Q: Which of the following best describes the staggered conformation of ethane in terms of its dihedral angle and energy?

Dihedral angle $$\theta = 60^\circ$$; minimum energy due to staggered bonds

Question 1

Which of the following best describes the staggered conformation of ethane in terms of its dihedral angle and energy?

Accepted Answer

Dihedral angle $$	heta = 60^\circ$$; minimum energy due to staggered bonds

Question 2

What is the primary reason for the higher energy of the eclipsed conformation in ethane compared to the staggered conformation?

Accepted Answer

Torsional strain caused by repulsion of bonding electrons in adjacent C-H bonds

Question 3

For n-butane, which conformation is the most stable and why?

Accepted Answer

Anti conformation; because methyl groups are 180° apart, minimizing steric repulsion

Question 4

Which factor contributes LEAST to the rotational barrier in ethane?

Accepted Answer

Steric interactions between hydrogen atoms

Question 5

Why does 1,2-difluoroethane prefer the gauche conformation over the anti conformation, despite dipole repulsion?

Accepted Answer

Hyperconjugative $$\sigma_{CH} \rightarrow \sigma^*_{CF}$$ interactions stabilize the gauche form

Question 6

Which conformation of cyclohexane is the most stable and why?

Accepted Answer

Chair; because it minimizes both angle and torsional strain

Question 7

A monosubstituted cyclohexane with a methyl group prefers which position and why?

Accepted Answer

Equatorial; because it minimizes steric strain from 1,3-diaxial interactions

Question 8

Which of the following statements about enantiomers is TRUE?

Accepted Answer

Enantiomers rotate plane-polarized light in opposite directions

Question 9

A compound was isolated in the lab and the observed rotation was $$+15^\circ$$ when measured in a 1 dm tube containing 2.0 g of sample in 10 mL of water. What is the specific rotation?

Accepted Answer

$$[\alpha]_D = +75^\circ$$

Question 10

How many stereoisomers are possible for a molecule with 3 chiral centers?

Accepted Answer

8

Question 11

Which of the following is NOT a characteristic of a meso compound?

Accepted Answer

Is non-superimposable on its mirror image

Question 12

In the Cahn-Ingold-Prelog system, which group receives the lowest priority when assigning R/S configuration?

Accepted Answer

The group with the lowest atomic number

Question 13

Which reaction mechanism results in inversion of configuration at a stereogenic center?

Accepted Answer

SN2

Question 14

Which conformation of cyclopentane is most stable?

Accepted Answer

Puckered envelope conformation

Question 15

Which of the following statements about optical activity is FALSE?

Accepted Answer

Diastereomers always have identical optical rotation

Question 16

Which is the major product in an E2 elimination reaction of a 2-bromobutane with a strong base?

Accepted Answer

The E-alkene

Question 17

Which of the following is TRUE about the Diels-Alder reaction?

Accepted Answer

The reaction is stereospecific and favors the endo product

Question 18

Which of the following is a symmetry-allowed pathway in an electrocyclic reaction?

Accepted Answer

In-phase overlap of orbitals (con-rotation)

Question 19

Which of the following statements about atropisomers is correct?

Accepted Answer

They are conformationally locked enantiomers due to steric hindrance

Conformations and Conformational Analysis in Organic Chemistry

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