orgo chap3 PP1

the reactivity and properties of a compound depend to some extent on the

shape of its molecules

conformation

The shape of a molecule with respect to bond rotations and other “easy” processes

ROTATING BONDS!!!

three conformations of hydrogen peroxide

Three conformations of hydrogen peroxide (H2O2)are shown. Conformation C, with an H–O–O–H angle of 115 degree, is the most stable

it is staggered, the oxygens are the farthest away in 3D space

ethane has an ___ number of CONFORMATIONS related by..

Ethane (H3CCH3) an infinite number of confirmations related by rotation about the C–C bond

which conformations of ethane are the most important?

staggered and eclipsed

stagged conformation of ethane

each C-H bond bisects an H-C-H

eclipsed conformation of ethane

each C-H bond is aligned with an adjacent C-H bond

staggered conformation of ethane: wedge and dash

staggered conformation of ethane: sawhorse

staggered conformation of ethane: newman projection

eclipsed conformation of ethane: wedge-and-dash

eclipsed conformation of ethane: sawhorse

eclipsed conformation of ethane: newman projection

torsion or dihedral angle

the angle between two bonds in a conformation (for example, the angle between adjacent C–H bonds in ethane)

eclipsed torsion angle

0 degrees

gauche torsion angle

60 degrees

anti torsion angle

180 degrees

which conformers are least stable with regards to energy?

eclipsed

which conformers are most stable with regards to energy?

staggered

Why are most molecules in the staggered conformation at any given time?

Because staggered conformations are lower in potential energy due to minimized steric interactions and hyperconjugative stabilization, making them overwhelmingly favored.

Why is the eclipsed conformation higher in energy?

Two reasons:

1. destabilization of eclipsed conformer due to nonbonding (steric) interactions

2. stabilization of staggered conformer due to hyperconjugative σC−H → σ*C−H interactions

How fast is rotation around a C–C single bond?

Extremely fast — t₁/₂ ≈ 10⁻⁶ seconds, meaning molecules constantly interconvert between conformations.

Are the energy differences between staggered and eclipsed conformations large?

No — they are relatively small, but still large enough that staggered conformations dominate.

What is hyperconjugation and how does it stabilize staggered conformations?

Hyperconjugation is the donation of electron density from a filled σC–H bond into an adjacent antibonding σ*C–H orbital.

This interaction is maximized in staggered conformations, lowering energy.

conformations of butane

two staggered conformations, unlike ethane

the terminal CH3 groups can be gauche or anti

gauche conformation of butane

Steric interaction between the gauche methyl groups is called a gauche interaction

anti conformation of butane

In the anti-conformer, the CH3 groups do not engage in steric interactions with one another

anti vs gauche conformation of butane

anti: lower in energy than gauche due to a destabilizing steric interaction in gauche

eclipsing CH3 groups in butane cause greater destabilization than H-H or H-CH3 eclipsing

staggered conformers

energy minima

eclipsed conformers

energy maxima

conformers of larger alkanes: in general, which conformer is the most stable?

all-anti conformer

all-anti conformer

zig-zag appearance, this is why we draw alkyl chains as zig-zags

the most stable cycloalkanes have bond angles of what?

near 109.5 degrees

we measure stability of cycloalkanes using what?

enthalpies of combustion

heat per CH2 combusted more specifically.

the cycloalkane with the smallest heat per CH2 combusted will be the most stable

which cycloalkane is the most stable?

cyclohexane

are cycloalkanes planar?

no. with the exception of cyclopropane

cyclopropane

extremely severe angle strain

its internal C-C-C bond angle is only 60 degrees

the C-C bonds in cyclopropane are..

bent and derived from the angled overlap of sp3 hybrids

rotation is impossible

strong sp3 sp3 sigma bonds are not possible. orbitals cannot be properly aligned

in cyclopropane, the internal C-C bonds are

weak and liable to break

torsional strain in cyclopropane

due to eclipsing C-H’s

cyclobutane

Angle strain is not quite as bad as cyclopropane (90 degree versus 60 degree)

the cyclobutane ring “puckers” to minimize torsional or eclipsing strain- adjacent CH bonds are twisted away from each other

nonetheless, cyclobutane is still somewhat unstable