Q: If the energy of a CH$$_3$$–CH$$_3$$ eclipsing interaction is 2.5 kcal/mol and a CH$$_3$$–CH$$_2$$ gauche interaction is 0.95 kcal/mol, what is the total energy for a conformation with two CH$$_3$$–CH$$_3$$ eclipsing and one CH$$_3$$–CH$$_2$$ gauche interaction?

$$2 \times 2.5 + 0.95 = 5.95$$ kcal/mol

Question 1

In the reaction $$\mathrm{C_2H_5OH} + \mathrm{NO_2$^-} \rightarrow \mathrm{C_2H_5O^-} + \mathrm{$$HNO_2$$}$$, which species acts as the Brønsted-Lowry acid?

Accepted Answer

$$\mathrm{C_2H_5OH}$$

Question 2

Given the p$$K_a$$ values $$\mathrm{C_2H_5OH}$$ ($$pK_a = 16) $$and $$\mathrm{$$HNO_2$$}$$ ($$pK_a = 3.3$$), will the forward reaction $$\mathrm{C_2H_5OH} + \mathrm{NO_2$^-} \rightarrow \mathrm{C_2H_5O^-} + \mathrm{$$HNO_2$$}$$ occur to a significant extent?

Accepted Answer

No, because the reaction favors formation of the weaker acid.

Question 3

Which of the following C$$_4H$_{10}$$ isomers has the highest boiling point, and why?

Accepted Answer

n-butane, because it has less branching and more surface area.

Question 4

A molecule contains 4 degrees of unsaturation, 13 sp$$^2$$ carbons, and 5 sp$$^2$$ oxygens. Which functional groups are most likely present?

Accepted Answer

Aromatic ring, cyclic ether/lactone, alcohol, and carboxylic acid

Question 5

How many stereoisomers are possible for a molecule with 5 stereogenic centers?

Accepted Answer

$$2^5 = 32$

Question 6

Given the Fischer projection of 2,3-dihydroxybutanedioic acid, how many chiral centers does it have?

Accepted Answer

2

Question 7

Which of the following is the correct IUPAC name for the compound with the structure $$\mathrm{CH_3CH(CH_3$)CH_2CH_3}$$?

Accepted Answer

2-methylbutane

Question 8

For the cis-1-chloro-4-methylcyclohexane, which conformation is most stable?

Accepted Answer

Both substituents equatorial

Question 9

In a Newman projection along the C2–C3 axis of 2,3-dimethylpentane, which conformation is lowest in energy?

Accepted Answer

Anti conformation

Question 10

If the energy of a CH$$_3$$–CH$$_3$$ eclipsing interaction is 2.5 kcal/mol and a CH$$_3$$–CH$$_2$$ gauche interaction is 0.95 kcal/mol, what is the total energy for a conformation with two CH$$_3$$–CH$$_3$$ eclipsing and one CH$$_3$$–CH$$_2$$ gauche interaction?

Accepted Answer

$$2 	imes 2.5 + 0.95 = 5.95$$ kcal/mol

Question 11

Which compound will show the greatest number of signals in its $$^1H $$NMR spectrum?

Accepted Answer

2-methylbutane

Question 12

A radical chlorination of 2,2-dimethylbutane produces three monochlorinated isomers. Which isomer is formed in the highest percentage?

Accepted Answer

3-chloro-2,2-dimethylbutane

Question 13

How many chiral centers are present in 2-chloro-2,2-dimethylbutane?

Accepted Answer

0

Question 14

Which of the following best explains why n-butane has a higher boiling point than 2-methylpropane?

Accepted Answer

n-Butane has more surface area and stronger van der Waals forces.

Question 15

What is the correct IUPAC name for the compound with the structure $$\mathrm{CH_3CH_2CH(CH_3$)CH_3$}$$?

Accepted Answer

2-methylpentane

Question 16

Which of the following statements about stereoisomers is correct?

Accepted Answer

Enantiomers have identical physical properties except for their interaction with plane-polarized light.

Question 17

In a molecule with 4 chiral centers, how many possible stereoisomers exist?

Accepted Answer

$$2^4 = 16$

Question 18

Which of the following is NOT a functional group present in the structure shown in question 3 of the exam?

Accepted Answer

Amine

Question 19

Which of the following best describes the most stable conformation of trans-1-chloro-4-methylcyclohexane?

Accepted Answer

One substituent axial, one equatorial

Organic Chemistry Exam Practice: Acid-Base, Stereochemistry, NMR, and Conformational Analysis

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