Acidity of Alcohol: Definition and Properties

Acidity of Alcohol: Alcohol is a class of organic compounds having at least one hydroxyl functional group \({\rm{( – OH)}}\) attached to the alkyl-substituted carbon atom. In alcohol, the hydroxyl group is polar in nature due to the electronegativity difference between oxygen \((3.44)\) and hydrogen atoms \((2.2).\) Due to this electronegativity difference, alcohols donate a proton \(\left( {{{\rm{H}}^{\rm{ + }}}} \right)\) and form alkoxide ion \(\left( {{\rm{R – }}{{\rm{O}}^{\rm{ – }}}} \right)\) in the presence of a strong base. As a result, they are slightly acidic. This article gives more details about the Acidity of Alcohol.

Acidity of Alcohol: Description

Before understanding the acidity of alcohol, we should think about the Bronsted-Lowry concept of acid and base. According to this, all proton donors are acids and all proton acceptors are bases. In accordance with this, alcohols are Bronsted acid. Alcohols are acidic due to the polarity of the \({\rm{O – H}}\) bond. Owing to this, the shared pair of electrons shifts towards the \({\rm{O}}\) atom, and the \({\rm{O – H}}\) bond becomes weak. This facilitates the release of a proton from the alcohol molecule making them acidic.

What is the Order of Acidity of Alcohols?

The strength of alcohol depends on the strength of the corresponding conjugate base, i.e. alkoxide ions formed. It means the stronger the conjugate base of alcohol, the weaker will be the acid. Thus it can be concluded that a more stabilised alkoxide ion is a weaker conjugate base and hence the alcohol will be more acidic as the stable alkoxide ion will easily release \({{{\rm{H}}^{\rm{ + }}}}\) ion. 

In the case of simple alkyl alcohols, primary alcohols \(\left( {{1^ \circ }} \right)\) are more acidic than secondary alcohols \(\left( {{2^ \circ }} \right)\) which are more acidic than tertiary alcohols \(\left( {{3^ \circ }} \right)\).

To understand and evaluate the strength of the alkoxide ion we will look into the two factors:

1. Steric Factors – More substituted alkyl groups mean that the alkoxide ion is bulkier and it would be harder 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. Hence the acid, from this conjugated base, is weak.
2. Electronic Factors – When there are more Electron Donating Groups attached to the carbon having a hydroxyl group, the electron density on the O atom increases, and the alkoxide is consequently less stable and more reactive making it a strong conjugate base. Hence, more substituted alcohols are less acidic.

Hence considering both electronic and steric factors, it can be concluded that primary alkoxide ions are the most stable and tertiary alkoxide ions are the least stable. Hence, primary alcohols are the most acidic and tertiary alcohols are the least acidic in nature. 
Thus decreasing order of acidity of alcohol is:

\({\rm{Primary}}\,{\rm{alcohol}}\left( {{\rm{1}}^\circ } \right) > {\rm{Secondary}}\,{\rm{alcohol}}\left( {{\rm{2}}^\circ } \right) > {\rm{Tertiary}}\,{\rm{alcohol}}\left( {{\rm{3}}^\circ } \right)\)

Acidity of Alcohols and Phenols

Alcohols and Phenols both are very weak acids but phenols are more acidic than alcohols due to the stability of phenoxide ions. The main reason for its stability is the delocalisation of electrons in the benzene ring. The negative charge on oxygen gets delocalised in the entire benzene ring and hence becomes very stable. 

Thus phenols easily release protons in the presence of a base as the phenoxide ion is very stable thus, Phenol is more acidic. The resonating structures of phenoxide ions are given below:

Effect of Substitution on Acidity of Phenol

Phenol substituted with Electron-withdrawing groups such as Nitro and Chloro group makes it more acidic by the formation of stable phenoxide ion through delocalization of the negative charge and inductive effects i.e. \({\rm{ – I/ – m}}\) effect. Whereas phenol substituted with Electron-donating groups such as methyl or methoxy group makes it less acidic due to destabilisation of phenoxide ion due to \({\rm{ + I/ + m}}\) effect. Decreasing acidity order of para-substituted phenols of EWG and EDG are given below:

Effect of Position of Substituents on Acidity of Phenol

In the resonating structure of phenoxide ion, we can see that in the resonating structure of phenoxide ion, a negative charge is concentrated on the ortho and para positions. Thus it can be concluded that Electron Withdrawing Groups at the ortho and para position of phenol increase its acidity. Decreasing order of ortho, para, and meta substituted Nitro-phenol is given below:

Acidity of Alcohols and Water

Water is more acidic than alcohol because hydroxyl ion is more stable than alkoxide ion, thus water will release proton easily as compared to alcohol. Let’s understand this in terms of polarity. In alcohol, the \({\rm{ – OH}}\) group is less polar than the \({\rm{ – OH}}\) group of water because the alkyl group in alcohol decreases the polarity of the \({\rm{O – H}}\) bond by +I effect, decreasing the acidity of alcohol.

For example, The \({\rm{pKa}}\) value for methanol and water is \(16\) and \(15.7\) respectively. As we can see that \({\rm{pKa}}\) value of water is less than methanol, the dissociation of methanol is less labile than water. Hence, water is a stronger acid than methanol. 

Acidity of Alcohols in the Gas Phase

In the presence of the solvent, we have learned that the acidity of alcohol decreases with an increase in alkyl substituent due to the destabilisation of alkoxide ions. But this entirely reverses in the gaseous phase. More alkyl-substituted alcohols are more acidic than less alkyl-substituted alcohols. This is because, in the gas phase where there is no solvent, the overpowering effect is the polarizability of the anion, i.e., the conjugate base, as we go on adding more and more larger alkyl groups they take up more space, and the electrons are easily moved in the electric field, therefore, they are more polarizable, this polarizability stabilizes the anion in the gas phase making them more acidic.

Acidity of Alcohols, Phenols and Carboxylic Acids

An acid is a substance that easily releases \({{\rm{H}}^ + }.\) The strength of an acid is decided by its ability to give away \({{\rm{H}}^ + }\) easily. The stronger the acid, the more easily it can release \({{\rm{H}}^ + }.\) 

Carboxylic acids are stronger acids than corresponding alcohols and even phenols because it loses its 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 has a carbonyl group that is an electron-withdrawing group making it more acidic as compared to the phenol which is stabilized by resonating structures of phenoxide ions. Also, in phenol, only carbon will carry the negative charge, while in carboxylic acid the negative charge is spread between two oxygen atoms. Thus, on the other hand, regular alcohol is not resonance stabilized upon losing a proton, therefore weaker acid. Therefore, the order of stability of these ions is Carboxylate ion > Phenoxide ion> Alkoxide ion. Let us see how to convert Alcohol to Carboxylic acid:

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

Summary About Acidity of Alcohol

Finally, we can say that alcohols are very weak acids as they give off protons in the presence of a base forming a conjugate base. There are various factors on which the acidity of alcohol depends. As the presence of electron-donating groups destabilizes the conjugate base making them weaker acids. Thus acidity order of substituted alcohol is: \({\rm{Primary}}\,{\rm{alcohol}}\left( {{\rm{1}}^\circ } \right) > {\rm{Secondary}}\,{\rm{alcohol}}\left( {{\rm{2}}^\circ } \right) > {\rm{Tertiary}}\,{\rm{alcohol}}\left( {{\rm{3}}^\circ } \right)\)

When it comes to phenol and alcohol, phenols are more acidic because of the formation of resonance stabilized phenoxide ions. Since phenoxide ion is more stable than alkoxide ion, it is a stronger acid. The stability of phenoxide ions also depends on the type of substituents attached to the phenol ring. 

For example, the electron-withdrawing group stabilizes the phenoxide ion by delocalisation of negative charge in the ring, on the other hand, the electron-donating group pushes electrons to the negatively charged oxygen and destabilises the phenoxide ion. 

Thus, it can be concluded that the presence of the electron-withdrawing group increases the acidity of phenol and the presence of the electron-donating group decreases the acidity of phenol. Water is more acidic than alcohol due to the polar hydroxyl group. 

In the gas phase, the acidity order of alcohol is simply reversed, i.e., more alkyl substituted alcohols are more acidic than less alkyl-substituted alcohols because, in the gas phase there is no solvent, the polarizability of the anion plays the main role. As we go on adding more and more larger alkyl groups they become more polarizable, this polarizability stabilizes the anion in the gas phase. 

In comparing the acidity of carboxylic acid, phenol, and alcohol, Carboxylic acids are stronger acids than corresponding alcohols and even phenols because it loses their proton to form a stable conjugate base. Thus the final acidic order of the discussed compounds is: Carboxylic acid > Phenol > Water > Alcohol.

FAQs on Acidity of Alcohol

The most commonly asked questions about the acidity of alcohol are answered here:

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