Chapter 23 Aromatic Hydrocarbons Part 3 by TEACHING CARE Online coaching and tuition classes

 

 

• Methods of preparation
• Haworth synthesis

O                                               O

||                                                ||

C – CH                                  C

 

 

CH₂

CH

 

2

+ O                  |

AlCl₃

CH2

Zn(Hg)

CH₂

Conc. H₂SO₄                                       2

 

 

Benzene

C – CH₂

||

O

S

HO – C

||

|

CH₂

HCl

|

CH₂

||

heat (–H₂O)

C CH₂

||

O

 

uccinic

anhydride

O

b

-Benzoylpropionic

acid

O

g-phenylbutyric acid

 

 

Se

heat

CH₂

CH₂

CH₂

a-Tetralone

 

 

Zn(Hg) HCl

 

 

 

• By 4-phenyl-1-butene :

 

• Structure

CH₂

CH₂

|

CH

H₂C

 

 

 

CaO

heat

Naphthalene

 

 

+ 2H₂

Naphthalene

CH₂

Tetralin

 

• In naphthalene all carbon atoms are

sp ² – hybridized.

sp ² – hybrid orbital overlap with s- orbital of

 

hydrogen atoms forming C – C and C – H sigma bond.

• All carbon and hydrogen atoms lie in one plane in

sp²– sp²

H         H            C-C

H                                H s-bond

sp²–s

H                               H  C-H

s-bond

H         H

• According to resonance It is a resonance hybrid.

8           1

7                           2

6                           3

5           4

• (II) (III)                               Hybrid

(d) Position : 1,4,5,8 = a ;           2,3,6,7 = b

a           a                         8           1

b                           b         7                           2

b                          b         6                           3

5           4

• Resonance energy of naphthalene is 61 kcal/mol. Which is less than of benzene. So that naphthalene is less aromatic e. more reactive than benzene.
• In naphthalene C₁ – C₂ bond length is shorter (1.36Å) e. C = C and C₂ – C₃ is 1.40Å i.e. single bond.
• Physical properties : It is a colourless crystalline compound. It melts at 80.2° It is very volatile and sublimes slowly even at room temperature. It has strong characteristic odour. It is insoluble in water but very soluble in ether, benzene and hot alcohol. It burns with smoky flame.

 

 

• Chemical properties : It undergoes usual aromatic electrophilic substitution The product of monosubstitution is either a or b -depending on conditions, but the a -product always predominates.

E

 

+ E⁺

 

a-product

and/or

 

 

b-product

E

+ H⁺

 

Substitution at b -position occurs only when the reaction is carried at high temperatures or when bulkier

 

solvents are used.

 

 

250°C       SO2Cl 25°       SO2Cl 100-       HNO3/H2S 50°C     HNO3/H2S high temp.       H2SO 40°-     H2SO 160°C     C2H5B

Cl₂

 

 

 

 

 

 

 

 

 

 

 

 

 

Cl

Cl

+

 

Cl

 

 

Cl

 

 

Cl NO

 

Naphthalen

 

CH₃CO

AlCl₃ in CS₂ at

 

 

CH₂O +

ZnCl₂

 

 

 

CrO₃ CH₃COOH,

 

 

 

Na₂Cr₂O₇/H₂SO₄

or acid

 

 

 

 

O /V O ,

COC

 

 

CH₂Cl

 

O

 

 

 

O

COO

COO

O

||

C

 

NO         NO  NO

2      2    5                                                                                                         O

 

 

 

Naphthalene

+

 

NO

SO₃

 

or conc. H₂SO₄,

 

 

 

 

 

+

C

||

Pthalic aOnhydride

O             O

O                              CH

O                              CH

 

 

 

 

 

 

AlCl₃

SO₃ C₂H

 

 

Na/C_–

Reflux

DiozoOnide          O

 

(Dialine) 1, 4-Dihydro

Pthalaldehyd

 

 

Na/Isopentyl Reflux

 

 

(Tetralin) (1, 2, 3, 4-

 

 

 

H₂/Ni

Reflux

 

(Decalin) (Decahydronaphthale

 

 

• Uses
• As moth It is, however, now being replaced by more powerful insecticides such as p-

dichlorobenzene and DDT.

• For commercial production of phthalic anhydride, a –naphthol, b -naphthol,
• For manufacture of dyes, explosives and synthetic
• For increasing the illuminating power of coal

(2)  Anthracine

The hydrocarbon derives its name from the world anthrax (Greek = coal) as coal is the chief source from which it is manufactured. It is present in coal-tar (Less than 0.5%) and is obtained from the anthracene oil or green oil fraction (because of its dark green fluorescence) formed during coal-tar distillation. This fraction is collected between 270 – 360°C.

The anthracene oil fraction is cooled when crude anthracene crystallizes out. The crude product consists phenanthrene and carbazole as impurities. The crude product is successively washed with solvent naphtha as to remove phenanthrene and pyridine to remove carbazol. Finally, the solid is sublimed to get pure anthracene.

• Methods of preparation

Haworth synthesis

 

O                                                            O

||                                                             ||

C

O +

C

||                                               o-Benzoyl benzoic acid

O

 

 

Conc. H₂SO₄

(heat) (–H₂O)

O

||

C

Zn dust

distill.

C

||

O

Anthracene

 

Phthalic anhydride

 

• Structure

• Anthracine is tricyclic aromatic

9, 10-Anthraquinone

 

• All carbon atoms in anthracene are
• resonance hybrid are as follows

sp ² hybridized.

 

 

 

• (II) (III)                                          (IV)                                     Hybrid form

 

• It gives both addition and electrophilic substitution
• In anthracene the numbering of carbon atom is

 

 

8           9           1

7                                        2

 

6                                        3

5          10         4

Anthracene

a           g           a

b                                       b

 

b                                       b

 

Anthracene

 

 

• Physical properties : Anthracene is a colourless It melts at 217°C. It is insoluble in water but soluble in alcohol and ether in small amounts. It is comparatively more soluble in hot benzene. With picric acid, it

 

forms a red colour picrate.

(4) Chemical properties

 

 

Br₂ CCl₄

 

 

 

 

HNO₃ AC₂O

Br

 

 

9-bromoanthracene

NO₂

 

 

NO₂

9, 10-dinitroanthracene

SO₃H

 

 

 

 

 

 

 

Anthracene

H₂SO₄

low temp.

 

 

H₂SO₄

High temp.

 

SO₃H

 

 

 

Na/C₂H₅OH

 

 

 

 

 

(v) Uses : Anthracene is used

• For manufacture of
• For making dyes (Alizarin).
• In smoke

 

 

 

[O]

Na₂Cr₂O₇ + H₂SO₄

9, 10-dihydroanthracene

O

 

 

 

O

9, 10-anthraquinone

 

+ H₂O

 

 

These are cyclic compounds in which the ring includes in addition to carbon atoms at least one atom of another element (Hetero = other, different). The common hetero atoms present in the carbon rings are O,N and S.

(1)  Thiophene

It is found in the benzene fraction of coal-tar and petroleum. The benzene fraction is shaken with cold concentrated sulphuric acid. Thiophene present in the fraction combines with sulphuric acid more readily than benzene to form thiophene sulphonic acid which is separated with water being soluble. Thiophene sulphonic acid is treated with super-heated steam to recover thiophene.

 

 

or

 

 

Properties : It is a colourless liquid. Its boiling point is 84°C. Its odour is similar to that of benzene. It is insoluble in water but soluble in organic solvents. It is flammable and toxic in nature.

 

 

Its resonance energy is 31k cal mol ^-1 . Hence, it is more stable and resembles benzene more closely than

 

furan

(23 k cal mol ^-1 )

and pyrrole

(25 k cal mol ^-1 ). It does not show basic properties and does not undergo

 

Diels-Alder reaction.

.–.

 

 

.–.

 

.–.        +

S

.–.

 

Thiophene as a resonance hybrid (Resonance energy is 31 k cal /mol

 

CH                                 Al₂O₃              

2       +                 400°C

CH

Cl₂

Cl

2-

 

 

Br

Br

2-

 

 

I₂

HgO

 

 

 

F.HNO₃, –10°C

I

2-Iodothiophene

 

 

 

 

CH₂ – CH₂

Acetic anhydride

 

2-

Cold conc.

NO

 

 

SO₃H

 

|             |

CH       CH

+ 4S           650°C                 

Thiophene

 

HgCl₂

2-Thiophene sulphonic acid

 

CuO

HgCl

S

2-Chloromercurithiophene

 

 

 

CH₃COCl

AlCl₃

2-

COCH₃

 

 

 

HCHO + HCl

 

 

2-

CH₂Cl

 

 

Pd/H₂

or Na–Hg/C₂H₅OH

 

Tetrahydrothiophene (Thiophan)

 

CH₂COON

a

|

 

(2)  Furan

P₂S₃

[Laboratory

Raney heat

CH₃CH₂CH₂CH₃ +

NiS

 

sp²

 

 

 

sp²

 

Furan derives its name from furfur meaning bran in Greek which is the source of its aldehyde, furfural. It is present in pine-wood tar and may be                                                                  ⁴

extracted from it.                                                                                            5

¯

3              H –^s– C

2              H –^s– C        ..

¯

C –s– H

C –s– H

 

Furan shows aromatic behaviour because resulting p-molecular orbital             ₁

satisfies the Huckel rule ((4n + 2)p where, n = 1 .

­     O   ­

2

sp² ­ sp sp²

 

 

Furan is also considered as a resonance hybrid of the canonical forms. Out of which the first three are the

 

main contributing structures.

Furan     has      resonance     energy     about

.–.

 

 

.–.

 

23 k cal mol ^-1

which is less than benzene. However it

.–.

.–.        +

O

 

is less aromatic and more reactive than benzene.

Properties : Furan is a colourless liquid. Its

I                        II                       III                     IV                     V

Canonical forms of furan

 

boiling point is 32°C. It is insoluble in water but soluble in orgainic solvents. It is a reactive compound. It is a weak base. In Furan electrophilic substitution reactions take place preferably at 2 and 5 position where electron density is high. If these positions are occupied, substitution occurs at 3 and 4 positions. It undergo Diels-Alder reaction.

 

 

Br2

Dioxane, 0°C

O        Br

 

Pine wood

distillation

 

O

||

CH3 – C – ONO2 or HNO3

(CH3CO)2O, 10°C

• Bromofuran

 

 

 

 

NO2

• Nitrofuran

 

 

Furfural

[O]

CHO

 

O

Furoic acid

 

COOH

 

200°C

–CO2

 

 

SO3

Pyridine

 

O    SO₃H

 

2-Furan sulphonic acid

 

 

 

 

CHO

steam

Ag2O

 

 

O

Furan

 

(CH3CO)2O BF3

O

||

– C – CH₃

 

• Acetylfuran

 

 

 

 

2H /Ni

H C –– CH

HCl

 

 

CH₂ – CHO

CH = CHOH               P O

2

heat

2                                   2                           H₂C –––– CH₂

|          |                       |                |

 

|                                                     ⇌ |                                                                                   2    5

H₂C     CH₂

 

H C – Cl  CH  – OH

 

CH  – CHO             CH = CHOH                   ^{D                                                                                                                                    2               2}

 

2                                                                                                                                                                                                                                                                                                                                                      O

Tetrahydro furan (THF)

Tetra methylene chloroform

(4-chloro-1-butanol)

 

CHOH   ––  CHOH

|                                                    |

Dry distill.

Br

HgCl₂

 

H – C – OH H – C – OH

|                                                    |

COOH             COOH

– CO2

– 3H2O

CH3COON

HgCl

O

 

 

Mucic acid or Saccharic acid or Glucaric acid

• Chloro

 

2

3 2

CH  – CH(OCH )

|

COR

 

CH3OH + HCl

CH2 – CH(OCH3)2

Diacetal of succinic dialdehyde

 

 

 

 

• HCN + HCl, AlCl₃

O

||

– C – H

 

 

(2-Formyl furan) Furfural

 

NH₃+Al₂O₃

 

 

 

 

C H Li

N H

Pyrrol

 

4    9

 

 

Li

2-lithium furan

(i)

COO

Furoic acid

 

 

 

(3)  Pyridine

Pyridine is a six membered aromatic heterocycle with one nitrogen atom in the ring. It may be supposed to

 

have been derived by replacement of

= CH –

group of benzene by

= N – . Hence, it is isoster of benzene. Its

 

systematic name is azabenzene. (Prefix aza stands for nitrogen). The hybrid structure of pyridine is represented as:

sp²–s

 

 

 

CH

HC                    CH

 

HC                    CH

N

(C–C, s bond)       H H

 

1.37Å

H                  N

 

Completely filled

sp²-hybrid orbital

sp²–sp²

(C–C, s bond)

 

H

 

1.40Å

 

H

 

sp²–sp²

(C–N, s bond)

 

 

• Properties : It is a colourless liquid having an unpleasant It boils at 115°C. It is miscible with water and is hygroscopic. It is a good solvent for many organic compounds and inorganic salts.

 

• Aromatic character : Each carbon atom and nitrogen atom in the ring have

sp ²

hybridized and one

 

unhybrid p-orbital containing one electron. These orbitals overlap to form p molecular orbital consisting six

 

electrons. The p molecular orbital satisfies Huckel’s rule pyridine.

(4n + 2)

and thus aromatic properties are observed in

 

The resonance energy is 43 k cal/mol and bond length of C – C bond is 1.40 Å and

 

p electron cloud

C – N

1.39 Å.

 

 

 

 

4n + 2 = 6 ; n = 1

N:                 N:

 

.–.

 

.–.                                        ..

 

• Resonance structure : + +

 

+

Canonical form of pyridine

 

• Basic nature : Pyridine is basic in nature due to presence of lone pair nitrogen atom. It is more and more basic than pyrrole and less basic than aliphatic

 

 

Example :

.N.

Å 

+     HCl

Å           

[NH]Cl

Pyridinium hydrochloride

 

 

• Chemical properties :

 

 

CH CH NH HCl         ^N                                                                                                                  NO

 

2       2       2

H

H SO /300°C

KNO₃/H₂SO₄                                   2

 

CH₂

CH₂CH₂NH₂HCl

Piperidine  _{2          4}

300°C

• Nitropyridine

 

Penta methylene diamine dihydrochloride

F.H₂SO₄/HgSO₄

250°C

SO₃H

Electrophilic

 

3-Pyridine sulphonic acid substitution

 

2C₂H₂ + HCN

Br₂

300°C

Br

3-Bromopyridine

 

 

 

 

+ CH₂I₂

N H

 

 

 

 

CH₃ONa

200°C

 

 

 

 

 

 

 

Pyridine

 

NaNH₂

100°C

 

 

 

C₆H₅Li

100°C

 

NH₂

2-Aminopyridine

 

 

C H

 

 

 

 

 

 

 

Nucleophilic substitution

 

6  5

2-Phenylpyridine

 

 

 

 

2CH CHO + HCHO + NH

 

 

 

Al O

KOH

300°C

OH

2-Hydroxypyridine

 

2    3

3                                              3

 

 

Na/C₂H₅OH or

H₂/Ni

 

N H

Piperidine

Red

 

 

 

2CH

= CH. CHO

NH₃,

K₂Cr₂O₇/H⁺

^HI                 CH CH CH CH CH

+ NH

 

2                                     D                 [O]

300°C

3       2       2       2       3              3

 

n-Pentane

 

• Uses
• To denature
• As a basic solvent in organic
• For preparing sulpha-pyridine and vitamin B₆ .
• As a catalyst in many reactions, g., in the formation of Grignard reagent, in Perkin and Knoevenagel reactions.

 

 

 

(4)  Pyrrole

It occurs in coal-tar and bone oil and is found in many natural products including chlorophyll, haemoglobin and alkaloids.

 

or

 

|                                 |

H

H

Pyrrole

 

 

sp²–sp²

(C-C, s-bond)

 

H                H

 

H

N

2       2                      |

 

 

 

H

sp²–s

 

 

 

 

sp²–s

(C-H, s-bond)

 

|

 

 

 

.–.                                   

+                        +     .–.

N                       N

|                          |                          |                        |

 

 

 

.–.

 

sp –sp

(C-N, s-bond) H

(N-H, s-bond)

H                       H                       H                        H                      H

 

 

 

 

• Preparation :

C H NK + H O

steam

 

4  4                  2

 

2C₂H₂ + NH₃

 

 

H₂C O=C

CH₂

 

N C=O H

|

 

CHOHCHOHCOONH₄

|

CHOHCHOHCOONH₄

 

+ NH₃

 

 

 

 

 

400°C

H

Pyrrole

 

 

 

 

distillation     –H2         Zn dust distillation         200°C   Glycerol     Al2O3

Note : ® Furan needed for the process is obtained from agricultural waste materials which are rich in pentosans. The pantosans on acid hydrolysis yields furfural which is decarbonylated.

• Properties : Pyrrole is a colourless liquid. Its boiling point is 131°C. It is slightly soluble in water but highly soluble in alcohol and Its odour is similar to chloroform. It rapidly becomes brown when exposed to air. Vapours of pyrrole turn a pine splint moistened with HCl red. Pyrrole derives its name from this property.

 

 

 

 

Chemical properties :

 

Br₂

Br              Br

 

^KOH                   C H N^–K⁺

 

C₂H₅OH

Br              Br                                                                                              4  4

 

 

 

Tetrabromo pyrrole

HCl

Pyrrole potassium

C₄H₄NH.HCl

Pyrrole hydrochloride

 

I              I

I₂/KI

I               I

CH₃I

 

 

|

 

 

 

 

 

 

 

 

 

N H

Pyrrole

 

SOCl₂

 

 

 

 

 

SO₃

Pyridine, 100°C

 

 

 

 

HNO₃

(CH₃CO)₂O, 5°C

 

 

 

 

(CH CO) O, 200°C

Tetraiodo pyrrole (Iodole)

 

 

Cl

 

2-Chloro pyrrole

 

 

SO₃H

2-Pyrrole sulphonic acid

 

 

NO₂

2-Nitro pyrrole

 

 

 

 

 

 

 

N H

Pyrrole

 

 

CH₃COCl

 

 

 

 

 

 

 

HONO

 

 

 

 

 

 

 

Zn/CH₃COOH

(Mild reduction)

CH₃

N-Methyl pyrrole

|

COCH₃

N-Acetyl pyrrole

|

NO

N-Nitroso pyrrole

 

3                 2

AlCl₃

COCH₃

 

2, 5-Dihydro pyrrole

 

 

 

C₆H₅N₂Cl

diazotisation in acid solution

 

 

 

CHCl₃/KOH

Reimer-Tiemann

2-Acetyl pyrrole

 

 

N=NC₆H₅

2-Phenylazo pyrrole

 

 

CHO

 

H₂/Pt

heat, pressure

 

 

 

Cr₂O₃/CH₃COOH

3[O]

 

 

 

 

Pyrrolidine

 

 

OC N CO H

 

2-Formyl pyrrole (Pyrrole-2-aldehyde)

Maleic imide

 

 

• Uses : It is used as a commercial solvent, as an intermediate in the production of nylon and for making