Cycloalkanes and their Stereochemistry

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Cycloalkanes and their Stereochemistry

• Chapter 4

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Naming Cycloalkanes

• Saturated cyclic hydrocarbons are defined as a
  carbon ring which has hydrogen in every position
  except the attachment point. These are also
  called alicyclic compounds.

Cyclopropane Cyclo the cyclic ring propane
the number of carbons
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Examples of Cycloalkanes
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Basic Examples of Cycloalkanes
Number of Carbons
Example Cyclohexane a ring of six carbons
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Naming Cycloalkanes

• 1) Find the parent Count the number carbons in
  the chain and the substituents. If the
  substituents is longer then the ring then the
  parent name is the substituent. If the ring is
  longer then the ring is the parent.

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Examples of Rule 1

• If we look at the molecule on the left. A
  pentane ring has 5 carbons compared to its
  substituent (1 carbon) a methyl. So the parent
  is the cyclopentane. However on the molecules on
  the right butane (4 carbons) is larger than
  cyclopropane (3 carbons) so the butane is the
  parent name.

1-Methylcyclopentane
1-Cyclopropylbutane
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Rule 2

• Number the substituents, and write the name.
• Make sure that you follow the ring so that the
  substituents have the lowest numbers.
• If two or more alkyl chains can be the same
  number then number alphabetically.
• Treat halogens as alkyl chains (F, Cl, Br, I).

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Rule 2 Examples
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Stereoisomers

• Unlike Constitutional Isomers which have the same
  molecular formula but a different orientation.
  Stereoisomers have the same order of connection
  but differ in spatial (or three dimensional)
  orientation.

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Cis-Trans Isomerisms

• Because ring dont rotate freely in space like
  linear chains two different possibilities exist.
  Cis Isomers are when two atoms are on the same
  side of a ring. Trans Isomers are when two atoms
  are on different side of a ring.

Trans Isomers
Cis Isomers
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Types of Strain

• Angle Strain also called Ring Strain is the
  strain due to expansion or compression of bond
  angles.
• Torsional Strain the strain due to eclipsing of
  bonds on neighboring atoms.
• Steric Strain the strain due to repulsive
  interactions when atoms approach each other too
  closely.

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Angle (Ring) Strain

• Cyclopropane has angles of 90o when we know that
  SP3 carbons like to bond at 109.5o. In
  cyclopropane there is angle strain. Example
  cyclopropane and cyclohexane

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Torsional Strain

• Is the strain due to eclipsing of bonds on
  neighboring atoms.
• When atoms are eclipsed then there is a natural
  tendency to move away from each other this is
  called torsional strain.

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Steric Strain

• Steric strain focuses on the strain of functional
  groups bound due to size onto the cyclic ring.
  Because rings dont rotate freely in space like
  linear chains then energy is higher when they are
  on the same side. For example cyclopropane

Lower Energy
Higher Energy
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Stability of Cycloalkanes

• Because we know that SP3 carbons like to have
  bond angles of 109o when the angles are smaller
  or larger than 109o the energy increases. It
  does not like bond angles. The greater the angle
  difference the more it is ready to explode to
  alleviate the strain.

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Conformation (Cyclopropane)

• The most ring strained alkane ring 60o instead of
  109.5o.
• The two hydrogens eclipse each other.
• Bonds are weaker and more reactive than any other
  cycloalkanes.

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Conformation (Cyclobutane)

• The most torsional strained alkane ring due to
  the increase number of hydrogens.
• Because of the increased torsional strain the
  total energy of cyclopropane and cyclobutane are
  the same.

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Confirmation (Cyclopentane)

• Cyclopentane has no angle (ring) strain but has a
  lot of torsional strain so it twists to a
  puckered non-planar configuration.

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Conformation (Cyclohexane)

• Cyclohexane is strain free which means that it is
  stable.
• Cyclohexane is widely used in nature.
• Adopt 2 three-dimensional structures or
  conformations (Chair and Boat).

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Chair and Boat

• There are two chair forms and one boat form. The
  boat form is a transition between one chair form
  and the other. If you look closely then you will
  see that if the red carbons bend up you will have
  the boat. If the blue carbons bend down then you
  will have the second chair form.

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Cyclohexane Bonding Stability

• - Steric Strain occurs when substituents are
  either both axial (higher energy) or equatorial
  (lower energy).
• Mono substituted cyclohexanes are more stable
  when they are in the equatorial position.
• To get the lowest energy isomer always place the
  largest group in the equatorial position.

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Cyclohexane (Axial)
There are six axial positions that are
perpendicular to the ring in the chair
configuration.
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Cyclohexane (equatorial)
There are six axial positions that are roughly in
the plane of the ring, around the equator.
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Cis-Trans Isomers of Cyclohexane
Regardless of the position (axial and equatorial)
the if two atoms are bound to the top position it
is the Cis isomer. Alternatively, if the
positions are on the opposite side then it is in
the Trans position.
Cis Isomer
Trans Isomer
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Conformations of Monosubstituted Cyclohexanes

• Since the equatorial position is the most stable
  it is preferred over the axial position when
  flipping. Always look for the Mono-substituted
  in the equatorial position.

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Conformation of Polycyclic Molecules

• Polycyclic Molecules are the joining of two
  individual cycloalkane rings.
• There are commonly 2 types of polycyclic
  molecules
• The joining of two ring structures
• Bicyclic alkanes

Trans-Decalin
Norbornane
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Take Home Message

• Know how to name cycloalkanes.
• Know the difference between cis and trans.
• Know Angle Strain, Torsional Strain, Steric
  Strain.
• Know why is cyclopropane so high energy.
• Know the conformations of cyclohexanes (boat and
  chair) and what is axial and equatorial.
• Know the properties of cyclohexanes.