Tools of the trade: understanding acid-base reactions and resonance
Organic reactions involve the conversion of an organic compound, called the starting material or reactant, into a new organic compound, called the product.
When organic compounds react, covalent bonds are broken and formed. For example, when bromomethane is converted into methanol, the C–Br is broken and a new C–O bond is formed.
It is an important skill to be able to represent this, and give a step‐by‐step account of how the starting material is converted into the product, using accurate reaction mechanisms. Curly arrows are used to show the movement of electrons as bonds are broken or formed. A double‐headed curly arrow is used to show the movement of a pair of electrons – the tail shows where the electrons come from and the head shows where they go.
Electrons flow from electron‐rich areas to electron‐poor areas. For example, in an acid‐base reaction , the base has a pair of electrons ready to donate to an acid. In the reaction below, the hydroxide ion forms a new covalent oxygen‐hydrogen bond by donating a lone pair of electrons (two valence electrons that are not involved in bonding to another atom) to a proton – represented by a single double‐headed curly arrow.
When a base reacts with a neutral organic compound, such as ethanoic acid , it abstracts the most acidic hydrogen, to form a negatively charged organic product. One curly arrow shows the formation of a new bond between oxygen and hydrogen (where the lone pair of electrons on the base is donated to make the new bond), and a second shows cleavage of an oxygen‐hydrogen bond, to give a lone pair of electrons on oxygen.
This acid‐base reaction is reversible, it can go backwards and forwards (this is shown by two parallel-single headed arrows, with their heads pointing in opposite directions). Whether an acid‐base reaction favours the starting materials or products will depend on the strength of the base and on how acidic the organic compound is.
The acid‐base reaction of hydroxide ion with propanone involves the formation of a new bond between oxygen and hydrogen, and cleavage of a carbon‐hydrogen bond (with the same movement of electrons described for the previous reaction).
The lone pair of electrons end up on carbon and the anion (a compound that has gained electrons and has a resultant negative charge, shown by a negative sign in a circle) is called an enolate ion.
Interestingly, the lone pair can be moved from carbon to oxygen, as represented by two further curly arrows – as oxygen is more electronegative than carbon it pulls the pair of electrons towards it.
The first curly arrow shows the formation of a C=C double bond, the second shows cleavage of the C=O double bond. These curly arrows do not show a chemical reaction, they show that the lone pair can be spread over the organic compound – we call this resonance and a double‐headed straight line is used to connect the different resonance forms.
If you would like some further information on resonance you may find this YouTube clip of use.
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