Rates of Halogenoalkanes

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Rates of Reaction of Halogenoalkanes

I have been asked to design an experiment to examine how the rate of reaction of halogenoalkanes varies with respect to the C-X bond. I will be varying the C-X bond using the halogens chlorine, bromine and iodine (C-Cl, C-Br or C-I) to see which of these bonds gives the fastest rate of hydrolysis. Other factors that affect the rate of reaction are temperature, concentration of reagents, pressure, surface area and the use of a catalyst, however I will make sure that these remain constant, in order to ensure that only the chosen variable (the halogenoalkanes) is causing any difference to the rate of hydrolysis.

The hydrolysis of the halogenoalkanes involves a nucleophilic substitution reaction. The mechanism for the reaction is as follows. Firstly as the halogen atom is more electronegative than the carbon atom, the C-X bond is polarised to become, Cᵟ+ - Xᵟ- . The positive charge of the carbon atom promotes attack by a nucleophile (a chemical that donates a pair of electrons to form a new covalent bond) this causes heterolytic fission of the carbon-halogen bond to occur and the halogen atom is displaced as a halide ion. A new covalent bond is then formed between the nucleophile and the carbon. Suitable nucleophiles to substitute a halogenoalkane are the hydroxide ion, water and ammonia.

Chemical Equations:

Equation of hydrolysis reaction for 1-chlorobutane (C4H9Cl):
1-chlorobutane + water = butan-1-ol + hydrogen chloride
C
4H9Cl (aq) + H2O (l) = C4H9OH (aq) + HCl (aq)

Equation of reaction with aqueous silver nitrate solution:
hydrogen chloride + silver nitrate = silver chloride + nitric acid
HCl (aq) + AgNO
3 (aq) = AgCl (s) + HNO3 (aq)


Equation of hydrolysis reaction for 1-bromobutane (C
4H9Br):
1-bromobutane + water = butan-1-ol + hydrogen bromide
C
4H9Br (aq) + H2O (l) = C4H9OH (aq) + HBr (aq)

Equation of reaction with aqueous silver nitrate solution:
hydrogen bromide + silver nitrate = silver bromide + nitric acid
HBr (aq) + AgNO
3 (aq) = AgBr (s) + HNO3 (aq)


Equation of hydrolysis reaction for 1-iodobutane (C
4H9I):
1-iodobutane + water = butan-1-ol + hydrogen iodide
C
4H9I (aq) + H2O (l) = C4H9OH (aq) + HI (aq)

Equation of reaction with aqueous silver nitrate solution:
hydrogen iodide + silver nitrate = silver iodide + nitric acid
HI (aq) + AgNO
3 (aq) = AgI (s) + HNO3 (aq)

Predictions:
Firstly I predict that during warming, reactions will occur in all test tubes except the control. I predict that the liquid in these test tubes will change from transparent to cloudy causing the crosses on each test tube to no longer be visible. I predict that this will be due to an insoluble precipitate being formed in each test tube. Furthermore I predict that this precipitate will be either; silver chloride, silver bromide or silver iodide depending on the halogenoalkane used and that as these precipitates will be formed at different rates, meaning the crosses will disappear at different times.

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I predict that as the halogen is changed down the group, i.e. from chlorine to bromine to iodine, the rate of hydrolysis will increase. This is because the bond enthalpy of a carbon-iodine bond is lower than that of a carbon-bromine bond, which is subsequently lower than that of a carbon-chlorine bond. This is shown by the following extract from the data website

http://www.webelements.com/webelements/elements/text/C/enth.html:

C-X Bond Enthalpy (kJ mol-1):
C-Cl = 397±29
C-Br = 280±21
C-I = 209±21

The enthalpies of the C-X bonds decrease from C-Cl to C-I due to the difference in size of the halogen atoms. As 7 group ...

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