Conformations of Alkanes and Cycloalkanes Flashcards
What are the different rotational arrangements of alkanes called?
Conformations.
Molecules having different conformations are referred to as conformers or conformational isomers.
What is the definition of conformation in chemistry?
Any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond.
What is torsional strain?
Repulsions between groups on adjacent atoms.
What is the purpose of a Newman projection?
To view a molecule by looking along a carbon-carbon bond.
What characterizes a staggered conformation?
Bonds on adjacent C atoms are furthest apart, resulting in the least torsional strain.
What characterizes an eclipsed conformation?
Bonds on adjacent C atoms are closest together, resulting in the most torsional strain.
What is the torsional strain of the eclipsed conformation of ethane?
2.9 kcal/mol (12 kJ/mol).
Define steric strain.
Repulsive force that arises when atoms not bonded to each other are forced closer to one another.
What is the anti conformation in butane?
Substituents are 180° from one another, resulting in no torsional strain and no steric strain.
What is the gauche conformation in butane?
Substituents are 60° from one another, resulting in no torsional strain but steric strain (~3.8 kJ/mol).
What is the angle strain in cyclopropane?
Bond angles of 60°, which do not permit maximum orbital overlap.
What is the conformation of cyclopentane compared to cyclopropane?
Cyclopentane has no angle strain but has higher torsional strain.
What is the envelope conformation?
A non-planar conformation of cyclopentane that reduces torsional strain.
What are the two kinds of positions in chair conformation of cyclohexane?
Axial and equatorial positions.
What occurs during a chair flip in cyclohexane?
Axial positions rotate to equatorial positions and vice versa.
What are 1,3-diaxial interactions?
Type of steric strain between axial ring substituents.
How does the size and shape of a substituent affect steric strain?
Bulkier alkyl groups are more stable in the equatorial position.
What is the difference between cis and trans isomers in disubstituted cyclohexanes?
Cis: two groups on the same face of the ring; Trans: two groups on opposite faces.
What is the most stable conformation of cis-1,2-dimethylcyclohexane?
The conformation where both methyl groups are in equatorial positions.
What is the most stable conformation of trans-1,2-dimethylcyclohexane?
The conformation where one methyl group is axial and the other is equatorial.
