cycloalkanes

Cycloalkanes

Many organic compounds contain cyclic or ring structures: carbohydrates nucleotides in DNA and RNA antibiotics

testosterone

OH

O

penicillin G

CH2 C NH

O NS

CO2H

O

Cycloalkanes Cycloalkanes:

alkanes that contain three or more carbons arranged in a ring

CnH2n

Cycloalkanes

Cycloalkanes are named using: prefix “cyclo” alkane base name

Examples: a cycloalkane with 5 carbons in the ring:

cyclopentane

a cycloalkane with 10 carbons in the ring:

cyclodecane

Naming Substituted Cycloalkanes

To name substituted cycloalkanes: Use the cycloalkane for the base name

Identify substituents using name and position

no number is needed if only one substituent is present

CHCH3

CH3

isopropylcyclohexane

Cycloalkanes

For 2 or more substituents, number the ring carbons to give the lowest possible numbers for the substituted carbons If numbering could begin with either

substituent, start with the one that is first alphabetically.

CH3

CH2CH

3

3

1

24

56

1-ethyl-3-methylcyclohexane

Cycloalkanes

When the acyclic portion of the molecule contains more carbons than the cycloalkane, the cyclic portion is named as a cycloalkyl substituent.

3-cyclopropyl-2,6-dimethylheptane

Cycloalkanes

Example: Name the following cycloalkanes.

CH3

CH2CH

3

CH3

CH3

CH2CH

3

CH3

Cl

Cycloalkanes

Example: Draw the following cycloalkanes.

sec-butylcyclooctane

1,1,3,3-tetramethylcyclohexane

Cycloalkane Conformations

Cycloalkanes containing 3 - 20 carbons have been synthesized. Rings with 5 or 6 carbons are the most

common WHY?

Recall that all alkanes contain C - C single bonds that are formed by the overlap of sp3 hybrid orbitals tetrahedral geometry ideal bond angle = 109.5o


In cycloalkanes the best overlap (strongest bond) between the sp3 hybrid orbitals will occur when bond angle = 109.5o

In some cycloalkanes, bond angles other than 109.5o lead to angle strain and less than optimum overlap of the sp3 hybrid orbitals

Angle strain: the strain associated with bond angles that are smaller or larger than the ideal value


In addition to angle strain, some cycloalkane conformations lead to significant amounts of torsional strain due to eclipsing of bonds.

Ring strain: the extra strain associated with the

ring structure of a compound compared to a similar acyclic compound

angle strain torsional strain


The heat of combustion (Hcomb) is often used to measure the ring strain of a cycloalkane. The amount of heat released when a

substance is burned in the presence of excess oxygen.

As Hcomb increases, a substance contains a greater amount of potential energy. As potential energy increases, the

compound becomes less stable.


Cyclopropane: greatest ring strain per CH2

60o bond angles weaker overlap of sp3 hybrid orbitals

all bonds eclipsed more reactive than other alkanes


Cyclobutane: second largest amount of ring strain

slightly folded conformation instead of planar and square

Square/planar conformation would have less angle strain but more torsional strain


Cyclobutane: Slightly folded conformation

88o bond angle not quite eclipsed

less torsional strain


Cyclopentane: Five membered rings are very important

biologically ribose and deoxyribose have cyclopentane like conformations

If cyclopentane existed as a planar, regular pentagon:

bond angle = 108o

low angle strain all bonds eclipsed

high torsional strain


Cyclopentane: Actual shape = slightly puckered

envelop reduces eclipsing and torsional strain

Molecule constantly “undulates” envelop flap moves around ring


Cyclohexane: Most common cycloalkane Carbohydrates, steroids, and some

pesticides contain cyclohexane-like conformations

If cyclohexane had a planar, regular hexagonal conformation:

120o bond angles high angle strain

adjacent methylene groups eclipsed high torsional strain


Cyclohexane has no ring strain. Cannot be a planar, regular hexagon

Cyclohexane achieves tetrahedral bond angles and staggered conformation by assuming puckered conformations: chair conformation

most stable boat conformation half-chair conformation

highest energy


Chair conformation most stable lowest energy 109.5o

staggered


• Repulsive force between two groups that are in close proximity on opposite ends of a ring

Boat conformation: 109.5o

torsional strain due to eclipsing of bonds

actually exists as the twist boat conformation in order to eliminate the flagpole effect


The twist boat conformation reduces flagpole effect reduces eclipsing of bonds lower in energy than the boat

conformation higher in energy than the chair

conformation

Cyclohexane exists predominantly in the chair conformation constantly interconverting from chair to

half-chair to boat conformations and back


Each carbon in a cyclohexane ring has two different types of carbon - hydrogen bonds: axial bond

a bond that is parallel to the axis of the ring

directed up and down

equitorial bond a bond that is directed along the “equator” of the ring

pointed out from the ring


Axial bonds

ee

e

ee

e

aa

aa

a

aaxis

ee

e

ee

e

axis

Equitorial bonds

ee

e

ee

e

aa

aa

a

a

aa

a

a

a

a

e

ee

e

e

e

Axial and equitorial bonds

Monosubstituted Cyclohexanes

Substituents on a cyclohexane ring can occupy either an axial position or an equitorial position. Chair-chair interconversions that occur

at room temperature lead to an equilibrium mixture of both conformations.

CH3

H

CH3

H

CH3

H

CH3

H

CH3

H

CH3

H

CH3

H

CH3

H

CH3

H

Axial methyl equitorial methyl


During chair-chair interconversions (ring flip), substituents change from: axial equitorial

Conformations with the substituent in the equitorial position are lower in energy (more stable) and therefore favored: “anti” arrangement no 1,3-diaxial interaction

Monosubstituted Cycloalkanes

1, 3-diaxial interactions steric hinderance between groups in axial positions on carbons 1 and 3

H

HH

H

H

H

HH

H

H

C

H

HH

C

H

H

H

H

HH

H

H

H

HH

H

H

C

H

HH

C

H

H

H

1, 3-diaxial interaction

More stable conformer


Example: Draw the two possible chair conformers of t-butylcyclohexane. Which one is more stable? Why?

Cis & Trans Isomers

Cycloalkanes have two distinct faces. Di-substituted cycloalkanes can exist as

cis and trans isomers.

Cis cycloalkane two identical groups on the same face

of the ring

cis-1,2-dimethylcyclopropane

CH3

CH3H

H

Cis and Trans Isomers

Trans cycloalkane two identical groups on opposite faces

of the ring

H

CH3

H

CH2CH

3

trans-1-ethyl-3-methylcyclobutane

H3C

HCH2CH3

H


Example: Name the following compound.

CH3

H

H

CHCH3

CH3


Drawing cis/trans isomers of disubstituted cyclohexanes:

Positions Cis Trans

1,2a,e or e,a a,a or e,e

1,3a,a or e,e a,e or e,a

1,4a,e or e,a a,a or e,e


Disubstituted cyclohexanes can also exist as different chair conformations: Some conformations are lower energy

More stable Preferred

Some conformations are higher energy Less stable Not preferred


Points to remember about relative stabilities: Greatest stability is found in disubstituted

conformers with both substituents in equitorial positions.

Disubstituted conformers with both substituents in the axial positions are very unfavorable.

If the isomer requires an a,e conformer, then the most stable conformer will have the largest group in the equitorial position.


Example: Draw both chair conformations of cis-1,3-diethylcyclohexane. Which one is more stable?


Example: Draw both possible conformations of trans-1-t-butyl-3-methylcyclohexane. Which one is more stable?

Bicyclic Molecules

Two or more rings can be joined into bicyclic or polycyclic molecules.

Fused rings share 2 adjacent carbons and the bond between them.

bicyclo[4.4.0]decane

Bicyclic Molecules

Bridged rings share two nonadjacent carbons and one or more carbon atoms between them.

bicyclo[3.2.1]octane

bridgeheadcarbons

bridging carbon

cycloalkanes

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