Naming Halogenoalkanes Based on original alkane, with a prefix indicating halogen atom: Fluoro for F; Chloro for Cl; Bromo for Br; Iodo for I. C C H H Br H C H H H H 1-bromopropane Substituents are listed alphabetically C C C C Cl H C H H H H H H H H H H 2-chloro-2-methylbutane Classifying halogenoalkanes Haloalkanes can be classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group. C C H H Br H C H H H H C C H H H Br C H H H H C C C C Cl H C H H H H H H H H H H Reactions of Halogenoalkanes 1. Nucleophilic substitution reactions Nucleophile: electron pair donator e.g. :OH- , :NH3 , CNHalogenoalkanes undergo either substitution or elimination reactions Substitution: swapping a halogen atom for another atom or groups of atoms The nucleophiles attack the positive carbon atom The carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen The rate of these substitution reactions depends on the strength of the C-X bond The weaker the bond, the easier it is to break and the faster the reaction. Bond enthalpy / kJmol-1 C-I 238 C-Br 276 C-Cl 338 C-F 484 The iodoalkanes are the fastest to substitute and the fluoroalkanes are the slowest. The strength of the C-F bond is such that fluoroalkanes are very unreactive Organic reactions are classified by their mechanisms The Mechanism: We draw (or outline) mechanisms to show in detail how a reaction proceeds :Nu represents any nucleophile – they always have a lone pair and act as electron pair donators We use curly arrows in mechanisms (with two line heads) to show the movement of two electrons A curly arrow will always start from a lone pair of electrons or the centre of a bond H C C + XH H H H X Nu: – δ + δ – H C C H H H H Nu N Goalby chemrevise.org 1 Primary halogenoalkane One carbon attached to the carbon atom adjoining the halogen Secondary halogenoalkane Two carbons attached to the carbon atom adjoining the halogen Tertiary halogenoalkane Three carbons attached to the carbon atom adjoining the halogen 3.3 Halogenoalkanes 2 Comparing the rate of hydrolysis reactions Water is a poor nucleophile but it can react slowly with haloalkanes in a substitution reaction Hydrolysis is defined as the splitting of a molecule ( in this case a haloalkane) by a reaction with water CH3CH2X + H2O CH3CH2OH + X- + H+ Aqueous silver nitrate is added to a haloalkane and the halide leaving group combines with a silver ion to form a SILVER HALIDE PRECIPITATE. The precipitate only forms when the halide ion has left the haloalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different haloalkanes. CH3CH2 I + H2O CH3CH2OH + I – + H+ Ag+ (aq) + I – (aq) AgI (s) – yellow precipitate The iodoalkane forms a precipitate with the silver nitrate first as the C-I bond is weakest and so it hydrolyses the quickest The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane AgI (s) – yellow precipitate AgBr(s) – cream precipitate AgCl(s) – white precipitate forms faster The rate of these substitution reactions depends on the strength of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction. Nucleophilic substitution with aqueous hydroxide ions Change in functional group: halogenoalkane alcohol Reagent: potassium (or sodium) hydroxide Conditions: In aqueous solution; Heat under reflux Mechanism:Nucleophilic Substitution Type of reagent: Nucleophile, OH- + KOH C H H C C H H H H H OH C H H C C H H H H H Br + KBr 1-bromopropane propan-1-ol The aqueous conditions needed is an important point. If the solvent is changed to ethanol an elimination reaction occurs Alternative mechanism for tertiary halogenoalkanes Tertiary haloalkanes undergo nucleophilic substitution in a different way C CH3 H3C CH3 Br C + CH3 H3C CH3 :OH- C CH3 H3C CH3 OH The Br first breaks away from the haloalkane to form a carbocation intermediate The hydroxide nucleophile then attacks the positive carbon Tertiary halogenoalkanes undergo this mechanism as the tertiary carbocation is stabilised by the electron releasing methyl groups around it. (see alkenes topic for another example of this). Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane in the same way as the mechanism above You don’t need to learn this but there have been application of understanding questions on this H3C C H H Br H3C C H H OH -HO: + :Br δ – + δ – N Goalby chemrevise.org Nucleophilic substitution with cyanide ions Change in functional group: halogenoalkane amine Reagent: NH3 dissolved in ethanol Conditions: Heating under pressure (in a sealed tube) Mechanism: Nucleophilic Substitution Type of reagent: Nucleophile, :NH3 Change in functional group: halogenoalkane nitrile Reagent: KCN dissolved in ethanol/water mixture Conditions: Heating under reflux Mechanism: Nucleophilic Substitution Type of reagent: Nucleophile, :CN- + :CN- 1-bromopropane butanenitrile C H H C C H H H H H Br + Br C C – H H H H H C H CN H Note: the mechanism is identical to the above one This reaction increases the length of the carbon chain (which is reflected in the name) In the above example butanenitrile includes the C in the nitrile group Nucleophilic substitution with ammonia C + 2NH3 H H C C H H H H H Br C H H H C H H C H H NH2 + NH4Br propylamine Naming amines: In the above example propylamine, the propyl shows the 3 C’s of the carbon chain. Sometimes it is easier to use the IUPAC naming for amines e.g. Propan-1-amine Further substitution reactions can occur between the halogenoalkane and the amines formed leading to a lower yield of the amine. Using excess ammonia helps minimise this. Naming Nitriles Nitrile groups have to be at the end of a chain. Start numbering the chain from the C in the CN CH3CH2CN : propanenitrile H3C CH CH2 C N CH3 3-methylbutanenitrileNote the naming: butanenitrile and not butannitrile.
18.104.22.168 Nucleophilic substitution
Halogenoalkanes contain polar bonds.
Halogenoalkanes undergo substitution reactions with the nucleophiles OH– , CN– and NH3
Students should be able to:
• outline the nucleophilic substitution mechanisms of these reactions
• explain why the carbon–halogen bond enthalpy influences the rate of reaction.