Conformations of Alkanes and Cycloalkanes Flashcards
Week 4
Conformations
Different Rotational arrangements;
Any 3D arrangement of atoms in a molecule that result from rotation about a single bond.
When are bond rotations possible
Between sp^3-hybridized carbons
Newman projection
A way to view a molecule by looking along a carbon-carbon bond
Torsional Strain
repulsion between groups on adjacent atoms
Staggered conformation
bonds on adjacent C atoms furthest apart (least torsional strain)
Eclipsed conformation
bonds on adjacent C atoms closest together (most torsional strain)
Steric Strain
Repulsive force that arises when atoms not bonded to each other are forced closer to one another (repulsion between electron clouds)
Anti-conformation
Substituents 180 degrees from one another.
- no torsional strain
- no steric strain
Gauche Conformation
Substituents 60 degrees from one another
- No torsional strain
- Steric Strain
When are rings no longer flat
When they are larger than 3 atoms
Cyclohexane
Have less flexibility than open-chair alkanes
Cyclic molecules do what to minimize angle and torsional strain
They adopt non-planar conformations
Angle strain
arise from bond angles that do not permit maximum orbital overlap between the atoms of a molecule.
3-membered ring must be/have…
- Be flat with bond angles of 60 degrees
- All C-H bonds eclipsed (lots of torsional strain
- requires that sp^3 - sp^3 bonds are bent
4-membered rings
Butterfly conformation used to alleviate torsional strain
5-membered rings
Envelop strain to reduce torsional strain and create very little ring strain
6-membered rings
Chair conformations to free ring of angle strain and torsional strain
Axial vs Equatorial
Up and down = axial
in plane = equatorial
When doing a ring flip…
equatorial groups move to axial positions and axial positions move to equatorial positions
1,3-diaxial interactions
type of steric strain between axial ring substituents
Note: Size and shape of substituents affects the magnitude of the steric strain. Bulkier alkyl groups are more stable in the equatorial position.
Stereoisomers
compounds in which the atoms are connected in the same order but differ in their spatial geometry
Stereoisomerism
Exists in cycloalkanes with two or more substituents. Disubstituted cycloalkanes can exist as cis-trans stereoisomers.
Trans-
two groups on opposite face of ring.
Cis-
Two groups on same face of ring.
