Janarthenan Singarajah
AS Assessed Practical (Skill P)
Rates of reaction of halogenoalkanes
Aim:
To compare the rate of displacement of the halide ion varies with respect to the C – X bond. C is the carbon atom and X being the halogen. This is a nucleophilic substitution reaction.
Background Knowledge:
Halogenoalkanes are alkanes that have one of the H atom replaced with a halogen atom. They are classified according to the number of carbon atoms bonded to the carbon atom which has the halogen atom bonded to. They are primary, secondary and tertiary. A nucleophile is an electron donor; it has one or more lone pairs of electrons which it donates to the carbon atom which has a slightly positive charge due to the difference in electronegativity between the carbon and the halogen atom resulting in the C atom susceptible to attack by an electron rich species (a nucleophile). This results in the C – X bond breaking and a new bond forming between the carbon and the nucleophile. Hence it is a substitution reaction as the halogen is substituted with a new species. Halogenoalkanes undergo three main nucleophilic substitution reactions. They react with OH-, NH3 and CN- ions. The hydrolysis of the halogenoalkanes which involves the OH- will be used in this experiment. The rate of the reaction depends on the strength of the C – X bond. Stronger bonds need more energy to be broken; therefore they have a higher bond enthalpy value. From the three main types I have chosen the OH- reaction because. CN- is a very toxic substance which is very dangerous to be used in a laboratory. The reaction with NH3 will produce a wide range of results because it will constantly substitute the chemicals ending up in a big range of amines. The same reaction can be carried out with water, but the rate is very slow with water, therefore it will not be used.
