University of Texas, Rio Grande Valley - CHEM 2325

Homework - I (CHEM 2325-03) Fall 2016

[General Review, Aromaticity & Electrophilic Aromatic Substitution]

Short answer/Multiple type questions

1. In case of covalent non-polar bond the difference in electronegativity between two bonded atoms is………..
2. In case of covalent polar bond the difference in electronegativity between two bonded atoms is………..
3. If the electronegativity difference between two bonded atoms is higher than

……………., the bond is called ionic bond.

4. Higher the electronegativity difference between two bonded atoms, more ……………..

is the covalent bond.

5. Carbon is ……..hybridized in methane whereas... hybridized in ethylene.
6. Carbon is ……..hybridized in ethane whereas.... hybridized in acetylene.
7. How many σ-bond and π-bond are present in ethylene (C2H4) molecule?

8. How many σ-bond and π-bond are present in acetylene (C2H2) molecule?

9. In benzene, the carbons are................ hybridized.

10. As the number of nearly equivalent resonance contributors.. , the

resonance energy …………….

11. The carbonyl carbon in acetic acid is…......... hybridized whereas the carbon

in methanol is……………

12. The carbonyl carbon in acetic acid contains……% s-character whereas the carbon in methanol contains…    % s-character.
13. EDG stabilizes the……………making the reaction…. whereas EWG

destabilizes the carbocation making the reaction …………….

14. ……………..is an activating group in EAS whereas... is a deactivating group.
15. The halogens......... the benzene ring in electrophilic

……………………substitution.

16. All........ derectors deactivate the benzene ring.

17. Write the structure of TNT.

18. Write the name and formula of the catalyst for Fridel-Crafts alkylation and acylation.

19. What is the major product in the following reaction?

20. Why do atoms bond?
21. What is electronegativity?
22. What do you mean by dipole moment? Explain, considering water molecule as an example.
23. Why chloroform (CHCl3) is polar whereas carbon tetrachloride (CCl4) is non-polar?
24. NH3 is a polar molecule but BF3 is non-polar—justify.
25. What do you mean by (i) σ-bond and (ii) π-bond? Explain.
26. Explain the terms “resonance contributor” and “resonance hybrid” considering benzene as an example.
27. What makes resonance structure have decreased stability?
28. The double bond in cyclohexene can be oxidized by potassium permanganate (KMNO4), whereas the double bonds in benzene do not undergo this oxidation reaction — Justify.

29. Benzene is aromatic whereas 1,3,5-hexatriene is nonaromatic — Explain.
30. Cyclooctatetraene is a cyclic molecule with conjugated π-system having 8 π electrons but it is non-aromatic — Why?

31. Write the two aspects of the electrophilic aromatic substitution reaction that are affected by a substituent? OR how the rate of the reaction and orientation are affected in EAS?

32. Write the four requirements of a molecule to be aromatic.
    • Structure must be Planar, Cyclic with Conjugated π (pi) bonds.
    • Each atom in the ring must have an unhybridized p orbital.
    • The p orbitals must overlap continuously around the ring. (Usually planar structure)
    • Compound is more stable than its open-chain counterpart.

33. Explain Hückel’s Rule of aromaticity with examples.
34. Cyclobutadiene is not available to purchase directly from the market — Why?
35. Clarify the terms: (a) Bonding MO (b) Antibonding MO (c) HOMO (d) LUMO
36. Pyrrole is a heteroaromatic five membered (I) organic base (pKa = 0.4) whereas pyridine is a heteroaromatic six membered (II) organic base (pKa = 5.25). Explain why pyridine is 13 times more basic than pyrrole.
37. Explain with appropriate figures why o- and/or p- substitution are fovored for EDG whereas m-substitution is preferred for EWG in electrophilic aromatic substitution.
38. Halogens are electron withdrawing but are ortho/para director in electrophilic aromatic substitution-Explain.
39. What is electrophilic acylium cation? How does it form and affect Friedel-Crafts acylation reaction?

40. Write the _________________of anisole with acetic anhydride.
41. Discuss the limitations in electrophilic aromatic substitutions with examples.
42. Go over the examples of the four rules related to additivity of substituent effects in the case of EAS.
43. Ethyl benzene is treated with (i) Br2 and FeBr3 and (ii) Br2 and light or heat separately. Do you think the products will be same? Justify your answer.
44. Go through the oxidation and reduction products of substituted benzenes. Which reagents are used for (i) Clemmensen reduction and (ii) Wolff-Kishner reduction?
45. Two hydrogen atoms can combine to form H2 but two He atoms cannot form He2

— Justify from MO viewpoint.

46. What is ipso substitution? Give an example [structures need to be drawn].

47. Why aromatic compounds are more stable than their aliphatic counterpart? Draw the MO diagram of benzene.

48. What is the relation between numbers of node with energy of the orbital?
49. Explain the terms “Aromatic”, “Non-aromatic” and “Antiaromatic” considering the examples of Cyclopentadienyl anion, Cyclopentadienyl radical and Cyclopentadienyl cation respectively.

50. Cycloheptatrienylium cation is more stable than Cycloheptatriene — Why?
51. Cyclopropenyl cation is a planar molecule. Is it “Aromatic”, “Non-aromatic” or “Antiaromatic”?
52. Imidazole with 8 π electrons is aromatic — Why?
53. Furan with 8 π electrons is aromatic — Why?
54. Thiophene with 8 π electrons is aromatic — Why?
55. Pyrene with 16 π electrons is aromatic — Justify.
56. The compound [10] Annulene is not aromatic — Justify.
57. What do you mean by “Pericondensed” and “Catacondensed” polycyclic aromatic hydrocarbons (PAHs)? Give two examples in each case (structures are required).
58. EAS can be considered as Lewis acid-Lewis base reaction — Justify.
59. How the electrophilicity of bromine (a weak electrophile) can be increased by using FeBr3?
60. Write the name of the electrophiles in nitration and sulfonation reactions?
61. Write the difference between ‘reagent’ and ‘catalyst’ of a reaction.
62. Write the name and resonance structures of the electrophile in Friedel-Crafts acylation reaction.

63. What is the ‘missing reagent’ in the following reaction?
64. Write the mechanism of the following reaction:
65. Write the mechanism of the following reaction:
66. Write the mechanism of the following transformation:
67. Write the mechanism of the following transformation:
68. Write the mechanism of the following transformation:
69. Write the mechanism of the following transformation:
70. Write the mechanism of the following transformation:
71. Why n-Propylbenzene cannot be synthesized by Friedel-Crafts alkylation reaction?
72. Write the name and structure of the electrophile in (a) Diazotization (b) Vilsmeier–Haack (c) Gattermann–Koch (d) Reimer–Tiemann reactions.

Representative multiple choice questions

1. As energy of MO increases, the number of nodes
   1. decreases
   2. remains same
   3. increases
   4. either (a) or (b)
   5. none of the above

2. The name of the following compound is:
   1. Tetrabenzene
   2. Chrysene
   3. Coronene
   4. Anthracene
   5. Pyrene

3. To achieve noble gas configuration, hydrogen requires…. whereas

chlorine requires…........... in outermost shell.

1. 1. Octet, duet
   2. Singlet, duet
   3. Duet, octet
   4. Octet, singlet
   5. None of the above

4. The................. the number of relatively stable resonance contributors, the

…………… the resonance energy.

1. 1. Higher, lower
   2. Lower, higher
   3. Higher, higher
   4. Lower, lower
   5. None of the above

5. The name of the following compound is:
   1. Benzene
   2. Toluene
   3. Formaldehyde
   4. Benzaldehyde
   5. Crotonaldehyde

6. Phenanthrene is ----------------,---------- compound.
   1. Bicyclic, aromatic
   2. Substituted, antiaromatic
   3. Tricyclic, aromatic
   4. Alcoholic, nonaromatic
   5. Acidic, antiaromatic

7. The number of π (pi) electrons in anthracene is: a. 6; b. 8; c. 10; d. 12; e. 14

8. Thiophene is a--------------------- compound.
   1. Five membered carbocyclic
   2. Six membered heterocyclic
   3. Seven membered cyclic
   4. Five membered heteroaromatic
   5. Aliphatic

9. The name of the following compound is:
   1. Imidazole
   2. Indole
   3. Pyridine
   4. Thiophene
   5. Pyrrole

10. A four membered, cyclic, planar, completely conjugated compound with sp2 hybridized carbons and 4 (pi) electrons will be:

1. Heteroaromatic
2. Aromatic
3. Nonaromatic
4. Antiaromatic
5. None of the above

11. The following planar cation will be:

1. Aromatic
2. Heteroaromatic
3. Antiaromatic
4. Both (a) & (b)
5. Nonaromatic

12. As energy of MO increases, the number of nodes

1. decreases
2. remains same
3. increases
4. Either (a) or (b)
5. None of the above

13. The name of the following compound is:

1. Pyridine
2. Pyrrole
3. Pyrimidine
4. Indole
5. Imidazole

14. Atorvastatin (Lipitor), the top selling marketed drug, is a derivative of

1. Pyrrole
2. Indole
3. Thiophene
4. Imidazole
5. None of the above

15. The compound furan is ……………….

1. aliphatic nitrogen heterocycle
2. aromatic nitrogen heterocycle
3. carbocyclic sulfur heterocycle
4. aromatic oxygen heterocycle
5. aliphatic oxygen heterocycle

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