HU Organic Chemistry - Conformations of Alkanes

dihedral angle for eclipsed conformation

0

what are the two different eclipsed conformations?

1) one CH3-CH3 and two H-H 2) two CH3-H and one H-H

how much energy is needed to go from one staggered form to the next?

12.6 kJ/mol, this is known as activation energy (Eact)

energy cost of CH3-CH3 torsional, angle and steric strain (eclipsed)

13 kJ/mol

energy cost of CH3-CH3 angle and steric strain (gauche)

4 kJ/mol

energy cost of H-H torsional strain (eclipsed)

4 kJ/mol

energy cost of H-CH3 mostly torsional strain (eclipsed)

6 kJ/mol

dihedral angle for staggered conformation

60

when is there steric strain?

CH3-CH3 interaction in both staggered and eclipsed forms

torsional strain

Strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation. Torsional strain is also called eclipsed-interaction strain.

conformational isomers

a 3D arrangement of atoms that occurs as a result of rotation about a single bond; NOT breaking a bond

eclipsed conformation

a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as close as possible to the atoms or groups on an adjacent carbon

staggered conformation

a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from atoms or groups on an adjacent carbon (60 degrees apart)

gauche conformation (staggered)

a conformation with a 60 degree dihedral angle between the largest groups

Newman projection

a method of viewing a molecule by looking down a single C-C bond; the most useful way of visualizing conformational isomers

an increase in angle strain will do what to steric strain?

decrease

theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the _________ conformation but rather by stabilization of the _______ conformation due to hyperconjugation between bonding MO and antibonding MO

eclipsed, staggered

how is enough activation energy absorbed to go from one staggered form to the next?

energy absorbed through collisions with other molecules

what causes torsional strain?

hyperconjugation

steric strain

increase in energy resulting when atoms are forced too close to one another; strain that arises when non-bonded atoms separated by four or more bonds are forced closer than their atomic radii would allow

why is the staggered conformation more stable?

it is able to distribute the charge through torsional stabilization (strain) due to hyperconjugation interaction between bonding and antibonding orbitals three bonds away

can their be torsional strain in staggered conformation?

no

angle strain

results when bond angles deviate from their ideal values by being stretched or compressed

are conformational changes big or small energy changes?

small

hyperconjugation

spreading out charge by the overlap of an empty p orbital with an adjacent sigma bond

which is lower in energy? staggered or elipsed?

staggered, and is therefore more stable

when angle strain increases, this is to minimize what?

steric strain

dihedral angle

the angle between two specified groups in a Newman projection

anti conformation (staggered)

when the methyls are 180 degrees apart