Doc Brown's Advanced A Level Chemistry Advanced A Level Chemistry - Kinetics-Rates revision notes Part 7

7.1 The water/alkali hydrolysis of halogenoalkanes

(aqueous hydrolysis of alkyl halides/haloalkanes)

Advanced A Level Chemistry Kinetics Index

GCSE/IGCSE rates reaction notes INDEX

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Case study 7.1 The kinetics of the hydrolysis of halogenoalkanes

Explanation and derivation of orders of reactants and how to write the rate expression

There is also a series of pages entirely devoted to organic reaction mechanisms!

• HALOGENOALKANES (HALOALKANES) UNDERGO SUBSTITUTION BY ONE OF TWO POSSIBLE MECHANISMS AND KINETICS CAN SORT OUT WHICH ONE!

• Reaction kinetics: The possibility of two reaction mechanisms for the hydrolysis of halogenoalkanes (RX) with sodium hydroxide or water has consequences for the rate expressions.

  • (i) R₃C–Cl + 2H₂O ==> R₃C–OH + Cl^– + H₃O⁺ 

    • or more simply: R₃C–Cl + H₂O ==> R₃C–OH + Cl^– + H⁺ 

    • e.g. direct hydrolysis with water for tertiary halogenoalkanes, 3 R's = alkyl/aryl

  • (ii) R₃C–Cl + OH^– ==> R₃C–OH + Cl^– 

    • e.g. hydrolysis with sodium hydroxide for primary halogenoalkanes, 3 R's=2 H's and 1 alkyl/aryl.

    • The arguments present apply equally to bromoalkanes and iodoalkanes as well as chloroalkanes.

• The S_N1 mechanism is sometimes described as 'unimolecular' because the rate only depends on the concentration of the halogenoalkane.

• The mechanism for (i) has three steps of bimolecular collisions. Here the rate is only dependent on one reactant, the halogenoalkane, R₃C–X, shown in step (1), (but it still has to collide with the solvent, which never seems to be shown at AS–A2 level and whose concentration is essentially constant!)

• Experimental results produce the overall 1st order rate expression: rate = k₁[RX]

• 1st order kinetics suggests there is a rate determining step involving one of the reactants, irrespective of the total number of steps, which in this case is three.

• This is because the activation energy of the 1st step, forming the carbocation intermediate by heterolytic bond fission, is so high, that the speed is relatively low. Therefore step (1) alone determines the speed of the reaction. This is referred to as the rate determining step (or rds in shorthand!). Steps (2)/(3) have much lower activation energies and are much faster. You would register zero order for the order of reaction with respect to e.g. any hydroxide ion present or it might even hydrolyse just in water!

• Note the simplicity of the rate expression, despite the complexity of the mechanism!

  • The 1st diagram (mechanism 10) shows the full reaction mechanism.

  • The 2nd diagram (mechanism 42) shows the reaction profile with step (1) having much the bigger activation energy and hence acting as the rate determining step. The two 'troughs represent the formation of the intermediates or transition states whatever their lifetime maybe!

• The S_N2 mechanism below for reaction (ii), is referred to as 'bimolecular' because the rate depends on the concentrations of both reactants.

• Experimental results produce the overall 2nd order rate expression: rate = k₂[RX][OH],

  • and here the orders do indeed match the stoichiometry of the equation!

• 2nd order kinetics suggests there is a rate determining step involving both of the reactants, irrespective of the total number of steps, though in this case it is just one step.

• This is because it is a simple single step mechanism involving a bimolecular collision of the two reactant molecules/ions. The rate depends on both the halogenoalkane and hydroxide ion concentrations (1st order with respect to both reactants). 

  • The 1st diagram above (mechanism 2) shows the 'simple' mechanism.

  • The 2nd diagram (mechanism 33) shows the 'activated complex' or 'transition state'^* which is the peak of the potential energy of the system (see diagram 41 below it) where the 'incoming' hydroxide ion is half–bonded to the carbon and the 'outgoing' chloride ion is still 'half–bonded' to the carbon. No intermediate is formed.

  • The 3rd diagram (mechanism 41) shows the energy changes as a reaction profile.

  • ^* Note that an 'activated complex' or 'transition state' is not the same as an intermediate like a carbocation which is a definite entity in its own right, however short its lifetime.

• (see also nucleophilic substitution by water/hydroxide ion)

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