Org 1 Review - Conformations

Section 1

Question	Answer
What is a conformational isomer?	Conformational isomer - two molecules with identical structural and molecular formulas that can be interconverted through rotation of bonds.
The ______ angle is the angle formed by four bonds as we look along the central C-C bond in a ____ projection.	dihedral angle is anle formed by four bonds as we look along central C-C bond in a Newman projection.
The predominant source of strain in molecules such as ethane, butane, and other linear molecules is _____ strain. Why does it arise?	The predominant source of strain in these molecules is torsional strain. It arises through electrostatic repulsions of the electron clouds of atoms with each other. If possible, atoms will rotate along sigma bonds such that they will minimize electrostatic repulsions. This is why the "staggered" form of butane is more stable
"Eclipsed" is a conformation with dihedral angles of ____ degrees.	Eclipsed - dihedral angle of 0 degrees.
"Staggered" depicts a conformation with dihedral angles of ____ degrees.	Staggered - dihedral angle of 60 degrees (for ethane).
Gauche interactions exist when bulky groups (e.g. methyl) are at an angle of _____ with each other. They only exist in the _____ conformation.	Gauche interactions are repulsions between bulky substituents that are 60 degrees from one another. They only exist in the staggered conformation.
What are the two major conformations encountered in cyclohexane?	Two major conformations of cyclohexane - the chair and the boat.
The most stable conformation of ethane is the ____ conformation.	staggered
The least stable conformation of ethane is the _____ conformation	eclipsed
When one does a ring flip, all the substituents that are ______ become ______ and vice-versa.	When one does a ring flip, all the substituents that are axial become equatorial and vice versa
	memorize

Section 2

Question	Answer
What is a "flagpole" interaction?	In the boat conformation of cyclohexane there is a "flagpole" interaction, which is between the substituents on the 1 and 4 positions - these can come into close proximity and repel.
Bulky substituents on a cyclohexane prefer to be in the _____ position.	Bulky substitutents prefer to be in the equatorial position, there is less repulsion there.
Why is there a barrier to a "ring flip" of cyclohexane chair conformations?	The barrier to the ring flip is caused by the necessity of going through the boat form (actually "twist-boat") of the cyclohexane.
There are two staggered conformations possible for butane. What are they called, and which is lowest in energy?	For butane in the staggered conformation, the one with dihedral angle between methyl groups of 180 degrees is called "anti" and the one with dihedral angle between methyl groups of 60 degrees is called "gauche"
Which of the following will have two chair forms with significantly different energies: 1,4-trans-dimethylcyclohexane, 1,4-cis-dimethyl cyclohexane, 1,3-trans dimethylcyclohexane, 1,3-cis dimethyl cyclohexane, 1,2-trans dimethylcyclohexane, 1,2-cis dimethyl cyclohexane. 1	1,4-trans dimethyl cyclohexane, 1,3-cis dimethyl cyclohexane, 1,2-trans dimethyl cyclohexane will all have one form where both substituents are axial and one where both are equatorial.
How would you rank the steric "bulk" of alkyl groups, ethyl isopropyl t-butyl methyl	The bulkiest is t-butyl, followed by isopropyl, ethyl, and then methyl.
True or false: It is possible for a "wedge" bond to represent a substituent in the equatorial position of a cyclohexane.	True! "Up" and "down" on a cyclohexane refer to which face of the ring the substituent is on, not whether it is axial or equatorial.
Which is bulkier, a t-butyl group or an alkyne? Therefore, which will preferentially adopt the equatorial position on a cyclohexane?	t-butyl group is bulkier, as it contains 3 methyl groups that each sweep out a "cone" of space - taking up much more space than the small alkyne group.
Which are bulkier, in general: alkyl substitutents or halides?	Alkyl substitutents are much bulkier than halides - they sweep out a larger area of space.
	memorize

 

Org 1 Review - Thermodynamics
Org 1 Review - Free Radicals
Org 1 Review - Reactions of Alcohols
Org 1 Review - Alkynes
Org 1 Review - Substitution and Elimination
Org 1 Review - Reactions of Double Bonds
Org 1 Review - Stereochemistry
Org 1 Review - Conformations
Org 1 Review - Structure and Bonding