Cyclopropane

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2.6 Cycloalkanes
• Cycloalkanes are alkanes that have carbon
atoms that form a ring (called alicyclic
compounds)
• Simple cycloalkanes are rings of CH2
units, (CH2)n, or CnH2n
• Structure is shown as a regular polygon with
the number of vertices equal to the number of
C’s (a projection of the actual structure)
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Nomenclature of Cycloalkanes
IUPAC naming rules are followed with the
addition of the prefix cyclo-
cyclopropane
cyclobut ane
cyclohexane
With multiple substituents, use lowest numbering sequence
1-ethy l-3-methy lcy clohexane
not 1-ethy l-5-met hy lcy clohexane
2.6 Cycloalkanes
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Complex Cycloalkanes
• Naturally occurring materials contain
cycloalkane structures
• Examples:
– chrysanthemic acid (cyclopropane),
– prostaglandins (cyclopentane),
– steroids (cyclohexanes and cyclopentane)
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Complex Cycloalkanes
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Properties of Cycloalkanes
Melting points are
affected by the
shapes and the way
that crystals pack so
they do not change
uniformly
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2.7 Naming Cycloalkanes
• Count the number of carbon atoms in the ring and the number
in the largest substituent chain. If the number of carbon atoms
in the ring is equal to or greater than the number in the
substituent, the compound is named as an alkyl-substituted
cycloalkane.
• For an alkyl- or halo-substituted cycloalkane, start at a point of
attachment as C1 and number the substituents on the ring so
that the second substituent has as low a number as possible.
When two substituents are present, we number the ring
beginning with the substituent first in the alphabet.
• Wenn three or more substituents are present, we begin at the
substituent that leads to the lowest set of locants
• Number the substituents and write the name
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1. Find the parent:
or butylcyclopropane
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Number the
substituents &
write the name:
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Examples:
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Problem 2.15: IUPAC names?
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1-ethyl-2,3-dimethylcyclopentane
2
1
3
4
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3-ethyl-4-methyl-1cyclopropylcyclohexane
5-ethyl-3-cyclohexyl-4-methyloctane
2.8 Cis-Trans Isomerism in
Cycloalkanes
• Rotation about C-C bonds in cycloalkanes is limited
by the ring structure
• Rings have two “faces” and substituents are labeled
as to their relative facial positions
• There are two different 1,2-dimethyl-cyclopropane
isomers, one with the two methyls on the same side
(cis) of the ring and one with the methyls on opposite
sides (trans)
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Isomerization with Cycloalkanes
Isomers - molecules that are the same but yet different
C4H 10
Constitutional (structural) isomers - same formula but
different connectivity
cis -isomer
same side
Stereoisomers- same formula and
connectivity but different spatial
arrangement
CH3
trans - isomer
opposite sides
CH3
CH3
CH3
Cis/Trans Isomers
CH3
CH3
CH3
CH3
Cis-1,2-dimethylcyclohexane Trans-1,2-dimethylcyclohexane
Cis/Trans Isomers
Also written as:
CH3
CH3
CH3
CH3
Cis-1,2-dimethylcyclohexane
Trans-1,2-dimethylcyclohexane
2.8 Cis-Trans Isomerism in
Cycloalkanes
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Stereoisomers
• Compounds with atoms connected in the same
order but which differ in three-dimensional
orientation, are stereoisomers
• The terms “cis” and “trans” should be used to
specify stereoisomeric ring structures
• Recall that constitutional isomers have atoms
connected in different order
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Stereoisomers
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Practice Prob. 2.4: Name?
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Problem 2.18: IUPAC Name?
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Cis and trans isomerization of
cycloalkanes is only for
disubstitued
rings.
Multi-substituted requires additional nomenclature
Examples of disubstituted:
cis-1-ethyl-3-methylcyclohexane
trans-1,2-dimethylcyclopentane
Structure of Cycloalkanes
Ring strain accounts for the unstability in certain cycloalkanes
Remember - sp3 has 109.5° bond angle
60°
90°
105°
Ring Strain is combination of eclipsing strain and bond-angle strain
Each cycloalkane adopts a
conformation that tries to
alleviate ring strain as much as
possible
cylcopropane
“banana” bonds that do not have
end-to-end overlap of typical σbonds
Quic kTime™ and a A nimation decompr es s or are needed to s ee this pic ture.
Highly unstable ring-system that will
“open” or react readily
Cyclobutane
Instead of flat conformation, a “puckered” conformation
reduces eclipsing strain
QuickTime™ and a Animation decompressor are needed to see this picture.
Cyclopentane
An “envelope” conformation reduces eclipsing strain
Ring Conformation
• Cyclopentane puckers
Cyclohexane
The “chair” conformation eliminates all eclipsing strain and
nearly all bond-angle strain
Ring Strain vs. Cycloalkane
Size
Cyclohexane the most stable
kcal/mol
Cycloalkane Ring Size
Axial and Equatorial positions
are not energetically equivalent
H
H
More Stable
H
equatorial
axial
C
H
H
H
H
H 3C
1,3-diaxial interaction (steric hinderance)
The larger the group, the greater the 1,3-diaxial interaction
Cyclohexane prefers larger groups at equatorial positions
Cyclopropane
A ball-and-stick model of cyclopropane shows the ring
strain when the bond angles are deformed from their
normal 109.5° to 60°.
Ball-and-stick Model
of Cyclopropane
Cyclobutane
A ball-and-stick model of cyclobutane shows the ring
strain when the bond angles are deformed from their
normal 109.5° to approximately 90°.
Cyclopentane
The ball-and-stick model of cyclopentane reveals a
nearly planar molecule with bond angles of 108°.
Cyclohexane: Chair and Boat
Conformations
The chair conformation of cyclohexane can be twisted
into the boat conformation.
Cyclohexane: Chair and Boat Conformations
The chair conformation of cyclohexane can be twisted into the boat
conformation. Further twisting results in a second chair conformation.
By labeling one axial substituent with a white sphere you can see how
the axial position of one chair becomes equatorial by twisting into the
second chair conformation.
Chapter 11 Organic
Compounds: Alkanes
11.9
The Shape of Cycloalkanes
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Bond Angles
Ideal bond angle is 109º
• Cyclopropane - 60º
• Cyclobutane - 90º
• Strained bonds, unstable
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cumming
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Free Rotation
Free rotation is not possible in
cycloalkanes.
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Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
Chair and Boat
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Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
Geometric Isomers
Geometric isomers have restricted rotation
around C-C
bonds and differ in 3-dimensional
arrangement of atoms.
Named “cis-” and “trans– “
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Learning Check
Identify each of the following as cis- or
trans-
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Learning Check
Draw and name all the isomers of
dichlorolcyclobutane.
(There are 5.)
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Stereoisomers
Stereoisomers are compounds with the
same structural
formula but different spatial arrangements of
atoms
• Cannot superimpose.
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