Organic Chemistry
Isomerism
Isomers have identical
composition but different
structures
Two forms of isomerism
–Constitutional (or structural)
–Stereoisomerism
Map of Organic Isomerization
Isomers
same different
conn ectivity con nectivity
Stereoisomers
Cons titu tion al Is omers
w ithout with
s tereocenters stereocen ters
Achiral
Cis -Trans Is omers
Chiral
Enantiomers
Dias tereomers
Constitutional isomers
compounds that have the
same molecular formula
but different structural
formulas (connected
differently)
Constitutional Isomerism
CH3 CH2 CH2 CH3
Bu tane
(bp -0.5°C)
CH3
CH3 CHCH3
2-Methylp ropan e
(bp -11.6°C)
Stereoisomerism
Same atom-to-atom
connections but different
arrangement in space
Stereoisomers: Geometric
Geometric isomers can occur when
there is a C=C double bond.
Cis-2-butene
Trans-2-butene
Stereoisomers: Optical
Optical isomers are molecules with
non-superimposable mirror images.
Such molecules are called CHIRAL
Pairs of chiral molecules are
enantiomers.
Chiral molecules in solution can rotate
the plane of plane polarized light.
Chiral Compounds—Polarized Light
Chirality generally occurs when a C
atom has 4 different groups attached.
These molecules are nonsuperimposable mirror images.
Lactic acid
Lactic acid isomers are non-
These amino acids are nonsuperimposable mirror images.
Stereoisomers in Nature
Right- and lefthanded seashells
The DNA here is
right-handed
Hydrocarbons
Alkanes and
Cycloalkanes
Alkanes
hydrocarbons
containing only
carbon-carbon single
bonds
Hydrocarbon: a
compound composed
of only carbon and
hydrogen
Alkanes
–the first 10 alkanes with
unbranched chains
Generic Alkane Formula: CnH2n+2
Alkanes
Alkanes are colorless gases,
liquids, and solids
Generally unreactive (but
undergo combustion)
Not polar (or low polarity) and so
are not soluble in water.
Nomenclature
IUPAC Names
Alkyl group: a substituent group
derived from an alkane by
removal of a hydrogen atom
–commonly represented by the
symbol R–named by dropping the -ane from
the name of the parent alkane and
adding the suffix -yl
N ame
methyl
Con dens ed
Structu ral Formula
-CH3
ethyl
-CH2 CH3
propyl
-CH2 CH2 CH 3
isopropyl -CHCH3
CH 3
bu tyl
-CH2 CH2 CH 2 CH3
N ame
isobu tyl
sec-butyl
Con dens ed
Structu ral Formula
-CH2 CHCH3
CH3
-CHCH2 CH3
CH3
CH 3
t ert-bu tyl -CCH3
CH 3
IUPAC Names
The IUPAC name of an alkane
with an unbranched chain of
carbon atoms consists of two
parts:
–(1) a prefix: the number of carbon
atoms in the chain
–(2) the suffix -ane: shows that the
compound is a saturated
hydrocarbon
IUPAC Names
1. The name for an alkane with
an unbranched chain of
carbon atoms consists of a
prefix showing the number of
carbon atoms and the ending ane
IUPAC Names
2. For branched-chain
alkanes, longest chain of
carbon atoms is the
parent chain and its
name is the root name
IUPAC Names
3. Name and number each
substituent on the parent
chain; use a hyphen to
connect the number to the
name
Nomenclature Example
CH3
CH3 CHCH3
1
2
3
2-Methylprop ane
IUPAC Names
4. If there is one substituent,
number the parent chain from the
end that gives the substituent the
lower number
CH3
CH3 CH2 CH2 CHCH3
4
5
3
2
2-Methylpen tane
(n ot 4-methylpentan e)
1
IUPAC Names
5. If
the same substituent
occurs more than once,
–number the parent chain
from the end that gives the
lower number to the
substituent encountered first
IUPAC Names
5. If
the same substituent
occurs more than once,
–indicate the number of times the
substituent occurs by a prefix di-,
tri-, tetra-, penta-, hexa-, and so on
IUPAC Names
5. If
the same substituent
occurs more than once,
–use a comma to separate
position numbers
CH3 CH3
CH3 CH2 CHCH2 CHCH3
6
4
5
2
3
2,4-Dimethylhexan e
(n ot 3,5-d imethylhexan e)
1
IUPAC Names
6. If
there are two or more
different substituents
–list them in
alphabetical order
IUPAC Names
6. If
there are two or more
different substituents
–number the chain from the
end that gives the lower
number to the substituent
encountered first
IUPAC Names
6. If
there are two or more
different substituents
–if there are different substituents
in equivalent positions on
opposite ends of the parent
chain, give the substituent of
lower alphabetical order the
lower number
CH3
CH3 CH2 CHCH2 CHCH2 CH3
2
1
4
3
CH2 CH3
3-Ethyl-5-methylhep tane
(not 3-methyl-5-ethylheptan e)
6
5
7
IUPAC Names
7. Do not include the prefixes di-,
tri-, tetra-, and so on, or the
hyphenated prefixes sec- and
tert- in alphabetizing;
–alphabetize the names of
substituents first, and then insert
these prefixes
Nomenclature Example
CH3 CH2 CH3
6
4
2
1
5
CH3 CCH2 CHCH2 CH3
3
CH3
4-Eth yl-2,2-d imethylhexan e
(n ot 2,2-d imethyl-4-ethylhexane)
Cycloalkanes
Cyclic hydrocarbon: a
hydrocarbon that
contains carbon atoms
joined to form a ring
Cycloalkanes
Cyclopentan e
Cycloh exane
Cycloalkanes
Nomenclature
–to name a cycloalkane, prefix the
name of the corresponding openchain alkane with cyclo-, and
name each substituent on the
ring
–if there is only one substituent
on the ring, there is no need to
give it a location number
Cycloalkanes
All compounds are flexible.
Cyclohexane, C6H12, has
interconverting “chair” and
“boat” forms.
Axial H ato m
H
H
4
H
H
Eq u ator ial H
atom
H
H
5
2
3
H
H
6
H
1
HH
Chair form
4
H
H
1
H
6
5
H
3
H
2
H
H
H
H
H
H
H
Boat form
H
H
H
H
5
4
H
3
HH
H
H
1
6
H
H
2
H
Chair form
H
Alkenes
Alkene: a hydrocarbon that contains
one or more carbon-carbon double
bonds
– ethylene is the simplest alkene
H
H
C
C
H
H
Ethylene
(an alken e)
H-C
C-H
Acetylen e
(an alk yn e
Alkenes - IUPAC Names
To name an alkene
–use the ending -ene to show the
presence of the C=C
–branched-chain alkenes are named
in a manner similar to alkanes;
substituted groups are located and
named
Alkenes - IUPAC Names
Examples
5
6
5
6
2
4
3
1
5
4
2
4
3
1-Hexene
1
2
1
CH3
CH3 CH2 CH2 CH2 CH=CH2
3
CH3 CH2 CHCH2 CH=CH2
4-Meth yl-1-h exene
CH2 CH3
CH3 CH2 CHC=CH2
CH2 CH3
2,3-D ieth yl-1-pen tene
Alkenes
Cis-trans isomerism
because of restricted rotation about a carboncarbon double bond, an alkene with two
different groups on each carbon of the double
bond shows cis-trans isomerism
How many isomers are possible for a compound
with the formula C4H8?
3 4
CH2CH3
H
1
C
H
1
C
2
C
H
H
1-butene
3
CH3
H
1
H3C
2
C
4
CH3
2
C
CH3
2-methylpropene
(isobutene)
H
4
CH3
H
3
C
2
C
H
cis-2-butene
1
H3 C
3
C
H
trans-2-butene
C
Alkynes
Alkyne: a hydrocarbon that contains
one or more carbon-carbon triple
bonds
– Acetylene (ethyne) is the simplest alkyne
H
C
H
hylene
alken e)
H-C
C-H
Acetylen e
(an alk yn e)
Benzene and its
Derivatives
Benzene, C6H6, in the
top 25 chemicals
produced in the U.S.
Starting point for
hundreds of other
compounds.
Aromatic Compounds
Aromatic compound: a
hydrocarbon that contains one
or more benzene-like rings
–arene: a term used to describe
aromatic compounds
–Ar-: a symbol for an aromatic
group derived by removing an -H
from an arene
Kekulé structure for benzene
(1872)
The electrons of Benzene are
Delocalized
sp2 hybridized
Other Aromatic Hydrocarbons
CH3
Toluene
Naphthalene
Benzene Derivatives
Aniline
Phenol
C6H5NH2
C6H5OH
TNT
trinitrotoluene
C6H4CH3(NO2)3
Naming Benzene Derivatives
Cl
1
6
2
3
5
4
Ortho to Cl
Meta to Cl
Para to Cl
1,4-dimethylbenzene
Common name: p-xylene
Nomenclature Example
1
NO2
2
NO2
OH
CH3
3
4
Cl
4-Ch loro-2nitrotoluen e
Br
6
1
5
Br
2
3
4
Br
2,4,6-Trib romoph enol
4
3
2
Br
CH2 CH3
1
2-Bromo-1-ethyl-4n itroben zene