Chapt 4: Organic Compounds: Cycloalkanes and their Stereochemistry

Rules in naming cycloalkanes

1. Find the parent (count the number of carbons in the ring and count the number in the largest substituent)

2. Number the substituents, and write the name

Cycloalkanes or alicyclic compounds

saturated cyclic hydrocarbons

• C_nH_2n

• Can be represented using skeletal drawings

Cycloalkanes are less

flexible than open-chain alkanes and have lesser conformational freedom in cycloalkanes

Stereoisomerism

compounds which have their atoms connected in the same order but differ in 3D orientation

Stereochemistry

term used to refer to the 3D aspects of chemical structure and reactivity

Cis-trans isomers

stereoisomers that differ in their stereochemistry about a ring or double bond

Angle strain

induced in a molecule when bond angles are forcced to deviate from the ideal 109° tetrahedral value

Torsional strain

caused due to eclipsing of bonds between neighboring atoms

Steric strain

caused due to repulsive interactions when atons approach each other too closely

Larger rings have many _______ than smaller rings

more possible conformations

Cyclopropane

• most strained of all rings due to angle strain caused by its C-C-C bond angles of 60°

• has considerable torsional strain

• has bent bonds

• C-H bonds are eclipsed

• bonds are weaker and more reactive than typical alkane bonds

Cyclobutane

• Has less angle strain than cyclopropane

• More torsional strain because of larger number of ring hydrogens, and their proximity to each other

• Slightly bent out of plane, one carbon atom is about 25° above the plane

  • increases angle strain but decreases torsional strain

Cyclopentane

• no angle strain

• large torsional strain

• non planar conformations strike balance between increased angle strain and decreased torsional strain

• four carbon atoms are approximately in the same plane

  • fifth carbon atom is bent out of plane

Cyclohexane

• adopts chair conformation

• has neither angle strain nor torsional strain

Chair conformation

has neither angle strain nor torsional strain

Boat cyclohexane

conformation of cyclohexane that bears a slight resemblance to a boat

• no angle strain

• large number of eclipsing interactions

Twist-boat conformation

conformation of cyclohexane that is somewhat more stable than a pure boat conformation

• nearly free of angle strain

Ring-flip

Interconversion of chair conformations, resulting in the exchange of axial and equatorial positions

Gauche butane is less stable than anti butane by 3.8 kJ/mol due to

steric interference between hydrogen atoms on the two methyl groups