10.2a Common mechanisms

Every reaction that you see is named after

a bond-breaking type + a reactant type + a mechanism.
  

Or a combination of two of those above.

Example: homolytic substitution, heterolytic nucleophilic addition. 

Part 1. Two bond-breaking types

1. Homolytic fission (the fission part usually does not make it to the long name)

A covalent bond (consist of 2 electrons) is broken. One electron goes with one reactant, the other electron goes with the other reactant. (Greek - homo means "the same," lytic means "break apart")

This usually results in a free radical. Free radical is the molecule / atom with a single unpaired electron (Eg above), shown by a single dot in the diagram above. Free radical is very reactive.

Note: both SL and HL students will need to know the mechanism for homolytic fission and the name of heterolytic fission. All the AHL materials are about the heterolytic fission type of reactions as followed.

2. Heterolytic fission

A covalent bond (consist of 2 electrons) is broken, but both electrons go with one reactant, the other reactant has nothing (so unfair!).

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The reactant with two electrons becomes negative ion (anion), the other positive ion (cation)

Part 2. Two reactant types

Note: the name has to do with the "attacking group," not the molecule that receives the attachment  

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Part 3. Four different mechanisms

1. Addition: two things (one of them is an alkene) combine into one thing, result in breaking of one bond in the double bond (AHL: breaking Pi bond) 

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2. Substitution: one group (called an attacking group) replaces another group in a molecule

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3. Oxidation/ Reduction: oxidation is an increase in the number of bond with oxygen and a decrease in the number of bond with hydrogen (old definition of redox). Reduction is vice versa. 

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4. Condensation (esterification): a molecule with -OH (alcohol) combine with another molecule with
-COOH (carboxylic acid) forming an HOH (water) and an ester.

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