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CH 4: Organic Compounds: Cycloalkanes and their

Stereochemistry

Renee Y. BeckerCHM 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 >1 substituent.

• Number so that sub. have lowest numbers– Give first in alphabet lowest number if possible

CH2CH3CH2CH3

CH3

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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

CH3

CH3

CH2CHCH2CH3

CH3

CH3

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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 C’s 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

Long-chain

157.4 157.4

166.6 164.0158.7 158.6158.3

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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 it’s 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 (>99%) 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.

CH3

CH3

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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)cyclohexane

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Example 4

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

a) b)

c) d)

Table 4.2 Axial and Equatorial Relationship in Cis and trans Disub Cyclohexanes

Cis/trans pattern Axial/Equatorial Relationship1,2–Cis 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

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