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Quiz 2 Answer Key

Q1a

Which of the following will not react with a grignard reagent?

A) butyraldehyde

B) butan-2-one

C) 2-butanol

D) 1-butene

E) 1-butyne

S1a

D) 1-butene

Q1b

The addition of a grignard reagent to a carbonyl is based on collision of a nucleophile to fill what acceptor orbital?

A) a s bonding orbital

B) a s* antibonding orbital

C) an empty p orbital

D) a p  bonding orbital

E) a p* antibonding orbital

S1b

E) a p* antibonding orbital

Q1c

Which reactions will not give a product as a racemic mixture?

A) the addition of phenyl grignard to propionaldehyde

B) epoxidation of an allylic alcohol with a Ti(OiPr)4, tBuOOH, and (–)-diethyl tartrate

C) the addition of NaOCH3 to cyclohexene oxide

D) the acid-catalyzed addition of methanol to cyclohexene oxide

E) an SN1 reaction

S1c

B) epoxidation of an allylic alcohol with a Ti(OiPr)4tBuOOH, and (–)-diethyl tartrate

Q1d

Which of the following is not a true statement:

A) a silyl ether bond can be cleaved by a fluoride source, such as TBAF

B) LiAlH4 is an oxidizing agent

C) epoxide formation using mCPBA with an alkene proceeds via a concerted mechanism

D) H2CrO4 will oxidize a secondary alcohol to a ketone

E) arene oxides can react with DNA and initiate cancer-causing pathways

S1d

B) LiAlH4 is an oxidizing agent

Q1e

In the boxes provided, indicate whether each of the following reactive carbon atoms is considered to be electrophilic (E) or nucleophilic (N).  R is used to indicate either H or alkyl.

a carbon attached to a metal (Li, Pd, Mg, etc) will be electron rich (nucleophilic), while a carbon attached to an electronegative atom (O, Cl, N, etc) will have a partial positive charge (electrophilic)

 

Q2

2. (6 pts) Draw the resonance contributor for a ketone, showing any curved arrows for its formation. Briefly explain what this resonance indicates about the reactivity of a carbonyl.

The resonance structure of a carbonyl, which has a positive charge on carbon, indicates that the sp2carbon is a very good electrophile.  The oxygen also has enhanced nucleophilicity and basicity as indicated by the anion.

Q3

3. (8 pts) Ethylene oxide is the starting material for the synthesis of “Cellosolve”, an important industrial solvent.  Provide a curved arrow mechanism for this reaction.

 

Note, this was question 11.20 from recommended practice problems.

 

Q4

4. (8 pts) Provide a curved arrow mechanism for the following transformation.

The mechanism involves an SN2 reaction followed by an elimination.  There is no oxidation change of the carbon.

 

Q5a

(10 pts) Show how to synthesize the following alcohol where all carbons in the molecule must come from compounds with 3 or fewer carbon atoms.  Include all necessary reagents for the synthesis you propose.

 

This answer is the shortest synthesis, but there are other possible routes. 

 

Q5b

(10 pts) Explain why you would need to use a protecting group strategy in a synthesis and give an example of a synthesis that requires the use of a protecting group.  Your synthesis must have at least 3 steps to receive full credit.

 

Protecting groups are required when a functional group you don’t want to modify is incompatible with a given set of conditions (Oxidation, reduction, nucleophilic attack, acid base chemistry etc.).  Protecting groups mask a functional group until they can be removed.

 

There are many correct answers to this question.

Example:

Q6

6. (8 pts) An aziridinium ion follows a similar reactivity pattern to an epoxide.  The following aziridinium ion reacts with NaOCH3 to form compounds A and B.  Given what you know about the reactivity patterns of epoxides and the information given below, identify the structures of both compounds A and B (in boxes provided).

 

 

Compound A is a ring-opening product that is identical to the reactivity of an epoxide. 

 

Due to the positive charge on N, which makes a good leaving group, compound B is an elimination product and the alkene will react with Br2, which will change colors once the reaction takes place.