Aromatic Compounds - Organic Chemistry

AROMATICITY AND THE HUCKEL RULE Minimum requirements for aromaticity: 1. Huckel’s rule must be followed, i.e. the number of π electrons = 4n + 2 where n is a whole number (n=0, 1, 2….). 2. There must be a cyclic, planar array of π electrons. Note: - Examples may include bo th neutral molecules and molecular ions. If an ion confor ms to the minimum requirements for aromaticity, expect that it is stable enough to be easily prepared. - A heteroatom may utilize sp2-hybridized orbitals for bonding in apparent nonconformance to the rules of thumb for determining hybridization state if doing so will result in aromatic stability. Examples: n = 0: vs. 2 π e-s over 3 p orbitals n=0; aromatic

not aromatic; 3rd carbon does not contribute to the π system

n = 1:

not aromatic; not complete cyclic π array

O

N

4n + 2 = 6 π electrons n=1 aromatic

4n + 2 = 4 π electrons n = 1/2 not aromatic

N

N

N N

N pyrrole

furan

6 π electrons delocalized over 5 p orbitals; aromatic

cycloheptatriene not aromatic

imidazole

pyridine

pyrimidine

6 π electrons delocalized over 6 p orbitals; aromatic

8 π electrons delocalized tropylium cation aromatic over 7 p orbitals; not aromatic 6 π electrons delocalized over 7 p orbitals; aromatic

2 NOMENCLATURE OF BENZENOID COMPOUNDS I. Monosubstituted Compounds - may be named as derivatives of benzene. Examples include:

Cl

NO2

nitrobenzene

Br

chlorobenzene

bromobenzene

-special names also accepted by IUPAC for some compounds, such as: CH3

CH3

CH

OH

NH2 CH3

toluene

aniline

phenol

cumene

OCH3

COOH

SO3H

O C

anisole

benzoic acid

benzenesulfonic acid

O

CN

C

H

benzaldehyde

CH CH2 CH3

benzonitrile acetophenone

styrene

II. Disubstituted Compounds: - these are named as derivatives of benzene or the monosubstituted compounds with special names. Three positional isomers are possible, as shown below. These isomers have special designations, which appear as a hyphenated italicized prefix in the IUPAC name of the compound.

Y

Cl Z

1,2-disubstituted ortho (o)

CH3 Cl

o-dichlorobenzene

NO2

o-nitrotoluene

3

Y

Br

Cl

Z

Br m-dibromobenzene

OH m-chlorophenol

1,3-disubstituted meta (m)

Y

NO2

NO2

Z

F

NH2

1,4-disubstituted para (p)

p-fluoronitrobenzene

p-nitroaniline

III. More Highly Substituted Compounds For compounds having greater than two substituents, use numbers to indicate positions of substituents. The carbon bearing the substituent that corresponds to a special name is always assigned the number 1, otherwise the carbons bearing substituents are numbered so that alphabetical ordering is observed.

OH

CH3 O2N

NO2

NO2 2,4,6-trinitrotoluene

O2N

OH NO2

NO2 2,4,6-trinitrophenol

OCH3

CHO 4-hydroxy-3methoxybenzaldehyde

4 ELECTROPHILIC AROMATIC SUBSTITUTION General equation: H

Y + HZ

+ Y-Z

where YZ is the electrophilic reagent and Y + is the electrophile. General mechanism: H (1)

H

H

ADDITION

+ Y+

+

Y

Y

+

+

H Y

the benzenonium cation; a resonance stabilized reaction intermediate

+

H

(2)

Y + Z

Y

ELIMINATION + HZ

KINDS OF EAS REACTIONS (summary) A. Nitration OH +

HNO3

+ N O

Electrophile: nitronium ion +NO2

H2SO4

B. Sulfonation O H

S O H

fuming H2SO4

Electrophile: sulfur trioxide SO3

O

C. Aromatic halogenation H

Electrophile: halonium ion X+

X

X2 FeX3 X = Cl or Br

D. Friedel Crafts Alkylation H

Electrophile: carbocation R+

R

RCl AlCl3

E. Friedel Crafts Acylation H R

O

O

C

C

AlCl3

Cl

R

Electrophile: acylium ion R-C+=O

5 ELECTROPHILIC AROMATIC SUBSTITUTION OF SUBSTITUTED BENZENES Benzene is nitrated by a mixture of concentrated nitric acid and sulfuric acid at around 80 °C. NO2

HNO3 , H2 SO4 80 °C

Further nitration of benzene is considerably more difficult. temperature are required.

Strong acid and higher NO2

NO2 NO2

NO2 fuming HNO3 ,

NO2

+

+

H2 SO4 , 100 °C NO2

NO2 para 0.3 %

ortho 6.4 %

meta 93.2 %

On the other and, toluene undergoes nitration more rapidly than benzene. In this case, the predominant products are the ortho and para isomers. CH3 CH3

CH3

CH3

HNO3 , H2 SO4

+

30 °C

+

NO2

ortho 62 %

NO2

NO2

para 33 %

meta 5%

As shown by the above examples, when an electrophilic reagent attacks an aromatic ring, the group already attached to the ring determines how readily the attack occurs (i.e. reactivity of the ring) and where it occurs (i.e., orientation). CLASSIFICATION OF SUBSTITUENT GROUPS Nearly all groups fall into one of two classes, activating and ortho, para – directing, or deactivating and meta – directing. The halogens are in a class by themselves, being deactivating but ortho, para – directing.

6

CLASSIFICATION OF SUBSTITUENT GROUPS

Activating; directors

ortho,

para Deactivating: directors

Strongly activating O

meta Deactivating; directors

para

O

H

-F

N O

NHR,

NH2,

ortho,

-Cl

CH3

NR2

N

-Br CH3

-I

CH3

Moderately activating

N

O

O R O NH

C

C

C

O OH

C

OR

CH3 SO3H

Weakly activating

O C

O H

C

R

R

THEORY OF REACTIVITY AND ORIENTATION

Reactivity in electrophilic aromatic substitution depends upon the tendency of a substituent group to release or withdraw electrons. The substituent group may exert this effect either by resonance or induction. A group that releases electrons activates the ring. Although it activates all positions of the benzene ring, it activates the ortho and para positions much more than it does the meta position. Because of this, activating groups are also ortho- and para-directors. For example, aniline is highly activated towards electrophilic aromatic substitution and undergoes EAS reactions much faster than ordinary benzene. The reaction products are essentially ortho- and para-substituted. Draw the resonance structures of aniline to account for this behavior.

7

NH2

A

B

C

E

D

resonance structures of aniline A group that withdraws electrons deactivates the ring. Although it deactivates all positions of the benzene ring, the ortho and para positions are especially deactivated. Thus, the electron withdrawing group is also meta-directing because the meta position is least deactivated towards EAS. Draw the resonance structures of benzaldehyde to illustrate this. O C

H

A

B C D resonance structures of benzaldehyde

E

ORIENTATION OF SUBSTITUTION IN DISUBSTITUTED BENZENES The presence of two substituents on a ring makes the problem of orientation more complicated but certain definite predictions can usually be made. The two may be located so that the directive influence of one reinforces the other. This is clearly seen for compounds I, II, and III. The orientation of further substitution is clearly indicated by arrows. O CH3

SO3H

NHCCH3 NC

NO2 NO2 I

II

III

When the directive effect of one group opposes that of the other, it may be difficult to predict the major product. In some cases, complicated mixtures of several products may be obtained. However, the following generalizations may be made: 1.

Strongly activating groups generally win out over the deactivating or weakly activating groups. The sequence of directing power is as follows:

8

O NH2,

2.

OH >

OR,

NH

C

CH3 >

,

R > meta directors

There is often little substitution at the position between two groups that are meta to each other.

Exercises: I.

Predict the major product(s) of each of the following reactions:

1.

OCH3 HNO3, H2SO4

H 3C

O

2.

NHCCH3 Br2, FeBr3

H 3C

O Br2, FeBr3 H

3. OCH3

Cl HNO3, H2SO4

4. CH3

Cl HNO3, H2SO4

5.

Br

9 II.

Supply the missing reagents and intermediate products in each of the following two-step conversions. NO2

1. Br

SO3H

CH3

2.

COOH

3. NO2

SO3H

4. Cl

10 ARENES -

includes alkylbenzenes, alkenylbenzenes, alkynylbenzenes

REACTIONS: A. Free radical halogenation of alkylbenzenes – the preferred site is the sp3-carbon right next to the benzene ring, also known as the benzylic carbon. Cl CH2CH2Cl

CHCH3

benzylic

Cl2 hν

CH2CH3

44 %

56 % CHCH3

Br2 hν

Br exclusive product

reactive intermediate: the resonance-stabilized benzylic free radical

-

the 1° and 2° benzylic halides produced from this reaction are very reactive to both SN1 reactions because of the resonance-stabilization of the benzylic carbocation and S N2 reactions because of low steric congestion around the electron deficient carbon. Tertiary benzylic halides undergo SN1 reactions only. 1°benzylic CH2OH

CH2Cl aq NaOH SN1 or SN2 benzyl chloride

benzyl alcohol

11 B. Reactions of Alkenylbenzenes/Alkynylbenzenes 1. Hydrogenation – occurs rapidly in the side chain using ordinary catalysts. H2, Pd/C

x's H2 high T & P Rh cat.

2. Electrophilic addition - examples: a. Br Recall: Markownikoff's Rule

HBr

reactive intermediate: the resonance-stabilized benzylic carbocation

b. C

C CH3

aq H2SO4 HgSO4

1-phenylpropyne

O C

CH2 CH3

phenyl ethyl ketone the Markownikoff product

C. Oxidation of the Side Chain of Arenes – occurs only if benzylic H is present.

O CHR2

C aq KMnO4, heat

OH

12 CHEMISTRY 40 Problem Set – Aromatic Compounds I.

2.

Indicate whether each of the following compounds or ions is aromatic or not: + a.

c.

b.

d.

Write the structures of the products formed from the reaction of the following reagents with (i) benzene, (ii) ethylbenzene and (iii) benzoic acid. If no reaction occurs, write NR. a. b. c. d. e.

3.

.. O ..

fuming sulfuric acid cold, dilute KmnO4 CH3 COCl, AlCl3 hot KMnO4 Br2/CCl 4, light

f. g. h. i.

CH3Cl, AlCl3 H2O, H+ HNO3, H2SO 4 Cl2, FeCl3

Predict the major product(s) that will be obtained upon monobromination of each of the following compounds. Indicate whether the reaction will be faster or slower than the bromination of benzene. a. nitrobenzene b. benzaldehyde c. aniline

4.

Arrange the following compounds in the order of increasing reactivity towards ring nitration. a. benzene, iodobenzene, aniline, toluene b. O C

OCH3

Cl

CH3

H

5.

Suggest a scheme for the preparation of each of the following compounds from benzene. a. p-bromobenzenesulfonic acid b. 2,5-dichloronitrobenzene c. p-chlorobenzoic acid

d. o-nitrotoluene e. benzyl alcohol f. styrene