Cycloalkanes Many organic compounds contain cyclic or ring structures: carbohydrates nucleotides in DNA and RNA antibiotics penicillin G testosterone

Cycloalkanes Cycloalkanes: alkanes that contain three or more carbons arranged in a ring CnH2n

Cycloalkanes Cycloalkanes are named using: prefix “cyclo” alkane base name Examples: a cycloalkane with 5 carbons in the ring: cyclopentane a cycloalkane with 10 carbons in the ring: cyclodecane

Naming Substituted Cycloalkanes To name substituted cycloalkanes: Use the cycloalkane for the base name Identify substituents using name and position no number is needed if only one substituent is present isopropylcyclohexane

Cycloalkanes 1-ethyl-3-methylcyclohexane For 2 or more substituents, number the ring carbons to give the lowest possible numbers for the substituted carbons If numbering could begin with either substituent, start with the one that is first alphabetically. 3 1 2 4 5 6 1-ethyl-3-methylcyclohexane

Cycloalkanes 3-cyclopropyl-2,6-dimethylheptane When the acyclic portion of the molecule contains more carbons than the cycloalkane, the cyclic portion is named as a cycloalkyl substituent. 3-cyclopropyl-2,6-dimethylheptane

Cycloalkanes Example: Name the following cycloalkanes.

Cycloalkanes Example: Draw the following cycloalkanes. sec-butylcyclooctane 1,1,3,3-tetramethylcyclohexane

Cycloalkane Conformations Cycloalkanes containing 3 - 20 carbons have been synthesized. Rings with 5 or 6 carbons are the most common WHY? Recall that all alkanes contain C - C single bonds that are formed by the overlap of sp3 hybrid orbitals tetrahedral geometry ideal bond angle = 109.5o

Cycloalkane Conformations In cycloalkanes the best overlap (strongest bond) between the sp3 hybrid orbitals will occur when bond angle = 109.5o In some cycloalkanes, bond angles other than 109.5o lead to angle strain and less than optimum overlap of the sp3 hybrid orbitals Angle strain: the strain associated with bond angles that are smaller or larger than the ideal value

Cycloalkane Conformations In addition to angle strain, some cycloalkane conformations lead to significant amounts of torsional strain due to eclipsing of bonds. Ring strain: the extra strain associated with the ring structure of a compound compared to a similar acyclic compound angle strain torsional strain

Cycloalkane Conformations The heat of combustion (DHcomb) is often used to measure the ring strain of a cycloalkane. The amount of heat released when a substance is burned in the presence of excess oxygen. As DHcomb increases, a substance contains a greater amount of potential energy. As potential energy increases, the compound becomes less stable.

Cycloalkane Conformations Cyclopropane: greatest ring strain per CH2 60o bond angles weaker overlap of sp3 hybrid orbitals all bonds eclipsed more reactive than other alkanes

Cycloalkane Conformations Cyclobutane: second largest amount of ring strain slightly folded conformation instead of planar and square Square/planar conformation would have less angle strain but more torsional strain

Cycloalkane Conformations Cyclobutane: Slightly folded conformation 88o bond angle not quite eclipsed less torsional strain

Cycloalkane Conformations Cyclopentane: Five membered rings are very important biologically ribose and deoxyribose have cyclopentane like conformations If cyclopentane existed as a planar, regular pentagon: bond angle = 108o low angle strain all bonds eclipsed high torsional strain

Cycloalkane Conformations Cyclopentane: Actual shape = slightly puckered envelop reduces eclipsing and torsional strain Molecule constantly “undulates” envelop flap moves around ring

Cycloalkane Conformations Cyclohexane: Most common cycloalkane Carbohydrates, steroids, and some pesticides contain cyclohexane-like conformations If cyclohexane had a planar, regular hexagonal conformation: 120o bond angles high angle strain adjacent methylene groups eclipsed high torsional strain

Cycloalkane Conformations Cyclohexane has no ring strain. Cannot be a planar, regular hexagon Cyclohexane achieves tetrahedral bond angles and staggered conformation by assuming puckered conformations: chair conformation most stable boat conformation half-chair conformation highest energy

Cycloalkane Conformations Chair conformation most stable lowest energy 109.5o staggered

Cycloalkane Conformations Boat conformation: 109.5o torsional strain due to eclipsing of bonds actually exists as the twist boat conformation in order to eliminate the flagpole effect Repulsive force between two groups that are in close proximity on opposite ends of a ring

Cycloalkane Conformations The twist boat conformation reduces flagpole effect reduces eclipsing of bonds lower in energy than the boat conformation higher in energy than the chair conformation Cyclohexane exists predominantly in the chair conformation constantly interconverting from chair to half-chair to boat conformations and back

Cycloalkane Conformations

Cycloalkane Conformations Each carbon in a cyclohexane ring has two different types of carbon - hydrogen bonds: axial bond a bond that is parallel to the axis of the ring directed up and down equitorial bond a bond that is directed along the “equator” of the ring pointed out from the ring

Cycloalkane Conformations axis axis Equitorial bonds Axial bonds Axial and equitorial bonds

Monosubstituted Cyclohexanes Substituents on a cyclohexane ring can occupy either an axial position or an equitorial position. Chair-chair interconversions that occur at room temperature lead to an equilibrium mixture of both conformations. Axial methyl equitorial methyl

Monosubstituted Cyclohexanes During chair-chair interconversions (ring flip), substituents change from: axial equitorial Conformations with the substituent in the equitorial position are lower in energy (more stable) and therefore favored: “anti” arrangement no 1,3-diaxial interaction

Monosubstituted Cycloalkanes 1, 3-diaxial interactions steric hinderance between groups in axial positions on carbons 1 and 3 More stable conformer 1, 3-diaxial interaction

Monosubstituted Cyclohexanes Example: Draw the two possible chair conformers of t-butylcyclohexane. Which one is more stable? Why?

Cis & Trans Isomers cis-1,2-dimethylcyclopropane Cycloalkanes have two distinct faces. Di-substituted cycloalkanes can exist as cis and trans isomers. Cis cycloalkane two identical groups on the same face of the ring cis-1,2-dimethylcyclopropane

Cis and Trans Isomers trans-1-ethyl-3-methylcyclobutane Trans cycloalkane two identical groups on opposite faces of the ring trans-1-ethyl-3-methylcyclobutane

Cis and Trans Isomers Example: Name the following compound.

Cis and Trans Isomers Drawing cis/trans isomers of disubstituted cyclohexanes: Positions Cis Trans 1,2 a,e or e,a a,a or e,e 1,3 a,a or e,e a,e or e,a 1,4 a,e or e,a a,a or e,e

Cis and Trans Isomers Disubstituted cyclohexanes can also exist as different chair conformations: Some conformations are lower energy More stable Preferred Some conformations are higher energy Less stable Not preferred

Cis and Trans Isomers Points to remember about relative stabilities: Greatest stability is found in disubstituted conformers with both substituents in equitorial positions. Disubstituted conformers with both substituents in the axial positions are very unfavorable. If the isomer requires an a,e conformer, then the most stable conformer will have the largest group in the equitorial position.

Cis and Trans Isomers Example: Draw both chair conformations of cis-1,3-diethylcyclohexane. Which one is more stable?

Cis and Trans Isomers Example: Draw both possible conformations of trans-1-t-butyl-3-methylcyclohexane. Which one is more stable?

Bicyclic Molecules Two or more rings can be joined into bicyclic or polycyclic molecules. Fused rings share 2 adjacent carbons and the bond between them.

Bicyclic Molecules Bridged rings share two nonadjacent carbons and one or more carbon atoms between them.