Question 1

Which of the following best describes the difference between staggered and eclipsed conformations in ethane, as seen in a Newman projection?

Accepted Answer

In the staggered conformation, the dihedral angle between C-H bonds on adjacent carbons is $$	heta = 60^\circ$$, while in the eclipsed conformation it is $$	heta = 0^\circ.$$

Question 2

Which factor contributes most to the rotational barrier between staggered and eclipsed conformations in ethane?

Accepted Answer

Torsional strain due to repulsion between bonding electrons in adjacent C-H bonds.

Question 3

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

Accepted Answer

The anti conformation, because the two methyl groups are $$180^\circ$$ apart, minimizing steric and torsional strain.

Question 4

Which of the following statements about 1,2-dichloroethane in polar solvents is correct?

Accepted Answer

Polar solvents stabilize the gauche conformer, making it favored at equilibrium.

Question 5

What is the main reason the gauche conformation of 1,2-difluoroethane is more stable than the anti conformation?

Accepted Answer

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

Question 6

Which conformation of cyclohexane is the most stable and why?

Accepted Answer

Chair conformation, 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 is true about meso compounds?

Accepted Answer

They are achiral and have an internal plane of symmetry.

Question 9

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

Accepted Answer

8

Question 10

Which statement about enantiomers is correct?

Accepted Answer

They rotate plane-polarized light in equal and opposite directions.

Question 11

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 12

Which of the following best describes the Cahn-Ingold-Prelog rules for assigning R/S configuration?

Accepted Answer

Assign priorities based on atomic number; lowest priority group is placed away, and the order of remaining groups determines R (clockwise) or S (anticlockwise).

Question 13

Which of the following is NOT a factor affecting the stability of a conformer?

Accepted Answer

Bond polarity

Question 14

Which conformation of cyclopentane is most stable?

Accepted Answer

Envelope conformation, because it reduces torsional strain.

Question 15

Which of the following is true about SN2 reactions at a stereogenic center?

Accepted Answer

They result in inversion of configuration at the attacked stereocenter.

Question 16

In an E2 elimination reaction, which geometry is most favorable for the reaction to occur?

Accepted Answer

Anti-periplanar geometry

Question 17

Which statement about the Diels-Alder reaction is correct?

Accepted Answer

The diene must be in the s-cis conformation to react.

Question 18

Which product is favored in the Diels-Alder reaction due to secondary orbital interactions?

Accepted Answer

Endo product

Question 19

Which of the following is a characteristic of atropisomers?

Accepted Answer

They are conformationally locked and cannot easily convert to their mirror image.

Conformations, Conformational Analysis, and Stereochemistry in Organic Chemistry

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