Organic Reactions

In organic chemistry there are four general types of reactions that occur:

1) Addition reaction – This is a reaction where two reactants combine and form one single product.  No atoms are left over in an addition reaction. 

An example of an addition reaction:

The mechanism by which the reaction above occurs is the C-C π bond breaking; this results in one C with a formal charge of +1 and a vacant p orbital. The H-Br bond also breaks, which results in Br becoming a nucleophile* and one of the Carbons becoming an electrophile**.  The Br donates its electrons to the positively charged Carbon resulting in a C-Br bond, this makes Carbon happy because it has no formal charge and all of its valence shells are filled. 

This results in two molecules becoming one single molecule with no atoms “left over”.

*Nucleophile - a chemical species with enough electrons to donate, also considered a Lewis-base, a polar bond is formed when it donates a pair of electrons

**Electrophile – a chemical species that accepts an electron pair from a nucleophile, and forms a polar bond

2)  Elimination reaction – An elimination reaction is essentially the opposite of an addition reaction.  In this, a single reactant is split into two products.  Oftentimes the formation of water or HBr will occur.

In the reaction above an acid catalyst is used to break down 1 C2-H bond and 1 C1-O-H bond. Once this is done there are carbons with empty valences. This results in a π bond forming between C1 and C2. The H and the OH released earlier combine to form H2O.



3) Substitution reaction – In a substitution reaction, 2 or more molecules give way to an equal amount of new products. This is accomplished by part of reactant1 and reactant2 breaking off and exchanging places. This is a common type of reaction in biological pathways such as the metabolism if dietary fats.

In Methyl acetate the C-O bond breaks.  This frees the O-CH₃.  In H₂O the H-O σ bond breaks and the H-O and O-Ch₃ exchange, creating the substitution reaction.

4)  Rearrangement reaction – In a rearrangement reaction, one molecule turns into a different molecule through the rearrangement of bonds.  This yields an isomeric product since none of the elements composing the molecule have changed.

(The carbons labeled left to right C₁, C₂, C₃ in Dihydroxyacetone phosphate) The C₂-O π bond breaks, as do the O-H bond off of C₃ and the C₃-H bond.  The hydrogen from the broken O-H bond bonds to the O from the broken π bond. The hydrogen from the broken C₃-H bond bonds to C₂ to complete its valence.

Describing a Reaction

Each of these four types of reactions has an equilibrium constant K_eq.

Keq= [Products]/[Reactants]

The coefficients of the compounds become the exponents in the equilibrium equation shown above. The value of the equilibrium constants tells which side of the reaction arrow is energetically favored.

K_eq < 1 than the reactants are favored

K_eq  > 1 than the products are favored

K_eq ≈ 1 both concentrations are similar (neither favored)

In order for a reaction to be spontaneous the energy of the products must be lower than the energy of the reactants (energy must be released).

Gibbs free-energy change (ΔG)

            Energy of the products = G_products

                        Energy of the reactants = G_reactants

ΔG=G_products - G_reactants

If the reaction is favorable, ΔG<0, in this case energy is lost by the chemical system usually in the form of heat released to the surroundings. This type of reaction is classified as exergonic. If ΔG > 0 it is unfavorable and energy is absorbed from the surroundings. This type of reaction is classified as endergonic.

            ΔG⁰ means that the reaction is carried out under standard conditions.  In standard conditions pure substances are used in their most stable form while the pressure is at 1 atm and temperature is 298K.

            The K_eq and ΔG⁰ both measure whether a reaction is favorable therefore they are mathematically related by the equation below.

For the equation

CH₂ CH₂ +HBr ----> CH₃CH₂Br       K_eq = 7.1 * 10⁷ This can be used to calculate ΔG⁰

Knowing ΔG⁰ introduces the formula      ΔG⁰=ΔH⁰ – TΔS⁰

ΔH= enthalpy, which is also called the heat of reaction. If ΔH<0 the energy of the products is less than the energy of the reactants, and heat is released so the reaction is exothermic. If ΔH>0 the energy of the products is more than the energy of the reactants, heat is absorbed and the reaction is endothermic.

ΔS = entropy change.  Entropy is the measure of change in the amount of molecular randomness. If ΔS>0 the reaction is more favorable, if ΔS<0 the reaction is less favorable.