Alcohols and their physical properties: A Proper Guideline


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Alcohols and Their Physical Properties

Suppose you are sentenced to live on a desert island for hundred days. You are allowed to take some compounds with you. If you are intelligent enough, you will certainly choose alcohol without any delay.

Why? Because alcohols can be converted to almost every other aliphatic compounds!

What does the general formula look like?

Alcohols have the general formula ROH, where R is an alkyl or substituted alkyl group.

Let me explain this in an easier way.

We can say that when a hydrogen atom of an alkane is replaced by a -OH group, we get alcohol. Now after you take away one hydrogen atom from an alkane,  make a hydroxyl group take that empty position, dare you still call it by the same name? No, you have to get it a new name, the format of the name being (alka+nol).

This is how we get what are called the common names of alcohols.

For example, if you replace a hydrogen atom of propane by a -OH group, the alcohol you get would love to be called by the name propanol. Sounds sweet, doesn’t it?

Classification based on the position of the carbon bearing the -OH group

Now just like your schooling is divided into different stages such as primary, secondary and higher levels, alcohol, according to the kind of carbon that bears the hydroxyl group, is classified into three types – 1° or primary,  2° or secondary and 3° or tertiary.
Well, as you can see, all degrees certainly do not always represent temperatures.

The alcohol where the carbon atom adjacent to the hydroxyl group is bonded with only one other carbon atom is called 1° or primary alcohol. Similarly, the one where the carbon atom adjacent to the hydroxyl group is bonded with two other carbon atoms is 2° or secondary alcohol.

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Now you should be able to define the 3° or tertiary carbon atom. Let me leave this task for you, please.

Propan-1-ol, propan-2-ol and 2-methylpropan-2-ol are examples of the three above kinds of respective systems. These are named according to the IUPAC system, one of the most versatile and the smartest systems ever introduced in the field of science.

For the simpler alcohols, the common names are used often. See! We human beings are not the only ones who have got gorgeous names, alcohols have got some too!

Classification based on structure: 

On the basis of the structure, alcohols are divided into two kinds- aliphatic alcohols and aromatic alcohols.
Again, Aliphatic alcohols may be saturated and unsaturated according to the nature of C-C bond.

Classification based on the number of hydroxyl groups present:

(a) Monohydroxy alcohol: One -OH group present.

(b) Dihydroxy alcohol : Two -OH groups present.

Ethylene glycol is a good example of dihydroxy alcohols.

  1. c) Trihydroxy alcohol: Three -0H groups present. Glycerine is probably the most familiar example of this kind.
  1. d) Polyhydroxy alcohol: Four -OH groups present. Sorbitol is a familiar example of polyhydroxy alcohols.

Physical Properties Of Alcohols

In alcohols, the carbon and the oxygen atoms are in sp3 hybridized state.
Out of the four hybrid orbitals of oxygen atom, two sp3 orbitals are completely filled.
These two completely filled sp3 orbitals do not take part in the bond formation.
The other two half-filled sp3 orbitals of oxygen atom take part in the sigma bond formation.
Thus C-O bond in alcohol is formed by the overlapping of one half-filled sp3 orbital of oxygen and one sp3 orbital of carbon atom of the alkyl group.
The bond between carbon atom of alkyl group and oxygen atom is thus sp3-sp3 sigma bond.
The other half filled sp3 orbital of oxygen forms a bond with orbital of hydrogen atom.
Thus O-H bond angle in alcohol is sp3

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-s sigma bond. The C-O-H bond angle in alcohol is 105° which is less than the normal tetrahedral angle (109°28′).

This is because of the fact that the two unshared and completely filled sp3 orbitals of oxygen atom repel each other causing the reduction of C-O-H bond angle (VSEPR theory).

Common alcohols are liquids at room temperature. This must be a fact of no wonder to you as you have already watched a lot of movies where villains drink a lot of alcohol from glasses and you know what is drunk from glasses cannot but be liquids.

However, highly branched alcohols i.e. those with more than twelve carbon atoms are solid at room temperature.

Alcohols are colorless too. Oh! What a relief for those who are color blind!

Most of the common alcohols have got a fruit-like smell, making their presence quite easily detectable.

Let us look at the boiling points. Among hydrocarbons, the factors that are responsible for the determination of boiling points are mainly molecular weight and shape. Alcohols, as expected, show increase in boiling points with increasing carbon numbers and decrease in boiling points with branching. But what is unusual about alcohols is they boil so high, much higher than hydrocarbons with nearly similar molecular weights (Aristocracy, isn’t it?).

It is due to the greater energy required to break the hydrogen bonds that hold the molecules together.

The O-H bond in alcohols is highly polar which arises due to the high electronegativity difference between oxygen atom (3.5) and hydrogen atom (2.1). As a result, the oxygen atom of -OH group in alcohol carries a partial negative charge. Thus the polarity of -OH bond in alcohols gives rise to forces of attraction between partially negatively charged oxygen atom of one alcohol molecule and the partially positively charged hydrogen atom in another alcohol molecule. The force of attraction thus produced between hydrogen atom and another electronegative atom two molecules is known as hydrogen bond. Thus numerous hydrogen bonds are produced among the alcohol molecules. The alcohol molecules get associated with hydrogen bonding. Consequently, much higher energy is required to break these numerous H-bonding to bring the associated molecules in monomeric forms. As a result, the b.p. of alcohol becomes high. On the other hand, alkanes have no (-OH) group. So hydrogen bonds are not formed between the alkane molecules.

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As a result, the b.p. of alkanes are low.

The fact that the lower alcohols are miscible with water also reflects their ability to form hydrogen bonds with water. The higher alcohols have bigger insoluble alkyl groups of comparatively higher mass. So the hydroxyl (-OH) group can not form hydrogen bonding with water molecules pulling the bigger alkyl group.

Therefore, alcohols of higher mass do not dissolve in water.

We better be like alcohols and with our close ones form a bond too strong to be broken easily!

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