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Acidity and Basicity of Alcohol and Phenol: Acidity Order, Acidity of Substituted Phenol, Acidity of Alcohol, Phenol and Carboxylic Acids

Acidity and Basicity of Alcohol and Phenol: Acidity Order, Acidity of Substituted Phenol, Acidity of Alcohol, Phenol and Carboxylic Acids

Alcohols are organic compounds that contain at least one hydroxyl functional group (-OH) attached to an alkyl-substituted saturated carbon atom. Because of the electronegativity difference between oxygen (3.44) and hydrogen atoms (2.2), the hydroxyl group in alcohol is polar. Because of this difference in electronegativity, alcohols donate furnish a proton (H+) and form an alkoxide ion (R-O-) in the presence of a strong base. Hence, they have a slightly acidic nature.

We should consider the Bronsted-Lowry idea of acid and base before considering the acidity of alcohol. All proton donors are acids and all proton acceptors are bases according to this theory. According to this theory alcohols are Bronsted acids. Because of the polarity of the O–H bond. As a result, the shared pair of electrons shifts towards the O atom, weakening the O–H bond. This allows a proton to be released from the alcohol molecule, making them acidic.

We should consider the Bronsted-Lowry idea of acid and base before learning about the acidity of alcohol. This states that bases are the only proton acceptors and that all proton donors are acids. This means that alcohols are Bronsted acids. Because of the polarity of the (O - H) bond, alcohols are acidic. As a result, the shared pair of electrons moves closer to the O atom, weakening the O - H bond. This makes them acidic by facilitating the release of a proton from the alcohol molecule.

Table of content:

  • Acidity of alcohol
  • Order of acidity of alcohol
  • Acidity of alcohol in gas phase
  • Basicity of alcohol
  • Acidity of phenol
  • Acidity of substituted phenol
  • Acidity of alcohol, phenol and carboxylic acid
  • Practice problems
  • Frequently asked questions

Acidity of alcohol:

Alcohols, like water, are weak acids and bases. This is due to the polarization of the O–H bond, which makes the hydrogen partially positive. Furthermore, the electron pairs on the oxygen atom of alcohol make it both basic and nucleophilic. Protonation of an alcohol converts hydroxide, a poor leaving group, into water, a good one. The two acid–base equilibria for ethanol are shown below.

The only alcohol that is slightly stronger than water (pKa = 15.7) is methanol (pKa = 15.5). Weaker acids include ethanol (pKa = 15.9), tert-butanol (pKa = 18.0), and others. All alcohols, on the other hand, are far stronger acids than terminal alkynes, hydrogen, ammonia, and alkanes.

Order of acidity of alcohol:

The acidic strength of alcohol is determined by the strength of the conjugate base produced, i.e. alkoxide ions. It means the stronger the conjugate base of alcohol, the weaker will be the acid. 

It implies that the acid will be weaker the stronger the alcohol's conjugate base. As a result, a more stable alkoxide ion is a weaker conjugate base, and as a result, the alcohol will be more acidic because the stable alkoxide ion will easily release H+ ion.

Primary alcohols (1o) are more acidic than secondary alcohols (2o), which are more acidic than tertiary alcohols (3o), in the case of simple alkyl alcohols.

We'll look at two factors to better comprehend and evaluate the alkoxide ion's stability:

Steric Factors : The alkoxide ion is bulkier with more substituted alkyl groups, making it more difficult for the solvent to stabilize the alkoxide. Since alkoxide ion will be less stable, it will be more reactive as a conjugate base with higher basic strength. As a result, the acid produced by this conjugate base is weak.

The alkoxide ion is bulkier when there are more substituted alkyl groups, making it more difficult for the solvent to stabilise the alkoxide. As a conjugate base with a higher basic strength, alkoxide ion will be more reactive due to its decreased stability. Because of this conjugated base, the acid is weak.

Electronic Factors: As the number of Electron Donating Groups linked to a carbon with a hydroxyl group increases, the electron density on the O atom rises, making the alkoxide less stable and more reactive, making it a strong conjugate base. As a result, more substituted alcohols have a lower acidity.

The electron density on the O atom rises as the number of electron-donating groups linked to the carbon with a hydroxyl group grows, making the alkoxide less stable and more reactive and making it a potent conjugate base. Thus, alcohols with more substitutions are less acidic.

Taking into account both electronic and steric factors, primary alkoxide ions are the most stable, whereas tertiary alkoxide ions are the least stable. As a result, primary alcohols are the most acidic in nature, whereas tertiary alcohols are the least acidic.

As a result, the decreasing order of alcohol acidity is:

Primary alcohol (1o) >Secondary alcohol (2o) >Tertiary alcohol (3o)

Acidity of alcohol in gas phase:

We learned that in the presence of the solvent, the acidity of alcohol reduces as the alkyl substituent increases due to the destabilization of alkoxide ions. In the gaseous phase, however, this is completely reversed. Alcohols with more alkyl substitutions are more acidic than alcohols with fewer substitutions. This is because in the gas phase, where there is no solvent, the polarizability of the anion, i.e., the conjugate base, dominates. As we add more and more larger alkyl groups, the electrons are more easily moved in the electric field, making them more polarizable. This polarizability stabilizes the anion in the gas phase, making it more acidic.

In Solvent phase,

CH3OH >CH3CH2OH>CH3CH2CH2OH>CH3CH2CH2CH2OH

In gas phase,

CH3CH2CH2CH2OH >CH3CH2CH2OH>CH3CH2OH>CH3OH

Basicity of alcohols:

Because alcohols are less acidic than water, their conjugate base alkoxide ions are more basic than the hydroxide ions. Strong bases, such as sodium/potassium hydride or sodium/potassium metal, which react violently but controllably with alcohol, can be used to convert alcohol into metal alkoxide. Because of the steric effect, the alkoxide ion is not solvated sufficiently when the alkyl substitution is bulky, resulting in less stabilization. Destabilization is also aided by inductive effects. As a result, the equilibrium is skewed toward alcohol.

Alcohols can also function as bases, accepting protons from strong acids. Notably, conjugate bases of compounds with higher pKa than an alcohol will deprotonate that alcohol.

Acidity of phenol:

Alcohols and phenols are both very weak acids, however due to the stability of phenoxide ions, phenols are more acidic than alcohols. The delocalization of electrons in the benzene ring is the major cause for its stability. The negative charge on oxygen is delocalized within the benzene ring, making it extremely stable.

Both alcohols and phenols are extremely weak acids, however due to the stability of phenoxide ions, phenols are more acidic than alcohols. The delocalization of electrons in the benzene ring is the primary factor contributing to its stability. The entire benzene ring experiences a delocalization of the oxygen's negative charge, making it extremely stable.

As the phenoxide ion is relatively persistent, phenols rapidly release furnish protons in the presence of a base, making it more acidic. The following are the resonating structures of phenoxide ion:

Acidity of substituted phenol:

Effect of Substitution on acidity of phenol:

By forming stable phenoxide ions by delocalization of the negative charge and inductive effects -I/-M effect, phenol substituted with electron-withdrawing groups (EWG) such as Nitro and Chloro makes it more acidic. Due to the destabilization of the phenoxide ion due to the +I/+M effect, phenol substituted with electron-donating groups (EDG) or electron-releasing groups (ERG) such as methyl or methoxy group makes it less acidic. The order of decreasing acidity of para-substituted phenols of EWG and EDG is as follows:

By forming a stable phenoxide ion by the delocalization of the negative charge and inductive actions, or the (- I/ - m) effect, phenol substituted with electron-withdrawing groups like Nitro and Chloro group makes it more acidic. A phenol replaced with an electron-donating group, such as a methyl or methoxy group, becomes less acidic because the +I/+m action destabilises the phenoxide ion. The para-substituted phenols of EWG and EDG are listed below in decreasing acidity order:

Effect of Substituent Position on Acidity of Phenol:

We can see that a negative charge is concentrated on the ortho and para positions in the resonating structure of phenoxide ion. As a result, it can be deduced that phenol's acidity is increased by electron withdrawing groups in the ortho and para positions. The following is a list of ortho, para, and meta substituted Nitro-phenol in decreasing order of acidity:

We can see that the ortho and para places of the phenoxide ion's resonating structure are where the majority of the negative charge is located. Thus, it may be deduced that phenol becomes more acidic when electron-withdrawing groups are positioned at the ortho and para positions. The following lists ortho, para, and meta substituted nitro-phenol in decreasing order.

Acidity of alcohol, phenol and carboxylic acid:

A substance that readily releases H+ is known as an acid. The ability of an acid to easily give away H+ determines its strength. The more powerful the acid, the easier it is to liberate H+.

Carboxylic acids are stronger acids than corresponding alcohols and even phenols because they lose their proton to form a stable conjugate base, i.e., carboxylate ion, which is more resonance stabilized than alkoxide or phenoxide ion. Along with stable carboxylate ions, carboxylic acid contains a carbonyl group, which is an electron-withdrawing group, making it more acidic than phenol, which is stabilized by resonating structures of phenoxide ions. In addition, in phenol, only carbon carries the negative charge, whereas in carboxylic acid, the negative charge is shared by two oxygen atoms. Regular alcohol, on the other hand, is not resonance stabilized when a proton is lost, making it a weaker acid. As a result, the order of stability of these ions is Carboxylate ion > Phenoxide ion > Alkoxide ion.

Because they lose their proton to produce a stable conjugate base, the carboxylate ion, which is more resonance stabilised than the alkoxide or phenoxide ion, carboxylic acids are stronger acids than the equivalent alcohols and even phenols. Compared to phenol, which is stabilised by resonant structures of phenoxide ions, carboxylic acid possesses an electron-withdrawing carbonyl group in addition to stable carboxylate ions, making it more acidic. Additionally, while two oxygen atoms share the negative charge in carboxylic acid, just carbon will carry it in phenol. Regular alcohol, on the other hand, does not resonance stabilise after losing a proton, making it a weaker acid. As a result, these ions are more stable in the following order: carboxylate ion > phenoxide ion > alkoxide ion.

As a whole the comparison between the acidity of alcohol, phenol, water, and carboxylic acid is given below:

 RCOOH > C6H5OH > H2O > ROH

Practice problems:

Q 1. Which alcohol is more acidic?

(A) C2H5OH
(B) (C2H5)2CHOH
(C) (C2H5)3COH
(D) none of the above

Answer: (A)
Taking into account both electronic and steric factors, primary alkoxide ions are the most stable, whereas tertiary alkoxide ions are the least stable. As a result, primary alcohols are the most acidic in nature, whereas tertiary alcohols are the least acidic.

As a result, the decreasing order of alcohol acidity is:

Primary alcohol (1o) >Secondary alcohol (2o) >Tertiary alcohol (3o)

Q 2. Which of the following is more acidic?

(A) o-nitrophenol
(B) m-nitrophenol
(C) p-nitrophenol
(D) Phenol

Answer: (C)
NO2 group at ortho and para position withdraws electrons of the O-H bond towards itself by the stronger -R effect while the -NO2 group at m-position withdraws electrons of the O-H bond by the weaker -I effect. Thus, ortho and para nitrophenols are more acidic than m - nitrophenol. Among ortho and para nitrophenols, ortho nitrophenol is a little less acidic than para nitrophenol due to intramolecular H-bonding which makes loss of a proton a little more difficult.

Q 3. The correct order of acidic strength in water of the following given substituted phenols

(A) p-nitrophenol < p-fluorophenol <p-chlorophenol
(B) p-fluorophenol< p-chlorophenol< p-nitrophenol
(C) p-chlorophenol< p-fluorophenol<p-nitrophenol
(D) All of the above

Answer: (B)
The following is the order of electron withdrawing tendency from the benzene ring: -F < -Cl <-NO2

As electron withdrawing tendency increases acidic strength also increases.

Acid strength is determined by how stable the anion is after losing the hydrogen atom. The acidic strength of the compound is affected by both inductive and mesomeric effects.The three groups are all electron withdrawing.

In the case of halogens, the inductive effect is stronger, whereas in other cases, the mesomeric effect is stronger.Nitro group, which demonstrates that -M dominates all other effects. When this is added to any group, it greatly reduces the electron cloud.

In the case of fluoro and chloro, both exhibit the I effect. When the resonating structures of the conjugate bases of p-chlorophenol and p-fluorophenol are observed, we can see that the p-chloro phenoxide ion has an extra resonance structure. This occurs because chlorine has an extra vacant d-orbital for electron delocalisation, which fluorine atoms do not have. As a result, the conjugate base of p- chlorophenol is more stable and thus acidic.

The acidity of p-chlorophenol is higher than that of p-fluorophenol.

Q 4. Which of the following is more acidic?

(A) o-fluorophenol
(B) o-chlorophenol
(C) o-bromophenol
(D) o-iodophenol

Answer: (B)
o-chlorophenol > o-bromophenol > o-iodophenol > o-fluorophenol

That's because, in o-fluorophenol fluorine is highly electronegative, forming an H-bond with the H atom of OH group. So the acidic nature of this OH is lowest.

In the other three cases, the electronegativity is not so significant compared to F, so here we should look for the -I effect of the halogens.

Cl >Br>I, in terms of electronegativity and same order is followed in terms of -I effect also.

Frequently asked questions:

Q 1. Why is phenol more acidic than water?
Answer: When a phenol molecule loses a proton, it produces phenoxide ion, which is stabilized by resonance as the negative charge delocalized over the aromatic ring. When a water molecule loses a proton to form a hydroxide ion, no such resonance exists. As a result, phenol is more acidic than water.

Q 2. Why are alcohols less acidic than water?
Answer: Water is more acidic than alcohol because the hydroxyl ion is more stable than the alkoxide ion, and thus water releases proton more easily than alcohol. 

Let's think about it in terms of polarity. Because the alkyl group in alcohol decreases the polarity of the O-H bond via the + I effect, the -OH group in alcohol is less polar than the -OH group in water.

Q 3. Why is picric acid more acidic than benzoic acid?
Answer: Because picric acid includes three -NO2 groups, which are electron withdrawing groups, it is more acidic than benzoic acid. The presence of an electron withdrawing group on a ring increases the acidic character, while the presence of an electron donating group on the ring decreases the acidic character.

Q 4. Why does phenol not react with sodium bicarbonate?
Answer: Phenol is naturally acidic. However, it is a weak acid. The presence of -OH on phenol and electron conjugation in benzene rings both pull electrons away from the -OH group. Sodium bicarbonate is a weak base. It readily accepts electrons from stronger acids such as carbonic acid, but it lacks the strength to remove the proton from phenol. Hence, due to the weak acidic nature of phenol NaHCO3does not react with phenol .

Related Topics:

Ethers-physical and chemical properties

Ethanol

Ethers-classification and preparation

Phenol

Lucas test

Methanol

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