Reactivity and Mechanism

There are a very large number of chemical reactions that are used in the field of organic chemistry. This page aims to cover the most important and fundamental chemical reactions and the elementary steps and mechanisms that make each possible. Chemical reactivity is an important topic, especially in chemistry as they allow for the creation of industrially useful chemicals as well as chemicals and compounds created by the pharmaceutical industry for use in life saving or therapeutic drugs treatments.

Reaction Mechanisms

In organic chemistry, a reaction mechanism is a single step out of a number of elementary steps that occurs during a chemical reaction that involves either the breaking of bonds or the forming of new bonds, ultimately resulting in the desired product or products.

The four main reaction mechanisms are:

• Nucleophilic attack
• Loss of the leaving group
• Deprotonation
• Carbocation rearrangement

Understanding each of these elementary steps in terms of how and under what conditions they proceed is crucial to understanding the reactivity of molecules and being able to effectively predict the product of a given reaction.

Nucleophilic Attack

The general mechanism of a nucleophilic attack

Nucleophiles are chemical species that donate a pair of electrons to an electrophile. In other words, nucleophiles are Lewis bases and electrophiles are Lewis acids. Since almost all chemicals reactions involve an exchange of electrons, nucleophilicity and electrophilicity are a very important part understanding chemical reactions.

A nucleophilic attack often occurs when an electron-rich species (the nucleophile) "attacks" an electron-deficient species (the electrophile, usually a carbocation), forming a new bond between the nucleophile and the carbocation.

Factors Influencing Nucleophilicity

1. Charge

   Nucleophiles are often the anion within an ionic bond, or the electronegative species participating in a polar covalent bond (often possessing a $\delta ^{-}$ charge).

   As a general rule, the conjugate base is always the better nucleophile than the conjugate acid of a given substance. Consider the following examples:

   • $I^{-}>HI$
   • $Cl^{-}>HCl$
   • $HO^{-}>H_{2}O$
   • $HS^{-}>H_{2}S$

   
   

2. Electronegativity

   In general, the less electronegative an atom is, the more readily its electrons can be donated. Put simply: the less tightly electrons are held by an atom, the more freely they are able to move around. Since we know that electronegativity increases up and to the right on the periodic table, we then know that nucleophilicity must increase going down and to the right on the periodic table.

   • $HS^{-}>HO^{-}$
   • $I^{-}>Br^{-}>Cl^{-}>F^{-}$

   
   

3. Solvent

   For nucleophilic reactions the choice of the solvent is very important as different types of solvents can weaken nucleophiles. Namely, the hydrogen atoms on polar protic solvents will create weak hydrogen bonds with the negative charge on the nucleophile, creating a shield around the nucleophile. This is why nucelophiles work best in polar aprotic solvents, as these weak Van der Waals interactions do not take place to such a high degree.

4. Steric hinderance or bulk

   If a sterically bulky group is attached to an otherwise very strong nucleophile, the nucleophilicity of that nucleophile is decreased.

Loss of the Leaving Group

The general mechanism for loss of the leaving group during a step-wise reaction.

A leaving group is an atom on a molecule that departs from the molecule with a pair of electrons during the heterolytic cleavage of a bond. The leaving group can either leave as the nucleophile is attacking or it can leave before the nucleophile attacks, creating a carbocation intermediate.

Deprotonation

The general mechanism for deprotonation

Deprotonation is in essence a simple acid/base reaction where a nucleophile attacks a hydrogen atom. This most often occurs when OH (a bad leaving group) is the leaving group to create water (a better leaving group).

Carbocation Rearrangement

The general mechanism for a carbocation rearrangement.

Carbocation rearrangement occurs when a more stable carbocation can be created by rearranging the groups attached to an adjacent carbon.

Reaction Types

Substitution-Nucleophilic (S_N) Reactions

S_N1

It's involves a carbocation intermediate. The rate determining step is unimolecular has an intermediate carbocation favored by tertiary carbons which can support the carbocation works best in a polar protic solvent. (solvation effect)

S_N2

It involves a rear attack. The rate determining step is bimolecular no nucleophilic intermediate carbocation helps to leave the group working best in a polar aprotic solvent.

Elimination Reactions

E1

The mechanism begins with the dissociation of the leaving group of an alkyl, producing a carbocation on the alkyl group and a leaving anion, the mechanism more reliably produces products that follow Zaitsev's rule. Is unimolecular.

E2

The mechanism is concerted and highly stereospecific, as it can only occur when the H and the leaving group X are in the anti-coplanar position. Is bimolecular.