Chemical properties of any organic compound largely depends on the functional group attached to it. The hydroxyl group (-OH) determines
most of the chemical properties of alcohols and phenols. The following types of reactions
are seen in -OH derivatives :
Reactions involving the cleavage of the oxygen-hydrogen bond (R−O...H).
Reactions involving the cleavage of the carbon-hydroxyl bond (C...OH).
Dehydration and oxidation reactions.
Electrophilic substitution reactions in phenols.
The cleavage of O−H bond
Both alcohols and phenols readily release proton (the H+ ion attached to the oxygen atom).
Take a look at the reactions below :
In above reactions, alcohols and phenols are donating a proton which suggests that they are acidic in nature.
Acids are proton donors whereas bases are proton acceptors.
Arrange CH3CH2OH, CF3CH2OH, CCl3CH2OH in increasing
order of their acidic strength.
The −I-effect of the halogen atoms decreases
the electron density in the O-H bond that makes it easier for molecule to lose proton. Further, the −I-effect of fluorine is
higher than that of chlorine. Hence the order is :
CH3CH2OH < CCl3CH2OH < CF3CH2OH
Alcohols are weaker acids than water
Reason : Electron attracting groups
increase the acidity of compounds whereas electron releasing groups
decrease the acidity.
Alcohols are weaker acids than water because of the presence of electron releasing R (hydrocarbon part) group in alcohols. Phenols
,however, are stronger acids than water because they form phenoxide ion on losing a proton which is resonance stabilized.
Comparison of acidity of primary, secondary and tertiary alcohols :
Alkyl groups being electron donating in nature increase the +I-effect. Hence, +I-effect in alcohols follows the order :
Increasing order of +I-effect in alcohols
Since +I-effect reduces the acidity of compounds. Acidity of alcohols follows the order :
primary > secondary > tertiary.
Phenols are more acidic than alcohols
Phenols react with aqueous alkalies whereas alcohols do not.
Moreover, phenols turn blue litmus red while alcohols do not have any effect on litmus paper. This suggests
that phenols are stronger acids than alcohols.
Reason : The reason for the stronger acidity of phenols can be understood with the help of resonance structures given below :
As a result of resonance, the oxygen atom acquires a partial positive charge. Because of this, the O−H bond becomes weak and splits off
The phenoxide ion also exhibits resonance. Now, take a look at the resonance structure of the phenoxide ion.
Although both phenol and phenoxide ion are stabilized by resonance, phenoxide ion only carries a negative charge whereas
phenol involves a separation of negative and positive charge. Since resonance structures that involve separation of negative and
positive charge are less stable (discussed in
major and minor contributors in resonance),
phenoxide ion is more stable than phenols. In other words, phenol has a tendency to form a more stable phenoxide ion by losing a proton.
Lets take the case of alcohols
Alcohols release protons to form alkoxide ions and neither the alcohol molecule nor the alkoxide ion exhibits resonance.
Further, due to
the formal negative charge on the alkoxide ion, it has greater energy than alcohol which makes it less stable. In other words, alcohols
have a negligible tendency to form less stable alkoxide ion by releasing a proton.
Effect of substituents on the acidity of phenols
Electron withdrawing groups (EWG) such as -NO2, -CN, -X (halogen) increase the acidic strength of phenol because they stabilize
the phenoxide ion w.r.t phenol by dispersing the negative charge.
Electron donating groups (EDG) such as -NH2, -OR (alkoxy), -R (alkyl) etc., destabilize the phenoxide ion w.r.t phenol
by increasing the amount of negative charge. Hence, they decrease the acidic strength of phenol.
The effect in both the cases is seen more at ortho and para positions than at meta positions.
Arrange phenol; 2,4,6-trinitrophenol; 2,4-dinitrophenol; o-nitrophenol; p-nitrophenol in order of decreasing acidic
Electron withdrawing groups increase the acidity; therefore, 2,4,6-trinitrophenol with three electron withdrawing groups is the
most acidic whereas phenol having no electron withdrawing groups is the least. Among o-nitrophenol and p-nitrophenol, o-nitrophenol
is less acidic due to intramolecular H-bonding. The overall order is :
Arrange phenol, o-cresol (2-methylphenol), m-cresol (3-methylphenol), p-cresol in decreasing order of their acidity.
Electron donating groups decrease the acidity. Moreover, the effect is greater at ortho and para positions. The order is :
Phenol > m-cresol > p-cresol > o-cresol
The cleavage of Carbon-hydroxyl bond
Alcohols undergo a number of reactions involving the cleavage of carbon-hydroxyl bond. Phenols, however, do not undergo these
reactions because their is some double bond character between C-O bond of phenol due to resonance which is difficult to break.
Electrophilic Substitution Reactions in Phenols
The -OH group in phenol activates phenols towards electrophilic substitution reactions.
Question : Ortho and para-nitrophenols are more acidic than phenol. Explain why?
Answer : The presence of electron withdrawing group at ortho and para positions of phenol tend to
increase the acidity of phenol. Since nitro group (-NO2) is an electron withdrawing group, its presence at
ortho and para positions makes phenol more acidic.
Question : Phenol is a very weak acid. What substitutions in the molecule can make it a stronger
acid and a weaker acid and why?
Answer : Electron withdrawing groups such as -NO2 will increase the acidic character of
phenol because they stabilise the phenoxide ion whereas electron releasing groups such as -CH3 will decrease
the acidity of phenol because their presence destabilises the phenoxide ion.
Question : Out of benzene and phenol, which is more easily nitrated and why?
Answer : Phenol is nitrated more easily because the -OH group in phenol acts as electron releasing
group and increases the electron density at o- and p-positions of phenol.