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4. Organic Compounds: Cycloalkanes and Their Stereochemistry

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4. Organic Compounds: Cycloalkanes and Their Stereochemistry 4. Organic Compounds: Cycloalkanes and their Stereochemistry Fall, 2014 by Dr.Isam Al Naser Based on McMurry’s Organic Chemistry, 7th edition Why this chapter? Because cyclic molecules are commonly encountered in all classes of bimoleculars: - Proteins.,Lipids.,Carbohydrates.,Nucleic acids… 2 4.1 Naming Cycloalkanes Cycloalkanes are saturated cyclic hydrocarbons Have the general formula (CnH2n) 3 Naming Cycloalkanes Find the parent. of carbons in the ring. Number the substituents 4 5 6 Give IUPAC names for the following cycloalkanes? Draw structures corrsponding to the following IUPAC names. a)1,1-dimethylcyclooctane b) 3-cyclobutylhexane c) 1,2-dichlorocyclopentane c)1,3-dibromo-5- methylcyclohexane. 7 4.2 Cis-Trans Isomerism in Cycloalkanes Cycloalkanes are less flexible than open-chain alkanes Much less conformational freedom in cycloalkanes 8 Because of their cyclic structure, cycloalkanes have 2 faces as viewed edge-on “top” face “bottom” face - Therefore, isomerism is possible in substituted cycloalkanes - There are two different 1,2-dimethyl-cyclopropane isomers - Both isomers are stable compund, and neither can be converted into the other without breaking chemical bonds 9 Stereoisomerism or chemical isomers Compounds which have their atoms connected in the same order but differ in 3-D orientation of the atoms. Butane and isobutan.1,2-dimethylcyclopropanes 10 Name the following substances, including the cis-or trans-prefix: Draw the structures of the following molecules: a) trans-1-bromo-3-methylcyclohexane b) Cis-1,2-dimethylcyclobutane c) Trans-1-tert-butyl-2-ethylcyclohexane. 11 4.3 Stability of Cycloalkanes: Ring Strain Rings larger than 3 atoms are not flat Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering Larger rings have many more possible conformations than smaller rings and are more difficult to analyze Baeyer (1885): since carbon prefers to have bond angles of approximately 109°. 12 Stability of Cycloalkanes: The Baeyer Strain Theory The strain compound equal total energy of the compound subtract the energy of a strain-free reference compound. Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion pre CH2 and acyclic alkane heat of combustion Pre CH2,and multiplying by the number of CH2 units in a ring. Cyclohexane rings are strain –free. 13 Summary: Types of Strain Angle strain - expansion or compression of bond angles away from most stable Torsional strain - eclipsing of bonds on neighboring atoms Steric strain - repulsive interactions between nonbonded atoms in close proximity 14 4.4 Conformations of Cycloalkanes Cyclopropane 3-membered ring must have planar structure Symmetrical with C–C–C bond angles of 60° Requires that sp3 based bonds are bent (and weakened) All C-H bonds are eclipsed 15 Bent Bonds of Cyclopropane In cyclopropane, the C-C bond is displaced outward from internuclear axis Cyclopropane bonds are weaker and more reactive than typical alkane bonds. 16 Cyclobutane Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens The total strain for cyclobutane 110 kJ/mol and 115kJ/mol for cyclopropane. Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above 17 Cyclopentane Planar cyclopentane would have no angle strain but very high torsional strain, total strain energy of 26 kJ/mol 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 18 4.5 Conformations of Cyclohexane Substituted cyclohexanes occur widely in nature The cyclohexane ring is free of angle strain and torsional strain The conformation is has alternating atoms in a common plane and tetrahedral angles between all carbons This is called a chair conformation 19 How to Draw Cyclohexane 20 Twist-boat conformation cyclohexane 21 4.6 Axial and Equatorial Bonds in Cyclohexane The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring 22 Drawing the Axial and Equatorial Hydrogens 23 Conformational Mobility of Cyclohexane Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip 24 Axial bromocyclohexane becaomes equatorial bromocyclohexane after ring-flip. Sins the different energy to chair-chair interconversion is only about 45KJ/mol 25 Draw two different chair conformation of cyclohexanol (hydroxycyclohexanol).Identify each position as axial or equatorial. Draw the two different chair conformation of trans- 1,4-dimethylcyclohexane,and label all positions as axial or equatorial. Draw the two chair conformation of menthol, and tell which is more stable. 26 4.7 Conformations of Monosubstituted Cyclohexanes Cyclohexane ring rapidly flips between chair conformations at room temp. Two conformations of monosubstituted cyclohexane aren’t equally stable. The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol 27 1,3-Diaxial Interactions Difference between axial and equatorial conformers is due to steric strain caused by 1,3-diaxial interactions Hydrogen atoms of the axial methyl group on C1 are too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kJ/mol of steric strain 28 Relationship to Gauche Butane Interactions Gauche butane is less stable than anti butane by 3.8 kJ/mol because of steric interference between hydrogen atoms on the two methyl groups The four-carbon fragment of axial methylcyclohexane and gauche butane have the same steric interaction In general, equatorial positions give more stable isomer 29 1,3-Diaxial Interactions •For other monosubstitiuted cyclhexanes Is more stable in an equatorial position than in an axial position •It is Depends on the nature and size of substituent. •The values must be doubled to arrive at the amount of strain in a mono substitiuted cyclohexane 30 4.8 Conformational Analysis of Disubstituted Cyclohexanes In disubstituted cyclohexanes the steric effects of both substituents must 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 the ring, and compound can exist in two chair conformations Consider the sum of all interactions In cis-1,2, both conformations are equal in energy 31 Trans-1,2-Dimethylcyclohexane Methyl groups are on opposite faces of the ring One trans conformation has both methyl groups equatorial and only a gauche 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 diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformation trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation 32 Conformation of cis-1tert-butyl-4-chlorocyclohexane These conformations aren’t of equal energy because an axial tert- butyl substitiutent and an axial chloro subsitituent produce different amounts of steric strain. 1,3-diaxial interaction between a hydrogen and tert- butyl group cost 11 kJ/mol, the interaction between a hydrogen and chlorine only 1kJ/mol . The stable conformation with the chlorine axial and the tert-butyl equatorial. 33 4.9 Conformations of Polycyclic Molecules Decalin consists of two cyclohexane rings joined to share two carbon atoms (the bridgehead carbons, C1 and C6) and a common bond Two isomeric forms of decalin: trans fused or cis fused In cis-decalin hydrogen atoms at the bridgehead carbons are on the same face of the rings In trans-decalin, the bridgehead hydrogens are on opposite faces Both compounds can be represented using chair cyclohexane conformations Flips and rotations do not interconvert cis and trans Although steroids look complicated compared with cyclohexane or decalin. 34 35.
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