Chapter 1 Electronic Structure and Bonding Acids and Bases

Acids & Bases in Organic Chemistry

Dr. Ron Rusay Fall 2004

Models of Acids and Bases

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

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

•HCl + H 2 O  acid base Cl  + H 3 O +

Acid-Base Equilibrium

Conjugate Acid/Base Pairs

•HA(aq) + H 2 O(l)  H 3 O + (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.

Weak Acids

• Weak acids are only partially ionized in solution.

HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) HA(aq) H + (aq) + A (aq)

K a



[H 3 O



][A ] [HA]

or

K a



[H



][A ] [HA]

•

K a

is the acid dissociation constant.

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 ammonia.

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

Conjugates

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

K a x K b = ?

K w What do pK a and pK b refer to?

-log K a or -log K b pK a + pK b = ?

14 Remember: pH + pOH = pK w

Which is the stronger acid?

methanol Which is the stronger base?

methylamine

Acid and Base Equilibria

• Strong vs. weak: Weak is favored!

Acid-Base Strength & Reactions Consider the following pairs of equilibrium reactions. Identify the weaker of the two: acid vs. its conjugate acid in each reaction. Which reaction in each pair favors formation of product? O O CH 3 C p

K

a =4.7

OH + NH 3 CH 3 C O + NH 4 p

K

a =9 O CH 3 C OH + NH 3 p

K

a =36 O CH 3 COH 2 p

K

a =–6 + NH 2 HCl + CH 3 OH p a =15.5

HCl + CH 3 OH p

K

a =–7 CH 3 C  CH + NH 2 p

K

a =26 CH 3 CH 3 + NH 2 p

K

a =50 H–Cl–H + CH 3 O pK a < –15 Cl + CH 3 OH 2 p

K

a =–2.5

CH 3 C  C + NH 3 p a =36 CH 3 CH 2 + NH 3 p

K

a =36

• • • •

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.

For N: = 5-[1/2(4)+4] =

-1 Base

Formal Charge = # of valence e s [1/2(# of bonding e s ) + # of non-bonding e s ]

CH 3 ..

: : H 3 C H 3 C N CH 3 CH 3 ..

: : CH 3 CH 3 H 3 C : ..

N : CH 3 H 3 C H 3 C CH 3 C : N CH 3 CH 3 CH 3 H 3 C : : ..

: O : : H 3 C H : ..

O : : CH 3 C CH 3 H H : O H : H 3 C ..

O H CH 3 For O: = 6-[1/2(6)+2] H 3 C : : ..

CH 2 H 3 C : ..

O : : CH 2 H 3 C : O H CH 3 =

+1 Acid

Structure and Acid-Base Properties

Important factors that effect acidity in binary compounds, eg HCl (having only two elements): • Bond Length (

shorter

= stronger; favors

lower

acidities) • Bond Strength (

weak

bonds favor

higher

acidity: 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.

Strength of Oxyacids

• •

Name and rank

HBrO, HIO

and

HClO: (1)

in order of increasing pH and

(2)

in order of increasing acidity.

• HBrO ,

K a = 2.06 x 10 -8

• HIO ,

K a = 2.3 x 10 -11

• HClO ,

K a = 3.0 x 10 -8 Is

HClO 4

a stronger or weaker acid than

HClO?

• Will this help?

HClO 2 ,

K a = 1.2 x 10 -2

Strength of Acids

• Use the concept of push-pull. Rank the following organic acids in order of increasing acidity.

Br-CH 2 COOH, I-CH 3 COOH, CH 3 COOH • pKa= 2.69

pKa=3.12

pKa=4.75

Is chloroacetic acid more or less acidic than bromoacetic acid? Will its pKa be higher or lower than bromoacetic acid?

Strength of Acids

•Use the concept of push-pull. Rank the following acids in order of increasing acidity.

Cl-CH 2 COOH, Cl 2 -CHCOOH, Cl 3 CCOOH, CH 3 COOH Which is the stronger acid? Cl 3 CCOOH or F 3 CCOOH

An Organic Base in Context

Erythroxylon 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:

#2450

, 11th ed.:

Caution:

May be habit forming….

Merck Index,

CH 3 ..

N

Acid -Base Chemistry

(Physical Properties) CO 2 CH 3 O 2 C

"Crack" Cocaine

• m.p. 98 o C • b.p. (very volatile > 90 o C) Solubility: • Water: 1.67 x 10 -3 g/mL • CHCl 3 : 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

CH 3 + N Conjugate Acid of Cocaine (Physical Properties) • m.p. >195 o C H Cl Solubility: CO O 2 C 2 CH 3 • • • Water: 2.5 g/mL CHCl 3 : 0.08 g/mL Ether: insoluble Cocaine Hydrochloride What accounts for the differences in solubilities of the base and conjugate acid?

Acid -Base Reactions

CH 3 ..

N CO 2 CH 3 + H O 2 C Cl CH 3 + N H Cl CO 2 CH 3 O 2 C Cocaine Hydrochloride

Acid Base Reactions

CH 3 + N H Cl CO 2 CH 3 + O 2 C OH CH 3 ..

N CO 2 CH 3 O 2 C "Crack" Cocaine

Which form, Acid or its Conjugate Base?

The pH of the solution (surroundings) determines it.

p

K

a  pH  log • A compound will exist primarily in its acidic form if the pH is < than its p

K

a • A compound will exist primarily in its basic form if the pH is > than its p

K

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

A weak acid with a pKa = 5.2

Lewis Acids and Bases

• Lewis Acid : electron pair acceptor • Lewis Base : electron pair donor • Example: Al 3+ + 6 O H H Al O H H 6 3+

Acid-Base Reactions

Showing a reaction with arrows CH 3 SCH 3 + BF 3 CH 3 + H 2 O CH 3 O + CH 3 SH O CH 3 CCH 3 + H 3 O HC  C + H 2 C=O CH 2 =CH 2 + HCl I + CH 3 Br (CH 3 ) 2 S–BF 3 CH 3 OH 2 CH 3 OH + CH 3 S OH H 2 O + CH 3 CCH 3 HC  C–CH 2 –O CH 2 –CH 3 + Cl ICH 3 + Br

Acid-Base Reactions

Predicting Products O CH 3 C OH + HCO B(CH 3 ) 3 + (CH 3 ) 3 N 3 NH 3 + OH H 3 C H 3 C C O + MgCl 2 H 2 SO 4 + CH 3 OCH 3 BF 3 + O CH 3 C OH O + AlCl 3

Predicting if reactions occur and the products if they do

a.

HO + Br–CH 3 HO + Br–CH 3 b.

H–C  C + H 2 C=O H–C  C + H 2 C=O c, NH 2 + H–OCH 3 NH 2 + H–OCH 3 HO–Br + CH 3 HO–CH 3 + Br H –C  C –CH 2 -O H –C  C –O–CH 2 H 2 N–OCH 3 + H H 2 N–H + OCH 3