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Chemistry Plan Halogenoalkanes

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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). ...read more.


Bond Bond Enthalpy (KJ/mol) C - Cl + 346 C - Br + 290 C - I + 228 Prediction: As you go down from Cl to I the bond enthalpy decreases. The bond enthalpy of the C- Cl bond is the strongest and C - I the weakest. This means that less energy is needed to break the C - I bond than needed for C - Cl. Bromine is more reactive than C - Cl, but less reactive than C - I. Therefore C - I will react faster than C - Br which will react faster than C - Cl. Order of reactivity: (C - Cl) < (C - Br) < (C - I) Equation: R - X + OH- R - OH + X- (R - CnH2n-1, X - Halogen) R - OH + :X- Apparatus: - 1-chlorobutane - 1-bromobutane - 1-iodobutane - Aqueous silver nitrate - 0.02moldm-3 - Ethanol - Water bath (50oC) ...read more.


Note the results in the table: Halogenoalkane Precipitate colour Time taken (s) Average Time (s) First Second Third 1-chlorbutane Faint White 1-bromobutane White 1-iodobutane Yellow Why my method works: All the variables have been carefully controlled. At low temperatures the rate was slow, so the procedure was carried out in a higher temperature. The temperature was kept constant by using the water bath. Also the chemicals were left in the bath to reach the baths temperature. The volumes were carefully controlled by using precise apparatus. The nucleophile was reliable and less hazardous. Ethanol was used as a solvent. The OH- ions for the reaction are obtained from the aqueous silver nitrate. Hazards and precautions: Halogenoalkanes: - Highly Flammable - Irritant - Harmful Silver Nitrate - Harmful - Dangerous to the environment - Oxidising - Corrosive The chemicals are kept away from any source of fire. All handled in a safe manner. Skin and eye protection has to be worn i.e. lab coats, gloves and goggles. All the glassware should be handled safely and any breakages should be notified. ...read more.

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