Ch.2 Acids and Bases

There are many good reasons for understanding the concepts of acid/base chemistry, but for us, in organic chemistry, the concepts of Lewis acids and base and electron movement will be central.

2.1 Arrhenius definition

+ + Arrhenius definition of acids: substances produce H or increase the H3O ion concentration in water

O O All of these acids increase [H O+] in H Cl H O S O H C H 3 water H C O E.g O 3

Arrhenius definition of bases: substances produce –OH or increase the -OH ion concentration in water

C All of these bases N H O O O increase [-OH] in water H H H from NaOH from Na2CO3

2.2 Bronsted definition

Bronsted definition of an acid: a proton donor (no reference to the solvent)

E.g: HCl, H2SO4, CH3CO2H, HNO3, HCN, H2CO3, HClO4, H2O, etc…

+ + H3O H Cl + H2O Cl conjugated base conjugated acid base acid

Bronsted definition of a base: a proton acceptor (no reference to the solvent)

------E.g: OH, HSO4 , CH3CO2 , NO3 , NC , HCO3 , H , NH3, etc..

+ NH3 + H2O OH + NH4 conjugated base acid conjugated base acid

Chm 201 – Dang1 Neutralization reaction (various solvents)

Acid Base Salt Solvent

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) solvent = H2O

NH4Cl(aq) + NaNH2(aq) NaCl(aq) + 2NH3(aq) solvent = NH3

HCl(aq) + CH3CO2Na(aq) NaCl(aq) + CH3CO2H (aq) solvent = CH3CO2H

E.g Indicate which of the following can act as a Bronsted acid, base or both.

H -2 H O H H C H O

A H C B

2.3 and 2.4 Acid-base strength and position of equilibrium

* Two acids are in completion: forward and reverse reactions are in equilibrium

** Equilibrium lies in the direction of the ………………...... acid/base pair**

When a strong acid is dissolved in water, almost all the molecules dissociate, which means the products are favored at equilibrium.

H Cl + O Cl + H O H H H H an acid a base a base an acid

When a much weaker acid is dissolved in water, very few molecules dissociate, so the reactants are favored at equilibrium

H F + O F + H O H H H H an acid a base a base an acid

Acid dissociation constant is defined as the degree to which an acid (HA) dissociate in water Chm 201 – Dang2 H O+ + A HA + H2O 3

+ [H3O ] [A ] Larger the value of Ka, the stronger the acid is Ka = [HA]

For simplicity, pKa is used instead of Ka to describe the characteristic of a particular The acidity of an acid solution can be measured compound + from the concentration of H3O ion

pKa = -log (Ka) + pH = log [H3O ] p Ka < 1 very strong acid pH < 7 acidic p Ka = 1 – 3 moderately strong acid pH > 7 Basic p Ka = 3 – 5 weak acid p Ka = 5 – 15 very weak acid p Ka > 15 extremely weak acid

E.g If the pKa of an acid is 4.5 then what is its Ka value?

Chm 201 – Dang3 E.g Predict the products then use the pKa value table and quantitatively predict whether the equilibria below favor the reactants or the products a. CH3CH2 + H C C H

pKa = 25

b. CH NH + 3 2 CH3CO

pKa = 40

Organic acids and Bases

Common organic acids are carboxylic acids (pKa = 3-5)

or RCOOH or RCO2H C R OH

O O + + C O C H H H O H H3C O H C H 3 O H a base an acid a base an acid

Amine (R-NH2) is an organic molecule that can behave as an acid or a base

+ H O H O CH3NH 2 CH3N H +

H an acid

Chm 201 – Dang4 H + H O + CH3N H 2 CH3N H H O H H H H a base

E.g Write an equation for the acid-base reaction of the following acid-base reaction. Be sure to use arrows to show the movement of electrons.

+ H Cl CH2CCH3

2.5 How the structure of an acid affects its pKa?

Recall: The weaker the base, the stronger its conjugated acid is and the more stable the base, the stronger its conjugated acid is.

Three factors:

For acids of element in the same row of the periodic table, changes in H—A bond strength are smaller and the polarity of H-A bond is the most important determinant of acid strength

Electronegativities

A more electronegative element bears a negative charge more easily, giving more stable conjugated base and a stronger acid.

(least) C < N < O < F (most)

Relative acidities

(weakest acid) CH4 < NH3 < H2O < HF (strongest acid)

Stabilities

- - - - (strongest conjugated base) CH3 > NH2 > OH > F (weakest conjugated base)

Conclusion: the stronger acid is the one with the most stable (less reactive, weaker) conjugate base!

Chm 201 – Dang5 E

parent acids are ~ same energy HF (neutral, stable compounds) CH4 NH3 H2O

The size

When atoms are different in size, the strongest acid will have its hydrogen attached to the largest atom

pKa 3 -7 -9 -10

Relative stabilities

(least stable) F- < Cl- < Br- < I- (most stable)

(weakest acid) HF < HCl < HBr < HI (strongest acid)

Why? The degree of orbital overlap between the small Hydrogen 1s orbital with the halogen orbital is sometimes used to rationalize the given bond energies. Fluorine (n = 2 valence shell) is small with concentrated electron density, which can efficiently overlap with the small hydrogen 1s orbital iodine (n = 5 valence shell), has very larger and diffuse valence orbitals, which lead to poor overlap with the small hydrogen 1s orbital. We can therefore argue that the H-F requires a large amount of energy to break its bond, while the H-I bond is broken more easily.

H I H F

better electron overlap in smaller poorer electron overlap in large, diffuse fluorine orbital, leads to a iodine orbital, leads to a weaker bond stronger bond

Chm 201 – Dang6 E.g Which species is a stronger acid?

+ + a) H2S or HCl b) PH4 or NH4

E.g Which species is a stronger base?

a) - - - HO or HS b) CH3CN: or CH3O

E.g Which is the most acidic hydrogen in each of these compounds?

A) B) C) H CH SH CH3CH2OH 3 H3C N CH2CH3

Inductive effects: is a donation of electron density through the sigma bond that is a group can withdraw (pull) or donate (push) electrons through sigma framework, based on relative electronegativities.

CH3 H H H H H

H3C C O I O Br O Cl O H O

19 pKa = 11 8.7 7.5 16

O Cl O O O

OH OH OH OH

Cl Cl pKa = higher/lower higher/lower higher/lower 4.8 (reference)

Chm 201 – Dang7 Donating electron density will stabilize a positive or partial positive center and destabilize partial negative center. In the opposite sense, electron withdrawal will stabilize a negative center and destabilize positive center.

E.g Which is the stronger acid, CH3OH vs. CF3OH

Draw the conj. Base

Resonance effect

- H+ OH O pKa 5 CB

- H+ OH O

pKa 16 CB

Which conjugate is more stable than the other?

The Effect of pH on the structure of an Organic Compound

A compound will exist primarily in its acidic form (with its proton) in solution that are more acidic than the pKa value of the group that undergoes dissociation and primarily in its basic form (without its proton) in solution that are more basic than the pKa value of the group that undergoes dissociation.

acidic form basic form - + RCOOH RCOO + H RNH + H+ RNH3 2

Chm 201 – Dang8  If the pKa of an acid is greater than the pH of a solution, then it will exist in the basic form (without its proton)

 If the pKa of an acid is smaller than the pH of a solution, then it will exist in the acid form (with its proton)

E.g Draw the form in which the following compounds will predominate in a solution of pH 6.5 a. HNO2 (pKa = 3.4)

+ b. NH4 (pKa = 9.4)

2.6 Lewis Acids and Bases

The Lewis definitions provide a very useful way of viewing acids and base in chemistry. In organic chemistry, Lewis acid will also be called electrophiles (electron or negative loving) and Lewis base will also be called nucleophiles (nucleus or positive loving).

Lewis base = Nucleophile = electron pair donor (curve arrows start here)

Lewis acid = Electrophile = electron pair acceptor (curve arrows point here)

B + H+ B H nucleophile electrophile bond formed (Lewis base) (Lewis acid)

H H H H H N B H H N + B H H H H H nucleophile electrophile bond formed (Lewis base) (Lewis acid) H H H C O C H H3C O + H C Cl 3

nucleophile electrophile bond formed (Lewis base) (Lewis acid)

Chm 201 – Dang9