NEXT STEPS
Organic Acids and Bases
WHY STUDY ACIDS AND BASES?

Studying acids and bases allows us to predict
reactivity.
Remember – all chemicals react to
become more stable!!
 “Strong” compounds react ‘quickly’
because they are unstable the way they
are. The faster they can get rid of the
thing making them unstable, the better!
 “Weak” compounds react ‘slowly’
because they are already stable! They
don’t want to change anything!

“STRONG” VS. “WEAK” – RELATIVE TERMS


Strong acids are those that react (ionize)
completely: Strong Acids include

HCl, HBr, HI, HNO3, HClO4, and H2SO4

Strong baes include LiOH, NaOH, KOH, Ca(OH)2,
and Ba(OH)2
Most organic acids and bases are considered
“weak” because they do not ionize completely.
STRONG ACIDS AND BASES

Strong Acids and Bases react completely in one
direction
WEAK ACIDS AND BASES ARE IN
EQUILIBRIUM



If Molecules Were People
Our objective for this section is to determine
whether a reaction in equilibrium will be favored
to the right or the left direction.
The next steps will show us the patterns we need
to meet this objective!
WHAT DO WE ALREADY KNOW?
Brønsted-Lowry Acids and Bases
 Acids:


Hydrogen Ion (H+) donor


“Proton” is a synonym
Bases:

Accept hydrogen ion/proton
CONJUGATE ACIDS AND BASES

In all acid/base reactions, the product that results
when the acid loses a proton is a conjugate base
(Because it now has the ability to accept a proton)


Likewise, the product that results when the base
gains a proton is called the conjugate acid (Because it
now has the ability to donate that proton)
Relationship:
Strong acids always yield a weak CB; Weak acids yield
strong CB
 Strong bases always yield a weak CA; Weak bases yield
strong CA

MEASURING STRENGTH OF ACIDS

Different acids have different levels of reactivity
that allows us to assess their strength.


This strength is expressed as an acidity constant, Ka
For the reaction of any general acid, HA, with
water, the acidity constant is:
Remember [__] =
molar
concentration
WHAT’S IT MEAN?


Think: If a lot of hydronium ion is produced, it’s
because HA was a STRONG acid. So, a larger
concentration of H3O+ will yield a larger Ka
value.
Relationship:
Large Ka = Strong Acid
Small Ka = Weak Acid
KA VS. PKA

Because acid ionization constants for acids are
numbers with negative exponents we often
express them as pKa values instead.
pKa = -log Ka
For Example: If an acid has Ka = 1.0 x 10-4, it is
much easier to say that the pKa = 4
For a stronger acid, the Ka value would be higher;
perhaps Ka = 1.0 x 10-2, so pKa = 2
WHAT’S THE RELATIONSHIP
The SMALLER the
pKa, the STRONGER
the acid!
Do not forget this
relationship!
TABLE 2.2 PAGE 47 (BLACK BOOK)
PREDICTING THE REACTION DIRECTION

Remember: Chemical reactions occur so that
unstable chemicals (strong) can become more
stable (weak)

Acid base reactions will always occur in the direction
that produces the weaker acid
ORGANIC ACIDS

Characterized by a positively polarized hydrogen
atom
ORGANIC BASES

Characterized by the presence of an atom with a
lone pair of electrons that can bond to H+
YOU TRY IT:

Acetone has a pKa = 19 while ammonia has a
pKa = 36. Will this reaction happen in the
forward direction?
ONE MORE…
C6H5OH +
Phenol
pKa = 9.95
HCO3-
Bicarbonate Ion
?
⇌ C6H5O- + H2CO3
Phenoxide Ion
Carbonic Acid
pKa = 6.36
QUALITATIVE ASSESSMENT OF ACID
STRENGTH


Using pKa values is great, but we need to be able
to assess the strength of organic acids and bases
without always needing to refer to a pKa table.
Major factors affecting acid strength
Atom: Electronegativity vs. Size
 Resonance
 Induction
 Orbitals

THE METHOD TO THE MADNESS
ATOM: ELECTRONEGATIVITY

Compare CH4, NH3, H2O, and HF: Which is the
stronger acid?

Examine the conjugate bases:

The atom with the higher electronegativity is more
stable when holding a negative charge. So…
HF > H2O > NH3 > CH4
Stronger
Weaker

Electronegativity may only
be used to compare anions
that are in the same period!
ATOM: SIZE


For comparing anions that are in the same
family, we evaluate the size of the anion.
Generally, a negative charge is more stable when
spread over a larger area. Compare HF, HCl,
HBr, and HI

Conjugate Bases
The Iodide ion is larger, therefore it is the most
stable
 HI > HBr > HCl > HF

YOU TRY IT!
Which Hydrogen is more acidic?

O
S
H
H
H
H
C
H
H
N
H
RESONANCE

A negative charge is stabilized by resonance.
Therefore, conjugate bases that can be stabilized
by resonance come from stronger acids.
Stronger Acid
YOU TRY IT!

Which proton is more acidic?
OH
HO
O
H3C
H
O
O H
INDUCTION

Compare Acetic Acid to Trifluoroacetic Acid
H
H
C
H

O
F
F
OH
O
C
F
OH
Negative charge is stabilized by the inductive
effect of the electronegative fluorine atoms
NOTE:

Compare Acetic Acid to Trimethylacetic Acid:
H
H
C
H

O
H3C
OH
O
C
CH3
OH
Electronegative atoms will pull electron density
away, but alkyl (carbon-containing) groups will
donate electron density toward the negative
charge. (The reasoning is due to hyperconjugation – if you care
to know more, you can Google it!)

CH3
So, which is more acidic?
YOU TRY IT!

Which proton is more acidic?
H
O
Cl
Cl
O
O
F
F
O
H3C
H
H
CH3
H
O
O
ORBITALS
Which type of orbital holds electrons closer to the
nucleus? sp3, sp2, or sp
 Why?

Do electrons closer to the nucleus have greater or
less stability?
 Why?

ORDER OF IMPORTANCE: ARIO

Always follow this order of importance: Atom,
Resonance, Induction, Orbital
Except when you are comparing a negative charge
on an sp3 hybrid Nitrogen to a negative charge
on an sp hybrid carbon.
See pages 66-67 in Handout.
Try problems 3.19 – 3.33 in the handout. Also, tell
which factor allowed you to make your decision.
YOU TRY IT!

Predict the position of equilibrium (left or right)
for each of these reactions:
-
+
CH Na
H2O
+
+
H3C
O
+
OH
-
O
+
+
HO Na
H3C
-
H3C
+
O Na
-
O Na+
OH
+
H3C
OH
-
OH
O Na+
+
-
+
O Na