Nucleophilic Substitution Reactions (edexcel)

Nucleophilic Substitution Reactions Relative bond strengths of halogenoalkanes in kJ mol-1: In all Nucleophilic substitution reactions the carbon-halogen bond must be broken. It is quite clear from the bond strengths which reactions will be faster and which will barely happen at all. Therefore the general trend is that the lower down the group of halogens in the substance the faster it will react.

A nucleophile is a species (ion/molecule) that is strongly attracted to an area of positive charge in another species. Examples of nucleophiles are OH- and H2O as they either have a negative charge or a strongly negative end to the molecule (polar molecules). Each of these contains at least one lone pair of electrons, either on an atom carrying a full negative charge, or on a very electronegative atom carrying a substantial - charge As Halogens have a higher electronegativity than Carbon it creates an induced dipole in the bond, leading Carbon open to ‘attack’ from the nucleophiles. For example:

SN2 - Primary This is named so as the S stands for Substitution, the N for Nucleophilic and 2 as the initial stage involves two species. An example of this reaction is the addition of a nucleophile to bromoethane. The bromine in the molecule will be considerably larger than the hydrogens bonded to the carbon. This is important as the nucleophile attracted to the carbon will be ‘attacking’ from the other side of the molecule as the bromine will effectively repel it (important as this is why tertiary halogenoalkanes have a different mechanism). The diagram above represents this and shows how a bromine ion is one of the products as it has been repelled from the molecule. There is also a transition state where the nucleophile is partially bonded to the halogenoalkanes and the halogen is being repelled, the diagram showing this is below: Make sure the difference between half made half broken bonds are different to the ones going back into the page. SN1 mechanism is not used for this as a primary carbocation would be formed which is much more energetically unstable.

Tom Baker

12ST1

Chemistry

SN1-Secondary This mechanism differs from the previous one as the positive carbon atom is blocked by the methyl groups that are joined onto it as shown: As a result a different mechanism occurs that is much slower than an SN2 reaction. What happens is the halogenoalkane ionises for form a carbocation and a bromide ion. When this happens the nucleophile is able to ‘attack’ the positive carbon atom and will do so immediately when they come into contact.

The speed of this reaction depends on how quickly the halogenoalkane will ionise and is initially slow as it only involves one species.

In secondary species both mechanism are possible as the back of the molecule is open to ‘attack’ and also a carbocation can be formed that is stable (at least more than a primary one carbocation but not as stable as tertiary as the charge is still spread in a lesser area).

http://www.chemguide.co.uk/mechanisms/nucsub/whatis.html#top Reflux is the process of boiling reactants while continually cooling the vapor returning it back to the flask as a liquid

Tom Baker

12ST1

Chemistry