1. Cycloalkanes Synthesis and Properties
ORGANIC CHEMISTRY- 1
Cycloalkanes Synthesis and Properties By
Dr. Sulaiman Al Sulaimi
Assistant Professor
UNIVERSITY OF NIZWA

2. Cycloalkanes Cycloalkanes are aliphatic saturated hydrocarbons.
These are also commonly known as alicyclic compounds. Sometimes called carbocyclic compounds to differentiate them from heterocyclic compound. They have general Formula CnH2n represented by polygons in skeletal drawing.

3. Cis–Trans Isomerism in Cycloalkanes
Due to the cyclic structures, cycloalkanes have two faces “top” face and a “bottom” face. For example, in 1,2-dimethylcyclopropane isomers, Cis isomer when two methyl groups on the same face of the ring while in trans two methyl groups on opposite faces.
cis-1,2-Dimethylcyclopropane
trans-1,2-Dimethylcyclopropane

4. Cycloalkanes
An alkane that contains a ring of carbon atoms. Ring sizes from 3 carbons to 30 or higher are known.

5. Line structure: A shorthand way of drawing structures in which atoms aren’t shown; instead a carbon atom is understood to be at each corner and hydrogens are “understood”.

6. Drawing and Naming Cycloalkanes
Cycloalkanes are represented by polygons. A triangle represents cyclopropane, a square represents cyclobutane, a pentagon represents cyclopentane, and so on.

7. Additional Rules for Naming Cyclics
Step 1: Use the cycloalkane name as the parent chain.

9. Cycloalkanes with Side Groups
Step 2: Number the substituents starting at the group that has alphabetical priority, and proceed around the ring in the direction that gives the second substituent the lower possible number.

10. (or equal number)

12. Physical Properties
In general cycloalkanes have similar properties to alkanes in their behavior however the ring structures do influence their properties.
Pack more tightly in to a crystal lattice, hence have high Melting point and Boiling point than comparable n-Alkanes.

13. Stability of CycloAlkanes
Heat of combustion
It is the amount of heat evolved when one mole of the substance is completely burned to carbon dioxide and water.
Higher heat of combustion of a compound means its higher energy content and higher the energy of a compound, the less its stability, since different cycloalkanes cant be compared directly with each other thus they can be compared with corresponding alkanes.

14. Heat of Combustion (kJ/mol)
No of C atoms
Cycloalkanes
n-Alkanes
Difference 3
2091.0
1975.8
115.2 4
2744.0
2634.4
109.6 5
3320.0
3293.0
27.0 6
3952.0
3951.6
0.4 7
4634.7
4610.2
24.5 8
5310.3
5268.8
41.1

15. Beyer Strain Theory (Angle strain)
According to this theory (Adolf von Baeyer in 1885), any deviation from the normal bond angle cause strain, called angle strain in the molecule, and greater the deviation form the normal angle, the greater the strain and the less stability.
In cycloalkanes, since each carbon atom is sp3 hybridization, the C-C-C bond Angle should be tetrahedral (109.5o),

16. • Angle strain— the strain due to expansion or compression of bond angles
• Torsional strain —the strain due to eclipsing of bonds on neighboring atoms
• Steric strain —the strain due to repulsive interactions when atoms approach each other too closely.

17. Conformations of Cyclohexane
23KJ/mol higher Energy than
Chair.

18. Cyclic Alkanes

19. PREPRATION OF CYCLOALKANES
Dehalogenation of α, ω dihalides
When α, ω dihaloalkanes are treated with sodium or better with Zn dust in warm alcohol. The corresponding Cycloalkanes are formed. C H 2 - B r + N a
Cyclopentane

20. Catalytic Hydrogenation of Cyclic Alkenes
Cyclo alkanes may also be prepared conveniently by the catalytic hydrogenation of cycloalkenes.

21. Reduction of Cyclic Ketones
Cycloalkanes may also be readily formed by the Clemensen reduction of the corresponding cyclic ketones.
Wolff-Kishner Reduction of cyclohexanone under basic Conditions (NH2NH2,NaOH)also gives cyclohexane.

22. Diels-Alder Reaction
Cyclo addition reaction which takes place when a conjugated diene and a compound containing a double bond, called dienophile are heated together or heated in some solvent eg. Benzene. The product of the reaction usually a six membered ring system, is often called Adduct

23. Chemical Properties of Cycloalaknes
Halogenation
Cycloalkanes, reacts with halogens either on heating or in the presence of light to give halogenated Cycloalkanes, through a free radical mechanism.

24. Sulfonation
Cycloalkanes on treatment with oleum are converted to their sulfonic acid derivatives
Addition Reaction (Ring Opening)
Cyclo propane and cyclo butane undergo addition reaction e.g

25. It is the addition of CH2I2 in an alkene
Simmon Smith Reaction
It is the addition of CH2I2 in an alkene
in the presence of Zn-Cu couple, e.g
Chemical Properties of Cycloalaknes
Substitution Reactions
Cycloalkanes including cyclopropane and cyclobutane undergo substitution reaction in a similar way as the open chain alkanes

26. ii) Reaction with H-Br
Substituted cyclo alkanes give different products

27. Oxidation
Cyclo alkanes are converted to dicarboxylic acids with alkaline KMnO4
Hydration Cyclopropane reacts with conc. Sulfuric acid to give di-n-propyl sulfate which is hydrolyzed to n-propyl alcohol

28. iii) Hydrogenation