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cycloalkene cycloalkene Presentation Transcript

  • Organic Compounds:Cycloalkanes and theirStereochemistry
  •  We’ve discussed open-chained compounds up to this point Most organic compounds contain rings of carbon atomse.g.- Prostaglandins- Steroids
  • Why this chapter? Because cyclic molecules are commonlyencountered in all classes of biomolecules:- Proteins- Lipids- Carbohydrates- Nucleic acids
  • 4.1 Naming Cycloalkanes Cycloalkanes are saturated cyclic hydrocarbons Have the general formula (CnH2n)
  • Naming Cycloalkanes Find the parent. # of carbons in the ring. Number the substituents
  • 4.2 Cis-Trans Isomerism inCycloalkanes Cycloalkanes are less flexible than open-chainalkanes Much less conformational freedom incycloalkanes
  •  Because of their cyclicstructure, cycloalkaneshave 2 faces as viewededge-on“top” face “bottom” face- Therefore, isomerism ispossible in substitutedcycloalkanes- There are two different1,2-dimethyl-cyclopropaneisomers
  • Stereoisomerism Compounds which have their atomsconnected in the same order but differ in 3-Dorientation
  • 4.3 Stability of Cycloalkanes:Ring Strain Rings larger than 3 atoms are not flat Cyclic molecules can assume nonplanarconformations to minimize angle strain and torsionalstrain by ring-puckering Larger rings have many more possible conformationsthan smaller rings and are more difficult to analyze
  • Stability of Cycloalkanes: TheBaeyer Strain Theory Baeyer (1885): sincecarbon prefers to havebond angles ofapproximately 109°,ring sizes other thanfive and six may be toostrained to exist Rings from 3 to 30 C’sdo exist but arestrained due to bondbending distortions andsteric interactions
  • Summary: Types of Strain Angle strain - expansion or compression of bondangles away from most stable Torsional strain - eclipsing of bonds on neighboringatoms Steric strain - repulsive interactions betweennonbonded atoms in close proximity
  • 4.4 Conformations of CycloalkanesCyclopropane 3-membered ring must have planar structure Symmetrical with C–C–C bond angles of 60° Requires that sp3based bonds are bent (andweakened) All C-H bonds are eclipsed
  • Bent Bonds of Cyclopropane In cyclopropane, the C-C bond is displacedoutward from internuclear axis
  • Cyclobutane Cyclobutane has less angle strain than cyclopropanebut more torsional strain because of its larger numberof ring hydrogens Cyclobutane is slightly bent out of plane - one carbonatom is about 25° above The bend increases angle strain but decreasestorsional strain
  • Cyclopentane Planar cyclopentane would have no angle strain butvery high torsional strain Actual conformations of cyclopentane are nonplanar,reducing torsional strain Four carbon atoms are in a plane The fifth carbon atom is above or below the plane –looks like an envelope
  • 4.5 Conformations of Cyclohexane Substituted cyclohexanes occur widely in nature The cyclohexane ring is free of angle strain andtorsional strain The conformation is has alternating atoms in acommon plane and tetrahedral angles between allcarbons This is called a chair conformation
  • How to Draw Cyclohexane
  • 4.6 Axial and Equatorial Bonds inCyclohexane The chair conformationhas two kinds ofpositions for substituentson the ring: axialpositions and equatorialpositions Chair cyclohexane hassix axial hydrogensperpendicular to the ring(parallel to the ring axis)and six equatorialhydrogens near theplane of the ring
  • Axial and Equatorial Positions Each carbon atom in cyclohexane has oneaxial and one equatorial hydrogen Each face of the ring has three axial andthree equatorial hydrogens in an alternatingarrangement
  • Drawing the Axial and EquatorialHydrogens
  • Conformational Mobility of Cyclohexane Chair conformations readily interconvert,resulting in the exchange of axial andequatorial positions by a ring-flip
  • 4.7 Conformations ofMonosubstituted Cyclohexanes Cyclohexane ring rapidly flips between chairconformations at room temp. Two conformations of monosubstituted cyclohexanearen’t equally stable. The equatorial conformer of methyl cyclohexane ismore stable than the axial by 7.6 kJ/mol
  • 1,3-Diaxial Interactions Difference betweenaxial and equatorialconformers is due tosteric strain caused by1,3-diaxial interactions Hydrogen atoms of theaxial methyl group onC1 are too close to theaxial hydrogens threecarbons away on C3and C5, resulting in 7.6kJ/mol of steric strain
  • Relationship to Gauche ButaneInteractions Gauche butane is less stable than anti butane by 3.8 kJ/molbecause of steric interference between hydrogen atoms on thetwo methyl groups The four-carbon fragment of axial methylcyclohexane andgauche butane have the same steric interaction In general, equatorial positions give more stable isomer
  • 4.8 Conformational Analysis ofDisubstituted Cyclohexanes In disubstituted cyclohexanes the steric effects of both substituentsmust be taken into account in both conformations There are two isomers of 1,2-dimethylcyclohexane. cis and trans In the cis isomer, both methyl groups are on the same face of thering, and compound can exist in two chair conformations Consider the sum of all interactions In cis-1,2, both conformations are equal in energy
  • Trans-1,2-Dimethylcyclohexane Methyl groups are on opposite faces of the ring One trans conformation has both methyl groups equatorial and only agauche butane interaction between methyls (3.8 kJ/mol) and no 1,3-diaxial interactions The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions Steric strain of 4 × 3.8 kJ/mol = 15.2 kJ/mol makes the diaxialconformation 11.4 kJ/mol less favorable than the diequatorialconformation trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in thediequatorial conformation
  • 4.9 Conformations of PolycyclicMolecules Decalin consists of two cyclohexane rings joined to share twocarbon atoms (the bridgehead carbons, C1 and C6) and acommon bond Two isomeric forms of decalin: trans fused or cis fused In cis-decalin hydrogen atoms at the bridgehead carbons are onthe same face of the rings In trans-decalin, the bridgehead hydrogens are on oppositefaces Both compounds can be represented using chair cyclohexaneconformations Flips and rotations do not interconvert cis and trans