4. Organic Compounds:

Cycloalkanes and their

Stereochemistry

Fall, 2014 by Dr.Isam Al Naser

Based on McMurry’s Organic Chemistry, 7th edition

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Why this chapter?

Because cyclic molecules are commonly encountered in all classes of bimoleculars:

- Proteins.,Lipids.,Carbohydrates.,Nucleic acids…

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4.1 Naming Cycloalkanes

Cycloalkanes are saturated cyclic hydrocarbons

Have the general formula (CnH2n)

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Naming Cycloalkanes

Find the parent. of carbons in the ring.

Number the substituents

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Give IUPAC names for the following cycloalkanes?

Draw structures corrsponding to the following

IUPAC names.

a)1,1-dimethylcyclooctane b) 3-cyclobutylhexane

c) 1,2-dichlorocyclopentane c)1,3-dibromo-5-

methylcyclohexane. 7

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4.2 Cis-Trans Isomerism in

Cycloalkanes

Cycloalkanes are less flexible than open-chain

alkanes

Much less conformational freedom in

cycloalkanes

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Because of their cyclic structure, cycloalkanes have 2 faces as

viewed edge-on

“top” face “bottom” face

- Therefore, isomerism is possible in substituted cycloalkanes

- There are two different 1,2-dimethyl-cyclopropane isomers

- Both isomers are stable compund, and neither can be

converted into the other without breaking chemical bonds

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Stereoisomerism or chemical isomers

Compounds which have their atoms connected in the

same order but differ in 3-D orientation of the atoms.

Butane and isobutan.1,2-dimethylcyclopropanes

Name the following substances, including the

cis-or trans-prefix:

Draw the structures of the following molecules:

a) trans-1-bromo-3-methylcyclohexane

b) Cis-1,2-dimethylcyclobutane

c) Trans-1-tert-butyl-2-ethylcyclohexane.

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4.3 Stability of Cycloalkanes:

Ring Strain Rings larger than 3 atoms are not flat

Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering

Larger rings have many more possible conformations than smaller rings and are more difficult to analyze Baeyer (1885): since carbon prefers to have bond angles of approximately 109°.

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Stability of Cycloalkanes: The

Baeyer Strain TheoryThe strain compound equal total energy of the compound subtract the energy of a

strain-free reference compound.

Cycloalkane strain energies, calculated by taking the difference between

cycloalkane heat of combustion pre CH2 and acyclic alkane heat of combustion

Pre CH2,and multiplying by the number of CH2 units in a ring.

Cyclohexane rings are strain –free.

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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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4.4 Conformations of Cycloalkanes

Cyclopropane 3-membered ring must have planar structure

Symmetrical with C–C–C bond angles of 60°

Requires that sp3 based bonds are bent (and weakened)

All C-H bonds are eclipsed

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Bent Bonds of Cyclopropane

In cyclopropane, the C-C bond is displaced outward from

internuclear axis

Cyclopropane bonds are weaker and more reactive than typical

alkane bonds.

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Cyclobutane

Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens

The total strain for cyclobutane 110 kJ/mol and 115kJ/mol for cyclopropane.

Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above

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Cyclopentane

Planar cyclopentane would have no angle strain but very high torsional strain, total strain energy of 26 kJ/mol

Actual conformations of cyclopentane are nonplanar, reducing torsional strain

Four carbon atoms are in a plane

The fifth carbon atom is above or below the plane –looks like an envelope

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4.5 Conformations of Cyclohexane

Substituted cyclohexanes occur widely in nature

The cyclohexane ring is free of angle strain and torsional strain

The conformation is has alternating atoms in a common plane and tetrahedral angles between all carbons

This is called a chair conformation

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How to Draw Cyclohexane

Twist-boat conformation cyclohexane

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4.6 Axial and Equatorial Bonds in

Cyclohexane The chair conformation

has two kinds of positions for substituents on the ring: axialpositions and equatorialpositions

Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorialhydrogens near the plane of the ring

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Drawing the Axial and Equatorial

Hydrogens

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Conformational Mobility of Cyclohexane

Chair conformations readily interconvert,

resulting in the exchange of axial and

equatorial positions by a ring-flip

Axial bromocyclohexane becaomes

equatorial bromocyclohexane after ring-flip.

Sins the different energy to chair-chair

interconversion is only about 45KJ/mol

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Draw two different chair conformation of

cyclohexanol (hydroxycyclohexanol).Identify each

position as axial or equatorial.

Draw the two different chair conformation of trans-

1,4-dimethylcyclohexane,and label all positions as

axial or equatorial.

Draw the two chair conformation of menthol, and

tell which is more stable.

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4.7 Conformations of

Monosubstituted Cyclohexanes Cyclohexane ring rapidly flips between chair

conformations at room temp.

Two conformations of monosubstituted cyclohexane aren’t equally stable.

The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol

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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

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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Relationship to Gauche Butane

Interactions Gauche butane is less stable than anti butane by 3.8 kJ/mol

because of steric interference between hydrogen atoms on the two methyl groups

The four-carbon fragment of axial methylcyclohexane and gauche butane have the same steric interaction

In general, equatorial positions give more stable isomer

1,3-Diaxial Interactions

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•For other monosubstitiuted

cyclhexanes Is more stable in

an equatorial position than in

an axial position

•It is Depends on the nature

and size of substituent.

•The values must be doubled

to arrive at the amount of

strain in a mono substitiuted

cyclohexane

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4.8 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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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

Conformation of cis-1tert-butyl-4-chlorocyclohexane

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These conformations aren’t of equal energy because an axial tert- butyl

substitiutent and an axial chloro subsitituent produce different amounts

of steric strain.

1,3-diaxial interaction between a hydrogen and tert- butyl group cost 11

kJ/mol, the interaction between a hydrogen and chlorine only 1kJ/mol .

The stable conformation with the chlorine axial and the tert-butyl

equatorial.

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4.9 Conformations of Polycyclic

Molecules Decalin consists of two cyclohexane rings joined to share two

carbon atoms (the bridgehead carbons, C1 and C6) and a common bond

Two isomeric forms of decalin: trans fused or cis fused

In cis-decalin hydrogen atoms at the bridgehead carbons are on the same face of the rings

In trans-decalin, the bridgehead hydrogens are on opposite faces

Both compounds can be represented using chair cyclohexane conformations

Flips and rotations do not interconvert cis and trans

Although steroids look complicated compared with cyclohexane or decalin.

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