H ALOGENOALKANES OR ALKYL HALIDES

S ITES : D Halogenalkanes+Sn1+Sn2.pdf Animation:

Halogenoalkanes contain F, Cl, Br, I bonded to an alkyl group. CH 3 CH 2 F CH 3 CH 2 Cl CH 3 CH 2 Br CH 3 CH 2 I Bond C-F C-Cl C-Br C-I Bond Enthalpy (kJmol -1 )

P RIMARY,S ECONDARY,T ERTIARY H ALOGENOALKANES Primary halogenoalkanes: In a primary halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group.

Secondary: Tertiary:

HALOGENOALKANES UNDERGO SUBSTITUTION. NUCLEOPHILIC SUBSTITUTION

N UCLEOPHILE A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else. Nucleophiles are either fully negative ions, or have a lone pair of electrons. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia.

SUBSTITUTION NUCLEOPHILIC BIMOLECULAR – SN 2 Primary halogenoalkanes undergo SN2: Because the mechanism of the reaction involves 2 species, it is known as an S N 2 reaction.

P AGE 226

++ -- CH 3 H Br C H - OH CH 3 H HO C H Br - HYDROXIDE ION WITH BROMOETHANE SN2 Mechanism ethanol reaction equation 2 (species reacting in the slowest step) SN2SN2 S (substitution) N (nucleophilic) CH 3 H Br C H HO -

SN1 – S UBSTITUTION N UCLEOPHILIC U NIMOLECULAR (CH 3 ) 3 CBr + OH - => (CH 3 ) 3 COH + Br - 2 bromo 2 methylpropane reacts with warm dilute aqueous sodium hydroxide solution to form the tertiary alcohol. The rate of reaction shows that the rate depends only on the concentration of the halogenoalkane and does not depend on the concentration of OH -

G ENERAL RULE Primary and secondary halogenoalkanes tend to undergo S N 2 reactions Tertiary halogenoalkanes tend to undergo S N 1 reactions S N 2 reactivity rates follow the trend: CH 3 X > primary > secondary > tertiary Why???? PAGE 227

S TABILITY OF C ATIONS

Alkyl groups tend to push the bonding pair of electrons towards the carbon they are bonded to. This is known as the positive inductive effect. The positive charge can be spread over more atoms, increasing the stability of the ion. It will more likely be formed.

SN1 X SN2

D IAGRAM OF COMMON ORGANIC REACTIONS. 21

A NSWERS

N UCLEOPHILIC SUBSTITUTION propanenitrile CH 3 CH 2 I (ethanol)+ CN - (aq)CH 3 CH 2 CN + I - cyanide ion with iodoethane cyanide ion with 2-bromo,2-methylpropane 2,2-dimethylpropanenitrile (CH 3 ) 3 CBr (ethanol) + CN - (CH 3 ) 3 CCN + Br - ( aqueous ) mechanism

++ -- CH 3 Br C CH 3 Br - CN - ION WITH 2- BROMO,2- METHYLPROPANE (S N 1) Nucleophilic substitution mechanism 2,2-dimethyl propanenitrile 1 (species reacting in the slowest step) SN1SN1 S (substitution) N (nucleophilic) Br - CH 3 C + CN C CH 3 CN - reaction equation

E LIMINATION

P RINCIPLES OF ELIMINATION Elimination of a hydrogen halide produces an alkene In general, in aqueous solution substitution takes place In ethanolic solution, elimination takes place

M ECHANISMS E1 E2 The particular mechanism depends on the strength of the base and the environment of the halide atom

E2

E1