haloalkanes (alkyl halides)

structure

R-X

X

F, Cl, Br or I

solvent

a liquid medium which moderates the reaction and dissipates heat - allows manipulation

by product

something that is inevitably formed in a reaction

side product

can be avoided by changing the reaction conditions

haloalkanes

insoluble in water

group 7 - monovalent

no hybridisation

fluorine

more electronegative

not a base or nucleophile

bonding pairs are low in energy

fluoroalkanes are chemically unreactive

nucleophilic substitution

halide is replaced by other atoms or groups

carbons bonded to halogens act as electrophiles

why are they reactive

Cl → I : halogen is getting larger - longer and weaker bonds

halide ions are quite stable species - formation is energetically favourable

amines and alcohol substitution

do not undergo substitution as the leaving groups are bad and bonds are strong

Sn in biological chemistry

methylation of biomolecules involved enzyme-catalysed reaction with S-odenosyl methionine

haloaromatics - bad electrophiles

properties differ when functional groups are directly attached as opposed to alkyl groups

elimination reactions

reaction of haloalkanes with strong bases can give elimination reactions

primary halides

give Sn unless the base is not nucleophilic

secondary halides

give E or Sn depending on solvent and temperature

tertiary halides

give E because substituted alkenes are more stable unless there is no base

classic elimination conditions

hot alcoholic KOH