Organic Chemistry I

Chem 334 - Fall 2009

Chapter 3 Notes

Professor Carl C. Wamser

Conformations of Alkanes and Cycloalkanes

Conformations

Ethane Conformations

C-H bonds on the two carbons may or may not align

eclipsed: C-H bonds are aligned

staggered: C-H bonds fit in between

Newman Projections

Potential Energy Diagrams

Butane conformations

Molecular mechanics

Types of strain:

Ring strain

Heats of combustion (normalized per CH2 unit)

Ring Size
Heat of combustion (kJ/mol)
Heat of combustion per CH2 (kJ/mol)
3
2091
697
4
2721
681
5
3291
658
6
3920
653
7
4599
657
larger
653 - 659

Small cycloalkane conformations

Cyclohexane

Monosubstituted cyclohexanes

PRACTICE DRAWING ACCURATE CHAIR FORMS WITH SUBSTITUENTS

equatorial generally preferred over axial
see the table in the text with delta G for axial vs equatorial

Disubstituted cyclohexanes - cis & trans isomerism

Depending on the substitution pattern, disubstituted cyclohexanes have different combinations of axial and equatorial substituents.

substitution

cis

trans

1,2-

e,a <==> a,e

e,e <==> a,a

1,3-

e,e <==> a,a

e,a <==> a,e

1,4-

e,a <==> a,e

e,e <==> a,a

Example - the two chair conformations for cis-1-chloro-3-methylcyclohexane involve either two axial substituents or two equatorial substituents. The latter is more stable.

All substituents prefer equatorial over axial

Polycyclic Compounds

spirocyclic - one carbon atom in common to two rings

naming: spiro[m.n]alkane (m,n are bridge sizes, where m<n)
number starting next to the common (spiro) atom as #1, go around smaller bridge first

example: spiro[2.3]hexane

bicyclic - two carbon atoms in common to all three rings (bridgehead positions)

naming: bicyclo[x.y.z]alkane (x,y,z are bridge sizes, where x>y>z)
number starting at one bridgehead atom as #1, go around bridges in size order (x>y>z)

example: bicyclo[3.2.0]heptane

Heterocyclic Compounds

contain one or more atoms in the ring that are not carbon

naming: special names for every case (covered later with the corresponding functional groups)

example: piperidine