Organic Chem Test 3

Question 1

To be aromatic, a ring must..

Answer 1

1. Have one 2 p orbital on each of its atoms.
2. Be planar or nearly planar, so that there is continuous overlap or nearly continuous overlap of all 2 p orbitals of the ring.
3. Have 2, 6, 10, 14, 18, or so forth pi electrons in the cyclic arrangement of 2 p orbitals.

Question 2

What are the Monosubstituted Benzenes

Answer 2

Ethylbenzene (CH2CH3)
Toluene (CH3)
Styrene (CH CH2)
Phenol (OH)
Aniline (NH2)
Benzaldehyde (O=, H)
Benzoic acid (O=, OH)
Anisole (OCH3)

Question 3

What is different between alkylbenzenes and benzenes?

Answer 3

Alkylbenzenes, like other hydrocarbons, are nonpolar and thus have lower boiling points.

Benzenes have polar substituents such as phenol, aniline, and benzoic acid. (High boiling points)

Question 4

What does the melting point of a substituted benzene rely on?

Answer 4

Depend on whether or not their molecules can be packed close together.
Benzene, which has no substituents and is flat, can pack its molecules very closely, giving it a considerably higher melting point than many substituted benzenes.

Question 5

What makes a phenyl group (Ph)

Answer 5

The substituent group derived by the loss of an H from benzene.

Question 6

What makes a benzyl group (Bn)

Answer 6

That derived by the loss of an H from the methyl group (CH3) of toluene. (CH3->CH2)

Question 7

When two substituents occur on a benzene ring, how many constitutional isomers are possible.

Answer 7

Three constitutional isomers are possible.

Question 8

How do we locate substituents on a benzene ring

Answer 8

1,2‐ ortho
1,3‐ meta
1,4‐ para

Question 9

What is a Polynuclear aromatic hydrocarbons (PAHs). Give examples.

Answer 9

Contain two or more aromatic rings, each pair of which shares two ring carbon atoms.
ex. Naphthalene, anthracene, and phenanthrene

Question 10

What is a benzylic carbon?

Answer 10

The carbon immediately bonded to the benzene ring.

Question 11

What happens if a benzylic hydrogen exists?

Answer 11

Then the benzylic carbon is oxidized to a carboxyl group and all other carbons of the side chain are removed

Question 12

What happens if no benzylic hydrogen exists?

Answer 12

If no benzylic hydrogen exists, as in the case of tert‐butylbenzene,
then the side chain is not oxidized.

Question 13

What happens if more than one alkyl side chain exists?

Answer 13

Each is oxidized to COOH

Question 14

What is the most characteristic reaction of aromatic compounds? Give examples.

Answer 14

Substitution at a ring carbon
ex. halogens
-the nitro (NO2)
-the sulfonic acid (SO3H)
-alkyl (R)
-acyl (RC=O)

Question 15

What is Ortho–para directing

Answer 15

Any substituent on a benzene ring that directs electrophilic aromatic substitution preferentially to ortho and para positions.

Question 16

What is Meta directing

Answer 16

Any substituent on a benzene ring that directs electrophilic aromatic substitution preferentially to a meta position.

Question 17

What is a Activating group

Answer 17

Any substituent on a benzene ring that causes the rate of electrophilic aromatic substitution to be greater than that for benzene.

Question 18

What is a Deactivating group

Answer 18

Any substituent on a benzene ring that causes the rate of electrophilic aromatic substitution to be lower than that for benzene

Question 19

What occurs when an existing substituent influences a further electrophilic aromatic substitution?

Answer 19

1. Substituents affect the orientation of new groups. Certain substituents direct a second substituent preferentially to the ortho and para positions; other substituents direct it preferentially to a meta position.
2. Substituents affect the rate of further substitution. Certain substituents cause the rate of a second substitution to be greater than that of benzene itself, whereas other substituents cause the rate of a second substitution to be lower than that of benzene. (activating or deactivating)

Question 20

Is a nitro group activating or deactivating?

Answer 20

strongly deactivating

Question 21

What generalizations can be made if we compare ortho–para and meta directors for structural similarities and differences?

Answer 21

1. Alkyl groups, phenyl groups, and substituents in which the atom bonded to the ring has an unshared pair of electrons are ortho–para directing. All other substituents are meta directing.
2. Except for the halogens, all ortho–para directing groups are activating toward further substitution. The halogens are weakly deactivating.
3. All meta directing groups carry either a partial or full positive charge on the atom bonded to the ring. All meta directing groups are deactivating.

Question 22

What does the rate of electrophilic aromatic substitution determined by?

Answer 22

-By the slowest step in the mechanism
-In every reaction of an electrophile with the aromatic ring, the attack of the electrophile on the ring to give a resonance‐stabilized cation intermediate is the slowest step

Question 23

What is the rate‐determining step in nitration?

Answer 23

The reaction of the nitronium ion with the aromatic ring to produce a resonance‐stabilized cation intermediate.

Question 24

What way does nitration of anisole occur preferably?

Answer 24

-Since the cation formed by reaction at an ortho or para position on anisole has a greater resonance stabilization
-It has a lower activation energy for its formation
-Nitration of anisole occurs preferentially in the ortho and para positions.

Question 25

Which position yields a lower activation energy? (meta, para, ortho)

Answer 25

The activation energy for reaction at a meta position is less than that for reaction at a para position

Question 26

What positions are taken depending on the stabilization of the aromatic ring? (ortho, meta, para)

Answer 26

-any substituent on an aromatic ring that further stabilizes the cation intermediate directs ortho–para

-any group that destabilizes the cation intermediate directs meta

Question 27

What considerations must be taken into account for the activating–deactivating effects of substituent groups?

Answer 27

1. Any resonance effect of NH2, OH, and OR
   -Increase the rate of electrophilic aromatic substitution, compared with the rate at which benzene itself reacts.
   -Decrease the activation energy
2. Any resonance or inductive effect, of NO2, C=O, SO3H, NR3+, CCl3, and CF3
   -Decreases electron density on the ring, deactivates the ring to further substitution
   -Decrease the rate of further electrophilic aromatic substitution, compared with the rate at which benzene itself reacts.
3. Any inductive effect of CH3 or another alkyl group
   -Releases electron density toward the ring, activates the ring toward further substitution

Question 28

What are the resonance and inductive effects of halogens?

Answer 28

1. The inductive effect of halogens.
   -The halogens are more electronegative than carbon
   - Have an electron‐withdrawing inductive effect.
   -Aryl halides, react more slowly in electrophilic aromatic substitution than benzene does.
2. The resonance effect of halogens.
   -A halogen ortho or para to the site of electrophilic attack
   stabilizes the cation
   -ortho-para directing