Transcript CHM 103 Lecture 26 S07

Announcements & Agenda (03/28/07)
Exam Results: 75% Ave, 100% High
Quiz next Wednesday (Chirality & Ch 13)
Will hand out HW practice on Monday
Today
 Chirality (12.6)
 Carboxylic acids (13.1, 13.2)
 Esters (13.3)
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What to do if…
…this test didn’t go as planned…
Make a plan for preparing for the rest of 103
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DO practice problems and study!!!
ATTEND weekly reviews!!!
REVIEW scores from assignments with Bob to
estimate grade
DO the extra credit assignment
VISIT BOB
2
Will Do Tollens’ Test on Friday
• Tollens’ reagent,
which contains
Ag+, oxidizes
aldehydes, but
not ketones.
• Ag+ is reduced
to metallic Ag,
which appears
as a “mirror” in
the test tube.
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Chiral Molecules (12.6)
• Stereoisomers: have the same # of atoms connected
in the same order BUT are arranged differently in
space (just another kind of geometric isomer)
• chiral molecules have 1 or more chiral carbon atoms
(each of which is bonded to 4 different groups)
Lines are in plane of paper
H
H3C
Cl
Br
Wedges point out at us
Dashes point away from us
4
Identify the Chiral Carbons
Lipitor (atorvastatin)
HO
O
OOH
F
N
Ca2+
O
HN
2
5
Answer
Two
HO
O
*
OOH
*
F
N
Ca2+
O
HN
2
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Mirror Images
• Mirror images of chiral compounds cannot be
superimposed.
•
pairs of stereoisomers are called enantiomers
• Example: When the H & I atoms are aligned,
the Cl & Br atoms are on opposite sides.
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Achiral Structures are Superimposable
Rotation of the mirror image of an achiral
structure can be aligned with the initial
structure → superimposable
8
Chirality: Who Cares?
Tremendously important in all aspects of chemistry,
including biochemistry!!!
Most biological molecules are chiral (DNA, proteins,
etc.)
Different enantiomers often elicit diff. responses
Big $ in chemical industry for producing chiral drugs
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Chirality: Who Cares?
Enzymes, industrial catalysts, etc. are often
tailored to recognize a particular enantiomer and
produce chiral species
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Fischer Projections
• are a 2-dimensional representation of a 3-D
molecule.
• places the most oxidized group at the top.
• uses vertical lines in place of dashes for
bonds that go back.
• uses horizontal lines in place of wedges for
bonds that come forward.
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Drawing Fischer Projections
12
D and L Notations
• By convention, the letter L is assigned to the
structure with the —OH on the left.
• The letter D is assigned to the structure with the
—OH on the right.
• FYI: Other conventions exist too…
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Learning Check
Indicate whether each pair is a mirror image
that cannot be superimposed (enantiomers).
CH2OH
A.
B.
H
H
C
CH3
CH2OH
and
CH3 C
Br
Br
Cl
Cl
C
H
CH3
and
CH3 C
H
Yes
H
No
H
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Carboxylic Acids (13.1)
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Carboxylic Acids
A carboxylic acid contains a carboxyl group, which
• is a carbonyl group (C=O) attached to a hydroxyl
group (—OH).
• is found on carbon 1 in carboxylic acids.
O

CH3 — C—OH
hydroxyl group
carbonyl group
Will also see a carboxylic acid written as CH3COOH
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Models of Carboxylic Acids
The 3D models show the geometry of atoms
in carboxylic acid molecules.
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Recognizing Carboxylic Acids from Names
The IUPAC names of carboxylic acids:
• Replace the -e in the alkane name with -oic acid.
CH4
methane HCOOH
CH3—CH3 ethane
methanoic acid
CH3—COOH ethanoic acid
• Number substituents from the carboxyl carbon 1.
CH3
O
|
║
Don’t need to
CH3—CH—CH2—C—OH
know numbering
4
3
2
1
rules!!!
3-methylbutanoic acid
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Names & Sources of Some Carboxylic Acids
(Know These)
By the way, very common in nature:
Citric acid, tartaric acid, lactic acid, fatty acids, etc.
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Polarity of Carboxylic Acids
• strongly polar because they have two polar groups:
[hydroxyl (−OH) and carbonyl (C=O)]
• form dimers in which hydrogen bonds form
between two carboxyl groups.
O … H—O
||
|
CH3—C
C—CH3
|
||
O—H … O
A dimer of acetic acid
• have higher boiling points than alcohols, ketones,
and aldehydes of similar mass.
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Comparison of Boiling Points
Compound
O
║
CH3−CH2−C−H
Molar Mass
Boiling Point
58
49°C
CH3−CH2−CH2−OH
60
97°C
O
║
CH3−C−OH
60
118°C
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Solubility in Water
Carboxylic acids:
• form hydrogen
bonds with many
water molecules.
Water molecules
• with 1-4 carbon
atoms are very
soluble in water.
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Boiling Points and Solubility
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Acidity of Carboxylic Acids
Carboxylic acids
• are weak acids.
• ionize in water to produce carboxylate
ions and hydronium ions.
O
║
CH3−C−OH + H2O
O
║
CH3−C−O– + H3O+
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Neutralization of Carboxylic Acids
Carboxylic acid salts
• are a product of the neutralization of a carboxylic acid
with a strong base.
CH3—COOH + NaOH
CH3—COO– Na+ + H2O
acetic acid
sodium acetate
(carboxylic acid salt)
• are used as preservatives and flavor enhancers.
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Learning Check
Write the equation for the reaction of propanoic acid with
A. water
CH3—CH2—COOH + H2O
CH3—CH2—COO– + H3O+
B. KOH
CH3—CH2—COOH + KOH
CH3—CH2—COO – K+ + H2O
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Esters
In an ester, the H in the carboxyl group is replaced
with an alkyl group.
O

CH3 — C—O —CH3
ester group
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Esterification
Esterification: reaction of a carboxylic acid &
alcohol in the presence of an acid catalyst to
produce an ester.
O

CH3—C—OH + H—O—CH2—CH3
H+
O

CH3—C—O—CH2—CH3 + H2O
ethyl acetate (an ester)
FYI: The acid half of an ester is called an “acyl” group
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