CHEM 231
Acids and Bases; Functional Groups (Ch2)
Physical Properties and Molecular Structure:
Melting point, boiling point, solubility, crystallinity all depends on the strength of
interactions between molecules (intermolecular attractive forces)
Types of interactions:
1) ion-ion:
2) ion-dipole
3) dipole-dipole
4) induced dipole-dipole
5) instantaneous dipole-induced dipole (London Dispersion Forces)
*van der Waals forces (dipole-dipole, induced dipole-dipole, London Dispersion) are
between 0.5 (LD) to 5 kcal/mol (HB). To compare, C-C covalent bond ~ 100 kcal/mol
Hydrogen Bonding: δ+ H attracted to electronegative atoms of other molecules (O, N, F)
CHEM 231
Acid Base Chemistry
Proton Transfer Reaction:
hydrogen = proton
deprotonation = loss of hydrogen
1) Lowry-Brönsted Acid: proton donor
2) Lowry-Brönsted Base: proton acceptor (can be either a lone pair of a neutral atom
or it can be an anion with a negative charge)
3)
these are equilibrium arrows which indicate that the reaction is reversible
Example 1:
Is NH3 an acid or a base?
Mechanism of a reaction: shows the movement of electrons during the making and
breaking of bonds (book keeping device)
CHEM 231
Example 2:
In the following reaction label the Lowry-Brönsted acid, Lowry-Brönsted base, conjugate
acid and conjugate base. Also draw the mechanism.
Example 3:
What is the conjugate acid of CH3NH2?
Acid base reactions can also occur without proton transfer:
Lewis Acid: an electron pair acceptor; “electrophile” (electron poor species), “sink”
Lewis Base: an electron pair donor; “nucleophile” (electron rich species), “source”
CHEM 231
Example 4:
Indicate whether the following are Lewis acids, Lewis bases, or both.
Lewis Acid/Base reaction mechanisms and curved-arrow formalism:
pKa and acid/base strength:
pKa = -log (Ka) where Ka is the acidity constant.
For this class we just want to know given the pKa if a compound is it a weak or strong
acid (and visa versa for strong or weak bases).
Rules:
Strong acid
Ka > 1
pKa < 0
weak conjugate base
Benchmark pKas to Know
Acid
Weak acid
Ka < 1
pKa > 0
strong conjugate base
H 3O
pKa
-2
O
5
OH
OH
NH4
There are 3 handouts on the moodle page which summarize
some pKa values for common functional groups
10
10
H2O
15
MeOH
H
Et3N
15
25
35
CH4
50
CHEM 231
Example 5:
Indicate whether the following are strong acids or weak acids based on the pKa values.
*Important to know: the pKa of H2O is 15.7 any molecule with a higher pKa
typically acts as a base and any molecule with a lower pKa typically acts as an
acid*
Effects of other atoms on proton acidity:
Structural features that help us estimate the strength of an acid (HA) :
1) The increasing electronegativity of A as we proceed from left to right across a
row in the periodic table. The more electronegative the atom to which the acidic
proton is attached, the more polar the bond, and the more acidic the proton will
be.
CH4 < NH3 < H2O < HF
weaker acid
stronger acid
2) The increasing size of A as we proceed down a column in the periodic table.
The acid strengths of the hydrogen halides increase in the order. (The longer the
bond the easier the hydrogen is removed.)
HF < HCl < HBr < HI
weaker acid
stronger acid
3) The resonance in A− that allows delocalization of a charge over several atoms.
This effect is frequently enhanced by the presence of additional electron negative
atoms in A−.
CHEM 231
4) Nearby dipoles in a molecule can affect the acidity of a proton (inductive effects).
5) If an acidic hydrogen forms a hydrogen bond with another atom in the same
molecule it becomes less acidic because you now have to perform two reactions
1. Break the hydrogen bond then 2. deprotonate.
6) The fewer hybridized orbitals the more acidic the proton.
CHEM 231
CHEM 231
Functional Groups:
Components of organic molecules which have additional reactivity or functionality.
Also document on moodle summarizing these as well…