�

1.5 Cyclic Alkanes / Alkenes

1.5.1 StructureA hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon.

When all carbons of the ring are saturated (sp3 ), the hydrocarbon is called cycloalkane.

When a double bond (sp2 ), is part of the ring, the hydrocarbon is called cycloalkene.

�

1.5.2 Nomenclature

The system for naming members of this class is straightforward. Alkane/Alkene names are preceded by the prefix cyclo-

However, no numbering of the functional group is needed in a cyclic alkeneExamples

�

�

Some definitions:Angle strain: it is the strain induced in a molecule when the bond angles are different from the ideal tetrahedral bond angle of 109.5o.Torsional strain: it is caused by repulsion between the bonding electrons of one substituent and the bonding electrons of a nearby substituent.Steric strain: it is caused by atoms or groups of atoms approaching each other too closely.

�������������� ��������������������������

0Cyclohexane25.9Cyclopentane

110.9Cyclobutane 114.2Cyclopropane

Strain Energy (kJ/mol)Alkane

1.5.3 Ring Strain and the Structure of Cycloalkanes

�

Generally speaking, cyclic alkanes found in nature have five or six-membered rings. On the other hand, compounds with three and four-membered rings are found much less frequently. This observation suggested that alkanes with five- and six-membered rings must be more stable than those with three- or four-membered rings. It was proposed that such instability could be explained on the bases of angle strain. Ideally, an sp3 hybridized carbon has bond angles of 109.5. As a result, stability of a cycloalkane may be predicted by determining how close the bond angle of a planar cycloalkane is to 109.50. The angles of an equilateral triangle are 60o. Therefore, the bond angles in a planar cyclopropane are compressed from the ideal bond angle of 109.5o to 60o, a 49.5o deviation causing angle strain.

�

As described earlier, normal sigma bond between two carbon atoms are formed by the overlap of two sp3 orbitals that point directly at each other. In cyclopropane, overlapping orbitals cannot point directly at each other.

Therefore, the orbital overlap is less effective than in a normal C-C bond. Hence, the less effective orbital overlap causes the C-C bond to be weaker and could be easily broken i.e. reactive. For example, cyclopropane could be readily hydrogenated to propane.

H2

Cyclopropane Propane

Pt

�

Because the C-C bonding orbitals in Cyclopropane cannot point directly at each other, they have shapes that resemble bananas and, consequently, are often called banana bonds.

In addition to angle strain, three-membered rings have torsional strain as a result of the fact that all hydrogen atomsare eclipsed.

�

Similarly, the bond angles in planar cyclobutane would have to be compressed from 109.5o to 90o, the bond angle associated with a planar square. Planar cyclobutane would then be expected to have less angle strain than cyclopropane because the bond angles in cyclobutane areonly 19.5o away from the ideal angle.Considering angle strain as the only factor, it was predicted that cyclopentane be the most stable of cycloalkanes because its bond angles (108o) are closest to the ideal tetrahedral one.

In addition, it may be predicted that cyclohexane, with bond angles of 120o, would be less stable.

Because three points define a plane, the carbons of cyclopropane indeed lie in a plane as it cannot twist.

As a result cyclpropane is planar.

On the other hand, other cycloalkanes are not planar. They are capable of twisting and bend in order to attain a structure that minimizes the three different kind of strain (angle, torsional, and steric strains) that destabilize a cycliccompound.

Contrary to this prediction, it turned out that cyclohexane is more stable than the five-membered ring! Why?

The assumption that all cyclic molecules are planar is not accurate.

�

Although planar cyclobutane would have less angle strain than cyclopropane, it could have more torsional strain because it has eight pairs of eclipsed hydrogens, compared with the six of cyclopropane. Hence, cyclobutane is not planar molecule-it is bent molecule. Although this increases the angle strain never the less, the increase is more than compensated for by the decreased torsional strain.

110.9Cyclobutane

114.2CyclopropaneStrain Energy (kJ/mol)Alkane

Similarly, if cyclopentane were planar, it would have essentially no angle strain. In this case however, its 10 pairs of eclipsed hydrogens would be subject to considerable torsional strain. Consequently, cyclopentane puckers, allowing the hydrogens to become nearly staggered although in doing so it acquires some angle strain.

0Cyclohexane25.9Cyclopentane

Strain Energy (kJ/mol)

Alkane

��

Such form is called the envelope conformation as the shape resembles an envelope with the flap up.

In contrast to smaller rings, distortion from planarity in cyclohexane relieves both the angle and torsional strain of the planar structure. Once more, the internal angle in a planar hexagon is 120o, larger, not smaller, than the ideal sp3 angle. Deviation from planarity will decrease both this angle and torsional strain from the six pairs of eclipsed hydrogens in planar model.Remarkably, this relaxation produces a molecule in which essentially all of the torsional and angle strain is gone. This energy minimum cyclohexane is called the chair form. In the chair conformer of cyclohexane, all bond angles are 111o

and all the adjacent bonds are staggered.

��

DefinitionsEquatorial carbon-hydrogen bonds are parallel to the ring carbon-carbon bonds one bond away in chair cyclohexane.Axial carbon-hydrogen bonds are parallel and pointing either straight up or down in chair cyclohexane

��

Cyclohexane rapidly interconverts between two stable chair conformations because of the ease of rotation about its C-C bonds. Such process is called ring flip. When the chair conformers interconvert, bonds that equatorial in one chair conformer become axial in the other chair conformer and vice versa.

Definitions: Equatorial carbon-hydrogen bonds are parallel to the ring carbon-carbon bonds one bond away in chair cyclohexane. Axial carbon-hydrogen bonds are parallel and pointing either straight up or down in chair cyclohexane

��

Cyclohexane can also exist in a “boat conformation”

Similar to the chair conformer, the boat conformer is free of angle strain. However, the boat conformer is not as stable because some of its bonds are eclipsed, giving torsional strain to the molecule. In addition, the boat conformer is further destabilized by the close proximity of the “flagpole hydrogens” which causes steric strain.

��

��

H

HH

HH

HH

H

CH2

CH2

Newman projection of the chair conformer

In the boat form, the bonded atoms are inthe less stable eclipsed conformation,whereas in the chair form, they are staggered

1

4

1

4In the boat form, carbons 1, 4Are pulled toward each other,Causing steric interactions betweenThe “flagpole” hydrogens.In the chair form, these same carbons arebent away from each other, and thus arenot subject to mutual repulsion.

��

It should be noted that while cyclohexane interconverts from one chair conformer to the other, it can assume other conformations namely, half-chair and twist-boat. As expected, because the chair conformers are the most stable conformers, at any instant more molecules of cyclohexane are in chair conformations than in any other one.

Interesting to note that it has been calculated that, for every thousand molecules of cyclohexane in a chair conformation, no more than two molecules are in the next most stable conformations-the twist-boat. Cis trans in cycloalkanes

��

1.6 Functional groups1.6.1 DefinitionIt is the part of a molecule where most of its chemical reactions occur. Alkanes do not have a functional group. It is the part that effectively determines the compound’s chemical and most of its physical properties.

1.6.2 Most common functional groups

Aldehyde Ketone Carboxylic acidCarboxylic esterAlcohol

Ether Alkene Alkyne

R C

H

H

OHR C

H

O C OR

RC

O

OHR

R O R

R C O R

O

R NH2

R C NH2

OAmine

Amide

R C C RCH

HC

R

R