Monosubstituted Cyclohexanes

Monosubstituted Cyclohexanes

• Substituents can occupy an axial or equatorial position

• Equatorial is preferred by larger groups. Why?

17.3 Monosubstituted Cyclohexanes. Conformational Analysis

1,3-Diaxial Interactions

• 1,3-Diaxial Interactions: van der Waals repulsions between an axial substituents on a cycloalkane ring

2

4

Problems

1) What is the energy cost of a 1,3-diaxial strain in methylcyclohexane?

2) What about bromocyclohexane?3) Which of the two molecules above will have

a higher percentage of its molecules in the equatorial conformation?

More on Ring Flip

• Note that the “up” substituent remains up and the “down” substituent remains down after chair interconversion

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Disubstituted Cyclohexanes1-chloro-2-methylcyclohexane

• Trans: the two groups have an up-down relationship

• Cis: the two groups have a down-down (or up-up) relationship

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Conformational Analysis

• For disubstituted derivatives, the larger group will preferentially occupy the equatorial position

97.4 Disubstituted Cyclohexanes

Problem

1) Which subsituent will most likely occupy the equatorial position for a greater amount of time in cis-1-chloro-2-methylcyclohexane?

2) Prove this with calculation.

Polycyclic Molecules• Polycyclic molecules: compounds with two or

more rings fused together– Spirocyclic: two rings that have only a single

common atom– Bicyclic: two rings that share two or more

common atoms

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127.6 Bicyclic and Polycyclic Compounds

Classification and Nomenclature

• Fused and bridged bicyclic compounds

137.6 Bicyclic and Polycyclic Compounds

Naming Bicyclic Systems• Indicate the number of rings using the prefix “bicyclo-”• Indicate the bridge lengths– Number of atoms connecting one bridgehead atom to

another (excluding the bridgehead atoms)– Separate by full periods and place in square brackets.• Cited in decreasing order of size (e.g. [3.2.1])

• the name of the hydrocarbon indicating the total number of skeletal atoms

Problems• Name the following polycyclic molecules:

• If you were just given the IUPAC names for a couple of bicyclic molecules, how would you tell the difference between a fused and a bridged variety?

Norbornane

Decalin

Cis and Trans Ring Fusion: Decalin

167.6 Bicyclic and Polycyclic Compounds

• Each ring in cis-decalin can undergo ring flip

• Ring fusion causes trans-decalin to be conformationally locked

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• Bicyclic compounds with small rings are restricted to cis ring fusion

• Trans fusion would incur too much ring strain

Ring Fusion with Small Rings

187.6 Bicyclic and Polycyclic Compounds

Steroids

• Organic compounds with 20 carbon, tetracyclic core– Variations in substituents dictate biological

activity

197.6 Bicyclic and Polycyclic Compounds

Steroids

• Many consist of all trans-fused rings– No conformational change

• Many have methyl groups at C-10 and C-13

207.6 Bicyclic and Polycyclic Compounds

Stereochemistry of Cycloalkene ReactionsAddition Reactions

• Syn-addition:

• Anti-addition:

227.9 Stereochemistry of Chemical Reactions

Stereochemistry of Bromine Addition

• Addition of bromine to an alkene is a highly stereoselective reaction

• Exclusively anti-addition– Gives all trans products

237.9 Stereochemistry of Chemical Reactions

Stereochemistry of Halohydrin Formation

• Exclusively anti-addition– Gives all trans products

Stereochemistry of Hydroboration-Oxidation

• Hydroboration is a stereospecific syn-addition

267.9 Stereochemistry of Chemical Reactions

Stereochemistry of Hydroboration-Oxidation

• The oxidation of organoboranes is a stereospecific substitution reaction

287.9 Stereochemistry of Chemical Reactions

Stereochemistry of Hydroboration-Oxidation

• The two steps of hydroboration-oxidation result in net syn-addition of H-OH to the alkene

• Note that the trans designation of the name has nothing to do with the way H-OH added

297.9 Stereochemistry of Chemical Reactions

Stereochemistry of Hydrogenation

• Catalytic hydrogenation is a stereospecific syn-addition

307.9 Stereochemistry of Chemical Reactions

Problem

• Draw the products for the following reactions