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.
Addition and Elimination Reactions
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.
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|