Reaction Mechanism Rules and Patterns

The proton (H⁺) transfer step is a rapid step, especially when the proton is transferred from an acidic position to a basic position. A solvent is often used to aid proton transfer. Proton transfers involve sigma bonds breaking and sigma bonds forming. Sigma bonds are bonds formed when two orbitals overlap end to end. In the presence of water, a proton will transfer from sulfuric acid to the water molecule, creating the positively charged hydronium ion.

Although acids are defined as proton donors, the mechanism involves the base attacking and removing the hydrogen from the acid. The hydrogen does not attack or move to the base. A nucleophilic attack is a step where the nucleophile, which is rich in electrons, will attack an electrophile, which is electron deficient. The lone electron pair of a nucleophile, such as a bromide ion, will attack an electrophilic carbon, forming a new covalent bond. The carbon is electron deficient because it is next to an electronegative oxygen atom. This nucleophilic attack is often paired with another mechanistic step, such as the loss of a leaving group.
The loss of leaving group step occurs often but not always with a nucleophilic attack in one mechanistic step. As the nucleophile is attacking an electrophilic atom, the atom has to lose a leaving group to avoid violating the octet rule. In certain reactions, the loss of leaving group can precede the nucleophilic attack. The leaving group is a functional group that is able to leave a compound and usually forms a stable (weak) species. Halogens, such as bromine, chlorine, and iodine, make very good leaving groups, as do hydronium ions, mesylates, tosylates, and triflates. A good leaving group is a group that forms a very stable, weak conjugate base when it leaves. Whether initiated by the nucleophilic attack or on its own, leaving groups will leave the substrate based on their ability to form stable species. The more stable species the leaving group forms, the more likely the leaving group is to leave. The last of the four mechanistic steps is the rearrangement step. Rearrangement steps are very rare compared to the other three steps. Rearrangement steps are often intermediate steps in a mechanism where a positively charged carbocation is formed. Any time a carbocation forms, a rearrangement may occur. A carbocation is a positively charged carbon with three bonds and no lone pairs. Carbocations will always rearrange if they can form a more stable species. Carbocations are classified as primary, secondary, or tertiary based on the number of alkyl groups bonded to the positively charged carbon. Primary carbocations are bonded to one alkyl group and two hydrogens, secondary carbocations are bonded to two alkyl groups and one hydrogen, and tertiary carbocations are bonded to three alkyl groups. Tertiary carbocations are more stable than secondary carbocations, which are more stable than primary carbocations, due to the presence of alkyl groups, which donate electron density to stabilize the carbocation.

A reaction takes place in many stages, or steps. Proton transfer, nucleophilic attack, loss of a leaving group, and carbocation rearrangements are intermediate steps in the overall mechanism of a reaction. These steps can occur in any order, and often a mechanism will involve one of these steps repeating multiple times. Many mechanisms have multiple proton transfer steps within the overall mechanism. Many mechanistic steps are reversible and exist in a state of equilibrium. Most elementary steps are reversible, meaning the product or products revert back into the reactants. Reversible steps may be shown with a double-headed arrow or two arrows pointing in opposite directions. In the Haber-Bosch process for production of ammonia from nitrogen gas and hydrogen gas, the reaction is reversible, and conditions need to be adjusted to make this reaction produce ammonia.