alkanes and cycloalkanes conformations and cis-trans stereoisomers

most stable conformation of ethane

staggered conformaton is more stable than eclipsed- each C-H bond biscts an H-C-H bond

eclipsed is less stable due to electron-electron repulsion in aligned bonds- each C-H bond is aligned with an adjactent C-H bond

staggered is stable because of better electron delocalization

wedge and dash

lines lie in the plane

dashes lie in the back

wedges are coming out of the plane

conformation

the most stable conformer has the lowest energy

any time there is an eclipse in an atom it is high in energy making it very unstable

eclipsed conformers are less stable because of their steric effects- bulkiness (steric hinderance)

torsion angles

eclipsed = 0

gauche- 60

anti- 180

staggered conformations of butane

they are not equivalent

anti conformation is MORE STABLE than gauche

gauche is less stable because of Vanderwall strain involving methyl groups

cycloalkanes

are nonplanar with the exception of cyclopropane

cyclopropane is planar and is destabilized by angle and torsional strain

cyclopropane is non planar and is less strained than cyclopropane

cyclobutane

ring puckers to minimise eclipsing strain and is very unstable

cyclopentane

has 2 non planar conformations that are similar in stability

envelope and Half chair

cyclohexane

have tetrahedral angles and there are 3 conformations

chair (most stable no torsional strain), the boat, and the twist

when a cyclohexane ring is present it almost always adopts a chair conformation- most stable

C-H bonds (Axial vs equatorial) in the chair conformation of a cyclohexane

they are not all equivalent and are divided into two sets of 6 each

axial and equatorial

Stereoisomers

Compounds with the same structural formula but with a different arrangement of the atoms in space.

stereoisomer stability

cis-1,3-dimethylcyclohexane vs trans-1,3-dimethylcyclohexane

in this case the trans is more stable because there is one axial methyl group