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

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

sp2sp2

H         H            C-C

H                                H s-bond

sp2s

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 C1C2 bond length is shorter (1.36Å) e. C = C and C2C3 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

 

 

 

 

CH2CH2

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 NaHg/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              HsC

2              HsC        ..

¯

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

 

 

CH2CHO

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:

sp2s

 

 

 

CH

HC                    CH

 

HC                    CH

N

(CC, s bond)       H H

 

1.37Å

H                  N

 

Completely filled

sp2-hybrid orbital

sp2sp2

(CC, s bond)

 

H

 

1.40Å

 

H

 

sp2sp2

(CN, 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

CN

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

 

 

sp2sp2

(C-C, s-bond)

 

H                H

 

H

N

2       2                      |

 

 

 

H

sp2s

 

 

 

 

sp2s

(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 NK+

 

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
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