Organic Reaction Mechanism

Organic Reaction Mechanism

A reaction mechanism is a step-by-step specification of what happens during the course of a reaction. A reaction mechanism describes in detail how bonds are broken and formed as starting material is converted into products.

A reaction can occur either in one step or by a series of steps.

EDdeS

CH 2009 Lecture 10 2009

1

Regardless of how many steps are in a reaction, there are only two ways to break (cleave) a bond: the electrons in the bond can be divided equally or unequally between the two atoms of the bond.

Homolysis generates two uncharged species with unpaired electrons. These are called radicals. Heterolysis generates two ionic species – a positively charged cation and a negatively charged anion.

EDdeS CH 2009 Lecture 10 2009 2

Most organic reactions involve the cleavage of a C-Z bond.

Homolysis of a C-Z bond results in radical intermediates.

Radicals are highly unstable and reactive because they contain an atom that does not have an octet of electrons. Heterolysis of a C-Z bond generates a carbocation or a carbanion.

A carbocation contains a carbon surrounded by only six electrons and carbanion has a negative charge on a carbon, which is not a very electronegative atom.

EDdeS CH 2009 Lecture 10 2009 3

Radicals, carbocations and carbanions are highly reactive and unstable and are known as “reactive intermediates”. In most cases they are extremely short lived. Radicals and carbocations are electrophiles because they contain an electron deficient carbon. A carbon radical contains 7 electrons in the valence shell. A carbocation contains 6 electrons in the valence shell. Carbanions are strong bases (electron pair donors) and strong nucleophiles because they contain a negatively charged carbon with a lone pair of electrons. They seek either a H+ or other positively charged centres to neutralize their charge.

EDdeS CH 2009 Lecture 10 2009 4

Bond formation occurs in two different ways:

Two radicals...