Chapter 3- Acids and Bases

An Introduction to organic reactions and their mechanisms

Four Categories of Organic Reactions

1) Substitution- one group replaces another

2) Additions- All parts of the adding agent appear in the product. Two compounds become one.

3) Eliminations- One molecule loses the elements of another small molecule

4) Rearrangements-A molecule undergoes a reorganization of its constituent parts

Homolysis vs Heterolysis

• Heterolysis- one of the atoms in a bond get both shared electrons when the bond breaks. The products are ions.

• Homolysis- bonds break evenly, with each atom getting half the shared electrons. The products are radicals.

Heterolysis

• Heterolysis usually occurs on polarized bonds

• Often needs other molecules “help” to separate charges

Bronsted-Lowry Acids and Bases

• Acid- a substance that can donate, or lose, a proton.

• Base- a substance that can accept, or remove, a proton

Conjugates

• Conjugate base- molecule or ion that forms when an acid loses its proton

• Conjugate acid- molecule or ion that forms when a base accepts a proton

• Ex

Leveling Effect

• Hydronium ions and Hydroxide ions are the strongest acid/base you can have in aqueous solutions

• This does not mean there aren’t stronger acids/bases, but anything stronger when put into water, they will react with water to form hydronium/hydroxide

Solvation

• Water has the ability to solvate both cations and anions

• Spectator ions…

Lewis Acid and Base

• Lewis Acid- electron acceptors

• Lewis Base- electron donator

• Ex.

Heterolysis of Bonds to Carbon

• Leads to one of two ions:• Carbocation

• Carbanion

• Carbocations are electron deficient, aka a Lewis Acid

Important Definitions

• Electrophiles- reagents which in their reactions seek extra electrons that will give them a stable valence shell. Electron deficient

• Nucleophiles- reagents that seek a proton or some other positive center. Electron rich

Strengths of Acid/Bases: Ka and pKa

• Strong acids such as HCl and H2SO4 dissociate completely in water

• Weaker acids such as Acetic Acid do not proceed to completion

• 0.1M solution at 25oC only about 1% of the Acetic Acid molecules ionize.

The Acid Constant, Ka

• Because the dissociation is at equilibrium, we can describe it with an expression for the equilibrium constant

• For dilute solutions, the concentration of water is considered constant so we can rearrange the equation and establish a new constant.

• Generic Reaction:

• Generic Expression:

Relationship in Expresson

• Products are in the numerator• The more products that form, the stronger the

acid• So the Larger the Ka, the Stronger the acid• The Smaller the Ka, the weaker the acid

• As a general rule, if Ka>10, the acid completely dissociates in water

pKa

• Since Ka can be very large or very small, it is usually expressed as a negative logarithm, pKa

pKa = - log Ka• This is like expressing the hydronium

concentration as pH!

• pKa is inversely proportionate to strength of acid

Table 3.1 page 115

• The range of the table is in the order of 1062

• Everything above hydronium and below water are approximated

• Remember, pKa is a logrithmic scale, so one unit change is equal to a power of 10!

Water as a Weak Acid

Predicting Base Strength

• One Rule- The stronger the acid, the weaker the conjugate base.

• We can relate the strength of a base to the pKa of its conjugate base.

• The larger the pKa of the conjugate acid, the stronger the base.

Acid-Base Reactions

• Reactions always favor the formation of the weaker acid/base pair

• The reason for this is that the outcome is determined by the position of the equilibrium

• The reaction is under equilibrium control which always favors the formation of the most stable (lowest potential energy) species

Using Acid/Base reactions for Solubility

• Water insoluble compounds will react to form water soluble ions

Relationship between Structure and Acidity

• Two main factors to consider

1) Strength of A-H bond

2) Stability of anion (conj base)

Strength of the Acid-H bond• As we move down a group, the strength of the

H-A bond decreases– Due to weaker overlap with 1s orbital of Hydrogen– Weaker bond = stronger acid

• Acidity increases as we move from left to right– Due to electronegativity

Stability of Conjugate Base

• Hybridization effect- the more s character, the closer to the positive charge nucleus the negative charge of the conjugate base will be, thus more stable

• Inductive effect- the polarization of sigma bonds to delocalize, thus stabilize, the conjugate base anion.– It decreases as distance increases

Energy Review

• Two types of Energy:– Kinetic- energy of motion– Potential- stored energy

• Molecules posses P.E.– The more P.E. a molecule posses, the less stable it is

• We don’t know absolute P.E. (no absolute stability)– Instead we take about relative stability

Energy Review

• P.E. decreases as covalent bonds form• Can express this in Enthalpies, of heat

contents• The difference in the relative enthalpies of

reactants and products in a chemical change called Enthalpy change is symbolized by ΔHo

• ΔHo for exothermic reactions is negative• ΔHo for endothermic reactions is positive

Energy Review

• So, in exothermic reactions, atoms have smaller enthalpies as products than as reactants

• Opposite is true for endothermic reactions

Relationship between Keq and Standard Free-Energy Change, ΔGo

• Equation:

• A negative ΔGo means reactions favor products when equilibrium is reached because that means Keq > 1

• ΔGo < -13 kJ/mol means the reactions goes to completion (>99% of reactants go to products)

Energy Review

• A positive ΔGo means reaction favors reactants when equilibrium is reached because Keq < 1

• ΔGo has two components related by the following equation:

Energy Review

• Enthalpy change is associated with changing in bonding

• If stronger bonds are formed in products than were in reactants, ΔHo is negative (exothermic reaction)

• A negative ΔHo contributes to making ΔGo negative, thus favors products

• For ionizing of acid, the more negative the ΔHo the stronger the acid

Energy Review

• Entropy- changes in the relative order of a system

• The more random a system is, the greater the entropy

• + ΔSo = more ordered to less ordered• - ΔSo = less ordered to more ordered

• Since there is a minus sign, a + ΔSo makes ΔGo more negative, which favors products

Acidity of Carboxylic Acids

• Carboxylic Acids vs alcohols

• Carboxylic Acids are more acidic due to– Resonance effect– Inductive effect

Resonance Effect

• The resulting ion is stabilized through resonance (delocalization of charge)

Inductive Effect

• Polarization of the sigma bonds weakens the oxygen-hydrogen bond and helps stabilize the conjugate base by delocalizing the negative charge on the conjugate base.

Effect of Solvent on Acidity

• Protic Solvents- solvents which have a hydrogen bonded to an electronegative atom such as Oxygen or Nitrogen

• Protic solvents can solvate the acid and conjugate base to stabilize them, but not equally.

Organic Compounds as Bases

• If a compound has an atom with unshared electrons, it is potentially a base

• Proton transfer reactions like this are often the first step in many reactions of compounds containing oxygen and nitrogen

Strength of Acids Summary

Pi bonds as bases

• Much like lone pairs can be basic, Pi bonds can be basic as well

Mechanism Intro

• Mechanism- a description of events that take place on a molecular level as reactants become products.

Acid/Bases in Non-aqueous Solutions

• Earlier we described the Leveling Effect. • Water will always give a proton to any base

stronger than hydroxide• So it is impossible to use any base stronger

than hydroxide in water• We can use stronger bases, however, if we use

a solvent that is a weaker acid than water.

Acid/Bases in Non-aqueous Solutions

• For example, we can use Sodium Amide in hexane, diethyl ether, or liquid ammonia

• They are all weak enough acids so they won’t give a proton to the amide

• Ex.

Alcohols as Solvents

• Alcohols are often used as solvents because they dissolve less polar organic compounds

• Plus, we can use Alkoxides as a base.

• Ex.

Alkyl Metal reagents

• Alkyl Metals, such as alkyl lithiums, are some of the strongest bases we have.

• The bond is considered covalent but react as if it is ionic.

D and T Labeling

• Some Hydrogens can be exchanged for isotopes such as Deuterium and Tritium.

• Reasons for labeling:– Deuterium is NMR inactive– Tritium is radioactive

• Ex.

First Synthesis Problem!!

• Convert to H3C C C H

H3C C C T