Organic Chemistry - Acids and Bases

Bronsted-Lowry Acids and Bases

Brønsted-Lowry definition:
- Acids donate a proton.
- Bases accept a proton.

Conjugate Acids and Bases

Brønsted-Lowry definition:
- A conjugate acid results when a base accepts a proton.
- A conjugate base results when an acid gives up a proton.

Curved Arrows in Reactions / Introduction

The making and breaking of bonds involves electron movement.
Curved arrows describe the flow of electron density.
These are the same as curved arrows used to draw resonance structures.
Here, the curved arrows describe the physical movement of electrons.
Learning to draw mechanisms is a valuable skill.

Curved Arrows in Reactions / Single-Step Example

The base "attacks" the acid, using a pair of electrons.
The acid cannot lose its proton without the base taking it; all acid/base reactions occur in one step.
The mechanism shows two arrows indicating that two pairs of electrons move simultaneously (one shows a bond breaking, the other shows the bond being made).

Quantifying Acidity / Introduction

"Strong" acids/bases differ from "weak" acids/bases.
The strength of an acid or base helps to predict how reactions will progress.
Quantitative strength analysis uses pKa values to compare the strengths of acids.
Qualitative strength analysis compares the general stability of structures.

Quantifying Acidity / Ka

Quantitative strength analysis uses numerical data to compare how strong acids are.
$K_a$ is the acid dissociation constant of an acid dissolved in water.
If the acid is strong, $K_a$ will be bigger than 1.

Quantifying Acidity / pKa

$K_a$ values range from $10^{50}$ to $10^{-10}$ , which are hard to work with.
Taking the $-log$ of the $Ka$ focuses on the exponent of the $Ka$ value, which ranges from $-10$ to $50$ .
$pKa$ values range from $-10$ to $50$ . Lower $pKa$ = stronger acid. The formula is: $pKa = -log(Ka)$

Quantifying Basicity

The stronger an acid, the weaker its conjugate base.

Using pKa Values to Predict Equilibria

With the relevant $pKa$ values, you can predict which direction an acid/base equilibrium will favor. Higher $pKa$ = weaker acid.
Equilibrium favors the weaker acid and weaker base.

Using pKas to Analyze Equilibria

Subtracting the $pKa$ values (e.g., $50 − 15.7 ≈ 34$ ) tells you that there will be ~ $10^{34}$ more products than reactants.
This indicates an irreversible reaction.

Qualifying Acidity / Introduction

To determine the relative strength of two acids without knowing their $pKa$ values, compare the stability of their conjugate bases.
The stronger the acid, the more stable its conjugate base!
When an acid loses a proton, it forms the conjugate base, which has a lone pair of electrons.
To determine the stability of a conjugate base, look at the stability of the lone pair.

Qualifying Acidity / ARIO

The more effectively a conjugate base can stabilize its negative charge (lone pair), the stronger the acid.
Four main factors affect the stability of a negative charge, remembered with the acronym ARIO:
- Atom: The type of atom that carries the charge.
- Resonance.
- Induction.
- Orbital: The type of orbital where the charge resides.

Qualifying Acidity / Atom

ARIO - The type of atom that carries the charge
Draw and analyze the stability of the negative charge on the conjugate bases.

Qualifying Acidity / Atom, Example

ARIO - The type of atom that carries the charge
Determine whether an oxygen or a carbon will better stabilize a negative charge.
When moving down a column, size is the most important factor. The larger the atom, the more stable a negative charge will be.
For atoms in the same period, electronegativity is the most important factor. Since C and O are in the same period, they are similar sizes. In this case, the more electronegative atom will better stabilize the negative charge.

Qualifying Acidity / Atom, Carbon vs Oxygen

ARIO - The type of atom that carries the charge
The relative stability of the bases tells us the relative strength of the acids.

Qualifying Acidity / Resonance

ARIO - Resonance stabilizes a negative charge (lone pair) by spreading it out across multiple atoms.
Compare the acidity of the two compounds below by comparing the stabilities of their conjugate bases.

Qualifying Acidity / Resonance, Example

ARIO - Resonance Now we know the relative stability of the acids (which can be confirmed by looking up their $pK_a$ values).

Qualifying Acidity / Induction

ARIO - Induction can also stabilize a formal negative charge by spreading it out.
How is induction different from resonance?
Electron withdrawing atoms/groups inductively withdraw electron density from their surroundings, thus stabilizing a negative charge.

Qualifying Acidity / Induction, Examples

More electron withdrawing groups = more stable conjugate base.
The closer the electron withdrawing groups to the negative charge = more stable the conjugate base.

Qualifying Acidity / Orbital

ARIO - The type of orbital also can affect the stability of a formal negative charge.
The closer electrons are held to the nucleus, the more stable they are.
The shorter the atomic orbital, the closer to the nucleus.

Qualifying Acidity / Orbital, Introduction

ARIO - The type of orbital also can affect the stability of a formal negative charge.
Consider the relative stability of the H’s indicated below:
To predict which H is more acidic, we first have to draw the two possible conjugate bases.
Which carbanion is more stable?

Qualifying Acidity / Orbital, Example

ARIO - The type of orbital also can affect the stability of a negative charge. The more s-character in the orbital, the more stable the negative charge.

Qualifying Acidity / Orbital, Practice

Compare the acidity of the compounds below by comparing the stabilities of their conjugate bases.

Qualifying Acidity / Using ARIO

When assessing the acidity of protons, we generally use ARIO as the order of importance of these stabilizing effects:
- Atom: The type of atom that carries the charge.
- Resonance.
- Induction.
- Orbital: The type of orbital where the charge resides.
It is typically helpful to use this order of priority when comparing the stability of conjugate bases, but it isn’t 100% reliable: there are exceptions.

Qualifying Acidity / Assessing ARIO

Ethanol is more acidic than propylene. Therefore, the conjugate base of ethanol must be more stable.
The type of atom (O versus C) is consistent with this fact.
But propylene’s conjugate base is resonance stabilized, which would suggest it is more stable
So, in this case, our order of priority (ARIO) is accurate.

Qualifying Acidity / ARIO Sometimes Fails

ARIO is only a guideline of priority… it sometimes fails.
In this example, we know equilibrium lies to the right because we know the $pK_a$ values.
If we had judged the conjugate base stability, we would’ve concluded that negative charge on N is more stable than C, and predicted equilibrium to lie to the left, and we would’ve been wrong.
Conclusion: for some acids, we simply need to know the $pK_a$ values because they are exceptions to the ARIO priority rule.

Predicting Equilibrium Position

Consider any acid base reaction:
There are two distinct ways to predict which side is favored at equilibrium:
- The $pKa$ values of H—A and H—B (the higher $pKa$ will be favored).
- The relative stability of the bases, B- and A-.

Leveling Effect / Introduction

Another important skill is to be able to choose an appropriate solvent for an acid/base reaction.
The solvent should be able to surround the reactants and facilitate their collisions without itself reacting.
Because water can act as an acid or a base, it has a leveling effect on strong acids and bases:
- Acids stronger than $H_3O^+$ cannot be used in water.
- Bases stronger than $OH^-$ cannot be used in water.

Leveling Effect / Use of Water

Appropriate use for water as a solvent – when the base is not stronger than hydroxide:
With water as the solvent, the $CH3CO2^-$ will react with the water, but the equilibrium greatly favors the left side, so water is an appropriate solvent.

Leveling Effect / Strong Acids

Acids stronger than $H3O^+$ cannot be used in water. For example, water would react with sulfuric acid producing $H3O^+$ . Virtually no sulfuric acid would remain if we wanted it to be available to react with another reagent.

Leveling Effect / Strong Bases

Bases stronger than $OH^-$ can not be used in water. For example, we wouldn’t be able to perform the following acid- base reaction in water.

Solvating Effects / ARIO Is Insufficient

Because they are so similar, ARIO can not be used to explain the $pKa$ difference comparing ethanol and tert-Butanol.
As with all acids, the difference in acidity is due to the relative stability of their conjugate bases.
The ability of the solvent to stabilize conjugate bases comes into play for this example.

Solvating Effects / Sterics

The solvent must form ion-dipole attractions to stabilize the formal negative charge.
If the tert-butoxide is sterically hindered, it won’t be as well solvated as the ethoxide. That is why t-butanol is not as acidic as ethanol.

Counterions

Counterions are also known as spectator ions.
There are always present, because they are necessary to balance the overall charge of a solution.
Full reaction with counterion(s) included: $NaNH2 + H2O → NH_3 + NaOH$
We often do not include the counterions when writing the rxn:

Lewis Acids and Bases / Definitions

Lewis acid/base definition:
- A Lewis acid accepts a pair of electrons.
- A Lewis base donates a pair of electrons.
Acids under the Brønsted-Lowry definition are also acids under the Lewis definition.
Bases under the Brønsted-Lowry definition are also bases under the Lewis definition.
This reaction fits both definitions.

Lewis Acids and Bases / Examples

Lewis acid/base definition
- A Lewis acid accepts and shares a pair of electrons
- A Lewis base donates and shares a pair of electrons
Some Lewis acid/base reactions can not be classified using the Brønsted-Lowry definition.