Organic chemistry mechanisms
Organic reaction types and mechanisms
Organic compounds are predominantly covalent. This is a consequence of the similar electronegativity of the atoms involved in the structures. The bond energies tend to be large and reactions slow. This makes following the reaction easier in terms of the actual processes involved during the course of a reaction. The path reaction follows from reactant to products is called the mechanism. Paradoxically, although the majority of organic compounds are covalent, their reactions often proceed via ionic intermediates, which govern the speed of the reaction (kinetics) and nature of the products.
Note: To understand reaction mechanisms it is important to understand the definitions of “reactant” and “reagent”. A reactant is any substance on the left hand side of the reaction equation. The reagent is the substance (or mixture of substances) added to bring about the chemical change of the reactant that we are considering, i.e. the organic component in this case.
Naming mechanisms
Mechanisms are named according to two factors:
i) The overall change occurring from reactants to products:
|Name |Description |
|Addition |As expected, a molecule or group of atoms is added to the original organic structure |
|Elimination |A molecule or group of atoms is removed from the original organic structure |
|Substitution |An atom or atoms are replaces by others in the original structure |
|Addition - elimination (condensation) |A combination of the first two above |
|Rearrangement |The atoms of the structure are the same but now in a different arrangement. |
ii) The initiating factor from the point of view of the reagent molecule/ion (not from the point of view of the organic molecule itself).
The reagent is either attracted to the organic molecule by electrostatic attraction or there is a simple collision of particles (it should be remembered that the particles in any mixture are colliding millions of times per second under normal conditions due to kinetic activity).
For electrostatic attraction the reagent may carry a partial (or whole) positive charge and be attracted to a region of negative charge on the organic structure - in this case the reagent is said to be electrophilic (negative seeking).
If the opposite is true and the reagent has a lone pair acting as a region of negative charge and is attracted to a region of positive charge on the organic molecule, then the reagent is said to be nucleophilic (positive seeking).
Processes involving free radicals (neutral, highly reactive species) are classified simply as just that - free radical processes.
|Name |Description |
|Electrophilic attack |The reagent has a region of partial positive charge that is attracted to a pair of electrons on the organic molecule |
|Nucleophilic attack |The reagent has a pair of electrons that is attracted to a partially (or wholly) positive region on the organic |
| |molecule |
|Free radical process |The reagent is a free radical of breaks down to give free radicals which then attack |
|Acid/base catalysed |The reaction is initiated by protonation (addition of a hydrogen ion) or deprotonation (removal of a hydrogen ion) by |
| |an acid or base respectively. |
Table: Summary of initiating processes
Examples:
In the reaction of an alkene with hydrogen bromide:
CH2=CH2 + HBr ( CH3-CH2Br
The reagent has a dipole with a partial positive charge on the hydrogen. This is attracted to the “pi” electrons of the alkene double bond initiating the reaction, which is said to be electrophilic (from the point of view of the reagent).
The overall change from organic reactant to product is an addition of two atoms therefore addition
Mechanism name - Electrophilic addition
Drawing representations of reaction mechanisms
The processes occurring are represented by showing the movement of the electrons as curly arrows. It must be stressed that only electron pairs are shown to move (or single electrons in the case of free radical mechanisms) and never positive charges, even if we are dealing with attack by a partially (or wholly) positive reagent (electrophile).
Electrophilic addition of alkenes
CH2=CH2 H-Br ( CH3 - CH2+ -Br ( CH3 - CH2Br
Summary:
Reaction mechanisms may be divided into the following categories:
Electrophilic addition
Electrophilic substitution
Elimination
Nucleophilic substitution
Nucleophilic addition
Rearrangement - (not usually considered at IB level)
Free radical processes
Addition elimination (condensation)
Electrophilic addition
Reaction between alkenes and some suitably polar molecules eg HBr, HOBr (Br2 solution in water), H2SO4 etc. It is also possible for halogen molecules (Cl2, Br2, I2) to undergo this reaction; they are thought to develop an induced dipole on approaching the alkene, which promotes the reaction.
The polar molecule is the electrophile, which is attracted to the pair of electrons in the pi orbital of the double bond. The reaction takes place in two stages with an intermediate carbocation (positively charged ion also called a carbonium ion).
The pair of pi electrons reaches out to the partial positive region of the reagent weakening the bond in the reagent and leaving one of the carbons of the double bond system without its share of the pi electrons giving it a positive charge.
|Electrophilic attack |Intermediate carbocation |Final addition product |
| | | |
|CH2=CH2 H-Br ( |CH3 - CH2+ Br- ( |CH3 - CH2Br |
| | | |
Markovnikov addition
If the alkene is asymmetrical there is the possibility of two different products. In the addition of HBr the hydrogen (and of course the bromine) has two distinct carbons to attach to, each giving a different final product. The major product will be that whose intermediate was the most stable energetically (and therefore the most easily formed).
As the intermediate carries a formal positive charge, any atom or group of atoms that tends to spread this charge out will also stabilise the intermediate. If we compare an alkyl group and a simple hydrogen atom attached to the double bond carbon atoms then the alkyl group has an electron inducing effect compared to the hydrogen atom (ie it tends to push electrons towards the positive charge) stabilising the intermediate. Hence an intermediate with alkyl groups adjacent to the carbon carrying the positive charge will be relatively more stable and will guide the final product. This effect was first investigated by Markovnikov and goes by the name of Markovnikov addition.
“In the addition of HBr to an asymmetric alkene the hydrogen will add to the carbon that already has the most hydrogens attached”
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