9/30/2010. Chapter 4 Organic Compounds: Cycloalkanes and Their Stereochemistry. Cyclics. 4.1 Naming Cycloalkanes

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1 John E. McMurry Chapter 4 Organic Compounds: Cycloalkanes and Their Stereochemistry Richard Morrison University of Georgia, Athens Cyclics Most organic compounds contain rings of carbon atoms Chrysanthemic acid Occurs naturally as esters Active insecticidal constituents of chrysanthemum flowers Prostaglandins Potent hormones Control physiological functions in humans Steroids Contain four joined rings Naturally occurring hormones in plants and animals 4.1 Cycloalkanes or Alicyclic Compounds (aliphatic cyclic) Saturated cyclic hydrocarbons containing rings of carbon atoms (CH 2 ) n or C n H 2n 1

2 Rule 1 Find the parent by counting carbon atoms Named as alkyl-substituted cycloalkane The number of carbon atoms in the ring is equal to or greater than the number in the substituent Named as cycloalkyl-substituted alkane The number of carbon atoms in the largest substituent is greater than the number in the ring Rule 2 Number the substituents, and write the name Choose attachment point as carbon 1 and number carbon atoms on the ring so that second substitutent has lowest number possible. If ambiguity exists, number carbon atoms so that third or fourth substituent has lowest number possible. 2

3 When two or more substituents could potentially receive the same numbers, number by alphabetical priority: If halogens are present, treat them just like alkyl groups: Additional examples: 4.2 Cis-Trans Isomerism in Cycloalkanes Cycloalkanes vs. Open-chain Alkanes Similarities nonpolar; fairly inert Differences cycloalkanes are less flexible than openchain alkanes Rotation occurs around the carbon-carbon bond in ethane No rotation is possible around the carbon-carbon bonds in cyclopropane without breaking open the ring 3

4 Cis-Trans Isomerism in Cycloalkanes Cycloalkanes: Two faces (as viewed edge on) Top face Bottom face Isomerism possible in substituted cycloalkanes Cis-Trans Isomerism in Cycloalkanes Constitutional Isomers Isomers that have their atoms connected in a different order Stereoisomers Isomers that have their atoms connected in the same order but differ in three-dimensional geometry cis isomer trans isomer Cis-Trans Isomerism in Cycloalkanes cis-trans isomers Stereoisomers that differ in their stereochemistry about a double bond or ring cis- (Latin, on the same side ) trans- (Latin, across ) 4

5 Worked Example 4.1 Name the following substances, including cis- or transprefix 4.3 Stability of Cycloalkanes: Ring Strain Angle strain The strain induced in a molecule when bond angles are forced to deviate from the ideal 109º tetrahedral value (Adolf von Baeyer 1885) Stability of Cycloalkanes: Ring Strain Angle strain Experimental data show that Baeyer s theory is only partially correct Baeyer assumed all cycloalkanes to be flat Angle strain occurs only in small rings that have little flexibility 5

6 Stability of Cycloalkanes: Ring Strain The three kinds of strain that contribute to the overall energy of a cycloalkane: 1. Angle strain the strain due to expansion or compression of bond angles 2. Torsional strain the strain due to eclipsing of bonds on neighboring atoms 3. Steric strain the strain due to repulsive interactions when atoms approach each other too closely 4.4 Conformations of Cycloalkanes Cyclopropane Most strained of all the rings Angle strain caused by 60º C-C-C bond angles Torsional strain caused by the eclipsed C-H bonds on neighboring carbon atoms a) Structure of cyclopropane showing the eclipsing of neighboring C- H bonds giving rise to torsional strain b) Newman projection along a C-C bond of cyclopropane Conformations of Cycloalkanes Bent C-C bonds Orbitals can t point directly toward each other Orbitals overlap at a slight angle Bonds are weaker and more reactive than typical alkane bonds C-C bond: 255 kj/mol (61 kcal/mol) for cyclopropane 355 kj/mol (85 kcal/mol) for open-chain propane 6

7 Conformations of Cycloalkanes Cyclobutane Total strain is nearly same as cyclopropane Angle strain less than cyclopropane Torsional strain more than cyclopropane because of larger number of ring hydrogens Not planar (not flat) One carbon atom lies 25º above the plane of the other three Newman projection along C1-C2 bond shows that neighboring C-H bonds are not quite eclipsed Conformations of Cycloalkanes Cyclopentane Less strain than cyclopropane or cyclobutane Planar cyclopentane exhibits: Angle strain very minimal Torsional strain large amount Cyclopentane twists to a nonplanar (puckered) conformation C1, C2, C3 and C4 are nearly planar but C5 is out of the plane Balance between increased angle strain and a decreased torsional strain 7