cycloalkanes (5).pptx

Cycloalkanes

1. Shikshan Prasarak Mandal’s College of Pharmacy, Akluj Tal- Malshiras Dist- Solapur Department of Pharmaceutical Chemistry UNIT V Cycloalkanes Class : Second Year B. Pharmacy ( Sem III) Subject : Pharmaceutical Organic Chemistry II Prepared By : Mr. R. N. Jalkote

2. INTRODUCTION Cycloalkanes or cycloparaffins are saturated hydrocarbons in which the carbon atoms are joined by single covalent bonds to form ring. The prefix ali is added because of their similarity to aliphatic compounds. the unsubstituted Cycloalkanes form homologous series with the general formula CnH2n. The first member of the series is cylcopropane, C3h6.

3. NOMENCLATURE The IUPAC rules for naming Cycloalkanes are as follows. The name of an unsubstituted Cycloalkanes is obtained by attaching the prefix cyclo - to the name of the corresponding normal alkane having the same number of the carbon atoms in the ring. Substituents on the ring are named, and their corresponding positions are indicated by numbers. The ring is numbered so that the carbons bearing the substituents will have the lowest numbers.

4. SYMBOLS OF THE CYCLOALKANES For convenience and simplicity, Cycloalkanes are often represented by simple geometric figures: a triangle for cylcopropane, a square for cyclobutane, a pentagon for cyclopentane, a hexagon for cyclohexane and so on. It is understood that each corner represents one carbons and two hydrogens.

5. METHODS OF PREPARATION From dihalides: Terminal dihalides when treated with sodium or zinc form Cycloalkanes. This reaction is an extension of Wurtz reaction and is useful for the preparation of 3 to 6 membered rings.

6. 2. From calcium salt of carboxylic acid: When the calcium or barium salts of dicarboxylic acids are heated, cyclic ketones are formed. The cyclic ketones can be readily converted into the corrosponding cycloalkanes by clemmensen reduction.

7. 3. From esters of Dicarboxylic acids ( Dieckmann Reaction) Esters of Dicarboxylic acids when treated with sodium undergo intramolecular acetoacetic ester condensation and a beta ketoester is formed. The beta ketoester on hydrolysis give cyclic ketones. These on reduction yield the corresponding Cycloalkanes.

8. 4. From alkenes. (Simmons-Smith Reaction) When alkenes are treated with methylene iodide in the presence of a zinc copper couple, cylcopropane derivatives are formed.

9. 5. From aromatic hydrocarbons: Six membered Cycloalkanes can be prepared by the catalytic reduction of benzene and its derivatives.

10. PHYSICAL PROPERTIES Cylcopropane and cyclobutane are gases at room temperature; the remaining Cycloalkanes are liquids. Melting and boiling points of Cycloalkanes show a gradual increase with the increase in molecular weight. Cycloalkanes are insoluble in water but dissolve in ethanol and water. IR SPECTRUM: like alkanes, they show characteristic C-H streching absorption at 2850-3050 wave number. compound Boiling point (degree Celsius) Melting point (degree celsius ) Cylcopropane -32.8 -127 Cyclobutane 12.5 -90.7 cyclopentane 49.3 -13.9

11. CHEMICAL PROPERTIES: Cycloalkanes resemble alkanes in their chemical behavior. However, cylcopropane and cyclobutane are the exceptions. With certain reagents they undergo ring-opening and give addition products .

12. Substitution reactions Substitution with Cl2 and Br2: Cycloalkanes react with chlorine and bromine in the presence of UV light to give substitution product.

13. B. RING OPENING REACTIONS 2. Addition of Cl 2 and Br 2 . cyclopropane reacts with chlorine and bromine in the dark to form addition products. CCL 4 is used as a solvent. Cyclobutane and higher members do not give this reaction.

14. 3. ADDITION OF HBr AND Hi Cylcopropane reacts with concentrated HBr and Hi to yield 1- bromopropane and 1- iodopropane. Cyclobutane and higher members do not give this reaction.

15. 4. ADDITION OF HYDROGEN Cylcopropane and cyclobutane react with hydrogen in the presence of Hi catalyst to give propane and n-butane respectively. Notice that higher temperature is required for cyclobutane.

16. 5. OXIDATION Cycloalkanes undergo oxidation with hot alkaline potassium permanganate to form Dicarboxylic acid.

17. STABILITY OF CYCLOALKANES In 1885 Adolf Baeyer proposed a theory to explain the relative stability of the first few Cycloalkanes. He based his theory on the fact that the normal angle between any pair of bonds of a carbon atom is 109° 28´. Baeyer postulated that any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring.

18. He also assumed that all Cycloalkanes were planar and thus calculated the angles through which each of the valency bond was deflected from the normal direction in the formation of various ring. This he called as angle strain, which determined the stability of the ring.

19. The bond angle in cyclopropane is 60 °. That is why deviation= (normal tetrahedral bond angle)-9actual bond angle) Deviation = 109.5 °-60=49.5 ° The bond angle in cyclobutane is 90 ° The normal tetrahedral bond angle value is 109.5 °. That is why = (normal tetrahedral bond angle)-9actual bond angle) Deviation=109.5°-90= 19.5°

20. Deviation for cylcopropane is 49.5°. Deviation for cyclobutane is 19.5°. The deviation is higher for cylcopropane than cyclobutane therefore cylcopropane is more prone to undergo ring opening reactions. As a result of this, the strain is more in cylcopropane as compare to cyclobutane. it will make cylcopropane less stable than cyclobutane . So, cylcopropane easily undergo ring opening reaction as compare to the cyclobutane.

21. LIMITATION TO THE BAEYERS STRAIN THEORY Baeyer was not able to explain the effect of angle strain in higher Cycloalkanes. According to the Baeyer cyclopentane should be much stable than cyclohexane but practically it is reversed. Larger ring systems are not possible according to the Baeyer as they have negative strain but they exist and are much more stable. Larger ring systems are not planar but puckered to eliminate angle strain.

22. ANGLE STRAIN IN CYCLOALKANES Compound Bond angle Angle strain Cylcopropane 60 ° 24 °44´ Cyclobutane 90 ° 9 °44´ Cyclopentane 108 ° °44´ cyclohexane 120 ° -5 °16´

23. SACHE-MOHR THEORY In order to account for the stability of cyclohexane and higher members, sache and Mohr (1918) proposed that such rings can become free from strain if all the ring carbons are not forced into one plane, as was assumed by Baeyer. If the ring assumed a folded or puckered condition, the normal tetrahedral angles of 109° 28´ are retained and as a result, the strain within the ring is relieved. For example, cyclohexane can exist in two non-planar puckered conformations both of which are completely free from strain.

24. These conformations are called as Chair Form and the Boat Form because of their shape. Such non-planar strain-free rings in which the ring carbons can have normal tetrahedral angles are also possible for higher Cycloalkanes.

25. The chair form of cyclohexane is more stable than the boat form. Under ordinary conditions, cyclohexane molecules will mostly exist in the chair form. Examination of the chair form of cyclohexane reveals that the hydrogen atoms can be divided into two categories. Six of the bonds to hydrogen atoms point straight up or down almost perpendicular to the plane of the molecule. These are called Axial Hydrogens. The other six hydrogens lie slightly above or slightly below plane of the cyclohexane ring, and are called Equatorial Hydrogens .

26. Figure: axial and equatorial hydrogens in the chair form. Axial hydrogens are shown as H a. Equatorial hydrogens are shown as H e.

27. MOLECULAR ORBITAL THEORY Covalent bond between two atoms is formed by the overlap of orbitals of the atoms involved. The greater the extent of overlap the stronger is the bond formed. The atomic orbitals overlap to the maximum extent if they overlap along their axes. As the axes of sp³ orbitals are at angles of 109° 28´ to each other, the C-C bonds will have their maximum strength if the C-C-C bond angles have a value of 109° 28´.

28. Cylcopropane has C-C-C bond angles of 60 °. Cyclobutane has C-C-C bond angles have a value of 90 °. The higher the Cycloalkanes and alkanes have C-C-C bond angles of 109 °28´. The small bond angles of cylcopropane indicate the overlap of sp³ orbitals of carbon in alkanes. The bond angles of cylcopropane are less than the bond angles of cyclobutane, which in turn are less than the bond angles of higher Cycloalkanes of n-alkanes. Therefore, the overlap of orbitals in cylcopropane is less than cyclobutane.

29. Cyclobutane in turn less than that in higher Cycloalkanes or n-alkanes. Fig. overlap between sp³ orbitals in (a) propane (b) cylcopropane. Maximum overlap occurs in propane.

30. The overlap of sp³ orbitals of carbons in cyclopentane, higher Cycloalkanes or n-alkanes is maximum because inn these cases it is possible for the sp³ orbitals to overlap their axes, the bond angles being approximately equal to 109 °28´. This implies that C-C bonds in cylcopropane are weaker than the C-C bonds of cyclobutane, which in turn are weaker than the C-C bonds in higher Cycloalkanes and n-alkanes. Cylcopropane undergoes ring opening reactions very readily under drastic conditions cleavage of the cyclobutane ring takes place. Cyclopentane and higher Cycloalkanes does not undergo such reactions.

31. Figure: The C-C bonds in cylcopropane are weaker than the C-C bonds in propane. They are called as banana bonds .

32. CONFORMATION OF CYCLOHEXANE AND ITS DERIVATIVES The cyclohexane ring can assume many shapes. A single cyclohexane molecule is in a continuous state of flexing or flipping into different shapes or conformations. Some of the shapes are as below.

33. These conformations arise due to the rotation around carbon-carbon bonds. The chair conformation is the most stable while boat conformation is the least stable.

34. The above figure shows the energy requirements for the interconversion of the different conformations of the cyclohexane. Notice that the chair form has the lowest energy while half chair has the highest energy. At any given time we would expect most of the cyclohexane molecules to be in their chair form. Indeed it has been calculated that about 99.9 percent of cyclohexane are in their chair form.