Transcript 22.3 Isomers

Types of Organic Compounds
• Vast majority of over 20 million known
compounds are based on C: organic
compounds.
• Generally contain C and H + other elements
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Isomerism
• Isomers have identical composition but
different structures
• Two forms of isomerism
– Constitutional (or structural)
– Stereoisomerism
• Constitutional
– Same empirical formula but different atomto-atom connections
• Stereoisomerism
– Same atom-to-atom connections but
different arrangement in space.
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22.3 Isomers > Constitutional Isomers
Look below at the structures of butane and 2methylpropane.
Butane
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2-methylpropane
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22.3 Isomers > Constitutional Isomers
• Even though both compounds have the
formula C4H10, their boiling points and
other properties differ.
• Because their structures are different,
they are different substances.
• isomers have the same molecular
formula but different molecular
structures.
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22.3 Isomers > Constitutional Isomers
• Butane and 2-methylpropane
represent a category of isomers
called constitutional isomers, or
structural isomers.
• Constitutional isomers are
compounds that have the same
molecular formula but the atoms are
joined together differently.
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22.3 Isomers > Constitutional Isomers
Constitutional isomers differ in
physical properties such as boiling
point and melting point. They also
have different chemical reactivities.
In general, the more highly branched
the hydrocarbon structure is, the
lower the boiling point of the
isomer will be compared with less
branched isomers.
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22.3 Isomers >
Which of these molecules will have a lower
boiling point and why?
Butane
2-methylpropane
2-methylpropane will have a lower boiling point
because it has a more highly branched structure.
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Structural Isomers
CH3CH2 CH2CH2 CH3
Pentane
Hydrocarbons
& Structural
Isomerism
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CH3
CH3CHCH2CH3
2-Methylbutane
CH3
H3CCCH3
CH3
2,2-Dimethylpropane
Note names of isomers
Isomers of C5H12?
C5H12 has 3
structural
isomers.
22.3 Isomers > Stereoisomers
Stereoisomers
What are two types of stereoisomers?
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22.3 Isomers > Stereoisomers
• Because molecules are threedimensional structures, molecules with
the same molecular formula and with
atoms joined in exactly the same order
may still be isomers.
• Stereoisomers are molecules in which
the atoms are joined in the same order
but the positions of the atoms in
space are different.
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types
of stereoisomers are cis-trans
22.3Two
Isomers
> Stereoisomers
isomers and enantiomers.
Two arrangements are possible for the methyl
groups and hydrogen atoms with respect to the
rigid double bond in 2-butene.
Cis configuration
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Trans configuration
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22.3 Isomers > Stereoisomers
Cis-Trans Isomers
• Cis-trans isomers, also known as
geometric isomers, have atoms joined in
the same order but the spatial orientation
of the groups differs.
• The most common example of cis-trans
isomerism occurs in molecules with double
bonds.
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22.3 Isomers > Stereoisomers
Cis-Trans Isomers
In the cis configuration, similar groups are
on the same side of the double bond.
Cis configuration
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22.3 Isomers > Stereoisomers
But, when similar groups extend from
opposite sides of the double bond, the
isomer is in the trans configuration.
Trans configuration
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22.3 Isomers > Stereoisomers
Cis-Trans Isomers
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•
Cis-trans isomers have
different physical and
chemical properties.
•
You should be able to
identify cis-trans isomers
of alkenes when each
carbon of the double
bond has one substituent
and one hydrogen.
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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 and
Polarized Light
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22.3 Isomers > Stereoisomers
Enantiomers
• A carbon with four
different atoms or
groups attached is an
asymmetric carbon.
•
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In the compound shown
here, H, F, Cl, and Br
atoms are attached to a
single carbon atom, so
the carbon is an
asymmetric carbon.
CHFClBr
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22.3 Isomers > Stereoisomers
Enantiomers
•
The relationship between
the two molecules is
similar to the relationship
between right and left
hands.
•
Sometimes the terms
right-handed and lefthanded are used to
describe compounds
with an asymmetric
carbon.
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22.3 Isomers > Stereoisomers
Enantiomers
• By contrast, a pair of hands is distinguishable
even though the hands have identical parts.
• When you try to stack your hands on top of one
another, the thumb of one hand lines up with the
little finger of the other hand.
•
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No matter how you turn your hands, you
can’t get them to look exactly alike.
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22.3 Isomers > Stereoisomers
Enantiomers
Pairs of molecules that are mirror images
and not superimposable are called
enantiomers, or optical isomers.
These molecules are examples of
enantiomers.
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Stereoisomers Isomers
Chirality
generally occurs
when a C atom
has 4 different
groups attached.
Lactic acid
Stereoisomers Isomers
Lactic acid
isomers are
non-superimposable
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Chirality: Handedness in
Nature
These molecules are non-superimposable
mirror images.
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Chirality: Handedness in
Nature
These amino acids are nonsuperimposable mirror images.
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Stereoisomers in Nature
Right- and lefthanded seashells
The DNA here is
right-handed
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22.3 Isomers > Stereoisomers
Enantiomers
•
Unlike other isomers, enantiomers have
identical physical properties such as boiling
points and densities.
•
Enantiomers do, however, behave differently
when they interact with other molecules that
have asymmetric carbons.
• Many molecules in your body have
asymmetric carbons, so each enantiomer
can have a different effect on the body.
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22.3 Isomers >
Sample Problem 22.4
Identifying Asymmetric Carbon
Atoms
Which compound has an
asymmetric carbon?
a.
b.
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22.3 Isomers >
Sample Problem 22.4
1 Analyze Identify the relevant concepts.
An asymmetric carbon has four different
substituents attached.
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22.3 Isomers >
Sample Problem 22.4
2 Solve Apply the concepts to this problem.
Draw the structure in a way that makes it easier
to compare the four different groups attached to
the central carbon.
a.
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b.
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22.3 Isomers >
Sample Problem 22.4
2 Solve Apply the concepts to this problem.
Compare the groups. If all four groups are unique,
the central carbon is asymmetric. If any two are
the same, the central carbon is not asymmetric.
a.
b.
Compound (b) is asymmetric.
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22.3 Isomers > Key Concepts
Constitutional isomers differ in physical
properties such as boiling point and
melting point. They also have different
chemical reactivities.
Two types of stereoisomers are cis-trans
isomers and enantiomers.
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22.3 Isomers > Glossary Terms
• isomers: compounds that have the same molecular
formula but different molecular structures
• constitutional isomers: compounds that have the
same molecular formula, but whose atoms are
bonded in a different order
• stereoisomers: molecules that have atoms in the
same order, but which differ in the arrangement of the
atoms in space
• cis-trans isomers: compounds that have atoms in
the same order, but differ in the orientation of groups
around a double bond
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22.3 Isomers > Glossary Terms
• cis configuration: the configuration in which
substituent groups are on the same side of a double
bond
• trans configuration: the configuration in which
substituent groups are on the opposite sides of a
double bond
• asymmetric carbon: a carbon atom that has four
different atoms or groups attached
• enantiomers: molecules that differ from one another
in the way that four different groups are arranged
around a carbon atom
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