Cycloalkanes

Cycloalkanes Cycloalkanes: An alkane that contains a ring of carbon atoms. Ring sizes from 3 carbons to 30 or higher If the carbon chain that forms the straight-chain hydrocarbon - - is long enough, the two ends coming together to form a cycloalkane. - One hydrogen atom has to be removed from each end of the hydrocarbon chain to form the CC bond that closes the ring. - Cycloalkanes therefore have two less hydrogen atoms than the parent alkane and a generic formula of CnH2n.

Examples of cycloalkanes - The smallest alkane that can form a ring is cyclopropane, C3H6, in which the three carbon atoms lie in the same plane. - The angle between adjacent CC bonds is only 60° as seen in the model below:

Nomenclature of Cycloalkanes - The prefix cyclo- is added to the name of the alkane with the same number of carbons - When one substituent is present it is assumed to be at position one and is not numbered - When two alkyl substituents are present the one with alphabetical priority is given position 1 - Numbering continues to give the other substituent the lowest number Hydroxyl has higher priority than alkyl and is given position 1- If a long chain is attached to a ring with fewer carbons, the cycloalkane is considered the substituent Chapter 4

1. No number is needed for a single substituent on a ring 2. Name the two substituents in alphabetical order

Chapter 4

Summary: Types of Strain Angle strain - expansion or compression of bond angles away from most stable 109.5o Torsional strain - eclipsing of bonds on neighboring atoms Steric strain - repulsive interactions between nonbonded atoms in close proximity Total strain for cyclopropane = angle strain + torsional strain

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

Cyclobutane Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above The bend increases angle strain but decreases torsional strain