Drawing Organic Structures
• Organic compounds are held together by
covalent bonds
• a covalent bond may be represented by a
pair of dots, or by a single line
Molecular formula
• Use methane as an example
• CH4
–
–
–
–
molecular formula
shows relative numbers of atoms
conveys no sense of structure or bonding
conveys no sense of chemistry
Writing Molecular Formulas
• The number of carbon atoms is specified first
• The number of hydrogen atoms is specified next
• The remaining atoms are then specified in (usually
alphabetical) order
–
–
–
–
cyclohexane: C6H12
benzoic acid: C7H6O2
ethyl para-aminobenzoate: C9H11NO2
picryl chloride: C6H2ClN3O6
Full structural formula
H
H C H
H
H
H C H
H
• every atom specified
• every bond explicitly shown
– methane is simple
– two structures convey the same information
about as efficiently
Now lets try it with cholesterol as
the example…
• Molecular formula
– C27H46O
Full structural formula
H
H
H C
H
H
C
H
H
H H C
C
C
C
C
O
H
H
H
C
C
H H
C
H
H H
H
H C
H
H
H
C
C
H
H
C
H H
C
C
H H
C
C
H
H
C
H
H
C
C
C
H
H H
C H
H
H H
C
H
C H
C
H
H
H C
H H
How do we simplify this?
• recognize that every organic compound contains
carbon
• delete the symbol for carbon atoms except for
those explicitly involved in a functional group
(CHO, COOH, CN)
Lets do it…
H H
H
H
H
H
H H
H
H
H
H
H H
O
H H
H
H H
H
H
H
H
H
H
H H
H H
H
H
H
H
H
H
H
H H
H
H
H
H
H H
H
H
Can we make it simpler…?
• We have removed all the carbon symbols
(every organic compound contains carbon)
• Practically every carbon compound contains
hydrogen
– delete symbol for hydrogen except where it is
part of a functional group (e.g. CHO, OH)
Lets try that…
H
O
• Dramatic simplification
– structural features are visible
• three rings
• length and shape of side chains
Lets try that…
H
• Dramatic simplification
O
– structural features are visible
– functional groups are visible
• OH group of alcohol
• C=C double bond
Lets try that…
• Dramatic simplification
H
O
– structural features are visible
– functional groups are visible
– information is more available
• how long does it take to find
the important features?
The molecules of this experiment
• cyclohexane
– C6H12
CH3
• toluene
– C7H8
O
C
O H
H
N
H
O
O C2H 5
• benzoic acid
– C7H6O2
• ethyl para-aminobenzoate
– C9H11NO2
Properties
• all electrically neutral
• all soluble in non-polar solvents (e.g. ether)
• none soluble in water to an appreciable
degree
• cyclohexane & toluene are liquids
• benzoic acid and ethyl para-aminobenzoate
are solids
Benzoic acid
• a carboxylic acid
• has one acidic hydrogen
O
C
O H
– weak acid
– only slightly ionized
– pKa ≈ 4
O
C
O H
• reacts with base to give
benzoate anion and water
O
+ OH
C
O H
O
C
O
+ H
O
C
O
+ H 2O
O
C
O H
O
C
O
OH
H3O
O
C
O
O
C
O H
• benzoic acid molecules are converted by strong
base into benzoate anions
• benzoate anions are converted by strong acids into
benzoic acid molecules
A comparison
• benzoic acid
– neutral, covalent
molecules
– soluble in non-polar
solvents (e.g. ether)
– almost insoluble in
water
• sodium benzoate
– sodium cations and
benzoate anions
– almost insoluble in
non-polar organic
solvents (e.g. ether)
– soluble in water
Ethyl para-aminobenzoate
H
N
H
O
H
N
H
O
H
N
H
O
O C2H 5
+ H2O
O C2H 5
+ H3O
O C2H 5
H
H N
H
O
+ OH
O C2H 5
H
H N
H
O
O C2H 5
• has both ester (neutral) and amine (base) functional groups
• has one basic nitrogen
– weak base: gives practically no OH— in water
• reacts with acid to give alkylammonium cation
H
N
H
H
H N
H
O
H3O
O C2H 5
O
O C2H 5
OH
H
H N
H
O
H
N
H
O
O C2H 5
O C2H 5
• ethyl p-aminobenzoate molecules are converted
by strong acid into alkylammonium cations
• the alkylammonium cations are converted by
strong bases into ethyl p-aminobenzoate
molecules
A comparison
• ethyl p-aminobenzoate
– neutral, covalent
molecules
– soluble in non-polar
solvents (e.g. ether)
– almost insoluble in
water
• the alkylammonium
salt
– alkylammonium
cations and sulfate or
chloride anions
– almost insoluble in
non-polar organic
solvents (e.g. ether)
– soluble in water