Halogen compounds

Syllabus points

• Describe the nucleophilic substitution reactions of halogenoalkanes with hydroxide, cyanide and ammonia.
• Describe the elimination reaction with ethanolic hydroxide.
• Explain the relative rates of hydrolysis of fluoro, chloro, bromo and iodo compounds.
• Explain the difference in conditions that leads to substitution rather than elimination.
• Describe the uses of halogenoalkanes and the environmental concerns about CFCs.

Why halogenoalkanes react

The carbon-halogen bond is polar, because the halogen is more electronegative than carbon. The carbon carries a δ+ charge, and that is what attracts a nucleophile.

Everything in this chapter follows from that single feature.

Nucleophilic substitution

Three reagents, three products, and the conditions matter as much as the reagent:

The cyanide reaction deserves attention because it is the only way on the AS syllabus to lengthen the carbon chain, which makes it a key step in synthesis questions. 1-bromopropane gives butanenitrile, with four carbons from three.

With ammonia, excess ammonia is specified because the amine produced is itself a nucleophile and will react further, giving secondary and tertiary amines and eventually a quaternary salt. Using excess ammonia makes the first product the most likely.

The mechanism

For a primary halogenoalkane the mechanism is one step:

1. A curly arrow from a lone pair on the nucleophile to the δ+ carbon.
2. A curly arrow from the C-Br bond to the bromine.

Both happen together, so the nucleophile attacks as the halide leaves. The arrow must start at the lone pair, not at the negative sign, and that detail is regularly worth a mark.

Rates of hydrolysis

The order of reactivity is

iodo > bromo > chloro > fluoro

so an iodoalkane hydrolyses fastest.

The explanation is bond energy, not polarity, and this is the trap. The C-F bond is the most polar, so on polarity alone fluoroalkanes should react fastest. They are in fact the slowest by a wide margin, because the C-F bond is much the strongest and the rate depends on how easily that bond breaks.

The experiment

Warm each halogenoalkane with aqueous silver nitrate in ethanol. Ethanol is there because halogenoalkanes do not dissolve in water. As the halide ion is released it precipitates with the silver:

• Iodoalkane gives a pale yellow precipitate fastest
• Bromoalkane gives a cream precipitate more slowly
• Chloroalkane gives a white precipitate slowest
• Fluoroalkane gives essentially nothing

Elimination

Change the solvent and the same hydroxide ion does something entirely different:

CH₃CH₂CH₂Br + NaOH → CH₃CH=CH₂ + NaBr + H₂O

The hydroxide removes a hydrogen from the carbon next to the one bearing the halogen, and a double bond forms as the halide leaves.

The solvent is the whole answer to "how do you get one rather than the other", and it is a favourite question. Aqueous conditions favour substitution; ethanolic conditions favour elimination.

Where the halogenoalkane is unsymmetrical, more than one alkene can form. 2-bromobutane gives both but-1-ene and but-2-ene, and but-2-ene, the more substituted alkene, predominates.

Uses and the environment

Halogenoalkanes have been used as solvents, refrigerants, propellants and flame retardants, because they are unreactive, volatile and non-flammable.

Chlorofluorocarbons (CFCs) were valued for exactly that unreactivity, and it turned out to be the problem. Being unreactive, they survive long enough to reach the stratosphere, where UV light breaks the C-Cl bond by homolytic fission:

CCl₃F → •CCl₂F + Cl•

The chlorine radical then destroys ozone in a chain:

Cl• + O₃ → ClO• + O₂

ClO• + O₃ → Cl• + 2O₂

The chlorine radical is regenerated in the second step, so a single one destroys many thousands of ozone molecules before it is removed. Thinner ozone means more UV reaching the surface, and more skin cancer and cataracts.

CFCs have been phased out under the Montreal Protocol and replaced by HFCs, which contain no chlorine and so cannot produce chlorine radicals.

The episode is a useful case study in unintended consequences: the property that made CFCs safe to handle is the property that made them persistent enough to do damage.

Common mistakes

• Explaining the hydrolysis trend with polarity. It is bond energy, and polarity would predict the opposite.
• Forgetting that the solvent decides substitution against elimination.
• Drawing the nucleophile's curly arrow from the charge rather than from the lone pair.
• Forgetting excess ammonia, and so not explaining why further substitution is suppressed.
• Missing that KCN adds a carbon to the chain, which is usually the point of a synthesis question.
• Saying CFCs damage ozone because they are reactive. They cause harm because they are unreactive and persist.