Download Classes of organic acids and bases

CHE2060 Lecture 5: Acid-base chemistry
5.1 Acids & bases: overview & basics
5.2 Acid & base strength
5.3 Equilibrium acid-base reactions
5.4 The leveling effect of solvents
5.5 Estimation of acidity by conceptual knowledge
5.6 Classes of organic acids & bases
5.7 Functional groups: acid-base nature
Daley & Daley
Chapter 5:
Acid-base theory
Classes of organic acids & bases
Organic acids & bases
Organic acids & bases have carbon skeletons. We’ll look at these types:
• Neutral organic proton acids
• Neutral organic bases
• Positively charged carbon acids
• Negatively charged carbon bases
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Neutral organic proton acids: carboxylic acid
Three main types:
• Carboxylic acids
• Phenols
• Alcohols
OH bond polarity makes these
groups acidic & reactive.
OH-1 functional group
Degree of acidity depends
on the stability of the
conjugate base.
Carboxylic acids are the most acidic of the three although they are weak
acids. Acetic acids (2 carbons) has a pKa of 4.8.
Anion has 3 unhybridized
p orbitals w/ total of 4 e-.
They form a 3-atom π
molecular orbital system;
½ π connects C to each O.
The conjugate base (anion) is stabilized by
resonance. The (-1) is delocalized over 2 oxygen
atoms in the hybrid.
Therefore stronger acid than alcohol!
D&D, p.225 - 7
Neutral organic proton acids: phenol
Phenols are much less acidic than carboxylic acids: pKa of 10.
The phenolate anion is stabilized by resonance & therefore has some acidity.
But some of phenolate anion’s resonance structures disrupt aromatic resonance &
creates (-) carbon.
Because phenolate’s carbon can be charged:
1. the phenolate ion is less stable than the carboxylate ion; and
2. phenol is less acidic than carboxylic acid.
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Neutral organic proton acids: alcohols
Alcohols are alkyl molecules with OH-1 functional groups: R – OH.
• Alcohols are much less acidic than phenols.
• Alcohols are just a bit more acidic than water.
• The pKa range of alcohols are ~ 15 – 18.
Is there any resonance stabilization for this alcohol? Nope.
D&D, p.228 - 9
Neutral organic bases: amines
Neutral organic bases contain one or more pairs of nonbonding electrons.
• When acting as Lewis bases these e- pairs are donated.
• The more available the :, the more powerful the base.
• Any molecule with a pair (:) can act as a base.
We’ll look at just two of the many neutral organic bases:
• Amines; and
• Ethers.
Amines are derivatives of ammonia, NH3, and act as weak bases in water.
methyl
amine
methyl
pKa 10.6
ammonium
relatively
ion
strong base
Amines are strong bases because N holds its :
less strongly than O, S, halogens.
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(+) acids & (-) bases
Positively charged acids are e- deficient; carbons don’t have an octet
• Carbocation is a carbon with a full (+) charge
• Highly reactive and seldom seen since the act as intermediates
• Carbocations are Lewis acids (hard acids)
• React with the first Lewis base they “see”
• Prefer reacting with hard bases
Carbocations sp2
with empty p
orbital carrying the
+ charge.
Negatively charged bases are carbons bonded only to three other atoms *
carrying an unbonded electron pair (:).
• Carbanion is a carbon with a full (-) charge
• Looks very much like an amine
• Carbon holds the : loosely
since C is not very electronegative
• Thus carbanions are strong bases
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Example
Rank the acidity of these related carboxylic acids:
pKa: 4.75
2.86
1.26
0.63
Acidity increases as the number of EN subsitutents increases.
More substituents help to carry the : (or the negative charge) of the
acid’s conjugate anion.
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Examples
Rand the acidities of these two sets of molecules.
pKa: 5.2 vs. 11.1
Resonance stabilizes the (+) &
holds : closer to the nucleus
making them less available for
donation. (11.1 is the better base)
sp2
• Most stable cation
sp3
pKa: - 10 sp hybridization keeps : very close; less available
• Least stable cation
• More stable cation because the + is
delocalized over the CH3 group
pKa: 9.4
10.6
The CH3 alkyl group helps to delocalize (or share) the (+) charge; better base
However, adding more alkyl groups can decrease basicity by sterically
crowding the N & destabilizing the ion.
D&D, p.233 - 4