Chemical Properties of Alcohols

Chemical Properties of Alcohols: Alcohols are organic compounds with one or more hydroxyl groups\(\left({{\text{-OH}}} \right)\) directly attached to an aliphatic carbon chain. Similarly, when the \({\text{-OH}}\) group is attached to the aromatic ring, they are called phenols. Each molecule of alcohol contains two parts:  the alkyl group and the hydroxyl group.

The chemical and physical properties of the alcohols are primarily dependent upon the hydroxyl \(\left({{\text{OH}}} \right)\) group, i.e. on the functional group of the alcohol. The alkyl group (or the aryl group in phenol) present in alcohols only modifies the properties.

Alcohols Classification

Alcohols are organic compounds with one or more hydroxyl groups \(\left({{\text{-OH}}} \right)\) directly attached to an aliphatic carbon chain.  Alcohols are classified depending upon the number of hydroxyl groups in them (one, two, three or many) as:

Type of Alcohol	Number of Hydroxyl Group	Examples
Monohydric alcohol	One	\({\text{C}}{{\text{H}}_3}{\text{OH}},{\text{C}}{{\text{H}}_3}{\left({{\text{C}}{{\text{H}}_2}} \right)_2}{\text{OH}}\)
Dihydric Alcohol	Two
Trihydric Alcohol	Three

Properties of Alcohol

The physical and chemical properties of alcohol are as follows:

Physical Properties of Alcohols

(i) Physical State:  The lower members of alcohols are colourless liquids at room temperature and have a distinct smell and a burning taste. Higher members of alcohols are odourless and colourless, waxy solids.

(ii) The boiling point of alcohols: An increase in boiling point with the increase in the number of carbon atoms is observed in alcohols. This increase is accounted for due to Vander-Waals forces. Also, with branching in the carbon chain, the Vander-Waals forces decrease (because of a decrease in surface area), and therefore, the boiling point also decreases with branching in isomeric alcohols.

The reason for higher boiling point in both alcohol and phenols (in comparison with other compounds of similar molecular masses such as ethers, hydrocarbons, haloalkanes and haloarenes) is due to the fact that they form intermolecular hydrogen bonding and usually exist as associated (larger) molecules.

iii) Solubility of alcohols in water: Alcohols are soluble in water. The capability of the formation of hydrogen bonds in them allows the molecules to form \( {\text{H}}\)-bonds with the water molecules. However, the solubility of the alcohol in water decreases with an increase in the alkyl group because they are hydrophobic in nature. As the alkyl group branches out, the surface area in which the hydrophobic part is present decreases, and hence, the solubility of alcohols with branched alkyl groups in water increases. The lower molecular weight alcohols are readily soluble in water in all proportions.

Chemical Properties of Alcohols

Alcohols react both as nucleophiles and electrophiles in their reactions.
1. When reacting as nucleophiles, the bond between the \({\text{O-H}}\) group is broken as shown:

2. When acting as electrophiles, the \({\text{C-O}}\) bond is broken as shown to form products.

Depending upon which bond is broken, the reactions of alcohols are divided into two categories.

Depending upon which bond is broken, the reactions of alcohols are divided into two categories.

1. Reactions with cleavage of \({\text{O-H}}\) bond:

A. Reaction with Metals

Alcohols react with active metals such as sodium, potassium, etc., to form alkoxides. The reaction is accompanied by the release of hydrogen gas.

\({\text{2C}}{{\text{H}}_3}{\text{-C}}{{\text{H}}_2}{\text{-OH + 2Na}} \to {\text{2C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2}{\text{-ONa + }}{{\text{H}}_2} \uparrow \)

Sodium Alkoxide

B. The Acidity of Alcohols

The polarity of the \({\text{O-H}}\) bond present in alcohols is responsible for its acidic nature. An alkyl group, which is electron releasing, increases the density of electrons on oxygen, thereby decreasing the polarity of the bond and therefore decreasing the acidity of the alcohols. The acidity of alcohols decreases in the order:

\({1^ \circ }\) alcohol \( > {2^ \circ }\) alcohol \( > {3^ \circ }\)  alcohol

Alcohols are also weaker acids than water since a water molecule more readily donates protons than an alcohol molecule.

C. Esterification Reaction

When alcohols react with carboxylic acids or acid anhydrides, esters are formed. This reaction is known as an esterification reaction. The reaction takes place in the presence of concentrated sulphuric acid.

\({\text{R}} – {\text{COOH}} + {\text{R}}’ – {\text{OH}} + {{\text{H}}^ + }\left({{\text{catalyst}}} \right) \Leftrightarrow {\text{R}} – {\text{COOR}}’ + {{\text{H}}_2}{\text{O}}\)

\({\left({{\text{R}} – {\text{CO}}} \right)_2}{\text{O}} + {\text{R}}’ – {\text{OH}} + {{\text{H}}^ + }\left({{\text{catalyst}}} \right) \Leftrightarrow {\text{R}} – {\text{COOR}}’ + {\text{R}} – {\text{COOH}}\)

Since the reaction is reversible, water molecules formed in the reaction should be removed immediately to prevent the reaction from going backwards and for the ester to form.

2. Reaction with Cleavage of \({\text{C-O}}\) Bond

A. Action with Hydrogen Halides

Alkyl halides are formed when alcohols react with hydrogen halides.

\({\text{R}} – {\text{OH}} + {\text{H}} – {\text{X}} \to {\text{R}} – {\text{X}} + {{\text{H}}_2}{\text{O}}\)

\({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2}{\text{-OH}} + {\text{HCl + anhyd}}{\text{.ZnC}}{{\text{l}}_2} \to {\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{Cl}} + {{\text{H}}_2}{\text{O}}\)

This reaction is also called the Lucas test, and the way the three classes of alcohols (\({1^ \circ },{2^ \circ }\) and \({3^ \circ }\)) react distinguishes them from each other.

When alcohols are dissolved in Lucas Reagent \(\left({{\text{Conc}}.{\text{HCl}} + {\text{ZnC}}{{\text{l}}_2}} \right)\):

1. Tertiary alcohols react immediately to form halides and form turbidity in the solution.
2. Secondary alcohols produce turbidity after \(5\) minutes.
3. Primary alcohols do not produce turbidity at room temperature.

B. Dehydration of Alcohols

Alcohols, on treatment with a protic acid like sulphuric acid or \({{\text{H}}_3}{\text{P}}{{\text{O}}_4}\) or on treatment with catalysts like anhydrous \({\text{ZnC}}{{\text{l}}_2}\) or \({\text{A}}{{\text{l}}_2}{{\text{O}}_3}\) gives alkenes.

For example, ethyl alcohol, on heating in the presence of Concentrated \({{\text{H}}_2}{\text{S}}{{\text{O}}_4}\), at \(443\,{\text{K}}\) dehydrates to ethylene.

\({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{OH}} + {\text{conc}} \cdot {{\text{H}}_2}{\text{S}}{{\text{O}}_4}\left({443~{\text{K}}} \right) \to {\text{C}}{{\text{H}}_2} = {\text{C}}{{\text{H}}_2} + {{\text{H}}_2}{\text{O}}\)

Dehydration of secondary and tertiary alcohols take place in mild conditions. The ease with which alcohols dehydrate is as follows:

\({3^ \circ }\) alcohol \( > {2^ \circ }\) alcohol \( > {1^ \circ }\) alcohol

C. Oxidation of Alcohol

When alcohols oxidise, two bonds are broken, and one new bond is formed. Oxidation of alcohols is accompanied by:
1. Cleavage of \({\text{O-H}}\) and \({\text{C-H}}\)  bonds
2. Formation of \({\text{C=O}}\) bond

The oxidation reaction is also called a dehydrogenation reaction because it involves the loss of hydrogen atoms. The product of the oxidation reaction depends upon the oxidising agent used. Primary alcohols are oxidised to aldehydes and subsequently to carboxylic acid in the presence of the strong oxidising agent.

\({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{OH}} + \left[ {\text{O}} \right] + {{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}/{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{C}}{{\text{H}}_3}{\text{CH}} = {\text{O}}\)

\({\text{C}}{{\text{H}}_3}{\text{CH}} = {\text{O}} + \left[{\text{O}} \right] \to {\text{C}}{{\text{H}}_3}{\text{COOH}}\)

While primary alcohols are oxidised to aldehydes and subsequently carboxylic acids, secondary alcohols are oxidised to ketones in the presence of chromic anhydride, and tertiary alcohols do not undergo oxidation. Under extreme conditions (presence of acidified \({{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}\) and \({\text{KMn}}{{\text{O}}_4}\)) and at higher temperatures, cleavage of the \({\text{C-C}}\) bond takes place. A mixture of acid and ketones are obtained, each of them containing a lesser number of carbon atoms than the parent alcohol.

Isomerism in Alcohols

Alcohols undergo the following types of isomerism:

Structural isomerism:  Structural isomers are compounds that have the same molecular formula but different structural formulas. Alcohols undergo four types of structural isomerism:

i) Chain Isomerism: Chain isomers are those which have the same molecular formula, but the number of carbon atoms in the chain is different. The chain isomers are also called nuclear isomers, and the phenomenon is known as chain isomerism.

Alcohols exhibit chain isomerism, as shown below:

ii) Position Isomerism: Isomers with similar carbon chains but differ in the position of the multiple bonds, or the functional groups are called position isomerism.

Example: Propan \( – 1 – {\text{ol}}\) and propan \( – 2 – {\text{ol}}\) have the same number of carbon atoms in the chain, but the position of the functional group is different.

iii) Functional Isomerism: Alcohols also show functional isomerism. For example, two functional isomers have a similar molecular formula: \({{\text{C}}_2}{{\text{H}}_6}{\text{O}}\)

iv) Tautomerism: Tautomerism is a kind of functional isomerism wherein the isomers stay in dynamic equilibrium with each other. The hydrogen atom from a polyvalent atom migrates to another within the same molecule, thereby resulting in tautomerism. Two significant types of tautomerism are dyad systems and triad systems.

Alcohols exhibit a triad system of tautomerism where the hydrogen atom from one polyvalent atom migrates to a third polyvalent atom within the same molecule.  The triad system exhibited by alcohol is called keto-enol tautomerism. Here, one form or tautomer (isomer) has the keto group, while the other tautomer exhibits the enol form or has the enolic group. Alcohols exhibit keto-enol-tautomerism with ketones and aldehydes.

Summary

Alcohols are compounds with a functional group of  \({\text{-OH}}\) (hydroxyl group). Alcohols are classified as monohydric, dihydric and trihydric alcohols depending upon the number of \({\text{-OH}}\) groups present in them. Lower members are liquids, while higher members of alcohols are solids at room temperature. The van der-Waals force decides upon the boiling point in an alcohol molecule. Alcohols are reactive and show two types of reactions- one involving cleavage of the \({\text{C-O}}\) bond and the other, the breaking of the \({\text{O-H}}\) bond. Alcohols exhibit four types of isomerism: chain, position, functional and tautomerism.

FAQs

Q.1. What are the three important chemical properties of alcohol?

Ans: Alcohols react in two ways- either by cleavage of the \({\text{C-O}}\) bond or by the cleavage of the \({\text{O-H}}\) bond. The important chemical reactions of alcohols are as follows:

a. Esterification: When alcohols react with carboxylic acids and anhydrides, esters are formed. This reaction is called an esterification reaction.

\({\text{R}} – {\text{COOH}} + {\text{R}}’ – {\text{OH}} + {{\text{H}}^ + } \Leftrightarrow {\text{R}} – {\text{COOR}}’ + {{\text{H}}_2}{\text{O}}\)

b. Oxidation Reaction:

Oxidation of alcohols involves dehydrogenation or removal of hydrogen from them in the presence of oxidising agents like potassium dichromate and sulphuric acid.

\({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{OH}} + \left[{\text{O}} \right] + {{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}/{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{C}}{{\text{H}}_3}{\text{CH}} = 0\)

\({\text{C}}{{\text{H}}_3}{\text{CH}} = {\text{O}} + \left[{\text{O}} \right] \to {\text{C}}{{\text{H}}_3}{\text{COOH}}\)

c. Action with Hydrogen Halide:

Alkyl halides are formed when alcohols react with hydrogen halides.

\({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{OH}} + {\text{HCl}} + {\text{anhyd}}.{\text{ZnC}}{{\text{l}}_2} \to {\text{C}}{{\text{H}}_3} -{\text{C}}{{\text{H}}_2} – {\text{Cl}} + {{\text{H}}_2}{\text{O}}\)

Q.2. Are alcohols liquids or solids at room temperature?
Ans: The lower members of the alcohol series are colourless liquids, while the higher members are waxy solids at room temperature.

Q.3. Does alcohol show position isomerism?
Ans: Yes, alcohols show position isomerism. For example, Propan  \( – 1 – {\text{ol}}\) and propan  \( – 2 – {\text{ol}}\) have the same number of carbon atoms in the chain, but the position of the functional group \(\left({ – {\text{OH}}} \right)\) is different.

\({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{C}}{{\text{H}}_2} – {\text{OH}}\) and \({\text{C}}{{\text{H}}_3} – {\text{CH}}\left({{\text{OH}}} \right) – {\text{C}}{{\text{H}}_3}\)

Q.4. What are the four types of isomerism exhibited by alcohols?
Ans: Alcohols exhibit position isomerism, chain isomerism, functional group isomerism and tautomerism.

Q.5. What is functional group isomerism? Give an example.
Ans: When two isomers have the same molecular formula but different functional groups, the isomerism exhibited is called functional group isomerism.

Example: \({\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{OH}}\) and \({\text{C}}{{\text{H}}_{3 – }} – {\text{O}} – {\text{C}} {{\text{H}}_3}\)