Physical Properties of Alcohols: Formula, Structure, Physical & Chemical Properties

Physical Properties of Alcohols: The organic compounds in which hydrocarbon hydrogen atoms have been replaced by the hydroxyl \(\left( {{\rm{ – OH}}} \right)\) group are called alcohols. These are the most common and valuable compounds in nature, in industry, and around the house.  In this article, let’s learn everything about the physical properties of alcohol in detail.

Alcohol Representation

The general formula for the homologous alcohol series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{{\rm{2n + 1}}}}{\rm{OH}}\) where \({\rm{n}} = {\rm{ }}1,{\rm{ }}2,{\rm{ }}3,\) etc. The saturated carbon chain forms the alkyl group and is often designated by the symbol R. Hence, alcohols are commonly represented as shown below

Alcohols can be viewed as organic analogues of water in which one hydrogen atom is replaced by an alkyl group. The two simplest and most commonly used alcohols are methanol and ethanol. They occur widely in nature and have many industrial and pharmaceutical applications.

Aromatic compounds, which contain a hydroxy group on a side chain, behave like alcohols are called aromatic alcohol. In these alcohols, the \({{\rm{ – OH}}}\) group is attached to a \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised carbon atom next to an aromatic ring.

Physical Properties of Alcohols

Some of the physical properties of alcohols are:

Colour and Odour

The lower alcohols are colourless liquids with a characteristic smell and a burning taste. However, the higher members (with more than \(12\) carbons) are colourless wax-like solids.

Polarity

Alcohols are often referred to as “polar protic” solvents. What’s that about? It is one of the key properties of alcohols and the key to understanding alcohols’ physical properties. The \({{\rm{ – OH}}}\) bond in alcohols is polar. This is due to the high electronegativity difference \((1.3)\) between oxygen and the hydrogen atom.

Oxygen has an electronegativity of \(3.5,\) whereas the electronegativity of hydrogen is \(2.2.\) Oxygen, being more electronegative than hydrogen, will pull the shared pair of electrons towards itself. This means that oxygen will develop a slight negative charge \(\left( {{\rm{\delta – }}} \right)\) and will become more “electron rich” (more negative)  than the hydrogen atom. 

On the other hand, hydrogen will develop a slightly positive charge \(\left( {{\rm{\delta + }}} \right)\) and will become more “electron poor” (more positive), resulting in the formation of dipoles. Hence, the electron density in the \({\rm{O – H}}\) bond is strongly “polarised” towards the oxygen atom.  

Hydrogen Bonding

The \({\rm{ – OH}}\) bond is polar in nature, develops partial negative and positive charges over the oxygen and hydrogen atoms. Since opposite charges attract, these partial charges will line up in solution where the partially negative oxygen atom on one molecule will interact with the partially positive hydrogen atom of the other molecule. 

These interactions between the negative oxygen atom and positive hydrogen atom result in the formation of hydrogen bonds. These bonds are about \(1/10\) as strong as normal bonds. 

Effect of Hydrogen Bonding in Alcohols

1. The hydrogen bonding leads to strong intramolecular forces of attraction.
2. The presence of hydrogen bonding in alcohols accounts for the higher boiling points of alcohols than their analogous alkanes.

Boiling Points and Melting Points

The presence of hydroxyl groups in alcohols significantly increases the boiling points. This can be accounted for by the presence of intramolecular hydrogen bonding.

For example – In propane (an alkane) and Propanol, the molecular weights are the same, but there’s over a \({\rm{100}}\,^\circ {\rm{C}}\) difference in boiling point. The difference occurs by replacing a methyl group \(\left( {{\rm{ – C}}{{\rm{H}}_{\rm{3}}}} \right)\) with hydroxyl \(\left( {{\rm{ – OH}}} \right){\rm{.}}\)

Replacing both the \({\rm{ – C}}{{\rm{H}}_{\rm{3}}}\) groups on propane with an \({\rm{OH}}\) group gives us “ethylene glycol”, a “di-ol” also called a “vicinal diol” would be more polar than Propanol and thus have a higher boiling point. This is true – the boiling point of ethylene glycol is \({\rm{197}}\,^\circ {\rm{C}}.\)

Boiling Points of Isomers of Alcohols

Generally, the more the \({\rm{ – OH}}\) group is able to interact with others, the higher the boiling point is. This means the primary alcohols have higher boiling points than the secondary alcohol (\(2-\)butanol)  which has a higher boiling point than the tertiary alcohol (\({\rm{t – }}\)butanol).

For example, the isomers of butanol, \(1-\)butanol and \(2-\)methyl\(-1-\)propanol (primary alcohol) have a higher boiling point than \(2-\)butanol (secondary alcohol), which has a higher boiling point than \({\rm{t – }}\) butanol (tertiary alcohol).

Hydrogen bonding is not the only intermolecular force present in alcohols. Along with hydrogen bonding, alcohols also experience van der Waals dispersion forces and dipole-dipole interactions. As the alcohols get longer or bigger, the hydrogen bonding and dipole-dipole interactions remain the same, but there is a significant increase in the dispersion forces. 

As the molecules grow longer and have more electrons, the attraction between them becomes stronger. The size of the transient dipoles created grows as a result of this. This is why the boiling points grow as the number of carbon atoms in the chains increases. As more energy is required to overcome the dispersion forces, the boiling points rise.

Boiling Point of Alkane and Alcohols

Alkanes experience only one intermolecular force of attraction called the van der Waals dispersion forces. Hydrogen bonds in alcohols are much stronger than van der Waals dispersion forces, and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules. This accounts for the higher boiling points in alcohols.

The chart below shows the boiling points of some simple primary alcohols with up to \(4\) carbon atoms compared with those of the equivalent alkanes (methane to butane):

\(\mathop {{\rm{C}}{{\rm{H}}_3}{\rm{OH}}}\limits_{{\rm{Methanol}}} \quad \mathop {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}}\limits_{{\rm{Ethanol}}} \quad \mathop {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}}\limits_{{\rm{Propan – 1 – ol}}} \quad \mathop {{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{Ol}}}\limits_{{\rm{Butan – 1 – ol}}} \)

The boiling point of alcohol would be higher than its corresponding alkane, even without any hydrogen bonding or dipole-dipole interactions. This is because:

1. Alcohols are longer molecules than their corresponding alkanes.
2. The oxygen atom in alcohols consists of eight electrons, which increases the van der Waals dispersion forces in alcohols.

Hence, the boiling point of alcohols increases with:

1. An increase in the carbon atoms in the chain, and
2. The availability of the \({\rm{ – OH}}\) group to interact with other molecules \(\left( {{{\rm{1}}^{\rm{o}}}\,{\rm{alcohol}} > {{\rm{2}}^{\rm{o}}}\,{\rm{alcohol}} > {{\rm{3}}^{\rm{o}}}\,{\rm{alcohol}}} \right)\)

The longer the alcohols carbon chain, the better the chance that the alcohol will be a solid at room temperature.

Solubility

The presence of hydrogen bonding in alcohols imparts a much greater solubility of alcohols in water. That’s because water being a hydrogen-bonding solvent, interacts favourably with the dipole of the hydroxyl group present in alcohols.

Small alcohols are completely soluble in water; mixing the two in any proportion generates a single solution. However, as the chain gets longer (after butanol), the greasy alkyl chain starts interfering with water solubility.

For example, ethanol is completely miscible in water; however, pentanol forms a separate layer. In order to be miscible, the original hydrogen bonds present in ethanol and water must be broken to form new hydrogen bonds between water and ethanol molecules. 

The energy released when these new hydrogen bonds form approximately compensates for the energy needed to break the original interactions. In addition, there is an increase in the system’s disorder, an increase in entropy that favours this chemical processes to occur.

However, in pentanol, the long hydrocarbon chains are forced between water molecules. This breaks the hydrogen bonds present in water molecules. The \({\rm{ – OH}}\) end of the alcohol molecule forms new hydrogen bonds with water molecules, but the hydrocarbon “tail” does not form hydrogen bonds. Only van der Waals dispersion forces exist between the water molecule and the hydrocarbon “tails.” 

These attractions are weaker and unable to furnish enough energy to compensate for the broken hydrogen bonds. Hence, as the length of the alcohol increases, the process becomes less feasible, and the solubility decreases. This situation becomes more pronounced with the increasing carbon chain.

The solubility of isomeric alcohols increases with branching because the surface area of the hydrocarbon part decreases with branching.

Solubility :   Primary  <   Secondary  <   Tertiary.

Density and Viscosity

The viscosity of small alcohols is much higher than the viscosity of alkanes. Generally, alcohols are lighter than water, i.e., less dense than water. The density of alcohols increases with molecular mass.

Summary

Alcohols are an essential class of compounds that has a wide range of applications. From being used as a solvent in the cosmetic industry, fuels in automobiles to sanitisers in pharmacology, alcohols are vital to the daily life of individuals. Hence, it is essential to learn its properties. In this article, we learnt the basic physical properties of alcohols especially pertaining to their boiling point. 

FAQs on Physical Properties of Alochol

Q.1. Why does Propanol have a higher boiling point than that of butane?
Ans: The boiling point of Propanol is much higher than that of butane because the molecules of Propanol are held together by an intermolecular hydrogen bond.

Q.2. Why are alcohols comparatively more soluble in water than hydrocarbons of comparable molecular masses?
Ans: Alcohols tend to form \({\rm{H – }}\)bonds with water and break the already existing \({\rm{H – }}\)bonds between water molecules. Hence, they are soluble in water.

Hydrocarbons, on the other hand, cannot form \({\rm{H – }}\)bonds; hence, they are insoluble in water

Q.3. What are the physical properties of rubbing alcohol?
Ans: The chemical name of rubbing alcohol is \(2-\)Propanol, isopropanol, propan\(-2-\)ol, with a chemical formula \({{\rm{C}}_3}{{\rm{H}}_8}{\rm{O}}.\) It is Miscible with water, alcohol, ether, and chloroform. Thezing point of rubbing alcohol is \(- 89.{\rm{5}}\,^\circ {\rm{C}},\) and its boiling point is \(82.4\,^\circ {\rm{C}}.\) It is a colourless, volatile liquid with a sharp musty odour. Vapours are heavier than air and mildly irritating to the eyes, nose, and throat.

Q.4. Is solubility a physical or chemical property?
Ans: Properties that can be determined without changing the composition of a substance are referred to as physical properties. Characteristics such as melting point, boiling point, density, solubility, colour, odour, etc., are physical properties.

Q.5. Are alcohols acidic or basic?
Ans: By the Arrhenius Definition of an acid and base, alcohols are neither acidic nor basic when dissolved in water. It neither produces \({{\rm{H}}^{\rm{ + }}}\) nor \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions in the solution.

Study Preparation of Alcohols Here

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