Fall, 2013 Dr Seemal Jelani Chem-240 1. Nomenclature Properties Preparation Reaction Spring,...

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Fall, 2013Dr Seemal Jelani Chem-240 1

Nomenclature Properties Preparation Reaction

Spring, 2011Dr Seemal Jelani Chem-240 2

Considered derivatives of waterOH SH OH SH

alcohol thiol phenol thiophenol

ether sulfide

Pure phenol is a white crystalline solid, smelling of disinfectant

It has to be handled with great care because it causes immediate white blistering to the skin

The crystals are often rather wet and discolored.

The simple monohydric phenols are either liquid with low melting crystalline

Some chromophoric group is present in their molecules but phenols are generally colorless

What is the reason of What is the reason of coloration in phenolscoloration in phenols

The colour of many phenols may be due to the presence of chromophoric group and also due to the presence of their oxidation products as impurities because they are easily oxidized on exposure to air

It is useful to compare phenol's melting and boiling points with those of methylbenzene (toluene)

Both molecules contain the same number of electrons and are a very similar shape

That means that the intermolecular attractions due to Vander Waals dispersion forces are going to be very similar.

The symmetrical phenols generally have higher MP than unsymmetrical isomers

Examples Among the chlorophenols, the para isomer has the highest MP, whereas the ortho isomer is liquid at ordinary temperature

CC66HH55OHOH CC66HH55CHCH33

Melting point (°C)

40-43 °C

-95 °C

C)Boiling point (°C)

118 °C

111 °C

The reason for the higher values for phenol is in partly due to permanent dipole-dipole attractions due to the electronegativity of the oxygen - but is mainly due to hydrogen bonding.

Hydrogen bonds can form between a lone pair on an oxygen on one molecule and the hydrogen on the -OH group of one of its neighbors.

The O-H bond of phenols is more polar than that of alcohols

Phenols therefore form stronger HB than alcohols, resulting in higher BP and greater solubility in water

Compare the boiling point and solubility of phenol with that of Cyclohexanol ( a comparable alcohol)

Cyclohexanol boils at 161 degree C and has solubility of 3.6g/100 mL of water

According to the general rule, symmetrical molecules should have lower boiling points than the unsymmetrical molecules. This is not the case with phenols, particularly those having an electronegative substituent ortho to the -OH group

Because it has been seen that in many such cases the ortho isomers, despite their lack of symmetry, have BP much lower than the meta and para isomers

Compare the BP of o and p nitrophenols

The lower BP ( higher volatility) of o-nitrophenol can be attributed to intramolecular HB ( chelation)

Chelation of o-nitrophenol helps its separation from its m- and p isomers by steam distillation

Non- chelated m- and p-nitrophenols are not steam-volatile due to the intermolecular HB which inhibits their steam-volatility by lowering their vapour pressure

Phenol is moderately soluble in water - about 8 g of phenol will dissolve in 100 g of water.

If you try to dissolve more than this, you get two layers of liquid. The top layer is a solution of phenol in water, and the bottom one a solution of water in phenol

Solubility of phenols increases with an increase in the number of –OH which cause increased hydrogen bonding

However this effect is counterbalance by the symmetry of the molecules which helps them to stabilize by themselves

ExampleResorcinol has solubility of 123g/100 mL of water at 250

C, hydroquinone is soluble only to the extent of 8g/100 mL of water 250 C

o-cresol m-cresol p-cresol

catecholresorcinol

hydroquinone

Water is soluble in phenol to the extent of

29 g /100 mL of phenol Phenols and water are miscible in all

proportions above 65 0 C A small amount of water lowers the MP

of phenols, thus phenol which has the MP of 420 C often found as semi liquid because of the presence of small amount of water; it is completely liquified by the addition of about 5% of water

The liquid form of phenol, containing 5% of water is known as Carbolic acid and it is used as disinfectant and germicide

The solubility behaviour of phenol and water is complicated

Phenol is somewhat soluble in water because of its ability to form hydrogen bonds with the water.

Common natural source – coal tar Found in many plants

CO2CH3

OHcoal tar

willow bark

oil of wintergreen

bioactive ingredient poison ivy and oak

eugenol (oil of clove) isoeugenol (oil of nutmeg)

vanillintetrahydrocannabinol

Hydroxybenzene is used as a parent name in most of its derivatives

Name substituents on aromatic ring by their position from OH

NO2O2N

p-methylphenol o-chlorophenol 2,4-dinitrophenol

While naming the polyfunctional aromatic compounds the hydroxy function is generally placed low in the order of priority

Only amino and ether functions are placed lower than the hydroxy function

Examples:p-hydroxy benzoic acidp-Amino phenol

Many common names in use

o-cresol m-cresol p-cresol

catecholresorcinol

hydroquinone

Ar-OH is more acidic than ROH Soluble in dilute NaOH Anion is resonance stabilized EWG make phenols more acidic than phenol EDG make phenols less acidic than phenol

OH O O

NaOH -

Although phenols and alcohols have the same FG –OH, phenols are much more acidic than alcohols but weaker acids than carboxylic acids

The pKa values of alcohols, phenols and carboxylic acids are in the vicinity of 18,10 and 5

Methyl groupMethyl group HalogensHalogens

Electron-Electron-releasing releasing substituent substituent

Decreases acidity

Electron-Electron-withdrawing withdrawing substituent substituent

Increases acidity

Strongly EWG like NO2 have acid-strengthening effect

It is more acid-strengthening when it is ortho or para to –OH group

Additional nitro groups further increase the acid strength

Thus the acidity of dinitrophenols is comparable to that of carboxylic acids

The cumulative effect of three nitro groups makes 2,4,6-trinitrophenol nearly as strongly acidic as a mineral acid and it is known as picric acid

In general phenols are very acidic. They are caustic towards flesh

and very poisonous Phenol itself is used as

disinfectant either in solution or mixed with Ca(OH)2, for toilets, stables, floors and drains

It is also used as a caustic for animals bites

Reactions of Reactions of phenolsphenols

They give two types of reactionsa)The reactions due to the –OHb)The reactions due to the aromatic

ringOH-group reactions involve the

cleavage of the O-H bond, as e.g. In the formation of salts, ethers

and esters, so they resemble alcohols in this respect

Most of the reactions of phenols fall in the second category in which phenols distinctly differ from alcohols

The striking featureHigh reactivity of its ring towards ESR, due to the presence of the strongly activating -OH

Thus phenols undergoes ESR like nitration, halogenation, Friedel –Crafts reaction and sulfonation characteristics of aromatic compounds

A number of important ESR of phenols involve phenoxide ions which are formed under basic conditions

•This phenoxide is water soluble•The –O- of the phenoxide ions is strongly electron releasing and thus more strongly activating than the –OH group

OH O O

NaOH -

Phenoxide ion can therefore react with even weaker electrophiles like carbon dioxide

With phenols special care must be taken to prevent polysubstitution and oxidation

Phenols are stronger acid than alcohols therefore they can form salts even with weaker bases

The most phenols can be converted into their salts by aq. NaOH, a much weaker base than the sodium metal which is required to form salts with alcohols, as alcohols do not react with NaOH

Most phenols can dissolve in aq. NaOH by forming sodium phenoxides whereas

Most alcohols having more than five carbon atoms do not

This provides a convenient method of distinguishing as well as separating phenols from alcohols

Alcohols with five or fewer carbon atoms dissolve in aq. NaOH not by forming sodium salts but because they are soluble in water

2,4,6-ter-butylphenol does not form 2,4,6-ter-butylphenol does not form salt even with sodium metal, why?salt even with sodium metal, why?

Due to steric hindrance to approach Due to steric hindrance to approach of a reagent to the –OH group and of a reagent to the –OH group and to solvation of the phenoxide ionto solvation of the phenoxide ion

On treatment with alkyl halides in the presence of aq. NaOH, alkyl aryl ethers are formed

With aq. NaOH, a phenol is converted to a phenoxide ions which act as a Nu: and displaces halide ion from the alkyl halide in an SN2 reaction

Dialkyl sulphates ca also be used as alkylating agents under these conditions

Phenols can be used to form esters

The reaction is carried out with an acid chloride or anhydride in the presence of a base or an acid

Acetyl salicylic acid commonly known as Aspirin is prepared by Acetylation of salicylic acid

On treatment with conc. Nitric acid, phenol is converted into 2,4,6-trinitrophenol (picric acid)

Mononitrophenols are produced by using dilute nitric acid

Reacts readily with an excess of chlorine or bromine in aqueous solution to give 2,4,6-trihalophenols

Phenols have high reactivity therefore no Lewis acid is required

MonohalogenationMonohalogenation of phenol can be achieved by carrying out the reactions in a solvent of low polarity such as chloroform, carbon tetrachloride or carbon disulfide at

low temperature These conditions reduce the

electrophilic reactivity of halogens

Monochlorination gives a mixture of o and p-chlorophenols

They differ in BP by 410 C and easily can be separated by fractional distillation

Monobromination yields p- bromophenol exclusively

o-Bromophenol can be also prepared

Friedel Crafts reactions with phenols in the presence of aluminium chloride is complicated by an reaction of the phenol with the Lewis acid

Yield is poor However alkyl and acyl

derivatives of phenols can be prepared by other methods

Alkylation of phenols can be achieved by using an olefin as an alkylating agent in the presence of HF

Can be obtained by heating alkyl phenyl ethers

Catalyst is aluminium chloride Reaction is intermolecular

process involving substitution of the alkyl group

On treatment with conc. H2SO4

250 C Product is ortho However with increased

temperature i.e.100 0 C para product is

mainly formed

Introduction of Formyl group –CHO into the aromatic ring of phenols

Generally o-positions

Reaction with Br-water gives 2,4,6-tri bromophenol

Many phenols when treated with FeCl3 in dilute aqueous or alcoholic solution, give characteristics colour ranging from violet through blue and green to red

Phenols itself gives violet coloration and cresols give a blue colour

The color develops due to the formation of ferric phenoxides ( coordination complexes) which involve an extended delocalization of pi electrons of the aromatic rings through the iron atoms and thus absorb visible light

Phenols show distinct colors with ferric chloride why ?

Industrial process from readily available cumene Forms cumene hydroperoxide with oxygen at

high temperature Converted into phenol and acetone by acid

Cl OHNaOH/heat

From aromatic sulfonic acids by melting with NaOH at high temperature

Limited to the preparation of alkyl-substituted phenols

Fall, 2013 Dr Seemal Jelani Chem-240 1. Nomenclature Properties Preparation Reaction Spring,...

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