polar, would react fastest. It does not: the rate depends on breaking the C–X bond, and C–I is the weakest (Figure 1.2). Fluoroalkanes are almost unreactive.
Worked example
Problem. The initial rate of hydrolysis of 2-bromo-2-methylpropane was measured with different starting concentrations:
| Experiment | [RBr] / | [] / | initial rate / |
|---|---|---|---|
| 1 | 0.10 | 0.10 | |
| 2 | 0.20 | 0.10 | |
| 3 | 0.10 | 0.20 |
Deduce the rate equation and the mechanism, and calculate k.
Solution. Experiments 1 and 2: doubling [RBr] doubles the rate, so the reaction is first order in RBr. Experiments 1 and 3: doubling [] has no effect: zero order in . Rate : an SN1 mechanism, whose slow step involves only RBr. .
Worked example
Problem. Name the organic product of 1-bromopropane with: (a) NaOH(aq), warm; (b) KCN in ethanol, reflux; (c) excess concentrated ammonia in ethanol, heated in a sealed tube; (d) KOH in ethanol, heated strongly.
Solution. (a) propan-1-ol (substitution). (b) butanenitrile, : the nitrile carbon is counted in the name, so the chain is now four carbons long. (c) propan-1-amine, . (d) propene (elimination).
Warning
Potassium cyanide is highly toxic, and with acid it releases hydrogen cyanide gas. It is handled only by trained staff, in a fume cupboard, and never acidified. In mechanisms, remember that the SN2 transition state is not an intermediate: it cannot be isolated, and it is drawn in square brackets with ‡.
Tip
Primary → SN2 (bimolecular, back attack, inversion). Tertiary → SN1 (unimolecular, carbocation, racemic). The number in the name is the number of species in the rate-determining step.
Chemistry Student: SN1 and SN2 Mechanisms, A-level Chemistry (15 min). Watch the curly arrows for each mechanism, why primary and tertiary halogenoalkanes react differently, and how the rate equations follow.
Practice
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