Acid-Base Practice Problems

Organic Chemistry Jasperse T

Acid-Base Practice Problems

A. Identify each chemical as either an “acid” or a “base” in the following reactions, and identify “conjugate” relationships. -You should have one acid and one base on each side -You should have two conjugate pairs 1.

CH3CH2OH + NaOH

2.

CH3CH2NHLi + CH3OH

3.

CH3CH2CO2H + CH3MgBr

CH3CH2ONa + H2O

CH3CH2NH2 + CH3OLi

CH3CH2CO2MgBr + CH4

H2O + CH3OH2+

+ 4. CH3OH + H3O

5.

CH3CH2NH3+ + CH3OH

CH3CH2NH2 + CH3OH2+

T

B. Choose the More Basic for Each of the Following Pairs (Single Variable). You can use stability to decide. 6.

NH3

NaNH2

7.

NaOH

H2 O

8.

9.

NH

O

NHNa

ONa

Keys: 1. Charge 2. Elecronegativity 3. Resonance

O

10.

Ph

O

Ph

NHNa

NHNa

11.

O

O

T

C. Rank the basicity of the following sets: Multiple Variable Problems 12. CH3MgBr

CH3NHNa

CH3NH2

O

13.

O

O

14.

15.

OH

ONa

NHNa

O

NH

ONa OH O

O

OH O

T

D. Choose the More Acidic for Each of the Following Pairs: Single Variable Problems NH3

16.

NH4

OH2

17.

OH

18.

OH

NH2

CH3

NH2

19.

OH O

20.

OH

OH

NH2

NH2

O

21.

E. Rank the acidity of the following sets: Multiple Variable Problems T

22.

O

OH

NH2

O

23.

HF

O

NH3

H2O

O

OH

OH

NH2

O

24.

25.

NH2

NH2

NH3

OH

T

OH

OH2

OH

O

O

OH

HeO

26.

NH2

OH

O

F. Draw arrow to show whether equilibrium favors products or reactants. (Why?) O

O

27.

28.

OH +

OH

H

+

H2O +

OH

NH

H

O

O

+

Keys: 1. Charge 2. Elecronegativity 3. Resonance

NH2

G. For the following acid-base reaction, a. put a box around the weakest base in the reaction b. put a circle around the weakest acid c. draw an arrow to show whether the equilibrium goes to the right or left. (4pt) 29.

OH

+

NHNa

ONa

+

NH2

Chem 341 Jasperse Ch. 1 Structure + Intro

12

Acid-Base Chemistry (Section 1.13-18) Acidity/Basicity Table T

Entry

Class

1

Strong Acids

2

Hydronium

3

Carboxylic Acid

4

Ammonium Ion (Charged)

Structure

Ka

H-Cl, H2SO4

102

H3O+, ROH+ cationic

OH

R R

H N

Cl

,

Base Stability

H2O, HOR neutral O

10-12

R

Base Strength

O HO S O O

R

R

O R N

R

Neutral, but basic!

Charged, but only weakly acidic!

5

Water

HOH

10-16

6

Alcohol

ROH

10-17

7

Ketones and Aldehydes

O

10-20

T

T

Base

10-5

O R

100

Acid Strength

! H

HO RO O !

8

Amine (N-H)

(iPr)2N-H

10-33

9

Alkane (C-H)

RCH3

10-50

(iPr)2N Li RCH2

Quick Checklist of Acid/Base Factors 1. Cations more acidic than neutrals; anions more basic than neutrals 1. Charge T 2. Electronegativity 2. Carbanions < nitrogen anions < oxyanione < halides in stability e 3. Resonance/Conjugation 3. resonance anions more stable than anions without resonance x t  When neutral acids are involved, it’s best to draw the conjugate anionic bases, and then think from the anion stability side. •

The above three factors will be needed this semester. The following three will also become important in Organic II.

4. Hybridization 5. Impact of Electron Donors/Withdrawers 6. Amines/Ammoniums

Chem 341 Jasperse Ch. 1 Structure + Intro

13

More Detailed Discussion of Acid/Base Patterns/Factors to remember 1. Charge Factor: central atom being equal, cations are more acidic than neutrals (H3O+ > H2O, NH4+ > NH3), and anions more basic than neutrals (hydroxide > water). T

2. Electronegativity Factor: • Acidity

• • • • T

•

•

Anion Stability

•

Basicity

•

Electronegativity

H-C < H-N < H-O < H-X (halogen)

Why: All neutral acids produce an anion after losing an H The more stable the anion Z- that forms, the more acidic the parent H-Z will be. (The Product Stability/Reactivity principle). The anion stability correlates the love for electrons (electronegativity). Summary of Key Relationships: • ANION STABILITY and the ACIDITY of a neutral acid precursor. • ANION STABILITY and the BASICITY of the anion (inverse relationship) • ANION BASICITY and the ACIDITY OF THE CONJUGATE ACID are inversely related (the stronger the acidity of the parent acid, the weaker the basicity of the conjugate anion) KEY: WHEN THINKING ABOUT ACIDITY AND BASICITY, FOCUS ON THE STABILITY OF THE ANION.

3. Resonance/Conjugation: Anion resonance is stabilizing, so an acid that gives a resonance-stabilized anion is more acidic. And an anion that forms with resonance will be more stable and less basic. •

Oxygen Series Examples: Acidity: sulfuric acid > carboxylic acid > water or alcohol Anion Basicity:

T

Anion Stability:

•

O HO S O O O HO S O O


O

O

Note: Resonance is normally useful as a tiebreaker between oxygen anions, nitrogen anions, or carbon anions