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Advanced Level Organic Chemistry: The Chemistry of Phenols

Part 7. The chemistry of AROMATIC COMPOUNDS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry electrophilic substitution products of phenol bromine chlorine weak acid character physical properties

Part 7.9 The physical & chemical properties of phenol & selected derivatives & their uses

Sub-index for this page

7.9.1 The structure and physical properties of phenol and selected phenol derivatives

7.9.2 The structure of phenol and its electrophilic substitution reactivity and product orientation

7.9.3 The electrophilic substitution reactions of phenol with chlorine water, bromine water and dilute nitric acid

7.9.4 Reaction of phenols as an 'alcohol' e.g. esterification with acid anhydrides and acid chlorides (NOT Friedel-Crafts)

7.9.5 Reactions of phenol as a weak acid, comparing the acid strengths of alcohols, phenols and carboxylic acids plus a specific test for phenols

7.9.6 Examples of the uses of phenol and its derivatives


INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

[SEARCH BOX]



TOP OF PAGE and sub-index


7.9.1 The structure and physical properties of phenol and selected phenol derivatives

If the OH group (hydroxy) is directly attached to a benzene ring, the molecule is classified as a 'phenol'.

If not, the molecule is classified as an aliphatic alcohol.

Make sure you can distinguish the two because some aliphatic alcohols may have a benzene (or other) ring.

(c) doc b is classed as an aromatic phenolic compound and called 3-methylphenol and lots more examples of phenols illustrated in the table below.

(c) doc b phenylmethanol ('benzyl alcohol') is a primary aliphatic alcohol, an isomer of 3-methylphenol.

diols triols and cyclo-alcohols structure and naming (c) doc b is cyclohexanol and is a secondary aliphatic (alicyclic) alcohol.

Phenols might referred to as 'aromatic alcohols', do NOT do so, so beware how you use the term 'alcohol'

The 'phen' part of the name comes from 'phenyl' C6H5, and the 'ol' comes from the OH functional group, but these are NOT aliphatic alcohols - they share some chemistry, but there are significant difference with phenols due to the presence of the benzene ring.

Abbreviations used: mpt = melting point;  bpt = boiling point; sub. = sublimes,

dec.= thermally decomposes on heating at atmospheric pressure before it can boil

Name of 'phenol' Structure Mpt/oC Bpt/oC Comments
phenol (c) doc b 41 182 White crystalline solid that smells like a disinfectant - which it is!, but is harmful to touch - causes blistering to the skin. It's solubility is 8g/100 g cold water, much more soluble in hot water
2-methylphenol structural formula 2-methylphenol (o-methylphenol ortho-cresol) molecular structure advanced organic chemistry structural formula 3-methylphenol (m-methylphenol meta-cresol) molecular structure advanced organic chemistry structural formula 4-methylphenol (p-methylphenol para-cresol) molecular structure advanced organic chemistry 31 191 Three positional structural isomers.

Soluble in hot water, only slightly in cold water.

3-methylphenol 12 202
4-methylphenol 35 202
2-nitrophenol (c) doc b 45 216 Slightly soluble in hot water. 1st of 3 positional isomers.

Exhibits intramolecular hydrogen bonding

3-nitrophenol 97 high, dec. Slightly soluble in cold water, very soluble in hot water.
4-nitrophenol 114 dec. 279 Slightly soluble in cold water, very soluble in hot water.
2-chlorophenol structural formula 2-chlorophenol (ortho o-chlorophenol) molecular structure advanced organic chemistry structural formula 3-chlorophenol (meta m-chlorophenol) molecular structure advanced organic chemistry structural formula 4-chlorophenol (para p-chlorophenol) molecular structure advanced organic chemistry 9 175 Moderately soluble in cold water.
3-chlorophenol 33 214 Slightly soluble in cold water, soluble in hot water.
4-chlorophenol 43 220 Slightly soluble in cold water, soluble in hot water.
2-aminophenol (c) doc b structural formula 3-aminophenol molecular structure (c) doc b 174 dec.? All three isomers slightly soluble in cold water, more soluble in hot water
3-aminophenol 122 dec.?
4-aminophenol 187 284
1-naphthol

naphthalen-1-ol

  93 288 Insoluble cold water, slightly soluble in hot water.
2-naphthol

naphthalen-2-ol

122 295 Insoluble in water.
2,4,6-tribromophenol See diagram below ~92 ~292 Very low solubility 0.007g/100g water at 25oC.
2,4,6-trichlorophenol See diagram below 69 dec. Slightly soluble in cold water, 0.08/100g water.
2,4,6-trinitrophenol See diagram below  122 >300oC, explodes Closely related to TNT. Slightly soluble in cold water (1.27g/100g water), much more soluble in hot water. Yellow crystals known historically as 'Picric acid'
benzene-1,2-diol   105 240  
benzene-1,3-diol   111 280  
benzene-1,4-diol   170 286  
         

structural formula of 2,4,6-trichlorophenol 2,4,6-tribromophenol 2,4,6-trinitrophenol molecular structure advanced organic chemistry

Extra notes on physical data table for phenols

(a) Comparing the melting and boiling points of methylbenzene (c) doc b and phenol  (c) doc b .

C6H5CH3: mpt -95oC; bpt 111oC  and C6H5OH: mpt 41oC; bpt 182oC.

hydrogen bonding diagram in phenol crystals raising melting/boiling point compared with lower melting point and boiling point of non-polar methylbenzene advanced organic chemistry

Both phenol and methylbenzene have a similar size, shape, molecular mass and numbers of electrons in the molecule, but, they have very different melting points and boiling points.

Both molecules will exhibit similar instantaneous dipole - induced dipole forces, but whereas methylbenzene is a relatively non-polar molecule, phenol is a highly polar molecule due to the highly polar δ-O-Hδ+ bond.

(Pauling electronegativities: H = 2.1 and O = 3.5, a big difference).

Therefore, the intermolecular force between phenol molecules is much greater than that between methylbenzene molecules because of the extra contribution of permanent dipole - permanent dipole forces, which includes the spatially directed hydrogen bond  δ-O-Hδ+llll:Oδ-.

The hydrogen bond angle is usually ~170-180o. (see diagram above for phenol and methylbenzene)

This means higher kinetic energy molecules are required to weaken the intermolecular forces to melt or vapourise phenol compared to methylbenzene - higher temperature and higher enthalpies of fusion and vapourisation.

Property Methylbenzene Phenol
Structure C6H5CH3 C6H5OH
Melting point -95oC 41oC
Boiling point 111oC 182oC
Enthalpy of fusion 6.6 kJ/mol 11.5 kJ/mol
Enthalpy of vaporisation 37 kJ/mol 48.1 kJ/mol

(b) Comparing the solubility in water of methylbenzene (c) doc b and phenol  (c) doc b .

hydrogen bonding diagram of phenol dissolving in water hydrogen bonds between phenol and water molecules so phenol dissolves compare with insoluble non-polar methylbenzene advanced organic chemistry

Methylbenzene is a relatively non-polar molecules and is virtually insoluble in water - its hydrophobic nature disrupts the hydrogen bonds of water without compensation form other intermolecular forces i.e. it can't form hydrogen bonds with water molecules.

Methylbenzene is very soluble/miscible with other hydrocarbon solvents like benzene.

Phenol can hydrogen bond with water (diagram above), allowing solvation to take place (dissolving), and at room temperature phenol is quite soluble in water (~6.7 g/100g water), but the bulky hydrophobic benzene ring of phenol does restrict it's solubility at room temperature - too much disruption of hydrogen bonding between water molecules without similar compensating intermolecular forces between solute and solvent.

Lower alcohols like methanol, ethanol and propanols are miscible/very soluble with water due to the strong solvating hydrogen bonding between solute and solvent, but their much higher solubilities in water compared to phenol are also due to a smaller less bulky hydrophobic alkyl group, which is less disrupting to the hydrogen bonding holding water molecules together in the liquid.

Although the solubility at ~25oC is not that high, from ~70oC upwards (well above the melting point of phenol) the two liquids are fully miscible in any proportions.

Despite its polar nature, phenol is quite soluble in non-polar benzene (~8.3g/100g benzene), the instantaneous - dipole - induced dipole intermolecular forces between the benzene rings of phenol and benzene itself are great enough for appreciable solvation to happen.

(c) (c) doc b structural formula 3-aminophenol molecular structure (c) doc b The relatively higher melting points of aminophenols

This because there are two functional groups, both of which enable hydrogen bonding between the molecules δ-O-Hδ+  and  δ-N-Hδ+.

e.g. structural formula 2-methylphenol (o-methylphenol ortho-cresol) molecular structure advanced organic chemistry structural formula 3-methylphenol (m-methylphenol meta-cresol) molecular structure advanced organic chemistry structural formula 4-methylphenol (p-methylphenol para-cresol) molecular structure advanced organic chemistry aminophenols have a similar molecular mass, size, shape and numbers of electrons in the molecule compared to methylphenols.

BUT, the methylphenols melt between 12 and 35oC, whereas the aminophenols with extra intermolecular hydrogen bonding, melt between 122 to 187oC.

 

(d) An example of comparing intermolecular hydrogen bonding and intramolecular hydrogen bonding.

diagram of intramolecular hydrogen bonding in 2-nitrophenol compare contrast with intermolecular hydrogen bonding in 3-nitrophenol and 4-nitrophenol advanced organic chemistry

In most cases of hydrogen bonding you are dealing with is between molecules e.g. illustrated by phenol (a previous diagram) and 3-nitrophenol and 4-nitrophenol.

These intermolecular hydrogen bonds are shown in the diagram above .

However, 2-nitrophenol can hydrogen bond within the molecule itself, known as an intramolecular hydrogen bond.

A hexagonal arrangement of atoms is involved of five covalent bonds and one hydrogen bond.

In the case of 2-nitrophenol the intramolecular hydrogen bond angle is ~120o, whereas in 3-nitrophenol and 4-nitrophenol the intermolecular hydrogen bond angle is ~170-180o. (see diagram above).

As a consequence ...

(i) 2-nitrophenol is only slightly soluble in water, but 3-nitrophenol and 4-nitrophenol are very soluble because, unlike 2-nitrophenol, they can hydrogen bond with water, they are also very polar molecule.

(ii) 2-nitrophenol has a lower melting point than 3-nitrophenol and 4-nitrophenol, again because of the hydrogen bonding between the latter two molecules in their crystal structures.


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7.9.2 The structure of phenol and its electrophilic substitution reactivity and orientation

The presence of the hydroxy group (OH) directly attached to the benzene ring( making the molecule a 'phenol', increase the electron density of the pi orbitals of the ring and greatly increases the susceptibility of phenol towards electrophilic attack.

electrophilic substitution activating effect of hydroxy group OH on the reactivity of phenol increased electron density at 2 4 6 positions ortho para substitution

Phenol is much more reactive than benzene with respect to electrophilic substitution.

This is due, as argued via the diagram above, because of the increased electron density, specifically at the 2, 4 and 6 ring positions from the inductive effect of the hydroxy group of phenol (see also the resonance structure diagram too).

It appears that the non-bonding lone pair of electrons of the pz orbital of the oxygen atom interacts with the delocalised pi electrons of the benzene ring orbitals - which themselves are formed from the overlap of the pz orbitals of the 6 carbon atoms of the benzene ring - the result is increased electron density at the 2, 4 and 6 positions and the rapid reaction of phenol with chlorine water and bromine water (details in section 7.9.3).

 

the five resonance hybrid structures of phenol diagram for advanced A level organic chemistry

Another approach to explaining the extra reactivity at the 2, 4 and 6 positions of the benzene ring is to consider the three Kekule resonance structures of phenol, shown on the right of the diagram above.

You can draw structures that place a negative charge on carbon atoms 2, 4 and 6 of the ring, but not on ring carbons 3 and 5, where little substitution occurs with phenol.

Although these resonance structures contribute less than the two classic Kekule structures on the left, the electron density is sufficiently raised at the 2, 4 and 6 positions to dictate the most probable sites of electrophilic attack and there the orientation of the substitution positions.

The more negative the carbon atom, the more likely it is to allow the attacking electrophile to substitute the hydrogen atom.

If you imagine a blurred 'resonance hybrid' it would resemble to 'fuzzy' orbital diagram of phenol above!

% percent theoretical yields of disubstituted products from electrophilic attack on a monosubstituted benzene derivative ring positions 2 3 4 5 6

What you find in practice, fits in with the orientation of products theory and with phenol you get far less electrophilic substitution at the 3 and 5 positions compared to the 2, 4 and 6 positions of the benzene ring - see nitration of phenol in the next section 7.9.3.

See also section 7.14 Explaining orientation of products when substituting into a monosubstituted benzene derivative

The +I electron shift interaction of the non-bonding electrons of the oxygen atom interacting with the pi orbitals increasing the electron density of the benzene ring overrides any minus inductive effect from the more electronegative oxygen atom (electronegativities: C = 2.5, oxygen 3.5).

This discussion so far has focussed on potential electrophilic substitution reactions of phenol, but there is another consequence of the 'interaction' described above on the chemistry of phenols compared to aliphatic alcohols.

This interaction reduces the negative charge on the oxygen atom (decrease δ-) reducing its reactivity as an alcohol (less nucleophilic).

The pulling of oxygen's non-bonding electrons into the benzene ring makes the hydrogen of the OH group more positive (increase δ+) compared to aliphatic alcohols, enabling the proton to be lost more easily, so phenols are more acidic than aliphatic alcohols.

The acid-base character of phenols compared to aliphatic alcohols is discussed in detail in section 7.9.5.


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7.9.3 The reaction of phenol with chlorine water, bromine water and dilute nitric acid.

The substitution reactions of less reactive benzene require anhydrous conditions and often a catalyst too e.g.

7.3 Synthesis of arenes including alkylation - electrophilic substitution

7.5 Electrophilic substitution - nitration of benzene and methylbenzene

7.6 Electrophilic substitution - ring halogenation of benzene & methylbenzene

However, phenol is so reactive that undergoes electrophilic substitution reactions with an aqueous solution of the electrophile at room temperature e.g. with chlorine, bromine and nitric acid.

electrophilic substitution activating effect of hydroxy group OH on the reactivity of phenol increased electron density at 2 4 6 positions ortho para substitution

The increased electron density of the pi electron orbitals due to the +I effect of the OH group (O: lone pair specifically) causes chlorine and bromine molecules to be much easier polarised on collision with phenol i.e. Clδ+Clδ- and  Brδ+Brδ- and become effective electrophiles.

With excess chlorine water or bromine water room temperature, phenol gives an immediate white precipitate of 2,4,6-trichlorophenol or 2,4,6-tribromophenol, with almost zero substitution at the 3 and 5 positions of the benzene ring (structures in diagram below).

Both white crystalline solids have a 'disinfectant' odour.

 

balanced equation diagram of phenol reacting with chlorine water to form 2,4,6-trichlorophenol advanced organic chemistry

When chlorine dissolves in water, chloric(I) acid is formed - it is a very weak acid with little ionisation.

Cl2(aq)  +  2H2O(l)    HOCl(aq)  +  H3O+(aq)  +  Cl-(aq)

The polarised chloric(I) acid molecule can act as the attacking electrophile HOδ-Clδ+.

The simplified equation for the formation of 2,4,6-trichlorophenol is:

C6H5OH(aq)  +  3Cl2(aq)  ===>  Cl3C6H2OH(s)  +  3HBr(aq)

Again, the interaction of the OH group on the benzene ring of phenol (via non-bonding O electrons) increases the electron density of the benzene ring so much, that phenol's reactivity towards the chlorine electrophile is so increased that 2, 4 and 6 position substitution rapidly takes place at room temperature (diagram above showing the formation of 2,4,6-trichlorophenol).

The white solid rapidly forms and can be filtered off and purified by re-crystallisation.

The trichlorophenol has a strong antiseptic odour!

See also section 7.14 Explaining orientation of products when substituting into a monosubstituted benzene derivative

 

balanced equation diagram of phenol reacting with bromine water to form 2,4,6-tribromophenol advanced organic chemistry

Similarly, when bromine dissolves in water, bromic(I) acid is formed - it is a very weak acid with little ionisation.

Br2(aq)  +  2H2O(l)    HOBr(aq)  +  H3O+(aq)  +  Br-(aq)

The polarised bromic(I) acid molecule can act as the attacking electrophile HOδ-Brδ+.

The simplified equation for the formation of 2,4,6-tribromophenol is:

C6H5OH(aq)  +  3Br2(aq)  ===>  Br3C6H2OH(s)  +  3HBr(aq)

Again, the interaction of the OH group on the benzene ring of phenol (via non-bonding O electrons) increases the electron density of the benzene ring so much, that phenol's reactivity towards the bromine electrophile is so increased that 2, 4 and 6 position substitution rapidly takes place at room temperature (diagram above showing the formation of 2,4,6-tribromophenol).

As well as the precipitate of the tribromophenol, you observe the rapid decolourisation of the bromine water from re-brown to colourless solution of hydrogen bromide (actually hydrobromic acid).

The white solid rapidly forms and can be filtered off and purified by re-crystallisation.

The tribromophenol has a strong antiseptic odour!

See also section 7.14 Explaining orientation of products when substituting into a monosubstituted benzene derivative

 

diagram equation products % yield of isomers for reaction between phenol and dilute nitric acid 2-nitrophenol 3-nitrophenol 4-nitrophenol

Nitration is another examples of phenol increased reactivity compared to benzene.

Even at low temperature (< 20oC), with dilute nitric acid, electrophilic substitution readily takes place at the 2, 4 and 6 positions on the benzene rings.

The actual typical and theoretical yields of products from low temperature nitration of phenol with dilute nitric acid
Name of the isomer 2-nitrophenol 3-nitrophenol 4-nitrophenol
Actual yield of each isomer 53% <1% 47%
Actual yield of 2- and 4- isomers >99%
Theoretical yield of each isomer 40% 40% 20%
Theoretical yield of 2- and 4- isomers 60%

What you see is the vast majority of the product involves substitution at the 2 and 4 positions of the benzene ring in phenol. 

This fits in with the orientation of products theory discussed in section 7.9.2.

On a random basis you might expect 40% of 3-nitrophenol, when all you get is <1%.

Further substitution is inhibited by the 'deactivating' effect of the nitro group of highly electronegative atoms.

% percent theoretical yields of disubstituted products from electrophilic attack on a monosubstituted benzene derivative ring positions 2 3 4 5 6

See also section 7.14 Explaining orientation of products when substituting into a monosubstituted benzene derivative

 

balanced equation diagram of phenol reacting with concentrated nitric acid to form 2,4,6-trinitrophenol advanced organic chemistry

You can make 2,4.6-trinitrophenol using a concentrated mixture of nitric and sulfuric acids (equation above).


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7.9.4 Reaction of phenols as an alcohol e.g. esterification with acid anhydrides and acid chlorides (NOT a Friedel-Crafts reaction)

Phenols cannot be directly esterified with carboxylic acids, even with an acid catalyst, but they can be synthesised using acid chlorides and acid anhydrides.

(a) Reaction of a phenol with acid chlorides

Phenols tend to react quite slowly with acid chlorides where the a lone pair of non-bonding electrons on the oxygen atom enables a phenol to behave as a nucleophile.

See mechanism and reactions of acid chlorides with alcohols to form esters

You should note that aromatic acid chlorides like C6H5COCl  are less reactive than aliphatic acid chlorides like CH3COCl  - see notes on reaction (a)(iii) where the presence of sodium hydroxide generates a phenoxide ion - a much better nucleophile than a neutral phenol molecule.

(i) ethanoyl chloride  +  phenol  ===> phenyl ethanoate  +  hydrogen chloride

 (c) doc b  +  (c) doc b  ===>  (c) doc b  +  HCl

The ester, phenyl ethanoate, is an insoluble colourless liquid, boiling point 196oC.

(ii) 2-hydroxybenzoic acid  +  ethanoyl chloride  ===> 2-ethanoylhydroxybenzoic acid  +  hydrogen chloride

(c) doc b  +  (c) doc b  ===>  (c) doc b   +   HCl

2-ethanoylhydroxybenzoic acid is better known as acetylsalicylic acid, even better known as Aspirin !

It is a white solid, melting point 135oC.

(iii) benzoyl chloride  +  phenol  ===>  phenyl benzoate  +  hydrochloric acid

 (c) doc b  +  (c) doc b  ===>  (c) doc b  +  H+  +  Cl-

The ester, phenyl benzoate, is a white solid of melting point 71oC.

An acyl chloride and alcohol usually readily react at room temperature, especially if both are aliphatic.

However, phenols may require the presence of aqueous sodium hydroxide to facilitate the reaction, especially if the acyl chloride itself is itself a less reactive (than aliphatic) aromatic acid chloride like benzoyl chloride

The alkali generates a negative phenoxide ion (e.g. C6H5O from phenol C6H5OH), which is a more powerful nucleophile than the original neutral phenol molecule and the white solid of phenyl benzoate rapidly forms (this is an example of a Schotten–Baumann reaction - just reminding myself of student days in the 1960s when types of reaction were well remembered by the names of chemists!).

Therefore you can represent this reaction in a simplified way as:

C6H5O-  +  C6H5COCl  ===>  C6H5COOC6H5  +  Cl-

 

(b) Reaction of a phenol with an acid anhydride

Acid anhydrides are less reactive than acid chlorides, so an acid catalyst is used - a few drops of conc. sulfuric acid is added to the phenol and acid anhydride mixture.

(i) phenol  +  ethanoic anhydride  ===>  phenyl ethanoate  +  ethanoic acid

(c) doc b  +  (c) doc b  ===>  (c) doc b  +  (c) doc b

The ester formed is phenyl ethanoate

(ii) 2-hydroxybenzoic acid  +  ethanoic anhydride ===> 2-ethanoylhydroxybenzoic acid  +  ethanoic acid

 (c) doc b  +  (c) doc b  ===>  (c) doc b   +   (c) doc b

Another way of preparing 2-ethanoylhydroxybenzoic acid (also known as acetyl salicylic acid).

 


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7.9.5 Reactions of phenol as a weak acid, comparing the acid strengths of alcohols, phenols and carboxylic acids plus a specific test for phenols

Two reasons why aromatic phenols are stronger acids than aliphatic alcohols

(1) The effect of the benzene ring on the O-H bond

Apart from increasing the electron density of the benzene ring (increasing susceptibility to electrophilic attack), the interaction of the non-bonding electrons of the oxygen with the pi orbitals of the benzene ring has another important consequence.

This interaction reduces the negative charge on the oxygen atom (decrease δ-) reducing its reactivity as an alcohol, making phenols less nucleophilic and the conjugate base of phenols (e.g. phenoxide ion) is less strongly basic than the corresponding conjugate base of an aliphatic alcohol.

The net result of pulling of oxygen's non-bonding electrons into the benzene ring makes the hydrogen of the δ-O-Hδ+ group more positive (increase δ+) compared to aliphatic alcohols, enabling the proton to be lost more easily.

This makes phenols are more acidic than aliphatic alcohols.

(2) The resonance stabilisation of the anion

 Any anion derived from a phenol e.g. phenoxide ion, is stabilised by the charge being partially delocalised in the benzene ring.

The diagram below of the phenoxide ion illustrates the interaction of the pi orbital electrons of the benzene ring with the non-bonding electrons of the oxygen atom.

By delocalising the negative charge to some extent around the benzene ring, the potential energy of the phenoxide ion is lowered slightly, enough for a tiny concentration of it to exist in aqueous solution.

pi orbital diagram showing delocalised negative charge of the phenoxide ion interaction of non-bonding oxygen electrons

Another approach to understanding the stability of 'phenolic anions' is to consider the resonance structures of the phenoxide ion derived from phenol.

resonance structures of phenoxide ion resonance hybrid molecular structure of phenoxide ion advanced organic chemistry

You can see all the structures carry a negative charge at some point in the ion - the spreading out of the charge on a time averaged basis, but most of the negative charge is still carried by the oxygen atom.

If you imagine a blurred 'resonance hybrid' it would resemble to 'fuzzy' orbital diagram of the phenoxide ion above!

The same arguments apply to all phenols unless they have a basic group like an amino group attached to the benzene ring e.g. an amino-phenol.

All of this explains why 'aromatic' phenols are stronger acids than 'aliphatic' alcohols.

 

An aqueous solution of ethanol has a pH of ~7, the pH of a 0.10 mol dm-3 solution as a pH of ~5.4.

(c) doc b (aq)  +  H2O(l)    (aq)  +  H3O+(aq)

The acid dissociation constant: Ka ~ 1.02 x 10-10 mol dm-3 at 25oC, so phenol is a very weak acid, but it is a million times a stronger acid than ethanol which has a Ka of ~10-16 mol dm-3 in aqueous solution and barely alters the pH.

Strongly electronegative groups (-I effect) attached to the benzene ring increase the acidic nature of any phenol, enabling an energetically more favourable transfer of the O-H proton to a water molecule e.g.

Cl3C6H2OH  +  H2O(l)    Cl3C6H2O-(aq)  +  H3O+(aq)

 The Ka for 2,4,6-trichlorophenol is  2.5 x 10-8 mol dm-3

Here you see the effect of three quite electronegative chlorine atoms on the value of Ka.

(Pauling electronegativities:  H = 2.1;  C = 2.5;  N = 3.0; Cl = 3.0; O = 3.5)

(O2N)3C6H2OH  +  H2O(l)    (O2N)3C6H2O-(aq)  +  H3O+(aq)

 The Ka for 2,4,6-trinitrophenol is 1.38 x 10-1 mol dm-3

Here you see the effect of nine electronegative chlorine atoms on the value of Ka.

To put all the Ka values in a carboxylic perspective, Ka for ethanoic acid is 1.74 x 10-5 mol dm-3

This means 2,4,6-trinitrophenol is nearly 10,000 times more strongly acid than ethanoic acid AND will liberate carbon dioxide from sodium hydrogencarbonate - beware the results of simple tests! See table and comments further down in this section).

 

So, phenols are sufficiently acidic to form salts with strong bases e.g. the alkali sodium hydroxide gives ....

C6H5OH(aq)  +  NaOH(aq)    C6H5O-Na+(aq)  +  H2O(l)

sodium phenoxide (old name sodium phenate), the simplest 'phenolic' salt - a white crystalline solid.

Contrasting isomeric phenylmethanol with methylphenols

C6H5CH2OH(aq)  +  NaOH(aq)    no reaction, an aliphatic alcohol

CH3C6H4OH(aq)  +  NaOH(aq)    CH3C6H4O-Na+(aq)  +  H2O(l)

 

A comparison of some test reactions for aliphatic alcohols, phenols and carboxylic acids

The table illustrates the different/similar behaviour with three compounds containing a benzene ring.

Non of these tests are definitive, other types of compounds can give the same positive results.

However, although limited, the table is a useful comparison of the behaviour of three types of functional group based on -OH.

Compound Structure (a) Test with blue litmus (b) Effect of sodium hydroxide (aq) (c) Effect on sodium hydrogencarbonate (d) Add sodium (e) Add to iron(III) chloride
Phenylmethanol - an aliphatic alcohol (c) doc b No change No reaction No reaction Effervescence

H2

No colour change
Phenol - a phenol (c) doc b Turns pink Will form a salt

No reaction Effervescence

H2

Violet colour
Benzoic acid - a carboxylic acid (c) doc b Turns pink Will form a salt

Effervescence

CO2

Effervescence

H2

No colour change

Notes on the five above tests and results with respect to organic compounds listed

(a) Litmus Test on the sample mixed with water

Any acidic substance will give a blue to pink colour change with blue litmus paper.

No alcohol will change the colour of litmus, but most phenols and all carboxylic acids will give a pink colour, the pH of the mixture being lowered below pH 7 because of ionisation yielding hydrogen ions e.g.

C6H5CH2OH    no ionisation, no product, pH ~7

C6H5OH(aq)  +  H2O(l) C6H5O-(aq)  +  H3O+(aq)

C6H5COOH(aq)  +  H2O(l) C6H5COO-(aq)  +  H3O+(aq)

(b) Effect of aqueous sodium hydroxide

This isn't really a test, but if the compound is insoluble, dissolving in dilute sodium hydroxide may indicate an acidic nature. Of the three, only alcohols do not yield salts.

C6H5CH2OH(aq)  +  NaOH(aq)    no reaction

C6H5OH(aq)  +  NaOH(aq)    C6H5O-Na+(aq)  +  H2O(l)

C6H5COOH(aq)   +  NaOH(aq)    C6H5COO-Na+(aq)  +  H2O(l)

(c) Effect of sodium hydrogencarbonate

Acidic materials mixed with water and sodium hydrogencarbonate (solid or aqueous solution) should evolve carbon dioxide gas - can test with limewater - white milky precipitate.

C6H5CH2OH(aq)  +  NaHCO3(aq)    no reaction, to weak an acid

C6H5OH(aq)  +  NaHCO3(aq)     no reaction, to weak an acid

C6H5COOH(aq)  + NaHCO3(aq)  C6H5COO-Na+(aq) +  H2O(l) + CO2(g)

Note that some phenols with strongly electronegative substituent groups are acidic enough to liberate carbon dioxide from sodium hydrogencarbonate e.g. 2,4,6-trinitrophenol

(O2N)3C6H2OH  +  NaHCO3(aq)    (O2N)3C6H2O-Na(aq)  +  H2O(l) + CO2(g)

(d) Adding a tiny piece of fresh sodium metal to the material

If the organic compound is a liquid, no problem, but if its a solid, you must melt it first or dissolve it in a DRY solvent that does not react with sodium e.g. hexane.

You can test the gas with a lit splint - squeaky pop of hydrogen combustion.

2C6H5CH2OH  +  2Na    2C6H5CH2O-Na+  +  H2

2C6H5OH  +  2Na     2C6H5O-Na+   H2

2C6H5COOH   +  2Na   C6H5COO-Na+  +  H2O  +  H2O

(e) Effect of neutral iron(III) chloride solution ('Ferric chloride' test for phenols)

If no change, the iron(III) chloride solution remains yellowish.

(c) doc bIf a phenolic compound is present, it complexes with the iron(III) ion giving a variety of colours e.g. red, green or blue and violet with phenol itself.

Iron(III) ion forms several typical transition metals complexes with phenol and other substituted phenols.

However, bifunctional group molecule like 2-hydroxybenzoic acid will also give a positive test for a phenol as wells as positive tests for a carboxylic acid.


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7.9.6 Examples of the uses of phenol and its derivatives

Several chlorophenols are used as antiseptics. (need more diagrams)

Phenols are used in the synthesis of dyes. (need x-ref to phenylamine page)

molecular structure of 2,4-dichlorophenol skeletal formula structural formula advanced level organic chemistry doc brown2,4-dichlorophenol (on the right) is an intermediate in the manufacture of the herbicide 2,4-dichlorophenoxyethanoic acid (2,4-D). It is also used in antiseptics like TCP and Dettol.

2,4,5-trichlorophenol has been used as an fungicide and herbicide and intermediate in the manufacture of 2,4,5-trichlorophenoxyethanoic acid (2,4,5-T). These two were used as defoliants in the Vietnam War - the still lasting nasty, polluting and toxic effects of 'Agent Orange'.

2,4,6-trichlorophenol, also known as TCP is a chlorinated phenol that has been used as a fungicide, herbicide, insecticide, antiseptic, defoliant, and glue preservative. It is a white to yellowish crystalline solid with a strong, phenolic odour.

Many of theses chlorophenols are considered environmental pollutants and associated with certain types of cancer and their use is curtailed or strictly restricted and regulated (maybe!).

BUT, not all chlorophenols are bad for us and the environment.

4-chloro-3,5-dimethylphenol (diagram above), known as 'chloroxylenol' also branded as 'Dettol', is an important antiseptic and disinfectant which is used for skin disinfection, and together with alcohol for cleaning surgical instruments. It is also used within a number of household disinfectants and wound cleaners. It is thought to act by disrupting microbial cell walls and inactivating cellular enzymes. It is considered to be relatively non-toxic to us unless ingested but poisonous to many animals like cats. Never-the-less, it is on the World Health Organization's List of Essential Medicines

(c) doc bPhenol is used in the Kolbe synthesis of 2-hydroxybenzoic acid, an intermediate in the manufacture of aspirin.

2-hydroxybenzoic acid (on the right) has two functional groups, it is both a phenol and a carboxylic acid

Phenols are used in surfactants and detergents

hydrocarbon hydrophobic end R-C6H4-OH hydrophilic phenolic end, where R = a long chain alkyl group.

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