Recognizing the Four Main Mechanistic Steps
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.
Combining Mechanistic Steps
Summary of the Mechanistic Steps in Organic Reactions
|Mechanistic Step||Always Used In||May Be Used In||Arrows||Other Notes|
|Proton transfer||Acid-base reactions||Addition and elimination||2 arrows:
1st arrow: from lone pair (or bond) of base to hydrogen
2nd arrow: from hydrogen-atom bond to atom (atom is usually carbon, oxygen, nitrogen, or halogen)
|Proton transfer steps can occur in any reaction type.|
|Loss of leaving group||Addition and elimination reactions||Acid-base reactions||1 arrow: from bond between leaving group and atom (usually carbon) to the leaving group||Loss of leaving group often occurs in the same step as nucleophilic attack.|
|Nucleophilic attack||Acid-base, addition, and elimination reactions||Used in all ionic reactions||1 arrow: from lone pair of nucleophile to electrophilic atom (usually carbon)||Nucleophilic attack often occurs in the same step as loss of leaving group.|
|Rearrangement||None||Any reaction where a carbocation forms||1 arrow: from methyl or hydrogen bond to carbocation||Rearrangements occur when a carbocation can rearrange to a more stable carbocation.|