B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
Cyclic aliphatic compounds are organic compounds in which the carbon atoms are joined together to form a ring or closed-chain structure. They are also called alicyclic compounds. Although these compounds contain cyclic structures, they do not possess the aromatic character associated with benzene and other aromatic compounds.
The simplest cyclic aliphatic compounds are cycloalkanes, such as cyclopropane, cyclobutane, cyclopentane and cyclohexane. Depending on the nature of the bonds present in the ring, cyclic compounds may be saturated or unsaturated.
The chemistry of cyclic aliphatic compounds is particularly important because the physical and chemical properties of a ring compound depend strongly on ring size, ring strain, molecular geometry and the presence of functional groups.
Cyclic aliphatic compounds can be classified in several ways according to the number of rings, type of atoms present in the ring and degree of unsaturation.
Homocyclic compounds contain only carbon atoms in the ring.
Examples: Cyclopropane, cyclobutane, cyclopentane and cyclohexane.
Heterocyclic compounds contain one or more atoms other than carbon within the ring. Common heteroatoms include nitrogen, oxygen and sulfur.
Examples include pyridine, furan and thiophene. However, the major focus of alicyclic chemistry is generally on carbon-containing cyclic compounds.
Saturated cyclic compounds contain only carbon-carbon single bonds. Cycloalkanes are the main examples.
Unsaturated cyclic compounds contain one or more double or triple bonds in the ring.
Examples include cyclopropene, cyclohexene and cyclohexadiene.
Compounds containing only one ring are called monocyclic compounds.
Compounds containing two rings are bicyclic compounds, while compounds containing more than two rings are called polycyclic compounds.
| Type | Characteristic | Example |
|---|---|---|
| Monocyclic | One ring | Cyclohexane |
| Bicyclic | Two rings | Decalin |
| Polycyclic | More than two rings | Adamantane |
The IUPAC names of simple cyclic aliphatic compounds are generally formed by adding the prefix cyclo- to the name of the corresponding open-chain hydrocarbon.
| Number of carbon atoms | Name | Formula |
|---|---|---|
| 3 | Cyclopropane | C3H6 |
| 4 | Cyclobutane | C4H8 |
| 5 | Cyclopentane | C5H10 |
| 6 | Cyclohexane | C6H12 |
| 7 | Cycloheptane | C7H14 |
| 8 | Cyclooctane | C8H16 |
When a substituent is attached to a cycloalkane ring, the ring is numbered to give the substituent the lowest possible locant.
For a monosubstituted cycloalkane, numbering is generally unnecessary because the substituted carbon is automatically carbon-1.
In disubstituted and polysubstituted compounds, numbering is carried out according to IUPAC rules so that the substituents receive the lowest possible set of locants.
A monocyclic saturated hydrocarbon contains one ring and therefore has two fewer hydrogen atoms than the corresponding open-chain alkane.
For example, when n = 6:
The ring itself accounts for one degree of unsaturation. An additional double bond or ring increases the degree of unsaturation.
Dihaloalkanes can undergo intramolecular Wurtz reaction in the presence of sodium metal and dry ether to form cycloalkanes.
The reaction is particularly useful for the preparation of small and medium-sized cycloalkanes.
Cycloalkenes can be converted into cycloalkanes by catalytic hydrogenation using catalysts such as nickel, platinum or palladium.
For example, cyclohexene gives cyclohexane on hydrogenation.
Cyclic ketones can be reduced to corresponding cycloalkanes by suitable reducing methods.
Electrolysis of the salts of suitable dicarboxylic acids can produce cyclic hydrocarbons through intramolecular coupling.
This method involves anodic decarboxylation followed by carbon-carbon bond formation.
Ring strain is the increase in energy of a cyclic molecule caused by deviations from the ideal bond angles and by unfavorable interactions between atoms or bonds.
Ring strain strongly influences the stability and chemical reactivity of cycloalkanes.
Carbon atoms in saturated hydrocarbons are approximately sp3-hybridized and prefer a bond angle of about 109.5°.
In small rings, the bond angles are forced to deviate significantly from this ideal value. This produces angle strain.
| Compound | Approximate ring angle | Strain |
|---|---|---|
| Cyclopropane | 60° | Very high |
| Cyclobutane | 90° | High |
| Cyclopentane | Near 108° | Low |
| Cyclohexane | Near 109.5° in chair form | Very low |
Torsional strain results from eclipsing interactions between bonds on neighboring carbon atoms.
Steric strain occurs when atoms or groups are forced too close to each other in space.
Cyclopropane is the simplest cycloalkane and contains three carbon atoms arranged in a three-membered ring.
The carbon atoms of cyclopropane are approximately arranged in a triangular structure. The C–C–C bond angle is approximately 60°, which is considerably smaller than the ideal tetrahedral angle of 109.5°.
Cyclopropane can undergo hydrogenation in the presence of a catalyst.
Halogen addition can also result in ring opening because of the considerable strain present in the three-membered ring.
Cyclobutane contains four carbon atoms in a four-membered ring and has the molecular formula C4H8.
A completely planar cyclobutane structure would contain significant eclipsing interactions. Therefore, cyclobutane adopts a slightly folded structure, which reduces torsional strain.
Cyclopentane has the molecular formula C5H10. The five carbon atoms form a five-membered ring.
The planar structure is not the most stable form because it would contain considerable torsional strain. Cyclopentane therefore adopts non-planar conformations.
One of the important conformations of cyclopentane is the envelope conformation, in which one carbon atom lies approximately out of the plane formed by the remaining atoms.
Other puckered forms can also occur because of rapid conformational changes.
Cyclohexane is one of the most important cycloalkanes and has the molecular formula C6H12.
It does not remain planar. Instead, it adopts several conformations, among which the chair conformation is the most stable because it minimizes angle and torsional strain.
In the chair form, the carbon-carbon bond angles are close to the tetrahedral angle of 109.5°, and neighboring C–H bonds are largely staggered.
Therefore, the chair conformation has very little angle and torsional strain.
The boat form is less stable than the chair form because it contains eclipsing interactions and additional steric interactions between the two flagpole hydrogens.
The twist-boat conformation is more stable than the exact boat form because twisting reduces some of the unfavorable interactions.
| Conformation | Relative stability | Main reason |
|---|---|---|
| Chair | Highest | Minimum angle and torsional strain |
| Twist-boat | Intermediate | Reduced eclipsing interactions compared with boat |
| Boat | Lower | Eclipsing and flagpole interactions |
In the chair conformation of cyclohexane, each carbon atom has one approximately vertical bond and one approximately outward bond. These are called axial and equatorial positions respectively.
Axial bonds are directed approximately parallel to the vertical axis of the ring and alternate up and down around the ring.
Equatorial bonds extend outward from the ring and are generally preferred by larger substituents because they experience less steric interaction.
Cyclohexane can undergo a conformational change called a ring flip. During the ring flip, axial positions become equatorial and equatorial positions become axial.
Substitution on cyclohexane changes the relative stability of its conformations.
A substituent generally prefers the equatorial position because an axial substituent experiences unfavorable 1,3-diaxial interactions.
An axial substituent on carbon-1 experiences steric interactions with axial hydrogen atoms or substituents located on carbon-3 and carbon-5. These are called 1,3-diaxial interactions.
Larger substituents show a stronger preference for the equatorial position.
Cyclic compounds can show geometrical isomerism because rotation around carbon-carbon bonds is restricted by the ring.
In a disubstituted cycloalkane, substituents are cis when they are located on the same side of the average plane of the ring.
Substituents are trans when they are located on opposite sides of the ring.
| Term | Meaning |
|---|---|
| Cis | Substituents on the same side of the ring |
| Trans | Substituents on opposite sides of the ring |
Cis-trans isomerism is an important stereochemical feature of substituted cyclic compounds.
Baeyer proposed that the stability of cycloalkanes is related to the deviation of the ring bond angle from the ideal tetrahedral angle of 109.5°.
According to the simple planar-ring model, cyclopropane has an angle of approximately 60°, cyclobutane approximately 90°, and cyclopentane approximately 108°.
The theory successfully explains the high strain of small rings but has limitations because it assumes that all rings are planar.
Larger rings can adopt non-planar conformations, allowing them to reduce torsional and angle strain.
In cyclopropane, the geometry does not permit the carbon atoms to form normal straight-line overlap of sp3 orbitals. The C–C bonds therefore have unusual bonding characteristics.
The carbon-carbon bonds of cyclopropane are often described using the concept of bent bonds. The electron density is not concentrated along the internuclear axis to the same extent as in a normal carbon-carbon sigma bond.
This unusual bonding contributes to the characteristic reactivity of small ring compounds.
Cycloalkanes generally undergo reactions similar to alkanes, especially substitution reactions. However, small-ring cycloalkanes can show additional ring-opening reactions because of their high ring strain.
Cycloalkanes undergo combustion in oxygen to produce carbon dioxide, water and heat.
Cycloalkanes can undergo free-radical substitution with halogens in the presence of light or heat.
Unsaturated cyclic compounds can undergo hydrogenation to produce saturated cyclic compounds.
Cyclopropane and cyclobutane can undergo ring-opening reactions with suitable reagents. The driving force is the reduction of ring strain.
For example, cyclopropane can react with hydrogen halides under appropriate conditions to give open-chain products.
Cyclic hydrocarbons containing one or more carbon-carbon double bonds are called cycloalkenes.
The simplest examples include cyclopropene, cyclobutene, cyclopentene and cyclohexene.
A monocyclic compound containing one double bond has two degrees of unsaturation and generally follows:
Bicyclic compounds contain two rings within the same molecule. The rings may share one carbon atom, two carbon atoms and a bond, or two bridgehead atoms connected through different paths.
Two rings are fused when they share two adjacent carbon atoms and the bond between them.
In bridged systems, two bridgehead atoms are connected by three different carbon pathways.
In spiro compounds, two rings share only one common atom.
Adamantane is a highly symmetrical polycyclic hydrocarbon with a rigid three-dimensional structure.
Its carbon skeleton resembles the framework of diamond, and the molecule possesses considerable structural stability.
Adamantane and its derivatives have applications in medicinal chemistry and materials-related research.
| Compound | Formula | Ring size | Important feature |
|---|---|---|---|
| Cyclopropane | C3H6 | 3 | Very high ring strain |
| Cyclobutane | C4H8 | 4 | High ring strain |
| Cyclopentane | C5H10 | 5 | Puckered conformations |
| Cyclohexane | C6H12 | 6 | Stable chair conformation |
The physical properties of cycloalkanes depend on molecular mass, ring size and molecular structure.
The stability of a cyclic compound is closely related to its ring strain. Small rings generally possess greater strain, while rings such as cyclohexane can adopt conformations that minimize strain.
Cyclohexane is particularly important because its chair conformation allows the carbon atoms to maintain nearly ideal tetrahedral bond angles while keeping most bonds staggered.
Cyclic aliphatic compounds are important in organic synthesis, pharmaceuticals, fuels, polymers and industrial chemistry.
Cyclic aliphatic compounds: Non-aromatic compounds containing carbon atoms arranged in a ring.
Cycloalkanes: Saturated cyclic hydrocarbons with general formula CnH2n.
Ring strain: Increased molecular energy caused by angle, torsional and steric effects.
Cyclopropane: Three-membered ring with very high angle strain.
Cyclobutane: Four-membered ring with significant ring strain.
Cyclopentane: Five-membered ring that adopts puckered conformations.
Cyclohexane: Six-membered ring that mainly adopts the stable chair conformation.
Axial position: Approximately vertical position in cyclohexane chair form.
Equatorial position: Position directed outward from the ring and generally preferred by bulky substituents.
Cis: Substituents on the same side of the ring.
Trans: Substituents on opposite sides of the ring.
Major concept: Ring strain and conformational stability are central to understanding the chemistry of cyclic aliphatic compounds.