Question 1

Which of the following pairs of compounds are enantiomers?

$$\begin{array}{cc}
	ext{I:} & \ce{Cl-CH_2$-CH_2$-CH_3$} \
	ext{II:} & \ce{Br-CH_2$-CH_2$-CH_3$} \
	ext{III:} & \ce{Cl-CH_2$-CH_2$-CH_3$} \
	ext{IV:} & \ce{Cl-CH_2$-CH_2$-CH_2$-CH_3$} \
\end{array}

A) I $$and III
B) II and IV
C) I and II
D) None of these

Accepted Answer

None of these

Question 2

How many stereogenic centers are present in the following molecule?

$$\ce{\begin{array}{c}
\includegraphics[height=1.5cm]{pregnenolone_acetate.png}
\end{array}}

A) 8
B) 7
C) 6
D) 5$$

Accepted Answer

6

Question 3

Which of the following best describes the relationship between the two compounds below?

$$\ce{\includegraphics[height=1.5cm]{diastereomers.png}}

A) $$Identical
B) Enantiomers
C) Constitutional isomers
D) Diastereomers

Accepted Answer

Diastereomers

Question 4

Which of the following pairs of Fischer projections represent the same compound?

$$\begin{array}{cc}
	ext{I:} & \ce{CH_3$-CHOH-CH_2OH} \
	ext{II:} & \ce{CH_3$-CHOH-CH_2OH} \
	ext{III:} & \ce{CH_3$-CH(OH)-CH_2OH} \
	ext{IV:} & \ce{CH_3$-CH(OH)-CH_2OH} \
	ext{V:} & \ce{CH_3$-CH(OH)-CH_2OH} \
\end{array}

A) I $$and II
B) II and IV
C) III and IV
D) IV and V

Accepted Answer

III and IV

Question 5

Rank the following groups according to the Cahn-Ingold-Prelog R/S configuration rules from highest (#1) to lowest (#4) priority:

$$1.\ \ce{CH_2OCH_3}$$

$$2.\ \ce{CH=CH_2$}$$

$$3.\ \ce{C\equiv CH}$$

$$4.\ \ce{CH_2CH_2CH_3}

A) 2$$ \u003e 4 \u003e 3 \u003e 1
B) 1 \u003e 3 \u003e 2 \u003e 4
C) 4 \u003e 1 \u003e 3 \u003e 2
D) 3 \u003e 2 \u003e 1 \u003e 4

Accepted Answer

3 \u003e 2 \u003e 1 \u003e 4

Question 6

Assuming no other changes, what effect on the rate would result from simultaneously doubling the concentrations of both 2-iodopentane and sodium acetate in the following reaction?

$$\ce{CH_3CH_2CH_2CH_2CH_2I + NaOAc \rightarrow CH_3CH_2CH_2CH_2CH_2OAc + NaI}

A) No $$effect
B) It would double the rate
C) It would triple the rate of the reaction
D) It would increase the rate four times

Accepted Answer

It would increase the rate four times

Question 7

Reaction of (R,S)-3-bromo-3-methylhexane with excess NaI in acetone gives both (R)- and (S)-3-iodo-3-methylhexane (racemic). What is the explanation that best describes this transformation?

A) An $$S_N2$$ reaction has occurred with inversion of configuration
B) An $$S_N1$$ reaction has occurred due to carbocation formation
C) An $$S_N2$$ reaction has occurred resulting in inversion of configuration
D) Racemization followed by $$S_N2$$ 'backside' attack

Accepted Answer

An $$S_N1$$ reaction has occurred due to carbocation formation

Question 8

Which ion is the strongest nucleophile in aqueous solution?

A) $$\ce{Br^-}
B$$) $$\ce{I^-}
C$$) $$\ce{F^-}
D$$) $$\ce{Cl^-}$$

Accepted Answer

$$\ce{I^-}$$

Question 9

In $$S_N1$$ reactions, which of the following will increase the rate of reaction?

A) Use water as the solvent
B) Use hydroxide ion as the nucleophile
C) Limit the amount of substrate (1-bromopentane)
D) Use a polar aprotic solvent like DMSO

Accepted Answer

Use water as the solvent

Question 10

Which statement about meso compounds is TRUE?

A) All molecules with stereocenters (chiral carbons) are chiral
B) Two enantiomers always have the same boiling point
C) A solution that has a 50% enantiomeric excess has a 50% of one isomer and 50% of the other
D) Meso compounds have one stereocenter and are chiral

Accepted Answer

Two enantiomers always have the same boiling point

Question 11

Select the potential energy diagram that represents an $$S_N2$$ endothermic (endergonic) reaction.

A) I
B) II
C) III
D) IV

Accepted Answer

III

Question 12

Which of the following alkyl bromide isomers would most readily undergo an $$S_N2$$ reaction?

A) Bromocyclobutene
B) 1-bromo-1-butene
C) 1-bromo-1-methylcyclopentane
D) 2-bromo-2-butene

Accepted Answer

1-bromo-1-butene

Question 13

Which selection below would be most favorable for the conversion of 1-bromopentane to 1-pentanol?

A) Use water as the solvent
B) Use hydroxide ion as the nucleophile
C) Limit the amount of substrate (1-bromopentane)
D) Use a polar aprotic solvent like DMSO

Accepted Answer

Use hydroxide ion as the nucleophile

Question 14

Which of the following is NOT a characteristic of $$S_N2$$ reactions?

A) Bimolecular rate-determining step
B) Inversion of configuration
C) Formation of carbocation intermediate
D) Favored by polar aprotic solvents

Accepted Answer

Formation of carbocation intermediate

Question 15

Which of the following statements about optical activity is TRUE?

A) Racemic mixtures rotate plane-polarized light
B) Meso compounds rotate plane-polarized light
C) Enantiomers rotate plane-polarized light in opposite directions
D) Diastereomers always rotate plane-polarized light in the same direction

Accepted Answer

Enantiomers rotate plane-polarized light in opposite directions

Question 16

Which of the following is the best leaving group for an $$S_N2$$ reaction?

A) $$\ce{OH^-}
B$$) $$\ce{I^-}
C$$) $$\ce{Br^-}
D$$) $$\ce{$$OCH_3$$^-}$$

Accepted Answer

$$\ce{I^-}$$

Question 17

Which of the following carbocations is the most stable?

A) $$\ce{CH_3$^+}
B$$) $$\ce{(CH_3$)_3C^+}
C$$) $$\ce{CH_2$=CH-CH_2$^+}
D$$) $$\ce{C_6H_5^+}$$

Accepted Answer

$$(CH_3$)_3C^+$$

Question 18

Which of the following best explains why $$S_N2$$ reactions are faster in polar aprotic solvents?

A) Polar aprotic solvents stabilize the nucleophile, making it less reactive
B) Polar aprotic solvents do not solvate anions well, making nucleophiles more reactive
C) Polar aprotic solvents stabilize the leaving group
D) Polar aprotic solvents increase the concentration of the substrate

Accepted Answer

Polar aprotic solvents do not solvate anions well, making nucleophiles more reactive

Organic Chemistry Exam 3 Study Guide: Stereochemistry, Nucleophilic Substitution, and Alkyl Halides

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