Lecture 5, 'Acids, bases and their chemical reactions', introduces an organic chemist's view of acids and bases, their strengths and reactions. Mechanisms of simple acid-base reactions are introduced, and pKa values are used to predict the direction and strength of those reactions. The leveling effect of solvents on acid-base reactions are explained. And types of organic acids and bases are described.
Lecture 5 slide sets: Acids, bases and their chemical reactions
(Click on the links below to download each slide set for the lecture.)
5.1 Acids & bases: overview and basics
5.2 Acid & base strength
5.3 Equilibrium acid-base reactions
5.4 The leveling effect of solvents
5.5 Estimation of acidity using conceptual knowledge
5.6 Classes of organic acids and bases
5.7 Functional groups: acid-base nature
- Key concepts
1. Nucleophilic attack is the mechanism of most acid-base reactions: a free electron pair of the Nu: (LB)
attacks the E+ (LA), forming a dative bond to a hydrogen of the LA.
2. The equilibrium state of an acid-base reaction is determined by comparing the pKa values of the Lewis
acid (reactant) and the conjugate acid (product). The lower pKa pushes the reaction away from it.
a. Lewis acids accept electron pairs while Lewis bases donate electron pairs.
b. Conjugate pairs are pairs of molecules that differ by only a single hydrogen atom. When an acid loses its
proton it becomes the acid’s conjugate base.
c. Nucleophiles (Nu: or Nu:-) are electron rich molecules seeking positive electrophiles (LB), while
electrophiles (E+) are positively charged, electron-lovers seeking electron-rich nucleophiles (LA).
d. Nu: can act as bases when they steal a hydrogen atom, or as a nucleophile when they are added to the
e. Strong acids are weak bases. In other words, the conjugate base of a strong acid will be a weak base.
And vice versa.
f. Solvents can ‘level’ (or hijack) an acid base reaction when the solvent reacts with one of the reactants,
using it up. In essence, the solvent can compete with the second reactant. Leveling is not an issue if the
equilibrium between a reactant and solvent favors reactants.
g. pKa can be estimated from a molecule’s structure using a number of criteria.
i. Lewis acids don’t have to be hydrogen atoms. For example, iron can act as an acceptor of electron pairs.
ii. Dative bonds are formed when the free electron pair of a Nu: attacks an E+ like a polar hydrogen,
forming a new bond.
iii. Charged nucleophiles (Nu:-) are stronger than neutral nucleophiles (Nu:).
iv. Some molecules, like water or alcohols, can act either as LB (Nu:) or LA (E+).
v. Carbonyl groups often have a dipolar positive carbon that acts as an E+.
vi. Acidity: sp (alkyne) > sp2 (alkene) > sp3 (alkane). Free electron pairs are closer to their nucleus in sp
orbitals, polarizing covalent bonds to hydrogen atoms and thus making that hydrogen atom more
vii. pKa is a measure of acid strength. pKa = -log(Ka). Strong acids have low pKa values or high Ka values.
viii. Water has a pKa value of 15.7 and can act either as an acid or as a base, depending on the nature of
the other reactant.
ix. Difference in acidity can be calculated. Subtract the two pKa values. Raise 10 to the power of the
x. Acid-base equilibrium can be quantitated using the difference in pKa values of the Lewis acid (reactant)
and the conjugate acid (product). Calculate the difference in pKa and raise ten to the power of that
difference to calculate Keq.
xi. When water is used as a solvent, no acid will be stronger than hydronium and no base will be stronger
xii. Estimating pKa looks for: 1) similar functional groups with known pKa; 2) inductive effects; 3) positive
charge; 4) stability of the anion (aka conjugate base) formed when the acid reacts; 5) size of the anion or
conjugate base; 6) orbital hybridization of the conjugate base; and 7) delocalization of an acid’s positive
xiii. Acid strength increases: alcohol < phenol < carboxylic acid.
xiv. Base strength increases as the electronegativity of the atom with the : decreases, because we want
the atom to give up, donate, the :.
xv. Carbocations & carbanions are very unstable and therefore powerful LA and LB,
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