reactions of haloalkanes.. nucleophilic substitution halogens are relatively electronegative. so the...

of 20 /20
Reactions of haloalkanes .

Author: gilbert-mckinney

Post on 04-Jan-2016




7 download

Embed Size (px)


  • Reactions of haloalkanes.

  • Nucleophilic substitutionHalogens are relatively electronegative.So the carbon/halogen bond will be polarised.-+The + on carbon will be attractive to nucleophiles, resulting in nucleophilic substitution.(CH3)2CHCl + Nuc: (CH3)2CHNuc + Cl_

  • Nucleophilic substitution

  • Reactions with sodium hydroxide. If haloalkanes are heated with aqueous sodium hydroxide the corresponding alcohol is formed. CH3CH2Cl + :OH- CH3CH2OH + :Cl_Chloroethane ethanolNB As haloalkanes are not very soluble in water they are dissolved in alcohol. Water by itself can also act as a nucleophile, but at a slower rate;CH3CH2Cl + :OH2 CH3CH2OH + HCl

  • Reaction with cyanideCyanide ions can act as a nucleophile to form the corresponding nitrile. CH3CH2Cl +:CN- CH3CH2CN + :Cl_Chloroethane propanitrileAn alcoholic solution of the reactant is refluxed with potassium or sodium cyanide.

  • NB The product has one more carbon than the haloalkane.

    So this reaction is important to organic synthesis by increasing chain length. +

  • Reaction with ammonia.Ammonia has a lone pair of electrons and can act as a nucleophile.CH3CH2Br + :NH3 CH3CH2NH2 + HBr_Bromoethane ethylamineAn alcoholic solution of the reactant is heated with excess ammonia under pressure.

  • If ammonia is not in excess further reactions are possible.The nitrogen of a primary amine itself has a lone pair and can act as a nucleophile, producing a secondary amine.(CH3CH2)2N(H): This secondary amine also has a lone pair and can act as a nucleophile producing a tertiary amine.(CH3CH2)3N:The tertiary amine still has a lone pair, and soacts as a nucleophile to give a quarternary ammonium salt.(CH3CH2)4N+The salt has no lone pair, so the reaction stops.

  • Write equations for the following reactions;1) 1 bromobutane with sodium hydroxide.2) 2 chloro 2 methyl propane with sodium cyanide.3) 2 iodo propane with ammonia.1) C4H9Br + NaOH C4H9OH + NaBr2) CH3CCl(CH3)2 + NaCN CH3CCN(CH3)2 + NaCl3) CH3CHICH3+ NH3 CH3CH(NH2)CH3+HI

  • Mechanisms of nucleophilic substitutionThere are two types of mechanism;1) SN1 (Substitution nucleophilic 1)2) SN2 (Substitution nucleophilic 2) SN1 occurs in tertiary haloalkanes.SN2 occurs in primary haloalkanesSecondary haloalkanes react with either, but not both, mechanism.

  • SN1 Reactions have two steps;Step 1 The C/halogen bond breaks heterolytically forming a carbocation;+XX-+

  • Step 2The carbocation reacts with the nucleophile.+XX+

  • -++X-xxxxNuc+Step 1Step 2

  • Substitution Nucleophilic 2 reactions SN2 reactions have only one step.The C/halogen bond breaksAs the C/nucleophile bond forms.

  • 1) A nucleophile attacks the haloalkane2) forming an intermediary complex3) which breaks down to complete the reaction. +-

  • Why the difference?Carbocations are unstable, but can be stabilisded by alkyl groups via their electron releasing inductive effect.+Tertiary haloalkanes have three alkyl groups.Giving three +I inductive effects.

    The combined +I effect is sufficient to stablise the ion.So tertiary haloalkanes react SN1.

  • But primary haloalkanes have only one alkyl group on the carbon with the halogen;Primary haloalkanes have only one alkyl group.Giving one +I inductive effect.The +I effect is insufficient to stablise the ion.So primary alkanes react SN2.+

  • Elimination reactionsIf haloalkanes are refluxed with solid sodium hydroxide elimination occurs instead of substitution.CH2HCH2Br + OH_ 2HC=CH2 + HOH + Br-Effectively eliminating a small molecule, HBr.

  • Elimination

  • 1) The hydroxide ion acts as a base and accepts a proton.2) The electrons are passed on3) A halide ion leaves and a double bond is formed.

    Forming waterOverall a hydrogen halide molecule is elminated.