How halogen atoms make carbon compounds reactive
Haloalkanes contain a halogen bonded to an sp³ carbon, while haloarenes have the halogen bonded to an aromatic ring. They are named by adding prefixes such as chloro- or bromo-, and are classified as primary, secondary or tertiary. They are prepared from alcohols using HX, PCl₅ or SOCl₂, from alkenes by adding HX following Markovnikov’s rule, or by the Finkelstein and Swarts reactions that exchange halogens.
The C–X bond is polar, so the carbon is attacked by nucleophiles. In SN2 reactions the nucleophile attacks in one step from the opposite side, inverting configuration, and primary halides react fastest. In SN1 reactions a carbocation forms first, so tertiary halides react fastest and racemisation can occur. With strong bases such as alcoholic KOH, elimination gives alkenes, with the more substituted alkene as the major product by Saytzeff’s rule. Haloarenes resist nucleophilic substitution because of resonance, but undergo electrophilic substitution at ortho and para positions.
Why it matters
Chloroform and iodoform were used in medicine, and many pesticides and refrigerants contain halogens. Chlorofluorocarbons damaged the ozone layer, which is why they are now restricted, and DDT is banned in many countries because it builds up in food chains. Substitution and elimination mechanisms are core ideas for all of organic chemistry.
Key terms
- Nucleophilic substitution
- A reaction in which a nucleophile replaces a leaving group such as a halide ion.
- SN2 mechanism
- A one-step substitution in which bond-making and bond-breaking happen together, inverting configuration.
- Saytzeff’s rule
- In elimination, the more highly substituted alkene is formed as the main product.
- Haloarene
- An aromatic compound in which a halogen is bonded directly to the benzene ring.