Chap 2 Cyclic hydrocarbonmonocylic hydrocarbonpolycycli hydrocarbon cyclic hydrocarbon aromatic hydrocarbon alicyclic hydrocarbonsaturatedunsaturatedcycloalkenecycloalkynecycloalkane1Section 1 Alicyclic hydrocarbon. Classification and NomenclatureThese ring compounds are named by attaching the prefix “cyclo” to the name of the parent hydrocarbon. 2 Cycloalkane: The alicyclic hydrocarbons have the molecular formula CnH2n. These compounds share most chemical properties with the unsaturated alkenes.cyclopropanecyclobutanecyclohexanecyclopentane1-ethyl - 2-methylcylopentane3Cycloalkenes:cyclobutenecyclohexene5-ethyl-1,3-cyclohexadiene3-methylcyclohexene123456cyclopentadiene1,4-cyclohexadiene1234561234564. Structure of cycloalkane1. Thought of Adolph von Baeyer: The planar forms of ring compounds are particularly subject to torsional strain. In addition, rings contain varying amounts of angle strain , depending on the ring size. Any deviation of their internal angles from the ideal tetrahedral angle of 10928 will introduce angle strain. The further the angle from the tetrahedral, the more the angle strain, and the less stable the molecule. 51092810928A normal tetrahe-dron with a bond angle of 10928A small ring will constrict this an-gle to less than 10928 A large ring will expand this angle to more than 10928Ring compounds will have angle strain if angle are forced to significantly smaller or larger than the ideal 10928.6（10928-60 ）/2 = 24 24（10928-90 ）/2 = 9 44（10928-108 ）/2 = 0 44（10928-120 ）/2 = 5 16 （10928-128 34 ）/2 = 9 33（10928-135 ）/2 = 12 46angle of deflexion609010812012834135bond angle7strain ring compounds, in which angle is smaller than ideal 10928. The ring is less stable and easily opened in chemical reactions.cyclopropanecyclobutanestrain-free ring compounds, in which angle is near to ideal 10928. The ring is stable and not easily opened in chemical reactions.cyclopentanecyclohexane82. The thought of interorbital angle: In the cycloalkane molecule, the bonds between the carbons as “bent”. To cyclopropane, the internuclear angle in an equilateral triangle is 60 and cannot be otherwise. However, the interorbital angle is apparently quite a bit larger and is 10928. . 960( internuclear angle is 60 )( interrobital angle is 10928 )The internuclear angle (60) and the interrobital angle (10928) are different for cyclopropane.6010928.10Although the customary line drawings of simple cycloalkanes are geometrical polygons, the actual shape of these compounds in most cases is very different.111213Stability of cycloalkanes:14. Chemical properties of alicyclic hydrocarbon1) addition of hydrogenH2 , cat.CH3CH2CH3CH3CH2CH2CH3H2 , cat.high temp.CH3(CH2)3CH3CH3(CH2)4CH3152) react with halogensBr2BrCH2CH2CH2BrBr2BrCH2CH2CH2CH2BrBr2 , hBrBr2 , hBraddition:substitution:163) addition of hydrogen halidesHX1231231718. Stereoisomerism1. cis-trans isomerism 1,2-dimethylcyclopropane 1,3-dimethylcyclobutaneBoth methyl groups on the same side of the ring, called cis (顺顺); on the opposite sides,called trans (反反).H3CCH3cis-trans-19A planar structure for cyclohexane is clearly improbable. The bond angles would necessarily be 120, 10.5 larger than the ideal tetrahedral angle. Also, every carbon-carbon bond in such a structure would be eclipsed. The resulting angle and eclipsing strains would severely destabilize this structure. If two carbon atoms on opposite sides of the six-membered ring are lifted out of the plane of the ring, much of the angle strain can be eliminated. 2. Conformation (构象构象) of cyclohexane20Boat form12341456HHHHHHHH262111234456Chair formHHHHHHHH325622Boat formChair form23Boat formChair form24The boat structure still has two eclipsed bonds and severe steric crowding of two hydrogen atoms on the bow and stern of the boat. This steric crowding is often called steric hindrance.By twisting the boat conformation, the steric hindrance can be partially relieved, but the twist-boat conformer still retains some of the strains that characterize the boat conformer. 25Finally, by lifting one carbon above the ring plane and the other below the plane, a relatively strain-free chair conformer is formed. This is the predominant structure adopted by molecules of cyclohexane.26In a chair conformation of cyclohexane, the twelve hydrogens are not structurally equivalent. Six of them are located about the periphery of the carbon ring, and are termed equatorial(平伏键、横键平伏键、横键). The other six are oriented above and below the approximate plane of the ring (three in each location), and are termed axial (竖立键、竖键竖立键、竖键).272829Because axial bonds are parallel to each other, substituents larger than hydrogen generally suffer greater steric crowding when they are oriented axial rather than equatorial. Consequently, substituted cyclohexanes will preferentially adopt conformations in which large substituents assume equatorial orientation.3031axial bond: the set of six, straight up and down bonds in chair cyclohexane. Ring flip interconverts these bonds with the set of equatorial bonds. equatorial bond: the set of six, also “up and down”, but more or less in the plane of the ring in chair cyclo-hexane. These bonds are interconverted with the set of axial bonds through ring “flipping” of the chair.Summary:3233Substituted Cyclohexane CompoundsIn a sample of cyclohexane, the two identical chair conformers are present in equal concentration, and the hydrogens are all equivalent (50% equatorial & 50% axial) due to rapid interconversion of the conformers. 34When the cyclohexane ring bears a substituent, the two chair conformers are not the same. In one conformer the substituent is axial, in the other it is equatorial. Due to steric hindrance in the axial location, substituent groups prefer to be equatorial and that chair conformer predominates in the equilibrium.35Chair conformations are generally more stable than other possibilities. *Substituents on chair conformers prefer to occupy equatorial positions due to the increased steric hindrance of axial locations.*The following equations and formulas illustrate how the presence of two or more substituents on a cyclohexane ring perturbs the interconversion of the two chair conformers in ways that can be predicted.3637Conformational Structures of Disubstituted Cyclohexanes1,1-dimethylcyclohexane38In the case of 1,1-disubstituted cyclohexanes, one of the substituents must necessarily be axial, and the other equatorial, regardless of which chair conformer is considered. Since the substituents are the same in 1,1-dimethylcyclohexane, the two conformers are identical and present in equal concentration.391-t-butyl-1-methylcyclohexane40flipThis equatorial methyl becomes axialThis axial teriarybutyl becomes equatorial( less stable )( more stable )41In the cases of 1,2-, 1,3- and 1,4-disubstituted compounds the analysis is a bit more complex. It is always possible to have both groups equatorial, but whether this requires a cis-relationship or a trans-relationship depends on the relative location of the substituents. As we count around the ring from carbon #1 to #6, the uppermost bond on each carbon changes its orientation from equatorial (or axial) to axial (or equatorial) and back. It is important to remember that the bonds on a given side of a chair ring-conformation always alternate in this fashion.42ee bond, preponderant conformationaa bonda/ebond434445The molecule in which the large group on the e bond is stable. If there are some substitutents in the ring, the more groups on the e bond, the more stable the molecule.Summary:46StableInstableStableInstable47StableInstable48 Exercise page 31 2-11 2-13 49