Phenols Dr. Neeta Sinha - PowerPoint Presentation

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Phenols

Dr. Neeta Sinha

Jamshedpur Cooperative College Jamshedpur

B. Sc. (

Chem

Hons

) Semester III

Outline

NomenclatureStructureSynthesisPhysical and Chemical Properties

Nomenclature

Phenols are defined as compounds having hydroxyl group attached directly to a benzene nucleus. Its general formula is ArOH, where Ar is aryl group and - OH is functional group.

OH

Phenol

Or,

Benzenol

Or,

Hydroxybenzene

Nomenclature continued …..

OH

OH

OH

OH

CH3

2-Methylphenol

OH

1,2-

Benzenediol

Cl

2-Chlorophenol

NO2

4-Nitrophenol

Structure

OH

In phenol, C-O bond is formed by the overlapping of sp2 orbital of carbon of the benzene nucleus and sp3 orbital of oxygen whereas O-H bond is formed by the overlapping of sp3 orbital of oxygen and 1s orbital of hydrogen . The C-O-H bond angle in phenol is 109 degree. The c-o bond length is slightly less than single covalent bond because of the resonating structure in which C-O bond acquires some double bond character.

The oxygen atom is more electronegative than carbon and hydrogen and hence

aquires

slight negative charge where as carbon and hydrogen

aquires

slight positive charge. As a result phenol has a dipole moment of 1.54 D.

C6H5 – O – H

Angle C-O-H = 109 degree

1. From Chlorobenzene

( Dow’s process ) - When chlorobenzene is treated with 10% NaOH

solution at 3550 C

and

300

atm

pressure

using

Cu as catalyst to produce phenol after acidification

.

10%NaOH/ 3550/Cu

HCl

C6H5Cl

C6H5ONa C6H5OH

Chlorobenzene

Sodium

phenoxide

Phenol

2. From

sulphonic

acid – Phenols can be prepared by heating benzene

sulphonic

acid or their salts with

NaOH at

3000 C followed by acidification

.

NaOH fuse/300 deg.C

Dil. HCl

C6H5SO3Na C6H5ONa C6H5OH

Phenol

3. From the hydrolysis of diazonium salt - In this method

diazinium salt solution is added to boiling dilute sulphuric

acid, phenol is obtained.

H2SO4

/ Heat

C6H5N2Cl C6H5OH

4

Synthesis

Phenol

+ -

4. From Grignard reagent – Phenols are prepared by bubbling oxygen through etherial solution of Grignard reagent followed by acidification.

O2 H+ /H2O C6H5MgBr C6H5OMgBr C6H5OH

Phenol5. From aniline –

In this process aniline is first

diazotised

and then

hydrolised

to get phenol.

Diazotised

Dil. H2SO4

C6H5NH2 C6H5N2Cl C6H5OH

Synthesis continued

…….

+ -

Properties

Boiling points – Like alcohols, boiling point of phenol is also higher than the corresponding aromatic hydrocarbon. This is because like alcohols , phenols also form intermolecular hydrogen bond.

2. Solubility – Phenols are soluble in water, again here because of the formation of intermolecular hydrogen bond with water molecule

.

H – O ………..H –O ……….H-O

…….

I

I

I

C6H5 H C6H5

H – O ………..H –O ……….H-O ………

I

I

I

C6H5

C6H5

C6H5

Properties continued …..

3. Acidic nature - Phenols are weakly acidic because it

ionises

to give

phenoxide

ion and hydrogen ion. Because of the formation of H+ ion , this is acidic and also the

phenoxide

ion is resonance

stabilised

.

C6H5OH

C6H5O

-

+

H

+

Phenoxide

ion

Alcohols are weaker acid than phenol. Alcohol

ionises as R – O – H RO- + H+

Alkoxide ionDue to + I effect of alkyl gr. , the electron density increases around oxygen and hence tendency of ionisation of alcohol decreases to release H+ ion . Secondly the

alkoxide ion formed is not stabilised by resonance like phenoxide ion.

The acidic strength of phenol is also affected by the presence of substituent. The electron withdrawing groups like nitro, cyano

, chloro increases the acidic strength because of the further dispersal of negative charge in phenoxide ion and further stabilises it whereas electron releasing groups like alkyl,

alkoxy, decreases the acidity because these grs. Intensify the negative charge and destabilises

the phenoxide ion.

4. Acylation ( Fries rearrangement) - Phenol when reacts with acid chloride or acid anhydride corresponding esters are formed. C6H5OH + CH3COCl C6H5OCOCH3 +

HCl C6H5OH + ( CH3CO)2O C6H5OCOCH3 + CH3COOH5. Electrophilic

subtitution

reaction – Phenol is

ortho

,

para

directing because –OH group increases the electron density at these position.

i

) Nitration - Nitration of phenol with dilute nitric acid gives the mixture of ortho &

para nitrophenol.

C6H5OH

Halogenation

-

Halogenation

of phenol gives 2,4,6-

trisubstituted

product.

Br2/H2O C6H5OH

Properties

continued …..

OH

OH

OH

+

NO2

NO2

Br

Br

Br

If solvent is non-polar then 2-bromo & 4-bromophenol is obtained.

Properties continued ……….

iii) Sulphonation - At low temperature phenol gives ortho product whereas at higher temperature it gives para product with H2SO4. C6H5OH

C6H5OH

OH

OH

H2SO4

H2SO4

L

ow temp

High temp

SO3H

SO3H

Properties continued…….

iv) Friedel craft alkylation – Alkylation of phenol gives the mixture of o- & p- products with poor yield. CH3Cl /anhy AlCl3 C6H5OH

v) Kolbe’s reaction – When sodium salts of phenol is treated with carbon dioxide at 125 degree c under pressure followed by acidification gives salicylic acid. C6H5OHvi) Reimer – Tiemann

reaction – When phenol is treated with chloroform and

NaOH

, mixture of

ortho

and

para

hydroxy

benzaldehyde are formed.

OH

OH

OH

OH

OH

+

CH3

CH3

COOH

C6H5OH

CHO

CHO

+

Properties continued ….…

vii)

Gattermann

synthesis - Phenol on treatment with HCN/

HCl

in presence of

anhy

. AlCl3 produce

parahydroxybenzaldehyde

.

C6H5OH

viii)

Laderer

–

Manasse

reaction - In acidic or alkaline medium , phenol

condences

with aromatic or aliphatic

aldehyde

to produce

ortho

&

para

hydroxybenzyl

alcohol. HCHO / H+ or OH-

C6H5OHHauben –

Hoesch reaction – It involves acylation of highly reactive triphenol in presence of CH3CN,

HCl and anhy. AlCl3.

C6H5(OH)3

OH

CHO

OH

OH

OH

OH

OH

+

CH2OH

CH2OH

COCH3

x) Williamson’s synthesis - In Williamson’s synthesis ether is prepared by heating sodium or potassium salts of phenol with alkyl halide.

C6H5ONa + CH3I C6H5OCH3 + NaI Methylphenyl

etherC6H5OK + C2H5Br C6H5OC2H5 +

KBr

Ethylphenyl

ether

Properties

continued ..…..

Thank You