(f,g,h,i) Reaction mechanisms

Syllabus

(f) the different types of covalent bond fission:

(i) homolytic fission (each bonding atom receiving one electron from the bonded pair, forming 2 radicals)

(ii) heterolytic fission (one bonding atom receiving both electrons from the bonded pair)

(g) the term radical (a species with an unpaired electron) and use of ‘dots’ to represent them.

{Radical mechanisms will be represented by a sequence of equations.}

{Dots, •, are required in all instances where there is a single unpaired electron (e.g. Cl • and CH3•). }

{Dots are not required for species that are diradicals (e.g. O).}

(h) ‘curly arrows’ as the movement of an electron pair, for heterolytic fission or formation of covalent bonds

{‘Half curly arrows’ are not required, see 4.1.2 f.}

(i) reaction mechanisms showing movement of electron pairs with ‘curly arrows’ & relevant dipoles.

{Any relevant dipoles should be included.}

{Curly arrows should start from a bond, a lone pair of electrons or a negative charge.}

What does this mean?

Fission means to break.

A covalent bond can break homolytically or heterolytically.

Hetero- means different, Homo means the same.

If the bond pair between X and Y is split equally, then each atom will have one unpaired electron - these are then radicals.

Radicals are shown with dots to indicate the unpaired electron they have.

This is homolytic fission because both atoms got the same share of the bond pair.

Previously, this movement of a single electron was indicated by a curly arrow with half a head because a curly arrow with a full head means a movement of a pair of electrons.

You won't need to do this now.

More usually one atom gains both electrons and because a negative anion.

The other loses both electrons, becoming a positive cation.

The atoms are getting a different share of the bond pair so this is heterolytic fission.

A full curly arrow represent two electrons moving so it must start where two electrons can be found.

In practice, this means in a bond (as in the example above) where a bond pair is moving and a bond is being broken.

The arrow must start in the middle of the bond and move towards the atom that gains both electrons.

Or, as in the second step below where a lone-pair moves towards a positive charge (or partially positive charge) - forming a new bond.

It would have been acceptable to start the curly arrow at the negative charge of the Br- ion but it is better to draw on the lone pair and begin the curly arrow there.

Anywhere else is incorrect and would be marked wrong.

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