CHEM282 Midterm Answers

CHEM 282 Midterm

John Carran February 2012 2h 2 sections Multiple Choice 20 questions (2 marks each) Short Answer 5 questions (12 marks each)

Answer the multiple choice questions on the SCANTRON sheet provided Answer the short answer questions LEGIBLY and CLEARLY in the ANSWER BOOKLET provided.

Note that proctors are unable to answer questions on course content.

1. Which free radical would be most stable?

A)

B)

C)

D)

E) Ans: B allylic and most substituted radical

2. Which would be the best synthesis?

? CH 2   CHCH 3

CH 2CHCH2Cl Br

A)

Propene

B)

Propene C)

D)

E)

Propene Propene Propene

Cl2, hν

Br 2, CCl4

Cl2, 400 oC HCl  NBS, CCl 4 Cl2, CCl4

Br  

Br 2, CCl4 Br 2, hν Cl2, CCl4  NBS, CCl 4

Ans: B

3. 2-Bromo-5-nitroaniline is:

Br 

Br  NH2 Br 

NO2

NO2 NH2

NO2

I

II

H2N

III

OCH3 Br 

Br  H3CO

NO2

NO2

IV

V

A) I B) II C) III D) IV E) V Ans: B 4. We now know that the two Kekule structures for benzene are related in the following way: A) They are each equally correct as a structure for benzene. B) Benzene is sometimes one structure and sometimes the other. C) The two structures are in a state of rapid equilibrium. D) Neither of the two structures adequately describes benzene; benzene is a resonance the two. E) None of the above Ans: D

5. Which of the following statements regarding the cyclopentadienyl cation is correct? A) It is aromatic. B) It is not aromatic. C) It obeys Hückel's rule. D) It undergoes reactions characteristic of benzene. E) It has a closed shell of 6 pi-electrons. Ans: B 6. Which of the following would you expect to be aromatic?

H

H

I

II

III

IV

A) I B) II C) III D) IV E) None of these Ans: B 7. How many resonances would be expected in the broad-band decoupled C-13 spectra of:

A) 1 B) 2 C) 3 D) 4 E) 5 Ans: D 8.

Which of these is the rate-determining step in the nitration of benzene?

A)

Protonation of nitric acid by sulfuric acid

B)

Protonation of sulfuric acid by nitric acid

C)

Loss of a water molecule by the protonated species to produce the nitronium ion

D)

Addition of the nitronium to benzene to produce the arenium ion

E)

Loss of a proton by the arenium ion to form nitrobenzene

Ans: D 9. Which of the following reactions could be used to synthesize tert-butylbenzene? A)

C6H6 + CH 2=C(CH3)2 B)

C6H6  + (CH3)3COH C)

C6H6  + (CH 3)3CCl D)

All of the above

E)

None of the above

H2SO4 H2SO4 AlCl3

Ans: D 10.

Undesired polysubstitution of an aromatic nucleus is most likely to be encountered in the case of:

A)

Friedel-Crafts alkylation

B)

Friedel-Crafts acylation

C)

Nitration

D)

Sulfonation

E)

Chlorination

Ans: A 11.

What would you expect to be the major product obtained from the following reaction?

O

excess

HN

I

B)

II

C)

III

D)

IV

E)

V

?

CH3Cl

O

O

O

HN

HN

HN

I

II

III

O

O

HN

HN

IV

A)

AlCl3

Ans: E which is the activated ring?

V

12. Cl

The reaction of benzene with

in the presence of anhydrous aluminum

chloride produces principally which of these?

II

I

IV

A)

I

B)

II

C)

III

D)

IV

E)

V

III

V

Ans: D Remember rearrangement of carbocations

13. What would be the product of the following reaction sequence? OH O

i) PCl5 ii) C6H6, AlCl3 iii) Zn(Hg), HCl, heat

O

?

O

OH OH

I

II

IV

A)

I

B)

II

C)

III

D)

IV

E)

V

III

V

Ans: E 14.

(Trifluoromethyl)benzene, C6H5CF3, will

A)

nitrate rapidly in the ortho-para positions.

B)

nitrate slowly in the ortho-para positions.

C)

nitrate rapidly in the meta position.

D)

nitrate slowly in the meta position.

E)

fail to nitrate under any conditions.

Ans: D

15. What is the product of the following reaction?

CH

CH2

O

+

CH3C

O C

C

?

CCH3

COCH3

COCH3

COCH3

COCH3

COCH3

COCH3

I

II

III

COCH3

COCH3

 

COCH3 COCH3

IV A) I B) II C) III D) IV E) V Ans: A ……………Diels alder reaction with a slight twist

V

16.

Which would be the best synthesis of the following compound?

CO2CH3 CO2CH3

CO2CH3

CO2CH3

CO

+

+

+ CO2CH3

CO2CH3

I

CH3O2C

II

III

CO2CH3

+

+ 2 HCO2CH3 CO2CH3

IV

A)

I

B)

II

C)

III

D)

IV

E)

V

Ans: B

V

17. What is the major product of the following reaction sequence?

HCN

LiAlH4

HCl

H3O+ ?

O

H2O heat

OH

OH

OH CN

NH2

CN OH

III

II

I

IV

OH

COOH

V

A)

I

B)

II

C)

III

D)

IV

E)

V

Ans:

D

18.

Ignoring stereochemistry, the 1:1 reaction of bromine with 1,3-cyclohexadiene at 25C in the dark and in the absence of peroxide forms which of these?

Br

Br

Br

 

Br 

Br 

I A)

I

B)

II

II

Br

III

Br  

IV

C)

III

D)

IV

E)

Both I and II

Ans: E

19.

Identify the reagent(s) that would bring about the following reaction:

CH3CH2CH2COCl -> CH3CH2CH2CHO A) H2/Ni B) Li/liq.NH3 C) LiAlH[OC(CH3)3]3, ether D) NaBH4, CH3OH E) LiAlH4, ether Ans: C…sterically bulky selective reduction 20. The product, K, of the following sequence of reactions would be:

MgBr 

AlCl3

I

J

H3O+

K 

heat

ether  Cl

(-H2O)

O O HO

I

III

II

OH

V IV

A)

I

B)

II

C)

III

D)

IV

E)

V

Ans: D

1.

How would you synthesise the following molecule starting from benzene and selecting from

among any of the following reagents: CH 3Cl, AlCl3, KMnO4, NaOH, H2SO4, Zn/Hg, HCl, SOCl2, PCl5, NBS, peroxides, CH3COCH3.

Hint: Not all of the reagents listed are necessary but many are. This is a multistep reaction. You have an abundant supply of benzene. You may use reagents more than once if necessary. Assume any solvent/temperature/light conditions you need are available. Mechanisms and solvents are not necessary but the reaction scheme and reagent combinations are. If

temperature is an important consideration in the reaction this should be listed too (eg high temp, or low temp). Answer: Best synthesis and one of the only ones possible with this list of reagents is: Benzene + CH3Cl with AlCl3 first to do a Friedel Crafts reaction, makes toluene. Oxidize toluene with KMnO4 and NaOH, and then add HCl to make benzoic acid, react benzoic acid with SOCl2 to make the acid chloride (PCl5 would have worked too), react the acid chloride with benzene and AlCl3 to make the diphenyl ketone C6H5COC6H5, then a Clemmenson reduction with Zn/Hg/HCl to form the product. There may be variations of this reaction but this is the main one. Attempts were given marks where correct reactions were used. Note….I listed the reagents used in order at the start of this list.

2. Why does the NH2 group direct ortho/para when it is a substituent on a benzene ring in an electrophilic aromatic substitution reaction? For your answer give a mechanistic/resonance explanation for the directing effects of this group and show why me ta substitution is not favoured.

Answer: Read the question very carefully, a mechanism requires a reaction, thus a mechanistic/resonance explanation means a description of arenium ion stabilisation from the EAS reaction. The solution to this question is given in the textbook and from my notes, you did not have to give both ortho and para substitution explanations, one would do along with the meta substitution. Arenium ions should have been shown, and extra resonance structures pointed out for ortho/para. If you only gave a resonance explanation without any EAS reaction or arenium ions you only gained a couple of marks IF your textual description also gave extra reaction information. Careful attention to charges and arrows should have been paid. 3. When toluene reacts with Cl2, the identity of the major product(s) obtained depends on whether the reaction is carried out in the presence of catalytic amounts of FeCl3, or without catalyst, at high temp. Explain clearly. Ans: Reaction with Cl2/FeCl3 results in electrophilic aromatic substitution at the o- and  ppositions, since the methyl group in toluene is an activating,

o-,p-

director. In contrast,

when the reaction is carried out without catalyst, at high temperature, the methyl group undergoes free-radical halogenation via the benzyl radical intermediate.

Cl2 FeCl3

Cl

+ Cl

Cl

Cl2 heat Extra structures for the chlorination should also have been given for the free radical reaction as chlorination is non-selective.

4. Give an example of a diene which cannot react in a Diels Alder reaction. Explain why. Using cyclopentadiene and the following dienophile:

Show the products of the Diels Alder r eaction between the two and explain how the major product is favoured over the minor.

Answer: Any diene that was FIXED in the S-trans conformation would have been accepted, you would not have gotten marks for any diene with free rotation that could adopt an S-cis conformation as this would react. For the second part of the question you need to show secondary orbital overlap with the sp2 hybridised CARBONYL carbons on the dienophile leading to a stabilised transition state favouring the endo major product over the exo. Both structures should have been shown. Many students mentioned a secondary orbital overlap but neglected to say exactly what was overlapping with what. Structures in textbook. 5. Diazotisation is a reaction used in the manufacture of certain dyes and antibiotics. Using a molecule of aniline and a molecule of benzene show how the reaction can be used as part of a synthesis to link these two molecules together with a diazo linkage (-N=N-). A MECHANISM FOR THE FIRST STEP OF THE REACTION IS NOT NECESSARY BUT YOU MUST GIVE A MECHANISM FOR THE STEP THAT LINKS THE TWO MOLECULES TOGETHER. For all steps give a reaction scheme with reagents and structures of the

intermediate and the product. List reagents (and conditions where necessary) on reaction arrows for reaction schemes (10 marks). Why is the diazotisation reaction carried out at low temperatures? (2 marks).

Answer: This is directly out of my notes. You could derive much of this reaction by working backwards given that an EAS reaction links the two aromatic molecules together . We wanted the reagents, the structure of the diazonium SALT (mark deducted for lacking the counterion) and a proper mechanism with the right arrows and charges. The salt is unstable at higher temps and decomposes, often violently.