Terms and Terminologies in Mechanistic Organic Chemistry Key words: Acids, bases, nucleophiles, electrophiles, Hardness-Softness (HSAB) concepts Introduction In this module, some very basic terms and terminologies are described. Historical perspectives and origin of some of the concepts are presented. Concepts such as electrophiles, nucleophiles, hardness and softness etc., are discussed. These concepts are of high significance to the following modules under reaction Mechanisms. The term acid • It comes from the Latin root ac, was first used in the seventeenth meaning sharp, as in acetum i.e. century. vinegar. • a characteristic sour taste Acids have long been recognized • ability to change the color of as a distinctive class of litmus from blue to red compounds whose aqueous • react with certain metals to solutions exhibit produce gaseous H2 the following properties. • react with bases to form a salt and water. First definition • According to his definition, acids of acid was are substances containing common given by element which gives that Antoine compound the acidic nature. Lavoisier, in • And he postulated that element to 1787. be oxygen. In 1811, • By 1830, dozens of oxygen free Humphrey Davy acids had been discovered. showed that muriatic acid • But it was only after 1840, the (HCl) does not hydrogen theory of acid became contain oxygen. generally accepted. In 1890, the • According to his theory, acid is a Swedish chemist substance containing hydrogen Svante Arrhenius atom, that can dissociate or ionize, formulated the when dissolved in water, first important producing hydrated hydrogen ion theory of acids. and an anion. However, there are substances • Hence, definition of acid was which do not contain H, but modified to “a substance that still yield H+ yields an excess of hydrogen ions ions when when dissolved in water.” dissolved in water. • HCl is a strong acid, it dissociates completely in water. H O 2 + - HCl H3O + Cl • H2SO4 is a polyprotic acid, it dissociates completely in water in two stages. H2O + - HCl H2SO4 H3O + HSO4 H2O - + 2- HSO4 H3O + SO4 • Acetic acid is a weak acid, it does not dissociate completely in water. H O 2 + - CH3COOH H3O + CH3COO ‘Base’ , as • The word alkali is synonymous defined by Arrhenius, is with base. It is of Arabic origin, but substance that the root word comes from the same yields excess of Latin kalium i.e., potash OH- ions when dissolved in • Alkali more specifically used for water. those containing OH- ions. • a bitter taste The name base • a soapy feeling when applied to the has long been associated with a skin class of • ability to restore the original blue compounds color of litmus that has been turned whose aqueous solutions are red by acids characterized by • ability to react with acids to form salts. H2O NaOH, KOH, + - oxides of certain NaOH Na + OH metal and hydrogen H2O compound of CaO Ca2+ + 2OH- certain nonmetals are classified as H O 2 + - Arrhenius bases. NH3 NH4 + OH Although Arrhenius gave useful definition of acid and base, • acidic nature of FeCl3 he could not • basic nature of NH , Na S justify nature of 3 2 certain substances. In 1923, a • Danish chemist J.N. Bronsted theory that is and English Chemist T.M.Lowry, both simple and put forward, independently, the more general was proposed by proton donor-acceptor concept. two chemists. According to this concept, acid is a proton • Definition makes no reference to donor whereas the environment in which proton base is a proton transfer takes place, so that it acceptor. applies to all kinds of reaction. Reaction • If the acid is denoted by AH and between acid and base is thus the base by B, then we can write a a proton generalized acid-base reaction as exchange reaction. AH + B A- + BH+ • It gives rise to a very important concept of conjugate acid-base In this reaction, pair. the protonated base formed is • Conjugate pair differ by one capable of proton. losing H+ ions in • Conjugate acid and base are in the solvent. equilibrium in solution. acid1 + base2 base1 + acid2 Some common conjugate acid-base pairs Substance Acid Conjugate Thus, the base protonated base Hydrochloric acid HCl Cl- - is another Acetic acid CH3COOH CH3COO potential acid Nitric acid HNO - and anion of 3 NO3 Ammonium + NH first acid is NH4 3 another potential chloride - base. Water H2O OH + Hydronium ion H3O H2O The stronger an acid is, the • Table showing examples of weaker its strong and week conjugate acid conjugate base will be and vice base pairs. versa. Many • For example water is a Bronsted- substances can Lowry base in its reaction with Proton act as an acid in Acid one reaction and HCl and an acid in its reaction as a base in with NH . another are 3 Base + - called HCl + H2O -----> H3O + Cl amphoteric. + - H2O + NH3 -----> NH4 + OH Another • In fact the amino acids usually important group of amphoteric species exist in zwitterion form, where the is the amino acids. proton has transferred from Each amino acid molecule contains the carboxyl to the amino group. an acidic carboxyl NH2 group and a basic NH3 R CH COOH amino group. R CH COO The Bronsted- • But still it fails to explain Lowry theory is reactions between substances that more general show similar features but no than any that protons are transferred in the preceded it. reaction. This deficiency was overcome by more general • According to this concept, acid is concept proposed by an electron pair acceptor while American base is an electron pair donor. chemist, G N Lewis in 1923. The acid-base • If acid is denoted by A and base reaction is by B:, then acid-base reaction sharing of between them is formation of electrons between the adduct A-B. two. A + B: A B The principal advantage of the • Therefore the number of acid- Lewis theory is the way it base reactions are also in majority expands the in relation to this theory. number of acids. Various molecules, ions, • A classical example is formation etc., can be of adduct between molecules BH3 grouped as acid and NH3. according to this theory. BH3 + :NH3 H3B-NH3 Like BH3, acidic • Al3+, thus exhibits acidic properties of various other properties as it can accept lone pair molecules and of electrons from water molecule ions can be in the hydration reaction. explained using 3+ 3+ Lewis theory. Al + 6H2O: [Al(H2O)6] The Lewis acid- base theory can • In the course of the reaction, also be used to water molecule acts as a base explain why nonmetal oxides donating a pair of electrons to such as CO2 carbon in CO2 molecule. dissolve in water to form acids. CO2(g) + H2O H2CO3(aq) + The proton (H+) • Ignoring the fact that H is is one of the solvated, acid-base reactions of H+ strongest but can be viewed as formation of also one of the adduct. most + - complicated H + OH H2O Lewis acids. + + H + NH3 NH4 Nearly all compounds of • Many Lewis bases are transition metals multidentate, i.e., they can form can be viewed several bonds to the Lewis acid. as a collections These multidentate Lewis bases of the Lewis bases. are called chelating agents. • Reactions like, The acid-base - - nomenclature FeCl3 + Cl ------> FeCl4 can create are a little confusing viewed as the confusion with acid (FeCl3) being a species Bronsted lacking an octet, (Fe3+) is a neutral nomenclature. species. Molecular orbital The reactions can be viewed as • a filled atomic or molecular interpretation involving the orbital on the base interaction of molecular orbitals • an empty atomic or molecular on the base and the acid. orbital on the acid. The filled orbital • Then we refer to the reaction would be the highest energy occupied simply as a filled-empty molecular orbital, the HOMO, and the interaction or a HOMO-LUMO empty orbital will be interaction. the lowest energy H unoccupied molecular orbital, H H B the LUMO. H HOMO 1s LUMO 2p Substance that yields H+ ACID is a proton donor. ACID is an electron pair + (H3O ) in solution is an BASE is a proton acceptor. ACID. acceptor. BASE is an electron pair donor. - Substance that yields OH 1. Acid ----> H+ + Base in solution is a BASE. 1 1 + A + B: -----> A-B 2. Base2 + H -----> Acid2 - - Acid + Base -----> BF3 + F -----> BF4 + - 1 2 AH ------> H + A - - Base1 + Acid2 I2 + I -----> I3 + - BOH ------> B + OH HCl -----> H+ + Cl- LEWIS THEORY LEWIS + + LOWRY THEORY LOWRY NH3 + H -----> NH4 Acid: species lacking an - + HCl + NaOH These are called octet: BH3, CH3 , etc. is actually conjugate acid-base Base: species with an ARRHENIUS THEORY H+ + OH- -----> H O pairs. unshared pair of 2 - electrons: H2O, NH3, H , Cl-, etc. ACID BASE Hydrogen halides : NaOH, KOH, LiOH HCl, HBr, HI BRONSTED Halogen oxyacids : Ba(OH)2, Ca(OH)2, HClO, HClO2, Mg(OH)2 HClO3, HClO4 H2SO4, HSO3F Al(OH)3 HNO3, H3PO4 Acetic acid, Benzenesulfonic acid, PTSA Oxalic acid In 1965, Ralph Pearson • He introduced the hard and soft attempted to explain the acid-base (HSAB) principle. differential affinity of Lewis • He classified Lewis acids and bases towards bases as hard, borderline or soft. Lewis acids. According to him, hard acids • This statement is neither an prefer to explanation or a theory. It is coordinate to simply a guideline that helps one hard bases and to qualitatively predict the relative soft acids to soft stability of acid-base adducts. bases. • The adjectives hard and soft doesn't mean strong and weak. Ralph G • Classification in the original Pearson, JACS, work was mostly based on 85, 3533(1963) equilibrium constants for reaction of two Lewis bases competing for a Lewis acid.