ALICYCLIC COMPOUNDS,B.Sc.II, ALIPHATIC.pptx

aliphatic cyclic compounds, alicyclic compounds, cyclic compounds, cycloalkanes, nomenclature, preparations and reaction, reactions of cycloalkanes, addition reactions of cyclopropane and cyclobutane, Baeyer's strain theory, angle strain, their heat of combustion and stabilities, Sachse and Mohr prediction, Pitzer's strain theory, torsional strain, cyclopropane, cyclobutane, cyclopentane, cyclohexane, chair form and boat form of cyclohexane, axial and equatorial hydrogen atoms,

1. CYCLIC ALIPHATIC COMPOUNDS CYCLOALKANES

2. Cyclic aliphatic compounds: Nomenclature, industrial source, preparation, reactions, reactivity of cyclopropane and cyclobutane by comparing with alkanes, stability of cycloalkanes–Baeyer’s strain theory, Sachse and Mohr prediction and Pitzer’s strain theory, factors affecting stability of conformations, conformational structure of cyclobutane, cyclopentane and cyclohexane, equatorial and axial bonds. 5 hrs SYLLABUS

3. CYCLOALKANES Cycloalkanes or cycloparaffins are saturated hydrocarbons in which the carbon atoms are joined by single covalent bonds to form a ring.They are also called alicyclic compounds. Chemists have divided all organic compounds into two broad classes: aliphatic(originated from Greek word ‘ aleiphar ’ meaning fat, as some of its important compounds are present in the fat.) compounds and aromatic(originated from Greek word aroma meaning sweet-smelling.) compounds.

4. The prefix ali -is added because they resemble with aliphatic compounds. The general formula of unsubstituted cycloalkanes is C n H 2n . The first member of the homologous series of cycloalkanes is cyclopropane, C 3 H 6 . The cyclic nature of some of cycloalkanes confers very special properties on them.

5. NOMENCLATURE OF ALIPHATIC COMPOUNDS The IUPAC rules for naming cycloalkanes are as follows: The name of an unsubstituted cycloalkanes is obtained by adding prefix, cyclo - before the name of normal alkanes having same no. of carbon atoms as in the ring.

6. EXAMPLES: Cyclopropane ( Common:Trimethylene ) Cyclobutane ( Tetramethylene ) A traingle A square A hexagon A pentagon Cyclopentane ( Pentamethylene ) Cyclohexane ( Hexamethylene ) ii. One or more substituents on the ring are named and their positions are indicated by numbers. The ring is numbered in such a way that carbon bearing substituents should be minimum.

7. EXAMPLES: CH CH 3 CH 3 Isopropyl cyclohexane Cl Chlorocyclohexane NO 2 CH 3 C 2 H 5 Nitrocyclohexane 1-Ethyl-3-methyl cyclohexane 1 2 3 Alphabetical rule O Cl CH 3 1 2 3 4 4-Chloro-4-methyl cyclohexanone CH 3 CH 3 Cyclopentanone 1, 1-Dimethyl cyclopentane

8. OH CH 3 1 2 2-Methyl cyclohexanol CH 3 Cl 1 2 1-Chloro-2-methyl cyclohexane Cl CH 3 1-Chloro-2-methyl cyclopentane 1 2 CH 3 C 2 H 5 Cl 4-Chloro-2-ethyl-1-methyl cyclohexane(not 1-chloro-3-ethyl-4-methyl cyclohexane) (sum of locants rule) CH 2 CH 2 CH 2 CH 2 CH 3 1-cyclobutyl pentane 1 2 3 4 5 iii. When a single ring system is attached at a single chain with a greater no. of carbon atoms or more than one ring system is attached to a single chain, then it is named as cycloalkyl alkanes. Examples: 1 3 2 1, 3-Dicyclohexylpropane

9. SOME MORE EXAMPLES: Cl CH 3 CH 3 1 2 3 4 1-Chloro-2,4-dimethyl cyclohexane Sum of locants rule OH C(CH 3 ) 3 1 2 3 3-(1,1-dimethyl ethyl) cyclohexanol

10. INDUSTRIAL SOURCES Ptroleum from certain areas(in particular California, US state) is rich in cycloalkanes known as naphthenes in petroleum industry. Among these are cyclohexane, methylcyclohexane , methylcyclopentane and 1,2-dimethylcyclopentane. By the elimination of hydrogen from aliphatic compounds yields aromatic compounds by the reforming or aromatization process. Similarly, addition of hydrogen to aromatic compounds yields cyclic aliphatic compounds, specifically cyclohexane derivatives.

11. An important example of this is the hydrogenation of benzene to yield pure cyclohexane. + 3H 2 Ni, 150-250 C 25 atm Cyclohexane. Benzene + 3H 2 Ni, 150-250 C 150 atm OH Cyclohexanol (aliphatic) Phenol (aromatic) (Hydrogenation of substituted benzene) From cyclohexanol , many other cyclic compounds containing a six- membered ring can be made.

12. PREPARATIONS OF CYCLOALKANES FROM DIHALIDES (BY COUPLING OF TWO ALKYL GROUPS THAT ARE PART OF THE SAME MOLECULE) i . The one of alkyl groups of the dihalide is converted into an organometallic compound. This particular method works only for preparation of cyclopropane. Example: CH 2 X CH 2 -ZnX CH 2 CH 2 X CH 2 -X ii. Terminal dihalides treated with sod. Or Zn form cycloalkanes. This reaction is an extension of Wurtz reaction & is useful for the preparation of 3- to 6-membered rings. CH 2 Zn, NaI -ZnX2 cyclopropane 1,3-Dihalopropane Example:

13. When the Ca or Ba salts of dicarboxylic acids are heated, cyclic ketones are formed. The cyclic ketones can be readily converted into the corresponding cycloalkanes by Clemmenssen reduction. Example 2. FROM CALCIUM SALTS OF DICARBOXYLIC ACIDS CH 2 CH 2 -COO - Ca 2+ ∆ Zn/Hg CH 2 CH 2 -COO - Conc.HCl -CaCO 3 Cyclopentanone Cyclopentane Calcium adipate CH 2 -Cl CH 2 + 2Na ∆ + 2NaCl CH 2 Cl Cyclopropane

14. 3. FROM ESTERS OF DICARBOXYLIC ACIDS ( Dieckmann reaction). Esters of dicarboxylic acids when treated with Na undergo intramolecular acetoacetic ester condensation and a β – ketoester is formed. β – ketoester on hydrolysis followed by decarboxylation give cyclic ketones which on Clemmenssen reduction give corresponding cycloalkanes. Example: CH 2 -CH 2 -CO OC 2 H 5 Na CH 2 CH 2 C=O CH 2 -CH H -C 2 H 5 OH CH 2 CH COOC 2 H 5 COOC 2 H 5 β -Ketoester Diethyl adipate α β

15. CH 2 -CH 2 ∆ CH 2 CH 2 CO C=O CH 2 -CH H 2 O/H+ CH 2 CH COOC 2 H 5 COOH β -Ketoester β - Ketoacid (Hydrolysis) ∆/ -CO 2 (Decarboxylation) Cyclopentanone Zn-Hg/ conc.HCl Cyclopentane (Reduction)

16. FROM ALKENES(Simmons Smith reaction, a cycloaddition reactions) The most important route to rings of many different sizes is through the important class of reactions called cycloadditions: reactions in which molecules are called together to form rings. When alkenes are treated with methylene iodide (CH 2 I 2 ) in presence of a zinc-copper couple, cyclopropane derivatives are formed. Examples: Zn-Cu couple CH 3 -CH=CH 2 + CH 2 I 2 CH 3 -CH-CH 2 Diiodomethane Ether(stirred) (Methylene iodide) CH 2 Methyl cyclopropane Zn-Cu couple CH 3 -CH=CH -CH 3 + CH 2 I 2 CH 3 -CH-CH – CH 3 Diiodomethane Ether(stirred) (Methylene iodide) CH 2 1,2-Dimethyl cyclopropane CH 2 I 2 + Zn(Cu) ICH 2 ZnI Carbene like species called a carbenoid The carbenoid then brings about the stereospecific addition of a CH 2 group directly to the double bond.

17. light CH 3 -CH=CH -CH 3 + CH 2 N 2 CH 3 -CH-CH – CH 3 2-Butene Diazomethane + N 2 CH 2 1,2-Dimethyl cyclopropane Methylene is formed by the photolysis of either diazomethane, CH 2 N 2 or ketene, CH 2 =C=O. CH 2 =N⁺=N̅ : CH 2 + N 2 Diazomethane Methylene (Very poisonous yellow gas) UV light CH 2 =C=O UV light or ∆ : CH 2 + CO Ketene Mthylene (Carbene)

18. CHEMICAL PROPERTIES OF CYCLOALKANES Clcloalkanes resemble alkanes in their chemical behavior. However, cyclopropane & cyclobutane are exceptions. With certain reagents, they undergo ring opening and give addition products. (A) SUBSTITUTION REACTIONS Like alkanes, cycloalkanes undergo chiefly free radical substitution. For example: substitution with Cl 2 & Br 2 . + Cl 2 UV light Cl Cyclopropane Chlorocyclopropane + Cl 2 UV light Cl + HCl + HCl Cyclohexane Chlorocyclohexane

19. Cyclopentane + Br 2 300 C + H Br Br Bromocyclopentane (B) RING OPENING REACTIONS (REACTIONS OF SMALL RING COMPOUNDS.CYCLOPROPANE & CYCLOBUTANE ) Besides the free-radical substitution reactions, cyclopropane and cyclobutane undergo certain reactions of a quite different type: addition. The addition reactions destroy the cyclopropane and cyclobutane ring system & yield open – chain products. EXAMPLES: Addition of Cl 2 & Br 2 . Cyclopropane reacts with chlorine and bromine in dark to form addition products. CCl4 is used as a solvent. Cyclopropane + Br 2 CCl 4 /Dark CH 2 – CH 2 – CH 2 Br Br 1,3-Dibromopropane

20. + Cl 2 FeCl 3 CH 2 – CH 2 – CH 2 Cl Cl 1,3-Dichloropropane NOTE: Cyclobutane & higher members do not give this reactions. Addition of HBr & HI. Cyclopropane reacts with conc. HBr & HI to yield 1-Bromopropane & 1-Iodopropane respectively. + HBr Conc. CH 2 – CH 2 – CH 2 - Br 1-Bromopropane NOTE: Cyclobutane & higher members donot give this reactions.

21. Addition of H 2 . Cyclopropane & cyclobutane reacts with hydrogen in presence of Ni catalyst to give propane & n-butane respectively. (NOTE: Higher temperature is required for cyclobutane) + H 2 Ni, 80 C CH 3 CH 2 CH3 Propane + H 2 Ni, 200 C CH 3 CH 2 CH 2 CH 3 n-Butane

22. Oxidation. Cycloalkanes undergo oxidation with hot alkaline KMnO 4 to form dicarboxylic acids Cyclohexane + 5[ O] KMnO 4 /OH̅ ∆ CH 2 CH 2 COOH CH 2 CH 2 COOH Adipic acid + H 2 O Ni, H 2 , 80 C Cl2, FeCl 3 Conc. H 2 SO 4 H 2 O CH 2 – CH 2 – CH 2 Cl Cl 1,3-Dichloropropane CH 3 CH 2 CH3 Propane CH 2 – CH 2 – CH 2 H OH n- Propyl alcohol

23. Cyclobutane doesnot undergo most of the ring opening reactions of cyclopropane; it is hydrogenated, but only under more vigorous conditions than those required for cyclopropane. Thus cyclobutane undergoes addition less readily than cyclopropane & with some exceptions, cyclopropane less readily than an alkene. The remarkable thing is that these cycloalkanes undergo addition at all.

24. BAEYER’S STRAIN THEORY (Stability of cycloalkanes) This theory was proposed by a German chemist Adolf von Baeyer of the University of Munich in 1885 to explain the relative stability of the first few cycloalkanes. His theory is based on the normal angle between any pair of bonds of a carbon atom is 109 28´, known as normal tetrahedral bond angle. The part of his theory that deals with the ring-opening tendencies of cyclopropane & cyclobutane is generally accepted today & other part of his theory is based on false assumptions and have been discarded. BAEYER’S ARGUMENTS (ASSUMPTIONS) ARE: 1). In general, the normal bond angle between any of bonds of a carbon atom is 109 28´, known as normal tetrahedral bond angle (of an SP 3 hybridized carbon atom). He postulated that any deviation of bond angles from the normal tetrahedral value will impose an internal strain on the ring.

25. Adolf von Baeyer of the University of Munich,Germany (1835-1917)

26. 2). He also assumed that all cycloalkanes are planar. He calculated angle between any pair of bonds of a carbon atom in various cycloalkanes assuming that all cycloalkanes are planar. He also calculated the angles through which each of the bond is deflected from the normal direction. This is called Angle Strain. Angle strain is the factor which determines the stability of the cycloalkanes. Formula to calculate angle strain( α ) is, α = ½ (109 28´ - bond angle of cycloalkane) Cycloalkanes Bond angle Angle strain, α Cyclopropane(n=3) 60 24 44´,1/2(109 28´-60 ) Cyclobutane(n=4) 90 9 44´,1/2(109 28´-90 ) Cycopentane (n=5) 108 44´,1/2(109 28´-108 ) Cyclohexane(n=6) 120 -5 16´,1/2(109 28´-120 ) Cycloheptane (n=7) 128 34´ -9 33´1/2(109 28´-128 34´) Cyclooctane (n=8) 135 -12 46´,1/2(109 28´-135 ) Cyclononane (n=9) 180 -35 16´,1/2(109 28´-180 )

27. The ring cyclopropane is a triangle with three angles of 60 & the ring of cyclobutane is a square with four angle of 90 . In these cycloalkanes, a pair of bonds does not assume the tetrahedral angle but is compressed to 60 or 90 to fit the geometry of the corresponding ring. Since in cyclopropane, the three carbon atoms occupy the corners of an equilateral triangle, cyclopropane has C – C – C bond angle of 60 . It shows that the normal tetrahedral angle of 109 28´ between the pair of bonds is compressed to 60 .Each of the two bonds in an equilateral triangle is compressed by 1/2(109 28´-60 )= 24 44´. This value represents the angle strain exhibited by the cycloalkane ring or deviation per bond from normal tetrahedral direction. 60 109 28´ 24 44´ 24 44´ (α) (α) The angle strain for other cycloalkanes can be calculated in the same way. Whether the angle strain( α ) is – or +, its magnitude indicates the extent of strain in the ring.

28. The value of angle strain is maximum in the case of cyclopropane. Thus according to the Baeyer strain theory, cyclopropane should be highly strained molecule & consequently most unstable. Thus it is expected for cyclopropane to open – up on slight effort & thus releasing the strain within it. This is actually so. It undergoes ring opening reactions with Br 2 , HBr & H 2 (Ni) to give open chain addition products. The value of angle strain in case of cyclobutane is less than that in case of cyclopropane. Thus it is expected that cyclobutane is more stable than cyclopropane. This is actually so. As expected, cyclobutane undergoes ring opening reactions but only under more drastic(vigorous) conditions.

29. The angle strain value is minimum in case of cyclopentane. Thus it is expected that cyclopentane should be under least strain and should be most stable. It is actually so. Cyclopentane doesnot undergo ring-opening reactions. The angle strain value in case of cyclohexane is higher than that in case of cyclopentane. The angle strain increases continuously with the increase of corbon atoms in the ring as shown in the table above (slide no.26). According to Baeyer strain theory, it can be precticted that cyclohexane and higher members should become increasingly unstable i.e., the order of instability of cycloalkanes higher than cyclopentane should be: cyclononane> cyclooctane >cycloheptane >cyclohexane. Hence Baeyer considered that rings smaller or larger than cyclopentane or cyclohexane are unstable.

30. But it is actually not so. Contrary to this prediction, cyclohexane and higher members are found to be quite stable as they do not give ring opening reactions but give substitution reactions. Thus they resemble open-chain alkanes in reactivity. Thus, Baeyer strain theory can satisfactorily explain the exceptional reactivity of cyclopropane, cyclobutane & cyclopentane only but not cyclohexane and higher members. In other words, this theory is valid for cyclopropane, cyclobutane & cyclopentane only but not valid for cyclohexane and higher members.

31. Sachse and Mohr prediction In order to explain the stability of cyclohexane and higher members, Sachse & Mohr(1918) has proposed that rings can become free from angle strain if all the carbon atoms in the ring are not forced into one plane as was assumed in Baeyer strain theory. If the geometry of cyclohexane and higher members are not planar & are assumed a ‘folded’ or ‘puckered’ structure, the normal tetrahedral angles of 109 28´ are retained. As a result, the molecules of cyclohexane & higher members get relieved from strain.

32. For example, cyclohexane can exist in two non-planar puckered conformations both of which are completely free from strain. These two puckered conformations are CHAIR FORM & BOAT FORM because of their shape as shown below: Chair form Boat form Such non-planar , ‘puckered’ rings are also possible for higher cycloalkanes in which ring carbons can have normal tetrahedral angles. The chair form of cyclohexane is more stable than boat form. In ordinary conditions, cyclohexane molecules mostly exist in chair form.

33. On examination of chair form, it is found that the hydrogen atoms can be of two categories. Six of the twelve C - H bonds point straight up or down generally perpendicular to the average plane of the ring. These six hydrogen atoms, by analogy with the earth, are called Axial Hydrogens. Rest six hydrogen atoms lie slightly above or slightly below the plane of the ring. These hydrogen atoms, again by analogy with the equator of the earth, are called Equatorial hydrogens as shown below: a a a a a a e e e e e e He=Equatorial hydrogens Ha=Axial hydrogens

34. RED= AXIAL Hydrogens BLUE=EQUATORIAL Hydrogens

35. PITZER’S STRAIN THEORY Any strain resulting from torsion is called Pitzer strain or eclipsing strain. In other words torsional strain is also called Pitzer strain.   Strain  caused by the close approach of atoms or  groups  separated by three  covalent bonds is called TORSIONAL STRAIN. In the  molecule  W-X-Y-Z, atoms W and Z may experience torsional  strain  in a particular  conformation  (such as an  eclipsed conformation ).

36. Let us see the eclipse conformation of 1,2-dichloroethane : Sawhorse projections : Newman projections : Eclipsed conformation More  strain Anti-staggered conformation Less  strain

37. When the chlorine atoms of 1,2-dichloro ethane  are aligned (an  eclipsed conformation ), the chlorine atoms experience torsional  strain . The  eclipsed  hydrogen atoms also experience torsional  strain  (but less than the chlorine atoms because hydrogen has a smaller  atomic radius  than chlorine). This torsional  strain  is relieved when carbon-carbon  bond rotation  changes the  molecule  into a  staggered conformation  (such as the  anti-staggered conformation  shown here).

38. STABILITIES OF CYCLOALKANES HEATS OF COMBUSTION AND RELATIVE STABILITIES OF CYCLOALKANES The heat of combustion is the quantity of heat evolved when one mole of a compound is completely burned to CO 2 and H 2 O. Let us see whether the heats of combustion of various cycloalkanes support or not support Baeyer’s proposal that rings smaller or larger than cyclopentane or cyclohexane are unstable. For open -chain alkanes, the heat of combustion per methylene group (-CH 2 - group ) is very close to 157.4Kcal/mol. Lists of heats of combustion measured for some of cycloalkanes are given below:

39. Ring size Heat of combustion per CH 2 , Kcal/mol. Ring size Heat of combustion per CH 2 , Kcal/mol. 3 166.6 10 158.6 4 164.0 11 158.4 5 158.7 12 157.6 6 157.4 (Most stable) 13 157.8 7 158.3 14 157.4 8 158.6 15 157.5 9 158.8 17 157.2 Open-chain 157.4 TABLE:

40. We notice that the heat of combustion per –CH 2 - group in cyclpropane is 9 kcal and in cyclobutane 7 Kcal higher than the open-chain value of 157.4. Whatever the compound in which it occurs, a –CH 2 - group produces the same products on complete combustion: CO 2 & H 2 O. -CH 2 - + 3/2 O 2 CO 2 + H 2 O + heat The value of heat of combustion per –CH 2 - should be the same whatever the compound in which it occurs. But it is not so, as shown in table above. The values of heat of combustion per –CH 2 - evolved by cyclopropane and cyclobutane are higher than open chain compound, it means that they contain more energy per –CH 2 -group.

41. The values of heat of combustion per –CH 2 - evolved by cyclopropane and cyclobutane are higher than open chain compound, it means that they contain more energy per –CH 2 -group. This result is in agreement with Baeyer’s strain theory that cyclopropane & cyclobutane are less stable than open chain compounds. It is reasonable to suppose that the tendency to undergo ring – opening reactions for these cycloalkanes is related to this instability that is, the ring opening reactions of cyclopropane and cyclobutane occur due to less stability than open chain compounds. According to Baeyer’s strain theory, rings larger than cyclopentane & cyclohexane also should be unstable due to higher angle strain & hence also should give high heats of combustion. This instability should increase steadily with increase of size of ring as angle strain increases in same order.

42. But, the heats of combustion of rings higher than 4-membered are not found to increase as expected on the basis of Baeyer’s strain theory and not deviate much from the open-chain value of 157.4. But, the one of the biggest deviation occurs in Baeyer’s ‘most stable’ compound, cyclopentane: 1.3 Kcal per –CH 2 - group, 5×1.3= 6.5 Kcal per molecule The seven member to eleven member rings have about the same value as cyclopentane. The rings containing twelve or more carbon atoms have indistinguishable heat of combustion from open chain value.

43. Contrary to Baeyer’s theory, none of these rings is appreciably less stable than open chain compounds and larger ones are completely free of strain. Furthermore, once they have been synthesized, these large ring cycloalkanes have little tendency to undergo ring-opening reactions characteristic of cyclopropane and cyclobutane. Then what is wrong with Baeyer’s theory that it does not apply to the rings larger than four members? Answer. Due to false assumption that all rings of cycloalkanes are planar or flat. Actually rings larger than four member are not planar but they are puckered so that they are free from angle strain.

44. A three- membered ring must be planar since ring of three point difines a plane. A four- membered ring need not be planar, but puckering of the ring would cause more angle strain. A five – membered also need not be planar, but puckering of the ring would increase angle strain. All rings larger than these (3-, 4- and 5-membered) are puckered. If large rings are stable, why are they difficult to synthesize? Answer – if a compound can not be synthesized, it does not necessarily mean that the compound is unstable. The synthesis of stable compound may be difficult due to other factors.

45. The closing of ring occurs only when the two ends of the chain come close enough to each other for the formation of bond. This opportunity to come two ends close enough to each other decreases with increase the chain length. Under these conditions the end of one chain is more likely to encounter the end of another chain to form an entire different product. C H 2 Y C H 2 Y C H 2 – CH 2 Fig. Ring closure

46. C H 2 Y C H 2 Y C H 2 Y C H 2 Y C H 2 Y C H 2 Y C H 2 - CH 2 C H 2 -CH 2 C H 2 Y C H 2 Y Fig. chain lengthening

47. This fact is taken into consideration to make large rings. Reactions are carried out in very dilute solutions so that collisions between two different chains are unlikely and ring- closing reaction, although slow, in principle is more favorable. Five- and six- membered rings are most commonly encountered in organic synthesis, because they are large enough to be free of angle strain and small enough that ring closure is likely.

48. FACTORS AFFECTING STABILITY OF CONFORMATIONS Any atom likes to have bond angles according to bonding orbitals : tetrahedral(109 28’) for sp 3 -hybridized carbon, for example. Thus any deviations from the staggered arrangement are accompanied by TORSIONAL STRAIN . Thus any deviations from the “normal” bond angles as required by the geometry of bonding orbitals are accompanied by ANGLE STRAIN . Any two of tetrahedral carbon atoms attached to each other like to have their bonds staggered.

49. Any atoms or groups that are not bonded to each other can interact in several ways, depending on their size, polarity and their closeness . These non-bonded interactions can be either repulsive or attractive, and as a result destabilization or stabilization of the conformation occurs. Any two non-bonded atoms or groups if just touch each other or in other words, that are about as far as the sum of their van der Waals radii- attract each other. Thus, if these two non-bonded atoms or groups come any closer than van der Waals radii, they repel each other. Such crowding together is accompanied by van der Waals strain( STERIC STRAIN )

50. Non-bonded atoms (or groups) like to take positions which give most favorable dipole-dipole interactions: that is, positions that minimize dipole-dipole repulsions or maximize dipole –dipole attractions.(A particular powerful attraction occurs due to the special kind of dipole-dipole interaction called the hydrogen bond.) The net stability of a conformation is determined with the help of all these factors(mentioned above), working together or opposing each other.

51. Energy contents of all possible combinations of bond angle, angles of rotation, and even bond lengths are calculated and the combination which gives the lowest energy content is considered. Such calculations have become quite feasible through the use of computers. Not only calculations but also experimental measurements show that the final result is a compromise, and that few molecules have the idealized conformations that we assign them and, for convenience, usually work with. For example, there is no tetrahedral carbon compound-except with four same substituents- has exactly tetrahedral bond angles: a molecule acquires a certain amount of angle strain to relieve van der Waals strain or dipole – dipole interaction.

52. In the Gauche conformer of n-butane, the dihedral angle between methyl groups is not 60 , but larger than 60 : in Gauche conformations, the two methyl groups are closer than the sum of their van der Waals radii causing steric strain, so the molecule accepts some torsional strain to relieve van der Waals (steric) strain between two methyl groups.

53. Remaining of syllabus: conformational structure of cyclobutane, cyclopentane and cyclohexane, equatorial and axial bonds. CONFORMATIONS OF CYCLOALKANES 1. CYCLOHEXANE The cyclohexane is the most important of the cycloalkanes. Let us examine different conformations of cyclohexane. The six- membered rings are, in general, more stable than five- membered ones as evidenced by its “normal” heat of combustion (i.e., 157.4Kcal/mol per CH 2 ).

54. There is considerable evidence that the “chair” form is the most stable conformation of cyclohexane. In this non-planar chair form, the carbon-carbon bond angles are all 109 28’ and therefore free of angle. The chair conformation is also free from of torsional strain. In chair conformation, the atoms are seen to be perfectly staggered(as shown in fig. below): Fig. A Newman projection of the chair conformation of cyclohexane

55. Moreover, the hydrogen atoms at opposite carbon atoms of the ring of cyclohexane are maximally separated. Fig. Illustration of large separation between hydrogen atoms at opposite corners of the ring (designed C3 & C6 ) when the ring is in the chair conformation. 3 6

56. The chair conformation is thus not only free of angle strain but also free of torsional strain. It lies at an energy minimum and is therefore a conformational isomer. The chair form is the most stable conformation of cyclohexane, and, indeed, of nearly every derivative of cyclohexane. By partial rotations about the carbon-carbon bonds of the ring (or by flipping the “right” end of the molecule up ) , the chair conformation gives another shape called the “boat” conformation. The boat conformation, like chair conformation, is free of angle strain.

57. In addition, two of the hydrogen atoms on C3 & C6 are close enough to each other to cause van der Waals repulsion. This effect is called the “flagpole” interaction of the boat conformation (as shown in fig. below): But the boat conformation, unlike chair conformation, is not free of torsional strain because in boat conformation, the atoms are seen to be eclipsed. This effect is called the “flagpole” interaction of the boat conformation (as shown in fig. below):

58. Torsional strain & flagpole interactions cause the boat to increase energy than the chair conformation. It has been calculated that the boat conformation is less stable by 7.1Kcal/mol than the chair conformation. The boat conformation is not a conformer but a transition state because it is believed that it does not lie at an energy minimum, but at an energy maximum. Now, what are those two conformers that lie on either side of the boat conformation?

59. Although the chair conformation is more stable, it is much more rigid than the boat form. In other words, the boat conformation is quite flexible. By flexing the boat conformation, a new conformation is obtained, known as the “twist” conformation. By flexing the boat conformation, it can relieve some of its torsional strain & at the same time, reduce the flagpole interactions as well. Thus, the twist conformation is more stable than the boat conformation i.e., the twist conformation has a lower energy than the boat conformation.

60. But, the twist conformation is, still, less stable than the chair conformation since the stability gained by flexing is insufficient. The twist boat conformation is also a conformer as it lies at an energy minimum. Between the two different conformers, chair and twist boat, there is another transition state known as half-chair conformation which , with angle strain & torsional strain, lies about 11Kcal above the chair form.

61. In the half –chair conformation, the carbon atoms of one end of the ring have become coplanar. The overall relationship are summerized in fig. below:

62. 11Kcal 1.6Kcal 5.5Kcal

63. As energy barriers between the chair, boat and twist conformations of cyclohexane are low, it is not possible to separate them at room temperature. At room temperature, the molecules have enough energies to cause approx, 1 million interconversions to occur per second. And more than 99% of the molecules are estimated to be in a chair conformation at any given moment due to its greater stability. At equilibrium between the chair and twist-boat forms, the more stable chair form is favored by the ratio 10,000:1 at room temperature.(from M&B)

64. Conformationally , the chair form of cyclohexane is the perfect specimen of a cycloalkane whereas the planar cyclopentane must be the the poorest. In planar cyclopentane, there is exact bond eclipsing between every pair of carbons, as a result there is much torsional strain and less angle strain. To partially relieve this torsional strain, cyclopentane takes on a slight puckered conformation, even at the cost of a little angle strain. The internal angle of a regular pentagon are 108 , a value very close to the normal tetrahedral bond angle of 109 28’. Therefore, if cyclopentane molecules were planar, they would have very little angle strain. 2. CYCLOPENTANE

65. But, planarity would cause considerable torsional strain because all ten hydrogen atoms would be eclipsed. Consequently, cyclopentane assumes a slight bent conformation in which one or two of the atoms of the ring are out of the plane of the others. Hence, with little torsional strain and angle strain, cyclopentane is almost as stable as cyclohexane.

66. 2. CYCLOBUTANE In planar cyclobutane, there is exact bond eclipsing between every pair of carbons, as a result there is much torsional strain and less angle strain. To partially relieve this torsional strain, cyclobutane takes on a slight puckered(or folded) conformation, even at the cost of a little angle strain. The internal angle of a square are 90 , thus planar cyclobutane has angle strain, lower than cyclopropane but higher than cyclopentane. In non-planar cyclobutane, the internal angles are 88 -deviation by more than 21 from the normal bond angle.

67. Fig. The “folded” or “bent” conformation of cyclobutane

68. Now, the question one may ask is: What is the stable conformation of a cyclohexane derivative in which one hydrogen atom has been replaced by an alkyl substituent? Let us consider methyl cyclohexane as an example of cycloalkane. Since the methyl group is the largest substituent on the ring, and hence subject to crowding, we must focus our attention on methyl in estimating stabilities of various conformations of the methyl cyclohexane. Methyl cyclohexane has two possible chair conformations (as shown below) . These two conformations are interconvertible the partial rotations that constitute a ring flip.

69. Fig. conformations of methyl cyclohexane

70. Fig. 1,3-diaxial interaction between the two axial hydrogen atoms & the axial methyl group in the axial conformation of methyl cyclohexane are shown with dashed arrows. Less crowing occurs in the equatorial conformation. Axial –CH 3 more crowded than equatorial –CH 3 1 2 3 5 4

71. Two conformations show that when equatorial –CH 3 is axial, it is so close to the two axial hydrogen atoms on the same side of the molecule that the van der Waals forces between them are repulsive. This type of steric strain, because it arises from an interaction between axial groups on carbon atoms 1 and 3 (or 5) is called a 1,3-diaxial interaction.

72. Studies indicate that the conformation with the methyl group equatorial is more stable than the conformation with methyl group axial by about 1.8 Kcal mol -1 . Thus in the equilibrium mixture, the conformation with the methyl group in equatorial position is the predominant one. Calculations show that it constitute about 95% of the equilibrium mixture at room temperature. Similar studies with other substituents show that there is generally less repulsive interaction when the groups are equatorial rather than axial.

73. As we know that conformation with the methyl group equatorial is more stable than the conformation with methyl group axial by about 1.8 Kcal mol -1 , this value is due to the 1,3-diaxial interaction of one -CH 3 group and two hydrogen atoms. On this basis, we assign a value of 0.9 Kcal /mol. to each 1,3-diaxial methyl-hydrogen interaction. Consequently, we can account well for the energy differences between conformations of a variety of cyclohexanes containing more than one methyl groups.

74. (Both calculations and experimental measurements show that the final result is a compromise, and few molecules have the idealzed conformations that we assign them and for convenience, usually work with. For example, probably no tetravalent carbon compound-except one with four identical substituents-has exactly tetrahedral bond angles: a molecule accepts a certain amount of angle strain to relieve van der Waals strain or dipole-dipole interaction. In the gauche conformer of n-butane, the dihedral angle between the methyl group is not 60 , but almost certainly larger: the molecule accepts some torsional strain to ease van der Waals strain (steric strain) between the methyl groups:)

75. CH 3 CH 3 H H H H Anti-conformation CH 3 CH 3 H H H H H H H H CH 3 CH 3 Gauche confor - mations (I) (II) (III) Staggered conformations of butane: