alkylhalides (halo alkanes) I

alkyl halides

halides - flourine, chlorine, bromine, iodine

alkyl group replaces H with a halide (CH₃, C₂H₅ etc.)

carbon - halide groups are polar

electronegativity values:

• F = 4.0

• Cl = 3.1

• Br = 3.0

• I = 2.6

carbon is partial positive

if tis situaution carbon is electrophilic and can react eith nucleophiles

nucleophiles

may have a negative charge

may be neutral (lone pair electrons)

nucleophiles form bonds by donating electrons towards an electrophilic atom

in general, strong nucleophile - if it contains a negative charge

weaker nucleophile - if neutral

nucleophilic substitution reaction mechanisms

two types of nucleophilic substitution reactions which are classified based upon the reaction mechanism (pathways0 that they undergo

SN2 reactions

nucleophile attacks from the backside of molecule (opposite from halogen)

transition state contains both nucleophile and substrate

no intermediate in the reaction mechanism

reaction rate is alpha [nucleophile][substrate]

steric hindrance raises the energy fo the S_N² transition state

primary is more reactive then secondary alkyl halides

tertiary alkyl halides do not react by S_N² mechanism

negatively charged nucleophiles are more reactive then neutral

good leaving group lower the energy of the transition state thereby the S_N2 reaction rate

inversion of configuration always takes place