CH 4: Organic Compounds: Cycloalkanes and their Stereochemistry

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CH 4 Organic Compounds Cycloalkanes and their
Stereochemistry

• Renee Y. Becker
• CHM 2210
• Valencia Community College

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Cycloalkanes

• Rings of carbon atoms (CH2 groups)
• Formula CnH2n
• Nonpolar, insoluble in water
• Compact shape
• Melting and boiling points similar to branched
  alkanes with same number of carbons
  

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Naming Cycloalkanes

• Cycloalkane usually base compound
• May be cycloalkyl attachment to chain
• It is off of a chain that has a longer carbon
  chain
• Number carbons in ring if gt1 substituent.
• Number so that sub. have lowest numbers
• Give first in alphabet lowest number if possible

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Naming Cycloalkanes

• Find the parent. of carbons in the ring.
• Number the substituents

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Example 1

• Give IUPAC names

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Example 2

• Draw the structure
• a) propylcyclohexane
• b) cyclopropylcyclopentane
• c) 3-ethyl-1,1-dimethylcyclohexane

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Stereoisomerism

• Compounds which have their atoms connected in the
  same order but differ in 3-D orientation

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Cis-Trans Isomerism

• Cis like groups on same face of ring
• Trans like groups on opposite face of ring
• Sub. Do not have to be on adjacent carbons of
  ring
  

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Cycloalkane Stability

• 5- and 6-membered rings most stable
• Bond angle closest to 109.5?
• Angle (Baeyer) strain
• Measured by heats of combustion per -CH2 -
• The more strain, the higher the heat of
  combustion, per CH2 group
• The energy released as heat when one mole of a
  compound undergoes complete combustion with
  oxygen.
  

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Stability of Cycloalkanes The Baeyer Strain
Theory

• Baeyer (1885) since carbon prefers to have bond
  angles of approximately 109, ring sizes other
  than five and six may be too strained to exist
• Rings from 3 to 30 Cs do exist but are strained
  due to bond bending distortions and steric
  interactions

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Summary Types of Strain

• Angle strain - expansion or compression of bond
  angles away from most stable
• Torsional strain - eclipsing of bonds on
  neighboring atoms
• Steric strain - repulsive interactions between
  nonbonded atoms in close proximity

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Heats of Combustion (per CH2 group) Alkane O2
? CO2 H2O
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Cyclopropane

• Large ring strain due to angle compression
• Very reactive, weak bonds

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Cyclopropane

• Torsional strain because of eclipsed hydrogens

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Cyclobutane

• Angle strain due to compression
• Torsional strain partially relieved by
  ring-puckering

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Cyclopentane

• If planar, angles would be 108?, but all
  hydrogens would be eclipsed.
• Puckered conformer reduces torsional strain.

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Cyclohexane

• Combustion data shows its unstrained.
• Angles would be 120?, if planar.
• The chair conformer has 109.5? bond angles and
  all hydrogens are staggered.
• No angle strain and no torsional strain.
  
  

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Chair Conformer
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Boat Conformer
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Conformational Energy
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Axial and Equatorial Positions
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Drawing the Axial and Equatorial Hydrogens
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Monosubstituted Cyclohexanes
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1,3-Diaxial Interactions

• Difference between axial and equatorial
  conformers is due to steric strain caused by
  1,3-diaxial interactions

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Hydrogen atoms of the axial methyl group on C1
are too close to the axial hydrogens three
carbons away on C3 and C5, resulting in 7.6
kJ/mol of steric strain
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Disubstituted Cyclohexanes
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Conformational Analysis of Disubstituted
Cyclohexanes

• In disubstituted cyclohexanes the steric effects
  of both substituents must be taken into account
  in both conformations
• There are two isomers of 1,2-dimethylcyclohexane.
  cis and trans
• In the cis isomer, both methyl groups are on the
  same face of the ring, and compound can exist in
  two chair conformations
• Consider the sum of all interactions
• In cis-1,2, both conformations are equal in energy

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Conformational Analysis of Disubstituted
Cyclohexanes
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Trans-1,2-Dimethylcyclohexane

• Methyl groups are on opposite faces of the ring
• One trans conformation has both methyl groups
  equatorial and only a gauche butane interaction
  between methyls (3.8 kJ/mol) and no 1,3-diaxial
  interactions
• The ring-flipped conformation has both methyl
  groups axial with four 1,3-diaxial interactions
• Steric strain of 4 ? 3.8 kJ/mol 15.2 kJ/mol
  makes the diaxial conformation 11.4 kJ/mol less
  favorable than the diequatorial conformation
• trans-1,2-dimethylcyclohexane will exist almost
  exclusively (gt99) in the diequatorial
  conformation

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Trans-1,2-Dimethylcyclohexane
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Cis-Trans Isomers

• Bonds that are cis, alternate axial-equatorial
  around the ring.

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Bulky Groups

• Groups like t-butyl cause a large energy
  difference between the axial and equatorial
  conformer.
• Most stable conformer puts t-butyl equatorial
  regardless of other substituents.

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Example 3

• Draw the most stable conformation
• a) ethylcyclohexane
• b) isopropylcyclohexane
• c) t-butylcyclohexane
• d) cis-1-t-butyl-3-ethylcyclohexane
• e) trans-1-t-butyl-2-methylcyclohexane
• f) trans-1-t-butyl-3-(1,1-dimethylpropyl)cyclohex
  ane

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Example 4

• Which of the following is the most strained ring?
  Least strained? Why?

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Table 4.2 Axial and Equatorial Relationship in
Cis and trans Disub Cyclohexanes
Cis/trans pattern Axial/Equatorial Relationship Axial/Equatorial Relationship
1,2Cis a,e e,a
1,2-trans a,a e,e
1,3-cis a,a e,e
1,3-trans a,e e,a
1,4-cis a,e e,a
1,4-trans a,a e,e