Aromatic Compounds - Organic Chemistry

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 sp

2

-hybridized orbitals for bonding in apparent

non-conformance to the rules of thumb for determining hybridization state if doing so

will result in aromatic stability.

Examples:

n = 0:

n = 1:

2 π e-s _over 3 p orbitals n=0; aromatic vs. not aromatic; 3rd carbon does not contribute to the π system not aromatic; not complete cyclic π array 4n + 2 = 4 π electrons n = 1/2 not aromatic 4n + 2 = 6 π electrons n = 1 aromatic N O N N N N N

pyrrole furan _imidazole 6 π electrons delocalized

over 5 p orbitals; aromatic

pyridine pyrimidine 6 π electrons delocalized over 6 p orbitals; aromatic

cycloheptatriene not aromatic

tropylium cation aromatic

6 π electrons delocalized over 7 p orbitals; aromatic

8 π electrons delocalized over 7 p orbitals; not aromatic

NOMENCLATURE OF BENZENOID COMPOUNDS

I. Monosubstituted Compounds

- may be named as derivatives of benzene. Examples include:

NO

₂ Cl Br

nitrobenzene chlorobenzene bromobenzene

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

CH₃ toluene CH CH₃ CH₃ cumene NH₂ aniline OH phenol OCH₃ anisole COOH benzoic acid SO₃H benzenesulfonic acid

C

O

H

benzaldehyde CN benzonitrile C O CH₃ acetophenone

CH CH

₂ 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

Z

1,2-disubstituted ortho (o)

Cl

Cl

o-dichlorobenzene

CH

₃

NO

₂ o-nitrotoluene

Y

Z

1,3-disubstituted meta (m)

Br

Br

m-dibromobenzene

Cl

OH

m-chlorophenol

Y

Z

1,4-disubstituted para (p)

NO

₂

F

p-fluoronitrobenzene

NO

2

NH

₂ 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.

CH₃ NO2 O₂N NO2 2,4,6-trinitrotoluene

OH

NO

2

O

₂

N

NO

2 2,4,6-trinitrophenol

OH

OCH

₃

CHO

4-hydroxy-3-methoxybenzaldehyde

ELECTROPHILIC AROMATIC SUBSTITUTION

General equation:

+ HZ Y + Y-Z H

where YZ is the electrophilic reagent and Y

+

is the electrophile.

General mechanism:

(2) (1) ELIMINATION + HZ + Z -Y + H Y + + H Y H Y + H Y ADDITION + Y+ H

the benzenonium cation; a resonance stabilized reaction intermediate

KINDS OF EAS REACTIONS (summary)

A.

+ H + HNO₃ H₂SO₄ N O O

-Nitration

B.

H fuming H2SO4 S O O O H

Sulfonation

C.

H X₂ FeX₃ X X = Cl or Br

Aromatic halogenation

D.

H RCl AlCl₃ R

Friedel Crafts Alkylation

E.

Friedel Crafts Acylation

R C O Cl C O R AlCl₃ H

Electrophile: nitronium ion

+NO

2

Electrophile: sulfur trioxide

SO

3

Electrophile: halonium ion

X+

Electrophile: carbocation

R+

Electrophile: acylium ion

R-C

+

=O

ELECTROPHILIC AROMATIC SUBSTITUTION OF SUBSTITUTED BENZENES

Benzene is nitrated by a mixture of concentrated nitric acid and sulfuric acid at around 80 °C.

Further nitration of benzene is considerably more difficult. Strong acid and higher temperature are required.

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

.

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.

NO₂ HNO₃, H₂SO₄ 80 °C + + HNO₃, H₂SO₄ 30 °C CH₃ NO₂ CH₃ NO₂ CH₃ NO₂ CH₃ ortho 62 % para 33 % meta 5 % ortho 6.4 % para 0.3 % meta 93.2 % + + fuming HNO₃, H₂SO₄, 100 °C NO₂ NO₂ NO₂ NO₂ NO₂ NO₂ NO₂ NO₂

CLASSIFICATION OF SUBSTITUENT GROUPS

Activating; ortho,

para Deactivating:

directors

meta

Deactivating;

directors

ortho, para

directors

-F

-Cl

-Br

-I

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.

O H NH₂, Strongly activating NHR, NR₂ Weakly activating R Moderately activating O R NH C CH₃ O N O O N CH₃ CH₃ CH₃ C N C O OH C O OR SO₃H C O H _C O R

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.

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.

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:

NH2

A B _{C D E}

C

A B C _{D E}

O _H

resonance structures of benzaldehyde

CH₃ NO₂ SO₃H NO₂ NC NHCCH₃ O I II III

2.

There is often little substitution at the position between two groups that are

meta to each other.

I.

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

Exercises:

1.

2.

3.

4.

5.

NH2, OH > OR, NH C CH3 O > _, R > meta directors Cl CH₃ HNO₃, H₂SO₄ Cl Br HNO₃, H₂SO₄ OCH₃ H₃C HNO3, H2SO4 NHCCH₃ H₃C Br2, FeBr3 O Br₂, FeBr₃ OCH₃ H O

II.

Supply the missing reagents and intermediate products in each of the

following two-step conversions.

1.

2.

3.

4.

NO2 Br SO3H CH₃ COOH NO2 Cl SO3H

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.

- 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 SN2 reactions because of low steric congestion around the electron deficient carbon. Tertiary benzylic halides undergo S_N1 reactions only.

CH₂CH₃ benzylic Cl₂ hν Br₂ hν CHCH₃ CH₂CH₂Cl CHCH₃ Cl 56 % 44 % exclusive product

reactive intermediate: the resonance-stabilized benzylic free radical Br

CH₂Cl 1°benzylic

aq NaOH

CH₂OH

benzyl chloride benzyl alcohol

B. Reactions of Alkenylbenzenes/Alkynylbenzenes

1. Hydrogenation – occurs rapidly in the side chain using ordinary catalysts.

2. Electrophilic addition - examples:

a.

b.

C. Oxidation of the Side Chain of Arenes – occurs only if benzylic H is present. H₂, Pd/C

x's H2 high T & P Rh cat.

reactive intermediate: the resonance-stabilized benzylic carbocation

HBr

Br

Recall: Markownikoff's Rule

C C CH3 C CH₂ CH₃

O aq H2SO4

HgSO₄

1-phenylpropyne phenyl ethyl ketone

the Markownikoff product

CHR

₂

aq KMnO

₄

, heat

C

O

CHEMISTRY 40

Problem Set – Aromatic Compounds

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

a. c.

b. d.

2. 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. fuming sulfuric acid f. CH3Cl, AlCl3 b. cold, dilute KmnO4 g. H2O, H+ c. CH3 COCl, AlCl3 h. HNO3, H2SO4

d. hot KMnO4 i. Cl2, FeCl3

e. Br2/CCl4, light

3. 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.

5. Suggest a scheme for the preparation of each of the following compounds from benzene.

a. p-bromobenzenesulfonic acid d. o-nitrotoluene b. 2,5-dichloronitrobenzene e. benzyl alcohol

c. p-chlorobenzoic acid f. styrene

+ O

..

..

C O H OCH_{3 Cl} CH₃