1 Nucleophilic substitution PAG7

Naming Haloalkanes 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 haloalkanes 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 PRIMARY Haloalkane One carbon attached to the carbon atom adjoining the halogen SECONDARY Haloalkane Two carbons attached to the carbon atom adjoining the halogen TERTIARY Haloalkane Three carbons attached to the carbon atom adjoining the halogenNucleophilic substitution reactions Nucleophile: electron pair donator e.g. :OH- , :NH3 , CNSubstitution: 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 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 2 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 H3C C H H Br H3C C H H OH -HO: + :Br δ – + δ – Comparing the rate of hydrolysis reactions Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reaction. Use reflux OR heat for more than 20 minutes 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.