Haloalkanes and Haloarenes: common doubts, answered
The questions students ask most often about Haloalkanes and Haloarenes, each with a short answer. For the full chapter, read the Haloalkanes and Haloarenes notes.
Classification
Read this section in the notes →Is benzyl chloride an aryl halide?
No. In benzyl chloride, C₆H₅CH₂Cl, the chlorine is attached to an sp³ CH₂ carbon next to the ring, so it is a benzylic halide and behaves like a reactive alkyl halide. An aryl halide, such as chlorobenzene, has the halogen bonded directly to an sp² carbon of the ring. Allylic halides are the comparable case next to a C=C bond.
What is the difference between vinylic and allylic halides?
In a vinylic halide the halogen sits on a carbon of the double bond itself, an sp² carbon, as in CH₂=CHCl. In an allylic halide it sits on the sp³ carbon next to the double bond, as in CH₂=CHCH₂Cl. Allylic halides react readily because their carbocation is stabilised by resonance, while vinylic halides resist substitution, much like aryl halides.
Nomenclature
Read this section in the notes →How are dihalides classified as geminal and vicinal?
In a geminal dihalide both halogens are on the same carbon, as in ethylidene chloride, CH₃CHCl₂, which is 1,1-dichloroethane; these are also called alkylidene halides. In a vicinal dihalide the halogens are on neighbouring carbons, as in ethylene dichloride, ClCH₂CH₂Cl, which is 1,2-dichloroethane. Common names differ from IUPAC names here, so read questions carefully.
The carbon-halogen bond
Read this section in the notes →Why does CH₃Cl have a larger dipole moment than CH₃F?
Dipole moment depends on both charge separation and bond length. Fluorine is more electronegative, so the C–F bond carries larger partial charges, but it is much shorter. The C–Cl bond is longer, and that greater distance outweighs its smaller charges, giving CH₃Cl the largest dipole moment among the methyl halides. The order runs CH₃Cl > CH₃F > CH₃Br > CH₃I.
Preparing haloalkanes
Read this section in the notes →Why is thionyl chloride preferred for making alkyl chlorides from alcohols?
With thionyl chloride, the by-products, sulphur dioxide and hydrogen chloride, are both gases that bubble out of the mixture. That leaves fairly pure alkyl chloride behind, with no need to separate it from other liquids or solids. Other reagents, such as PCl₃ or PCl₅, leave phosphorus acids or POCl₃ that must be removed afterwards.
Why is H₃PO₄ used instead of H₂SO₄ to make an alkyl iodide from an alcohol with KI?
The acid is needed to generate HI from KI. Concentrated sulphuric acid is an oxidising agent and would oxidise HI to iodine, so little alkyl iodide would form. Phosphoric acid is not oxidising, so the HI survives to react with the alcohol. This substitution of a non-oxidising acid is a standard reason-type question.
What are the Finkelstein and Swarts reactions?
Both swap one halogen for another. In the Finkelstein reaction, an alkyl chloride or bromide is warmed with NaI in dry acetone to give the alkyl iodide; it goes forward because NaCl and NaBr are insoluble in acetone and precipitate out. In the Swarts reaction, an alkyl chloride or bromide is heated with a metallic fluoride such as AgF or SbF₃ to give the alkyl fluoride.
Preparing haloarenes
Read this section in the notes →Why can't aryl halides be made from phenol and HCl?
The C–O bond in phenol has partial double-bond character because the oxygen's lone pair is delocalised into the ring, so it is very hard to break. Aryl halides are made instead by halogenating benzene with a Lewis acid catalyst such as FeCl₃, or from diazonium salts by the Sandmeyer reaction, using CuCl or CuBr.
Physical properties
Read this section in the notes →Why does p-dichlorobenzene melt at a higher temperature than its o- and m-isomers?
The para isomer is the most symmetrical, so its molecules fit together more neatly in the crystal lattice. Stronger packing means more energy is needed to break the solid apart, so it melts higher than the ortho and meta isomers. Their boiling points, which depend on attractions in the liquid rather than packing, are nearly the same.
Why are haloalkanes insoluble in water even though they are polar?
To dissolve, a haloalkane must break the hydrogen bonds between water molecules. The new attractions it forms with water are much weaker than those hydrogen bonds, so too little energy is released to pay for breaking them. Haloalkanes therefore barely dissolve in water, but they mix readily with organic solvents, where the attractions broken and formed are similar.
Nucleophilic substitution and SN2
Read this section in the notes →Why does KCN give an alkyl cyanide but AgCN give an isocyanide?
Cyanide is an ambident nucleophile that can attack through carbon or nitrogen. KCN is mainly ionic, so the free CN⁻ ion attacks through carbon, forming a C–C bond and an alkyl cyanide. AgCN is mainly covalent, so the carbon is tied to silver and only the nitrogen lone pair is free, giving an isocyanide. Nitrite behaves the same way: KNO₂ gives alkyl nitrites, AgNO₂ nitroalkanes.
Why is the SN2 reaction slowest for tertiary halides?
In SN2 the nucleophile attacks the carbon from the side opposite the leaving group, in a single step. Bulky groups around that carbon block the approach and crowd the transition state. A methyl halide is least hindered and a tertiary halide most hindered, so SN2 reactivity runs CH₃X > 1° > 2° > 3°.
SN1 and reactivity order
Read this section in the notes →Why do tertiary halides react fastest by SN1?
The SN1 reaction goes through a carbocation, formed in a slow first step that sets the rate, so rate = k[RX] and the nucleophile does not appear. A tertiary carbocation is stabilised most by the electron-releasing effect and hyperconjugation of three alkyl groups, so it forms fastest. Reactivity is 3° > 2° > 1°, and polar protic solvents help by stabilising the ions.
Why are benzylic and allylic halides very reactive in SN1 reactions?
The carbocations they form are stabilised by resonance: the positive charge spreads over the benzene ring or along the double bond. That makes them easy to form, so these halides react rapidly by SN1 even though they are primary. They react readily by SN2 too, so they are among the most reactive halides in substitution reactions.
Stereochemistry of substitution
Read this section in the notes →Why does SN2 give inversion of configuration while SN1 gives racemisation?
In SN2 the nucleophile enters from the back while the leaving group departs from the front, so the other three groups flip over like an umbrella turning inside out, giving inversion. In SN1 the carbocation is planar, and the nucleophile can attack equally from either face. An optically active halide therefore gives a mixture of both enantiomers, a racemic product.
What is the difference between enantiomers and a racemic mixture?
Enantiomers are a pair of molecules that are non-superimposable mirror images of each other; they rotate plane-polarised light by equal amounts in opposite directions. A racemic mixture contains equal amounts of both, so their rotations cancel and the mixture shows no optical activity. A molecule needs a chiral centre or overall asymmetry to have enantiomers.
Elimination and reactions with metals
Read this section in the notes →What does Zaitsev's rule predict in dehydrohalogenation?
When more than one alkene can form, the main product is the more substituted alkene, the one with more alkyl groups on the double-bond carbons, because it is more stable. So 2-bromopentane heated with alcoholic KOH gives mostly pent-2-ene rather than pent-1-ene. The hydrogen is removed from the neighbouring carbon that has fewer hydrogens.
Why must Grignard reagents be prepared in dry ether and kept away from water?
In a Grignard reagent, RMgX, the carbon bonded to magnesium carries a strong negative charge and behaves as a powerful base. Any source of an acidic hydrogen, even water or alcohol, protonates it immediately and turns it into an alkane. Dry ether is used because it has no such hydrogen and also stabilises the reagent by coordinating to magnesium.
Reactions of haloarenes
Read this section in the notes →Why are aryl halides much less reactive than alkyl halides towards nucleophiles?
The halogen's lone pair is delocalised into the ring, giving the C–X bond partial double-bond character. The carbon is also sp², which holds the bond more tightly, the phenyl cation that SN1 would need is very unstable, and the electron-rich ring repels incoming nucleophiles. So chlorobenzene gives phenol only with NaOH at about 623 K and high pressure.
Why does a nitro group at the ortho or para position make aryl halides more reactive?
A nitro group withdraws electrons strongly, and from the ortho or para position it can take the negative charge of the intermediate anion onto its own oxygens by resonance. This stabilises the intermediate, so nucleophilic substitution happens much more easily, and more nitro groups make it easier still. A meta nitro group cannot delocalise the charge this way and helps very little.
Polyhalogen compounds
Read this section in the notes →Why is chloroform stored in dark, completely filled bottles?
In light and air, chloroform is slowly oxidised to phosgene, COCl₂, a very poisonous gas. Dark bottles keep out light and filling them to the top leaves no room for air. Some ethanol is also commonly added, which converts any phosgene formed into harmless diethyl carbonate.
Why was DDT banned in many countries despite being an effective insecticide?
DDT breaks down very slowly in the environment and dissolves in fat rather than water, so it builds up in the fatty tissues of animals and becomes more concentrated at each step up a food chain. Many insects also became resistant to it, and it proved highly toxic to fish. In birds, high levels led to thin-shelled eggs, so its use was restricted in many places.
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