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Halogenoalkanes

Halogenoalkanes. AH Chemistry Unit 3(b). Background. Also known as haloalkanes or alkyl halides Rare in the natural world Widely used Synthesised in the lab. Nomenclature practise. 2,3-dichloropentane 3-bromobut-1-ene 2,3-dibromo-1-chloro-2-methylbutane. Primary, secondary, tertiary.

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Halogenoalkanes

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  1. Halogenoalkanes AH Chemistry Unit 3(b)

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

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

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

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

  6. Nucleophilic substitution

  7. Nucleophilic substitution hydroxide ion with bromoethane mechanism CH3CH2Br + OH- (aqueous) CH3CH2OH + Br- ethanol hydroxide ion with 2-bromo,2-methylpropane mechanism (CH3)3CBr + OH- (aqueous) (CH3)3COH + Br- 2-methylpropan-2-ol

  8. hydroxide ion with bromoethane (SN2) - H H - + HO HO Br H C C CH3 H CH3 H - Br Br C CH3 - H OH Nucleophilic substitution mechanism ethanol SN2 reaction equation S(substitution) 2(species reacting in the slowest step) N(nucleophilic)

  9. OH- ion with 2-bromo,2-methylpropane (SN1) CH3 CH3 CH3 - + OH + C C CH3 Br C CH3 CH3 CH3 CH3 CH3 - - Br Br - OH Nucleophilic substitution mechanism 2-methylpropan-2-ol SN1 reaction equation S(substitution) 1(species reactingin the slowest step) N(nucleophilic)

  10. General rule • Primary and secondary halogenoalkanes tend to undergo SN2 reactions • Tertiary halogenoalkanes tend to undergo SN1 reactions • Can you suggest why?

  11. Important substitution reactions

  12. Nucleophilic substitution cyanide ion with iodoethane CH3CH2I (ethanol) + CN-(aq) CH3CH2CN + I- propanenitrile cyanide ion with 2-bromo,2-methylpropane mechanism (CH3)3CBr (ethanol) + CN- (aqueous) (CH3)3CCN + Br- 2,2-dimethylpropanenitrile

  13. CN- ion with 2-bromo,2-methylpropane (SN1) CH3 CH3 CH3 - + CN + C C CH3 Br C CH3 CH3 CH3 CH3 CH3 - - Br Br - CN Nucleophilic substitution mechanism SN1 2,2-dimethylpropanenitrile S(substitution) 1(species reactingin the slowest step) N(nucleophilic) reaction equation

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