Acids & Bases/ Organic Chemistry

Dr. Ron Rusay

Fall 2009

Chapter 1

Structure and

Bonding

Acids and Bases

Models of Acids and Bases

•Arrhenius: Acids produce H+ & bases produce OH ion in aqueous solutions .

•Brønsted-Lowry: Acids are H+ donors & bases are proton acceptors.

•HCl + H2O Cl + H3O+

acid base

Comprehensive TutorialAcid & Base Principles in Organic Chemistry

Highly recommended viewing

Acid-Base EquilibriumConjugate Acid/Base Pairs

•HA(aq) + H2O(l) H3O+(aq) + A(aq)conj conj

acid 1 base 2 acid 2 base 1

•conjugate acid: formed when the proton is transferred to the base.

•conjugate base: everything that remains of the acid molecule after a proton is lost.

http://chemconnections.org//organic/Movies%20Org%20Flash/ConjugateAcidBaseActivity.swf

QUESTION: 1Aniline, C6H5NH2, was isolated in the 1800s and

began immediate use in the dye industry. What is the formula of the conjugate acid of this base?

A. C6H5NH2+

B. C6H5NH3+

C. C6H5NH–

D. C6H5NH+

E. C6H5NH2

Weak AcidsWeak Acids

• Weak acids are only partially ionized in solution.

or

• Ka is the acid dissociation constant.

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

HA(aq) H+(aq) + A-(aq)

[HA]

]][AO[H -3

aK

[HA]]][A[H -

aK

Organic Acids & Bases• Organic acids are weak acids, eg. Acetic

acid.• However, there can be substantial differences

in their relative strengths. What could you use to compare relative acidities?

• Organic bases are weak bases and relate to ammonia.

• However, there can be substantial differences in their relative strengths. What could you use to compare relative basicity?

Conjugates

Ka x Kb = ?

What do pKa and pKb refer to?pKa + pKb = ?

Kw

-log Ka or -log Kb

14

Remember: pH + pOH = pKw

Which is the stronger acid?

Which is the stronger base?

methanol

methylamine

QUESTION: 2

Use information on the table above to determine which of the following bases would have the weakest conjugate acid:

OC6H5–; C2H3O2

–; OCl–; NH3

A. OC6H5–

B. C2H3O2–

C. OCl– D. NH3

QUESTION: 3

Use information on the table above to determine the order of increasing base strength for the following bases:

OC6H5–; C2H3O2

–; ClO2–; NH3

A. OC6H5– < NH3 < C2H3O2

– < ClO2–

B. C2H3O2– < ClO2

– < NH3 < OC6H5–

C. ClO2– < C2H3O2

– < NH3 < OC6H5–

D. NH3 < OC6H5– < C2H3O2

– < ClO2–

Acid and Base Equilibria

• Weak is favored!

The equilibrium favors the weaker of the acid vs. its conjugate acid or base vs. its conjugate. Weak wins!

http://chemconnections.org/organic/Movies%20Org%20Flash/acid-base-eq.swf

QUESTION: 4

CH3CCH + NH2

pKa=26

CH3CC + NH3

pKa=36

CH3CH3 + NH2

pKa=50

CH3CH2 + NH3

CH3COH

O

+ NH3 CH3CO

O

+ NH4

HCl + CH3OH pKa=15.5

H–Cl–H + CH3O

HCl + CH3OH Cl + CH3OH2

pKa=36

pKa=4.7

pKa-=–7 pKa=–2.5

pKa=9

CH3COH

O

+ NH3

O

+ NH2

pKa=36 pKa=–6

CH3COH2

pKa < –15

Consider the following equilibria. Identify the weaker of the two: acid vs. its conjugate acid in each reaction. Which reactions favor formation of product?

1.

2.

3.

4.

5.

6.

A. All do: 1-6

B. 1, 4, 5

C. 2, 3, 6

D. 1, 5

E. None do.

Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets

Organic Acids & Bases• Organic molecules in context can be considered as

behaving relatively as weak acids or weak bases.• Formal Charge is important in considering which.• Knowing the Formal Charge allows a prediction.• (+) positive atoms behave acid-like, (-) negative

atoms behave base-like.

• This can be used in predicting how molecules will react--- or don’t react, and the products of reactions.

Formal Charge / Acids & BasesElectrophiles / Nucleophiles / Reactivity

CH3

NCH3CH3

CH3

CH3

NCH3

..

CH3 NCH3

CH3

O..

: :

:CH3

O

O

: :..

::::

:H O

..

::

CH3

CH3

CH3 C

CH3

CH3

CH3 C:CH3

CH3

CH3

O: :..

:H O H

H

:CH3 O H

CH3

..

CH3

O

CH2

: :..

:CH3 O

..

::

CH3

O

CH3

H:

CH2

-1

+1

Base /nucleophile

Acid / electrophile

Worksheet 1http://chemconnections.org/organic/chem226/226assign-09.html#Worksheets

Structure and Acid-Base PropertiesImportant factors that effect acidity in binary compounds,

eg HCl (having only two elements):• Bond Length (shorter = stronger bonds; lower acidity)• Bond Strength (longer = weaker bonds; higher acidity:

more dissociation, more protons [hydronium ions] in solution)

• Bond Polarity (smaller e.n. differences favor higher acidities)

Select & explain which is the stronger acid: HBr or HF.

Therefore HBr is a stronger acid than HF.

Strength of Oxyacids(Three atoms: ternary vs. binary)

Push-Pull electronic effects on the proton.

QUESTION: 5

• Rank 1.0M solutions of HBrO, HIO and HClO in order of increasing acidity.

HBrO , Ka = 2.1 x 10-8

HIO , Ka = 2.3 x 10-11

HClO , Ka = 3.0 x 10-8

A) HBrO < HIO < HClO B) HIO < HBrO < HClO

C) HClO < HBrO < HIO D) HIO < HClO < HBrO

Question: 6 True (A) / False (B)

• HBrO4 is a weaker acid than HBrO2.

• HClO2 , Ka = 1.2 x 10-2

• HBrO , Ka = 2.06 x 10-8

• HIO , Ka = 2.3 x 10-11

• HClO , Ka = 3.0 x 10-8

QUESTION: 7

• Rank 1.0M solutions of HBrO, HIO and HClO in order of increasing pH

HBrO , Ka = 2.1 x 10-8

HIO , Ka = 2.3 x 10-11

HClO , Ka = 3.0 x 10-8

A) HBrO < HIO < HClO B) HIO < HBrO < HClO

C) HClO < HBrO < HIO D) HIO < HClO < HBrO

QUESTION: 8• Use the concept of push-pull. Rank the

following organic acids in order of decreasing acidity.

1) Br-CH2COOH 2) I-CH2COOH 3) CH3COOH

• A) 2 > 1 > 3 B) 3 > 2 > 1

• C) 2 > 3 > 1 D) 1 > 2 > 3

QUESTION: 9Rank the following acids in order of decreasing

acidity.A) ClCH2COOH B) Cl2CHCOOH

C) Cl3CCOOH D) CH3COOH

1) A > B > C > D 2) C > B > A > D

2) D > C > B > A 3) B > C > A > D

Question: 10 True (A) / False (B)

Trichloroacetic acid, Cl3CCOOH, is more acidic than trifluoroacetic acid, F3CCOOH.

An Organic Base in ContextErythroxylon spp.

• It is very valuable. The leaves are chewed by indigenous tribes in the Andes to boost their energy.

• It has been used as a psycho-therapeutic, an opthalmic anesthetic and was purportedly used in a popular beverage that is at the heart of a $20 billion corporation.

• However, both its base and conjugate acid are currently controlled substances under U.S. Federal Regulations: Title 21 secs. 329.1 & 1308.12 (1987).

• Can you name the beverage and the base?

The beverage reportedly produced using the extract of leaves of Erythroxylon coca:

The compound: cocaine, is an organic base: Merck Index,

#2450, 11th ed.: Caution: May be habit forming….

Acid -Base Chemistry(Physical Properties)

NCO2CH3

O2C

CH3..

"Crack" Cocaine

• m.p. 98 oC• b.p. (very volatile

> 90 oC)

Solubility: • Water: 1.67 x 10-3

g/mL

• CHCl3: 1.43 g/mL

• Ether: 0.29 g/mL What structural feature makes cocaine a base? What simple compound can you relate it to?

“Regular” Cocaine

Conjugate Acid of Cocaine(Physical Properties)

CH3 NCO2CH3

O2C

H Cl -

Cocaine Hydrochloride

+• m.p. >195 oC

Solubility:• Water: 2.5 g/mL

• CHCl3: 0.08 g/mL

• Ether: insoluble

What accounts for the differences in solubilities of the base and conjugate acid?

Acid -Base Reactions N

CO2CH3

O2C

CH3

+ H Cl

..

CH3N

CO2CH3

O2C

H Cl -

Cocaine Hydrochloride

+

Acid Base Reactions CH3

NCO2CH3

O2C

H Cl -+

+ OH -

..

"Crack" Cocaine

CH3

NCO2CH3

O2C

Which form, Acid or its Conjugate Base?

• A compound will exist primarily in its acidic form if the pH of the solution is < than its pKa

• A compound will exist primarily in its basic form if the pH of the solution is > than its pKa

• NOTE: A buffer solution maintains a nearly constant pH within certain parameters.

A

HAlogpHp aK

The pH of a solution (its surroundings) determines which and is related by the following equation.

A weak acid RCO2Hwith a pKa = 5.2

99% acid1% base

90% acid10% base

50% acid50% base

10% acid90% base

1% acid99% base

QUESTIONS from Worksheet:

The pKa of a general anesthetic, sodium pentothal, is 7.4.  If a patient is given sodium pentothal orally instead of iv, will it put the patient to sleep?

What information is needed to answer this fundamental anesthesiology (“gas passer”) question?

A drug has a pKa of 7.8 and is a known teratogen.  If given iv to a pregnant woman whose blood pH is within normal levels, will this drug cross the placenta and affect the baby?

What information is needed to answer this anesthesiology (“gas passer”) question?

Lewis Acids and Bases

• Lewis Acid: electron pair acceptor

• Lewis Base: electron pair donor

• Example:

Al3+ + 6H

OH

Al OH

H6

3+

Question: 11

Select the chemical equation that depicts the correct movement of electrons in the reactionof ammonia with hydrogen chloride.

A) B)

C) D)

Acid-Base ReactionsShowing a reaction with arrows

CH3SCH3 + BF3 (CH3)2S–BF3

CH3 + H2O

O

CH3CCH3 + H3O

HCC + H2C=O HCC–CH2–O

I + CH3Br ICH3 + Br

CH3O + CH3SH CH3OH + CH3S

H2O + CH3CCH3

OH

CH2=CH2 + HCl CH2–CH3 + Cl

CH3 OH2

Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets

Acid-Base ReactionsPredicting Products

O

CH3COH

+ HCO3

O

CH3COH

+ AlCl3

B(CH3)3 + (CH3)3N

+ OH

C OH3C

H3C+ MgCl2

H2SO4 + CH3OCH3

BF3 + O

NH3

Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets

Predicting if reactions occur and the products if they do

a. HO + Br–CH3 HO–Br + CH3

HO–CH3 + Br

b. H–CC + H2C=O H–CC–CH2-O

H–CC–O–CH2

NH2 + H–OCH3 H2N–OCH3 + H

H2N–H + OCH3

HO + Br–CH3

c,

H–CC + H2C=O

NH2 + H–OCH3

Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets