Nucleophile are compounds which are attracted to positive compounds. They are a chemcial species that are able to donate an outer electron pair to an electron deficient species
Water has no charge hence it does not bond with compounds. Hydroxide ions are negative and using the knowledge of opposite attract, this shows hydroxide ions are better nucleophile than water.
20.2.2 Describe and explain how the rate of nucleophilic substitution in halogenoalkanes by the hydroxide ion depends on the identity of the halogen
Bond breaking is an endothermic process
The stronger the carbon - halogen bond, the more energy is required to break this bond, the less energetically favourable the reaction
The more mass the halogen have, the more electron shielding thus less attraction. Iodine bonds are weakest and easiest to break while fluorine has the strongest bonds.
20.2.3 Describe and explain how the rate of nucleophilic substitution in hydrogenoalkanes by the hydroxide ion depends on whether the hydrogenoalkane is primary, secondary or tertiary
The reaction involving the intermediate carbocations (SN1) reacts faster than the formation of the activated complex (SN2). This is because the formation of an activated complex requires a larger activation energy (collision energy)
Tertiary halogenoalkanes react faster than secondary and primary react the slowest
20.2.4 Describe, using equations, the substitution reactions of hydrogenoalkanes with ammonia and potassium cyanide
The ammonia molecule has a lone pair, but it is neutral and must also lose a hydrogen ion before arriving at the final product.
The problem with this reaction is that the primary amine that is formed can further react with other molecule of halogenoalkane, as it still has the lone pair on the nitrogen. To prevent this, they use excess nitrogen to prevent further substitution reaction.
The cyanide ion is a good nucleophile as there is a formal negative charge and a lone pair on the carbon atom.
The reaction is very useful as it adds a carbon atom to the chain.
20.2.5 Explain the reactions with primary hydrogenoalkanes with ammonia and potassium cyanide in terms of the SN2 mechanism
Primary halogenoalkanes are attacked by the lone pair of the nitrogen atom
This makes an unstable transition state with a five membered carbon atom
The halogen atom bond can then break in a concerted process in which the nitrogen atom always loses a hydrogen ion to restore the neutrality of the product molecule
The cyanide ion is provided by potassium cyanide by ensuring that it is in an alkaline medium (pH>7)
This then attacks the primary halogenoalkane in the same way as the hydroxide ion:
20.2.6 Describe, using equations, the reactions of nitriles using hydrogen and a nickel catalyst
Nitrile can be transformed into an anime by heating with hydrogen at 150 degree celcius in the presence of a nickel catalyst.
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