Reactions and Reaction Mechanisms

Types of Reactions

Acid-Base Reactions

Acid-base reactions involve the reaction of an acid with a base, which involves a proton transfer (Brønsted-Lowry acid-base reaction) or electron-pair transfer (Lewis acid-base reaction).

Acids and bases may be defined as Brønsted-Lowry acids and bases or Lewis acids and bases. A Brønsted-Lowry acid is a compound that can donate a proton (H+) to another compound in solution. A Brønsted-Lowry base is a compound that can accept a proton from another compound in solution. A Lewis acid is an electron-pair acceptor in a Lewis acid-base reaction. A Lewis base is an electron-pair donor in a Lewis acid-base reaction.

Therefore, a Brønsted-Lowry acid is considered a proton donor (H+), and a Lewis acid is an electron-pair acceptor, or electrophile. A Brønsted-Lowry base is a proton acceptor, and a Lewis base is an electron-pair donor, or nucleophile. An acid-base reaction involves the transfer of an electron pair or a proton. An electrophile is a molecule or ion that accepts electrons to form a covalent bond. The term is derived from the Greek for "electron loving." A nucleophile is a molecule or ion rich in electrons that donates a pair of electrons that forms a covalent bond. The term is derived from the Greek for "nucleus loving."

In general, if a proton is transferred from one molecule (acid) to another molecule (base), the reaction is considered a Brønsted-Lowry acid-base reaction. Ethanoic acid is commonly known as acetic acid. When acetic acid (CH3COOH) is combined with methylamine (CH3NH2), a Brønsted-Lowry acid-base reaction will occur, forming an acetate ion and a methylammonium ion.
C2H3OOH+CH3NH2C2H3OO+CH3NH3+{\rm {C}_2{H}_3{OOH}+{CH}_3{NH}}_2\leftrightarrows{\rm {C}_2{H}_3{OO}^-+{CH}_3{NH}_{3}}^+
The acid donates a proton to the base, and the base accepts the proton. So, acetic acid donates a proton to methylamine, which accepts the proton. The acid, HA{\rm {H{-}A}}, becomes a conjugate base, A, and the base, B, becomes a conjugate acid, HB+{\rm {H{-} B}^+}.
A base, methyl amine, removes a proton from an acid, acetic acid, to produce a conjugate acid and a conjugate base.
If electrons are transferred from one molecule (base) to another molecule (acid), the reaction is considered a Lewis acid-base reaction. Outside of a small set of reactions, most Lewis acid-base reactions are classified as nucleophile-electrophile reactions in organic chemistry. An example of of a Lewis acid-base reaction is the nucleophilic attack reaction of dimethylamine and boron trifluoride.
A lone electron pair from a Lewis base (diethylamine) attacks the electron-deficient boron atom of a Lewis acid (boron trifluoride) to complete a Lewis acid-base reaction.

Addition and Elimination Reactions

Addition and elimination reactions are two types of reactions that involve an alkene or alkyne. Addition reactions add a group in place of an alkene or alkyne, and elimination reactions remove a group to form an alkene or alkyne.
Addition reactions add a functional group to a double or triple bond. Elimination reactions remove a functional group to form a double or triple bond. A functional group is a group of atoms with specific physical, chemical, and reactivity properties. Halides (RX{\rm {R{-}X}}) and alcohols (ROH{\rm {R{-}{OH}}}) are examples of functional groups that are added in addition reactions. When bromine is added to ethylene, the ethylene is the nucleophile because it has a double bond between its carbon atoms. The double bond is electron rich, and it reacts with the electrophilic bromine.
Addition reactions, such as the addition of bromine to ethylene, cause a double bond to break and be replaced by a functional group.
Alkynes have a triple bond and react much the same way as alkenes. The triple bond of the acetylene acts as a nucleophile. The hydration of an alkyne is an electrophilic addition reaction. In the hydration of alkynes, water (H2O) and an acid are added to acetylene (C2H2). Mercury, acting as a catalyst, speeds up the reaction. This reaction occurs via a tautomerism of an intermediate. Tautomerism is the chemical equilibrium between a ketone or aldehyde and an enol.
Water adds to an alkyne to form a vinyl alcohol that rearranges to a ketone or aldehyde.
In elimination reactions, a functional group (and often a hydrogen) is removed from a molecule and replaced with a double or triple bond, which results in the formation of a new pi bond(s), which is a bond formed when two p orbitals overlap side by side on the same plane. Elimination reactions are the reverse of addition reactions. Elimination reactions replace a functional group(s) with a multiple bond, and substitution reactions replace a multiple bond with a functional group. Eliminations can occur through either a nucleophilic or an electrophilic initiation. Elimination reactions are a method to create alkenes and alkynes. 1,2-Eliminations occur when the atoms removed come from adjacent carbons. The steps in addition and elimination reactions often involve heterolytic cleavage. Heterolytic cleavage is a covalent bond that breaks and both electrons of the bond go with one atom and the other atom is left with none. An example of a simple elimination reaction is the reaction of isopropyl chloride (C3H7Cl) with sodium hydroxide (NaOH) to form propene (C3H6), water (H2O), and table salt (NaCl). The chlorine undergoes heterolytic cleavage from the carbon, which means both electrons from the carbon-chlorine bond leave with the chlorine to form the chloride anion, which coordinates with the sodium cation.
C3H7Cl+NaOHC3H6+H2O+NaCl{\rm{C}_3{H}_7{Cl}+{NaOH}\rightarrow\rm{C}_3{H}_6 + {H}_2{O} + {NaCl}}
In an elimination reaction, a reactant with a halide (fluorine, chlorine, bromine) leaving group will undergo 1,2-elimination in the presence of a base, such as sodium hydroxide (NaOH), resulting in the formation of an alkene or alkyne and water and a sodium halide (NaX).

Radical Reactions

Radical reactions involve homolytic cleavage, in which a bond breaks and each atom of the bond gets one electron.

All the steps in radical reactions involve homolytic cleavage. Homolytic cleavage is a covalent bond that breaks and each atom in the bond gets one electron from the bond breakage. Radical reactions are reactions that have an initiation step, one or more propagation steps, and a termination step. Radical reactions have a different set of mechanistic steps than the mechanistic steps of acid-base, addition, and elimination reactions.

An initiation step is a step in a radical reaction where a covalent bond breaks and produces two radical species. A propagation step is a step in a radical reaction where a radical reacts with another species to create a new radical species. A termination step is a step in a radical reaction where two radicals combine to form a covalent bond and no new radicals are formed.

A free radical is a reactive species with one or more unpaired electrons. Radicals are electron deficient and usually do not carry a charge. When chlorine is added to an alkane in the presence of light or heat (as a catalyst), a radical reaction occurs. In the initiation step ultraviolet (UV) light fractures the chlorine molecule homolytically, creating two chlorine free radicals. In one propagation step the free radical abstracts a hydrogen to form hydrogen chloride and a methyl radical. Another propagation step forms the product, CH3Cl, when the methyl radical reacts with Cl2 to form the product and a chlorine free radical. In the termination step, the product forms CH3Cl from two radicals combining to form a covalent bond.
The overall result of a free radical reaction is the replacement of a hydrogen on a carbon with a halogen. This occurs via an initiation step, a propagation step, and a termination step. In the initiation step a dihalide undergoes homolytic cleavage to form a chlorine free radical, which then abstracts a hydrogen from methane to form a methyl free radical. Then, the methyl-free radical reacts with another chlorine molecule to form the product.

Summary of Reaction Types

Reaction Type Bond Cleavage Species Involved Result
Brønsted-Lowry acid-base Heterolytic Proton (H+) Proton transfer
Lewis acid-base Heterolytic Electron pair Electron-pair transfer
Addition Heterolytic Pi bond Alkene to functional group
Elimination Heterolytic Leaving group Functional group to alkene
Radical Homolytic Free radical Alkane to alkyl halide

The table contains a summary of each reaction and what bond cleavage is used, what species are involved, and the result of those reactions.