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Compare the rate of hydrolysis of some halogeno-compounds.

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Introduction

Carmen Tse 7C 21 Chemistry Experiment No.19 Comparison of Rates of Hydrolysis of Halogeno-compounds Objective To compare the rate of hydrolysis of some halogeno-compounds. Introduction Halogeno-compounds are organic compounds composed of carbon, hydrogen and halogen. One of the characteristic reactions of haloalkanes is nucleophilic substitution reactions: Nu- + R-X --> R-Nu + X- In the above experiment, the nucleophile is H2O. OH- attacks the electropositive carbon centre and displaces a halide ion from the haloalkane. This kind of substitution is called hydrolysis. The halide ions substituted can be identified by silver nitrate solution. Ag+ (aq) + X- (aq) --> AgX (s) where X is the halide ions The overall equation is: OH- + R-X + Ag+ --> R-OH + AgX In this experiment, the rates of hydrolysis of halogeno-compounds are compared in 3 categories: 1. chloro-, bromo-, and iodoalkanes; 2. primary, secondary and tertiary haloalkanes; 3. haloalkane, halobenzene and (halomethyl)benzene Chemicals Ethanol about 20cm3 0.1M silver nitrate 10cm3 1-chlorobutane 5drops 1-bromobutane 15drops 1-iodobutane 5drops 2-bromobutane 5drops 2-bromo-2-methylpropane 5drops Bromobenzene 5drops Precaution Ethanol is volatile. It easily evaporates and the amount of ethanol in different test tubes would be less than 2cm3, especially when having water bath in partA. ...read more.

Middle

I- is said to be more polarizable. As the electrons are far away from the nucleus, the C-I bond is much weaker than C-Br and C-Cl bond and hence, I- is the best leaving group, followed by Br- and Cl- respectively. As a result, iodobutane is most reactive. Kinetic studies show that the nucleophilic substitution reactions proceed by two different reaction mechanisms. They are bimolecular nucleophilic substitution (Sn2) and unimolecular nucleophilic substitution. (Sn1) For Sn2, it just involved 1 step. There are no intermediates. The reaction proceeds through the formation of an unstable arrangement of atoms called the transition state. The rate of reaction depends on both concentration of haloalkane and hydroxide ion. Two species are involved in the rate determining step of the reaction. Rate = k [haloalkane] [OH-] For Sn1, it involved 2 steps and one intermediate is formed. The first step is the rate determining step and a carbocation is formed. The rate of reaction is independent of the concentration of hydroxide ion. Rate = k [haloalkane] In part B of the experiment, the experimental condition favours Sn1 reactions because the substrate bromobutane is rather bulky. Due to the effect of steric hindrance, bulky substituents on or near that carbon atom have a dramatic inhibiting effect. ...read more.

Conclusion

Ag+ (aq) + Cl- (aq) --> AgCl (s) Ag+ (aq) + Br- (aq) --> AgBr (s) Ag+ (aq) + I- (aq) --> AgI (s) Sources of error and Way of Improvement The water bath is set up by a Bunsen flame and the temperature is measured with thermometer. However, the temperatures always fluctuate. The temperature can influence the reaction rate greatly. A thermostatic bath can improve the accuracy of the experiment by keeping the temperature at 60C constantly. In the experiment, though it is difficult to keep the temperature at 60C constantly, the temperature in the three test tubes should keep the same as a fair test. A dropper is used when adding the halogeno-compound to the test tubes. However, dropper is not an accurate apparatus; the size of a drop can be very different. The amount of halogeno-compound added will directly affect the reaction rate thus the amount of precipitate. To improve, a pipette can replace the dropper. In using a pipette, more time will be need, it is quite impossible to compare the 3 test tubes at the same time. Some changes will be needed: 1. Use a timer to record the time for precipitates to appear. Or 2. Prepare the measured halogeno-compound before adding to ethanol and silver nitrate. ?? ?? ?? ?? 1 ...read more.

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