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2.6 Cycloalkanes

• Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds)

• Simple cycloalkanes are rings of CH2 units, (CH2)n, or CnH2n

• Structure is shown as a regular polygon with the number of vertices equal to the number of C’s (a projection of the actual structure)

Nomenclature of Cycloalkanes

IUPAC naming rules are followed with the addition of the prefix cyclo-

With multiple substituents, use lowest numbering sequence

cyclopropane cyclobutane cyclohexane

1-ethyl-3-methylcyclohexane

not 1-ethyl-5-methylcyclohexane

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2.6 Cycloalkanes

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Complex Cycloalkanes

• Naturally occurring materials contain cycloalkane structures

• Examples:

– chrysanthemic acid (cyclopropane),

– prostaglandins (cyclopentane),

– steroids (cyclohexanes and cyclopentane)

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Complex Cycloalkanes

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Properties of Cycloalkanes

Melting points are affected by the shapes and the way that crystals pack so they do not change uniformly

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2.7 Naming Cycloalkanes• Count the number of carbon atoms in the ring and the number

in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane.

• For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible. When two substituents are present, we number the ring beginning with the substituent first in the alphabet.

• Wenn three or more substituents are present, we begin at the substituent that leads to the lowest set of locants

• Number the substituents and write the name

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1. Find the parent:

or butylcyclopropane

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Number the substituents & write the name:

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Examples:

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Problem 2.15: IUPAC names?

1

2

3

4 5

1-ethyl-2,3-dimethylcyclopentane

3-ethyl-4-methyl-1-cyclopropylcyclohexane

5-ethyl-3-cyclohexyl-4-methyloctane

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2.8 Cis-Trans Isomerism in Cycloalkanes

• Rotation about C-C bonds in cycloalkanes is limited by the ring structure

• Rings have two “faces” and substituents are labeled as to their relative facial positions

• There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side

(cis) of the ring and one with the methyls on opposite sides (trans)

Isomerization with Cycloalkanes

Isomers - molecules that are the same but yet different

C4H10Constitutional (structural) isomers - same formula but different connectivity

CH3CH3

CH3

CH3

Stereoisomers- same formula and connectivity but different spatial arrangement

cis -isomersame side

trans - isomeropposite sides

Cis/Trans Isomers

CH3

CH3

CH3

CH3

Cis-1,2-dimethylcyclohexane Trans-1,2-dimethylcyclohexane

Cis/Trans Isomers

Also written as:

Cis-1,2-dimethylcyclohexane Trans-1,2-dimethylcyclohexane

CH3

CH3

CH3

CH3

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2.8 Cis-Trans Isomerism in Cycloalkanes

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Stereoisomers

• Compounds with atoms connected in the same order but which differ in three-dimensional orientation, are stereoisomers

• The terms “cis” and “trans” should be used to specify stereoisomeric ring structures

• Recall that constitutional isomers have atoms connected in different order

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Stereoisomers

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Practice Prob. 2.4: Name?

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Problem 2.18: IUPAC Name?

Cis and trans isomerization of cycloalkanes is only for

disubstitued rings.Multi-substituted requires additional nomenclature

Examples of disubstituted:

cis-1-ethyl-3-methylcyclohexane trans-1,2-dimethylcyclopentane

Structure of Cycloalkanes

Ring strain accounts for the unstability in certain cycloalkanes

Remember - sp3 has 109.5° bond angle

60°90° 105°

Ring Strain is combination of eclipsing strain and bond-angle strain

QuickTime™ and a Animation decompressor are needed to see this picture.

Each cycloalkane adopts a conformation that tries to

alleviate ring strain as much as possible

cylcopropane

“banana” bonds that do not have end-to-end overlap of typical σ-bonds

Highly unstable ring-system that will “open” or react readily

Cyclobutane

Instead of flat conformation, a “puckered” conformation reduces eclipsing strain

QuickTime™ and a Animation decompressor are needed to see this picture.

Cyclopentane

An “envelope” conformation reduces eclipsing strain

Ring Conformation

• Cyclopentane puckers

Cyclohexane

The “chair” conformation eliminates all eclipsing strain and nearly all bond-angle strain

Ring Strain vs. Cycloalkane Size

Cycloalkane Ring Size

kcal/mol

Cyclohexane the most stable

Axial and Equatorial positions are not energetically equivalent

H3C

H

H

H

HCH

H

H

equatorial

axial

1,3-diaxial interaction (steric hinderance)

More Stable

The larger the group, the greater the 1,3-diaxial interaction

Cyclohexane prefers larger groups at equatorial positions

A ball-and-stick model of cyclopropane shows the ring strain when the bond angles are deformed from their normal 109.5° to 60°.

Ball-and-stick Model of Cyclopropane

       

Cyclopropane

Cyclobutane A ball-and-stick model of cyclobutane shows the ring strain when the bond angles are deformed from their normal 109.5° to approximately 90°.

Cyclopentane

The ball-and-stick model of cyclopentane reveals a nearly planar molecule with bond angles of 108°.

                         

                                               

       

Cyclohexane: Chair and Boat Conformations

The chair conformation of cyclohexane can be twisted into the boat conformation.

Cyclohexane: Chair and Boat Conformations

The chair conformation of cyclohexane can be twisted into the boat conformation. Further twisting results in a second chair conformation. By labeling one axial substituent with a white sphere you can see how the axial position of one chair becomes equatorial by twisting into the second chair conformation.

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Chapter 11 Organic Compounds: Alkanes

11.9

The Shape of Cycloalkanes

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Bond Angles

Ideal bond angle is 109º

• Cyclopropane - 60º

• Cyclobutane - 90º

• Strained bonds, unstable

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cumming

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Free Rotation

Free rotation is not possible in cycloalkanes.

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Chair and Boat

Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

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Geometric Isomers

Geometric isomers have restricted rotation around C-C

bonds and differ in 3-dimensional arrangement of atoms.

Named “cis-” and “trans– “

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Learning Check

Identify each of the following as cis- or trans-

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Learning Check

Draw and name all the isomers of dichlorolcyclobutane.

(There are 5.)

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Stereoisomers

Stereoisomers are compounds with the same structural

formula but different spatial arrangements of atoms

• Cannot superimpose.