How to Draw Mechanisms: Fundamentals, Electrophilic Addition & Nucleophilic Substitution (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about drawing organic reaction mechanisms: the fundamentals, electrophilic addition and nucleophilic substitution.

The three are covered together because they are one skill used three times: a curly arrow shows a pair of electrons moving from an electron-rich site to an electron-deficient one.

A curly arrow represents the movement of a pair of electrons, and everything else follows from that. A nucleophile is electron-rich and donates a pair; an electrophile is electron-deficient and accepts a pair. Electrophilic addition and nucleophilic substitution use the same arrow rule applied to two different starting points. In one the electron-rich site is the carbon-carbon double bond of an alkene, in the other it is the nucleophile itself.

We'll start with the curly arrow convention and the terms examiners expect, then take electrophilic addition and the major and minor products it gives with unsymmetrical alkenes, and finish with nucleophilic substitution of halogenoalkanes.

In organic reaction mechanisms, curly arrows represent the movement of electron pairs. The arrow begins at a bond or at a lone pair of electrons, and it points to the species that accepts that pair. That is the whole convention, and it is the same in every mechanism on this course: the arrow always runs from where the electrons are to where they are going.

Examiners expect the vocabulary as well as the diagrams. A nucleophile is an electron-rich species that donates a pair of electrons; an electrophile is an electron-deficient species that accepts a pair. Those two words label the two ends of every curly arrow.

Bond breaking comes in two kinds. In heterolytic fission the more electronegative atom takes both electrons, giving a negative ion and a positive ion. In homolytic fission each atom takes one electron from the bond, giving two radicals. Everything in this video is heterolytic fission. Homolytic fission belongs to free radical substitution, which is covered in the Alkanes video on combustion and free radical substitution.

The reaction types matter too: addition combines two molecules into a single product, and substitution replaces one atom or group of atoms with another.

Alkenes are reactive because the carbon-carbon double bond is an electron-rich region of the molecule, readily attacked by positively charged electrophiles. That makes the double bond the source end of the curly arrow.

Take hydrogen bromide. The hydrogen-bromine bond is polar because bromine is more electronegative, so the hydrogen carries a partial positive charge and acts as the electrophile. The carbon-carbon double bond donates its pair of electrons to that hydrogen, the hydrogen-bromine bond breaks heterolytically to give a bromide ion, and a highly reactive carbocation intermediate. The bromide ion then bonds to the carbocation, giving bromoethane. Bromine does the same even though bromine-bromine bonds are non-polar, because the electron density in the double bond induces a dipole in it. Two reactants become a single product, so the atom economy is 100%.

With an unsymmetrical alkene two products are possible, and you can predict which one dominates. It depends on the stability of the carbocation formed as the intermediate, and the order is tertiary more stable than secondary, secondary more stable than primary. The more stable carbocation produces the major product and the less stable one produces the minor product. The curly arrows do not just show what happens, they show which of the two products you get more of.

Nucleophilic substitution runs the same arrow rule from the other end. Here the electron-rich site is the nucleophile itself, and the target is a carbon carrying a partial positive charge. Halogenoalkanes provide exactly that, because the large electronegativity difference between carbon and the halogen makes the carbon-halogen bond polar.

The nucleophile donates its pair of electrons to the partially positive carbon, the carbon-halogen bond breaks, and the halogen leaves as a negative ion. Three nucleophiles are named: hydroxide gives an alcohol, cyanide gives a nitrile and adds an extra carbon atom to the chain, and ammonia gives a primary amine, with excess ammonia used to prevent further substitution.

Two things earn the marks. Draw the arrows clearly curly, because an arrow that is not curly enough is not awarded the mark, and always include the dipoles, since they are sometimes needed to gain the mark. In nucleophilic substitution, the first arrow starts at the lone pair of the nucleophile and goes to the delta positive carbon; the second starts on the carbon-halogen bond and goes to the delta negative halogen.

Two things lose marks. An arrow must never start at an atom, only at a lone pair or a bond. And the arrows must be double-headed: half-headed fishhook arrows are only for free radical substitution and are penalised here.

A curly arrow shows a pair of electrons moving from an electron-rich site to an electron-deficient one, beginning at a bond or a lone pair. In electrophilic addition the electron-rich site is the alkene's carbon-carbon double bond, which attacks an electrophile and gives a carbocation whose stability decides the major product. In nucleophilic substitution the electron-rich site is the nucleophile, which attacks the partially positive carbon of a polar carbon-halogen bond and displaces the halogen. Same arrow rule, two mechanisms.

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