CH 320M 1st Edition Exam 2 Study Guide Chapter 4 Acids and Bases By the Arrhenius definitions an acid is a substance that dissolves in water to produce H1 H3O1 ions and a base is a substance that dissolves in water to produce OH2 ions Br nsted Lowry Acids and Bases A Br nsted Lowry acid is a proton donor and a Br nsted Lowry base is a proton acceptor Any pair of molecules or ions that can be interconverted by transfer of a proton is called a conjugate acid base pair Neutralization of an acid by a base is a proton transfer reaction in which the acid is transformed into its conjugate base and the base is transformed into its conjugate acid The more stable charged species is the one in which the charge is more delocalized Bronsted Lowry bases with two or more receptor sites will be protonated on the site that gives the more delocalized charge p electrons can act as Bronsted Lowry bases to give cations a reaction that is important in the chemistry of alkenes Acid Dissociation Constants pKa and the Relative Strengths of Acids and Bases A strong acid or strong base is one that is completely ionized in water and a weak acid or weak base is one that is only partially ionized in water The acid dissociation constant Ka is a quantitative measure of acid strength Acid strengths are generally reported as pKa values which are equal to 2log10Ka Stronger acids have larger Ka values and therefore smaller pKa values Among the most common weak organic acids are carboxylic acids compounds that contain the COOH carboxyl group The value of Ka for acetic acid a representative carboxylic acid is 1 74 3 1025 M the value of pKa for acetic acid is 4 The Position of Equilibrium in Acid Base Reactions The position of equilibrium in an acid base reaction favors reaction of the stronger acid lower pKa value with the stronger base to form the weaker acid higher pKa 1 value and the weaker base For quantitative calculations the pKeq for an acid base reaction equals the difference of the pKa for the stronger acid pKaHA and the protonated form of the stronger base pKaBH1 pKeq 5 pKa HA 2 pKa BH1 Keq for an acid base reaction is equal to 10 2pKeq A good rule of thumb is that an acid will be substantially deprotonated if its pKa is two or more units lower than the pH of an aqueous solution A base will be protonated if the pKa of its conjugate acid is two or more units higher than the pH of an aqueous solution carboxylic acids and phosphodiesters are generally deprotonated and anionic at pH 7 8 while amines and guanidinium groups are generally found in their protonated and positively charged forms Thermochemistry and Mechanisms of Acid Base Reactions A reaction mechanism describes in detail how a reaction occurs Most chemical reactions occur via collisions The reactants must collide with the proper orientation and enough energy to reach the transition state and proceed to products A reaction can be described by a reaction coordinate diagram which is a plot of energy versus the progress of the reaction Thermochemistry is the study of energy of the entire system at each moment of a reaction Thermodynamics is the study of the relative energies between any two states in wells on a reaction coordinate diagram If products are more stable than reactants the overall thermodynamics are favorable for reaction and the step is exergonic If products are less stable than reactants the overall thermodynamics are not favorable for reaction and the step is endergonic Kinetics is the study of rates of chemical reactions The lower the free energy of activation energy difference between the transition state and starting state the faster the rate of the corresponding reaction step and vice versa Enthalpy is the energy contained within chemical bonds and solvation If bonds formed in a product are stronger than those broken in the starting materials heat is given off and the reaction is exothermic If bonds formed in a product are weaker than those broken in the starting materials heat is absorbed and the reaction is endothermic Entropy measures chaos versus order and chaos is favorable For acid base reactions to occur the acid and base must collide with a geometry in which the proton to be transferred is between the proton donor and proton acceptor atoms more or less in a linear geometry 2 Molecular Structure and Acidity The acidity of an acid is determined by the stability of the anion formed on deprotonation according to the rule that more acidic molecules form more stable anions upon deprotonation Factors that influence the stability of an anion are Electronegativity of the atom bearing the negative charge because more electronegative atoms are more stable as anions Size of the atom bearing the negative charge because larger atoms can more easily accommodate a negative charge it is spread over a larger area Delocalization of charge in the anion usually described by resonance contributing structures because greater delocalization of charge is stabilizing The inductive effect because adjacent electronegative atoms such as the halogens will stabilize a nearby negative charge The hybridization of the atom bearing the negative charge because the greater the percentage of s character in a hybrid orbital the more stable the anion Lewis Acids and Bases A Lewis acid is a species that can form a new covalent bond by accepting a pair of electrons a Lewis base is a species that can form a new covalent bond by donating a pair of electrons All Bronsted Lowry acids proton donors are also Lewis acids and all Bronsted Lowry bases proton acceptors are also Lewis bases But the Lewis acid base model is far more general in that it applies to reactions beyond just proton transfers Chapter 5 Structure of Alkenes An alkene is an unsaturated hydrocarbon that contains a carbon carbon double bond The general formula of an alkene is CnH2n A carbon carbon double bond consists of one s bond formed by the overlap of sp2 hybrid orbitals and one p bond formed by the overlap of parallel 2p orbitals The strength of the p bond in ethylene is approximately 264 kJ 63 kcal mol which is considerably weaker than the carbon carbon s bond The structural feature that makes cis trans isomerism possible in alkenes is lack of rotation about the two carbons of the double bond Index of hydrogen deficiency is the sum of the number of p bonds and rings in a molecule Nomenclature of Alkenes According to the IUPAC system the presence of a carbon carbon double bond is 3 shown by changing