1. Halogenoalkanes AH Chemistry Unit 3(b)

3. Background Also known as haloalkanes or alkyl halides Rare in the natural world Widely used Synthesised in the lab

4. Nomenclature practise 2,3-dichloropentane 3-bromobut-1-ene 2,3-dibromo-1-chloro-2-methylbutane

5. Primary, secondary, tertiary Draw a primary halogenoalkane and name it Draw a secondary halogenoalkane and name it Draw a tertiary halogenoalkane and name it

6. Reactions Depends on two factors… –Type of halogen atom –Position in molecule Two characteristic reactions: –Nucleophilic substitution –Elimination

7. Nucleophilic substitution

8. hydroxide ion with bromoethane ethanol CH 3 CH 2 Br+ OH - CH 3 CH 2 OH + Br - ( aqueous ) Nucleophilic substitution mechanism hydroxide ion with 2-bromo,2-methylpropane 2-methylpropan-2-ol (CH 3 ) 3 CBr+ OH - (CH 3 ) 3 COH + Br - ( aqueous ) mechanism

9. ++ -- CH 3 H Br C H - OH CH 3 H HO C H Br - hydroxide ion with bromoethane (S N 2) Nucleophilic substitution mechanism ethanol reaction equation 2 (species reacting in the slowest step) SN2SN2 S (substitution) N (nucleophilic) CH 3 H Br C H HO -

10. ++ -- CH 3 Br C CH 3 - OH Br - OH - ion with 2-bromo,2-methylpropane (S N 1) Nucleophilic substitution mechanism 2-methylpropan-2-ol reaction equation 1 (species reacting in the slowest step) SN1SN1 S (substitution) N (nucleophilic) Br - CH 3 C + OH C CH 3

11. General rule Primary and secondary halogenoalkanes tend to undergo S N 2 reactions Tertiary halogenoalkanes tend to undergo S N 1 reactions Can you suggest why?

12. Important substitution reactions Water or aqueous alkali AlcoholsCan then be converted to aldehydes, ketones or carboxylic acids AmmoniaAmines Alcoholic alkoxidesEthersA sodium alkoxide is produced in the reaction of sodium with a dry alcohol Potassium cyanide (ethanolic solution) NitrilesIncreases length of carbon chain

13. Nucleophilic 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

14. ++ -- 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