AQA A Level Chemistry - Haloalkanes (organic)

• Nucleophilic substitution of a haloalkane using hydroxide ions can also be called hydrolysis, which is splitting molecules using water • Hydrolysis does not occur at the same rate (speed) for all the haloalkanes. To study the speed of hydrolysis we need to be able to spot when the hydrolysis has occurred CH3CH2X + OH- -> CH3CH2OH + Cl- The Cl- (or Br- or I-) ion lost from the haloalkane can be identified by using aqueous silver nitrate (AgNO3(aq)) in ethanol Ag+ (aq) + Cl- (aq) -> AgCl (s) Method: 1) Place a measured sample of each halogenoalkane, (equal molar amounts) into separate test tubes and place in to a water bath at 50 degrees celcius 2) Into another test tube place a solution of ethanol, water and aqueous silver nitrate and place in the same water bath 3) Once at equal temperature add equal volumes of the ethanol mixture and haloalkane solutions together 4) Time how long it takes for each precipitate to form AgCl - white precipitate AgBr- cream precipitate AgI - yellow precipitate - The rate of this reaction can be determined by calculating 1/time taken for the precipitate to occur

• As the bond enthalpy (strength or heat energy needed to break a bond) decreases it is easier to break the C-Halogen bond • The stronger the bond the harder it is to break and the slower the rate of reaction will be as more energy is required to break it • The weaker the bond the faster the rate will be • The C-I bond is the weakest bond (least energy required to break the bond) so has the fastest reaction • The C-Cl bond is the strongest bond (most energy required to break the bond) so has the slowest reaction