The aim of this experiment is to show how the rate of reaction of the halogenoalkanes changes in respect to the C-X bond

There are 3 main types of nucleophilic reaction, one involves reacting them with an excess concentrated solution of ammonia in ethanol under pressure, this one shall not be used since it will constantly substitute the chemicals and the end result will be a huge range of amines. Another is with cyanide ions, which is not being used due to cyanide being extremely dangerous. Finally, the reaction can be done using hydrolysis with hydroxide ions which is our preferred method. This is the chosen method and involves reacting the halogenoalkane with an OH molecule from sodium or potassium hydroxide solution. A similar reaction is with water (H2O) and is not being used due to its slower rate.

It is important to note that there are two different sorts of reaction that can occur depending on the conditions used and the type of halogenoalkane. Primary, secondary and tertiary halogenoalkanes behave differently in this respect. These two reactions are substitution and elimination. In all cases, a mixture of both reactions will happen, some substitution and some elimination. However a number of factors determine the proportion of each. The most important factor is the type of halogenoalkane used, primary halogenoalkanes result in a mainly substitution reaction while secondary halogenoalkanes result in a mix and tertiary halogenoalkanes result in a mainly elimination reaction. Due to my investigation requiring a substitution reaction to take place, I will need to use a primary halogenoalkane. Other less important factors include the solvent, temperature and concentration of the sodium or potassium hydroxide solution however I will be ignoring these as they carry less of an influence.

As I will be using hydrolysis, the reaction is shown in the following equation:

CnHn+X + OH¯ CnHn+OH + X¯

This reaction can take place at room temperature. Halogenoalkanes do not mix with water, so ethanol is used as a solvent in which the Halogenoalkane and the aqueous sodium or potassium hydroxide both mix.

There is no colour change but the presence of a specific halide ion, X-, can be detected by testing with aqueous silver nitrate mixed with ethanol. Halogenoalkanes are covalently bonded, so they give no precipitate with silver ions but as the reaction proceeds and halide ions are produced, a percipitate of silver halide gradually forms - white for chloride ions, cream for bromide ions and pale yellow for iodide ions. Halogenoalkanes are insoluble in water, so the reaction is carried out in the presence of ethanol, which acts as a mutual solvent for the halogenoalkanes, the water and the silver ions but will not interfere with the reaction in any way.This reaction can be used to compare the rates at which the halogenoalkanes react, by mixing the silver nitrate solution with the reaction mixture and timing how long it takes for a percipitate to form.

Each halogenoalkane has a different bond strength, bond polarity and varying electronegativities.

Halogenoalkane

Electronegativity (C=2.5)

Bond Length (nm)

Bond Enthalpy (kJ mol-1)

C?Cl

Cl = 3.0

0.177

346

C?Br

Br = 2.8

0.194

290

C?I

I = 2.5

0.214

228

As can be seen from the table above, the halogens decrease in electronegativity from Chlorine to Iodine, this forms a less polar molecule and therefore a smaller + charge on Carbon. However, the Carbon?Halogen bond length increases from Chlorine to Iodine and the bond becomes weaker and is consequently more easily broken which accounts for the lower enthalpy. This means polarity and bond enthalpy give contradicting predictions on the rates of reaction.

My prediction for which bond will react most vigorously with the nucleophile at the greatest rate is C?I. For the reason that it has the lowest bond enthalpy meaning it can be broken with ease as it requires less energy. Consequently C?Br will not be as easy to break as C?I because of its higher bond enthalpy and will react slower. C?Cl will therefore be the hardest bond to break because it has the highest bond enthalpy of all the bonds being tested and will react slowest. I have decided to go with bond enthalpy as I feel that it is the more important property. I have dismissed polarity even though that would tell us that C?Cl would react quickest due to the large difference of sizes between the carbon and iodine resulting in a bigger C???charge thus allowing the nucleophile to attack the bond much easier.

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Chemistry Coursework

AS - Assessed Practical - Rates of reaction of halogenoalkanes