20.2 - Nucleophilic substitution reactions
20.2.1: Explain why the hydroxide ion is a better nucleophile than water
Nuclophiles are reagents that 'seek out' regions of positive charge in their target molecules. To do this they must have a lone pair of electrons to bond to the atom that carries the positive or partial positive charge.
The strength of a nucleophile depends on the availability of this lone pair of electrons.
The hydroxide ion has a formal negative charge, meaning that its lone pair is readily available to attack positively charged atoms. However, water does not have a formal negative charge, meaning that the lone pairs on the oxygen atom are not so readily available.
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.
The halogen atoms play three roles in the mechanism of nucleophilic substitution.
- 1 The carbon halogen bond must be polarised by the electronegative halogen atom to give the carbon atom its partial positive charge. This is only important for the SN2 mechanism. In the SN1 mechanism the halogen leaves behind a positively charged carbonium ion.
- 2 The carbon - halogen bond must break. The ease with which it breaks is a function of the bond strength, which in turn is a function of the size of the halogen atom. The larger the atom the weaker the bond.
- 3 The halide ion must interact with the solvent (usually water/ethanol), i.e. become solvated, releasing energy.
This first factor is favoured by the more electronegative halogens, i.e. chlorine is more electronegative than bromine etc. This means that the carbon atom in chloroalkanes is more partially positive. This is important in SN2 reactions. Also, the less bulky chlorine allows easier access to the carbon atom than the more bulky halogens.
However, the bond between the carbon and the halogen must break and this is highly favoured by the weakness of the carbon - iodine bond.
In summary, the situation is not as simple as it seems, but the iodoalkanes are more reactive and hence faster than the bromo- and chloroalkanes.
In the aqueous/ethanolic conditions the SN1 mechanism is much faster than the SN2, so tertiary haloalkanes react much faster than primary.
As a rough rule of thumb the most reactive (and hence fastest) are tertiary iodoalkanes, while the slowest are primary chloroalkanes.
20.2.3: Describe and explain how the rate of nucleophilic substitution in halogenoalkanes by the hydroxide ion depends on whether the halogenoalkane is primary, secondary or tertiary.
This has been covered above.
Tertiary faster than secondary faster than primary.
20.2.4: Describe, using equations, the substitution reactions of halogenoalkanes with ammonia and potassium cyanide..
The ammonia molecule has the required lone pair, but 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 molecules of haloalkane, as it still has the lone pair on the nitrogen and now it is even more avaiable by +I effect of the attached alkyl group. This would give a mixture of products including secondary, tertiary and even quaterniary amine salts.
The products can be directed towards the primary amine by ensuring that there is a large excess of ammonia. This makes reaction of the haloalkane with ammonia much more likely.
The cyanide ion is a good nucleophile as there is a formal negative charge and a lone pair on the carbon atom, which is itself not electronegative.
The reaction is very useful as it affords a method of adding one carbon atom to a chain.
The resultant nitrile can be either hydrolysed by heating with dilute acid to a carboxylic acid:
or it can be tranformed into an amine by heating with hydrogen at 150ºC in the presence of a nickel catalyst:
20.2.5: Explain the reactions of primary halogenoalkanes with ammonia and potassium cyanide in terms of the sN2 mechanism.
Primary haloalkanes 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 also 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 haloalkane in the same way as the hydroxide ion:
20.2.6: Describe, using equations, the reduction of nitriles using hydrogen and a nickel catalyst.
Nitriles can be tranformed into an amine by heating with hydrogen at 150ºC in the presence of a nickel catalyst:
As mentioned above, this forms part of a scheme to introduce one more carbon into a chain.