Alcohols have a hydroxyl group (OH) attached to their aliphatic carbon atom. They are classified based on the numbers of the hydroxyl group (OH) present in the molecule. Alcohols with only one hydroxyl group (OH) are called monohydric alcohol. Dihydric and trihydric alcohols have two and three hydroxyl groups (OH) in their molecules.
On the basis of a hydroxyl group (OH) attached to a carbon atom in an alcohol molecule, they are called primary and secondary alcohols. In the case of a primary alcohol, the hydroxyl group is attached to the first or primary carbon atom in a molecule. However, secondary alcohol has a hydroxyl group attached to the second or secondary carbon atom in a molecule.
These are the key difference between primary and secondary alcohols:
|Primary Alcohols||Secondary Alcohols|
|In primary alcohols, a hydroxyl group (OH) is directly attached to the primary carbon atom||Secondary alcohols have a hydroxyl group (OH) directly attached to the secondary carbon atom|
|These alcohols have less stable carbocations||This type of alcohol has more stable carbocations|
|Primary alcohols are formed by the reaction of the Grignard reagent with formaldehyde||They are formed by the reaction of the Grignard reagent with aldehyde|
|They are more reactive with metals||They are less reactive with metals|
|Primary alcohols are more acidic||Secondary alcohols are less acidic|
|They don’t have steric hindrance||They have steric hindrance|
|Rate of esterification is higher and equilibrium shifted toward the product side||Rate of esterification is less and equilibrium shifted toward the reactant side|
|Oxidation of primary alcohols gives carboxylic acid as the final product||Oxidation of secondary alcohols gives a ketone|
Primary alcohols use acidic or basic potassium permanganate (KMnO4) as an oxidizing agent
Secondary alcohols use chromic acid (H2CrO4) as an oxidizing agent
They favor the SN2 mechanism
They favor the SN1 mechanism
These alcohols give a reaction with thionyl chloride (SOCl2) to give acyl chloride at 100 oC
Secondary alcohols do not give any reaction with thionyl chloride (SOCl2)
|They are less miscible with water||They are highly miscible with water|
|Primary alcohols are difficult to dehydrate||Secondary alcohols are easy to dehydrate|
|They do not give Lucas a test||They give Lucas a test within 5-6 minutes|
|Primary alcohols give Victor Meyer’s test and show red color||Secondary alcohols also give Victor Meyer’s test with blue color|
What is primary alcohol?
In primary alcohols as compared to secondary alcohols, the hydroxyl group (OH) is directly attached to the first carbon atom. This carbon atom is known as the primary carbon atom and the alcohol is called primary alcohol.
Primary and secondary alcohols can be prepared by Grignard reagents (RMgX). The reaction of the Grignard reagent with formaldehyde (HCHO) gives a larger yield of primary alcohols. These primary alcohols have only one carbon atom more than the alkyl group of Grignard reagents. The primary alcohols with two carbon atoms more than the alkyl group can be produced by the reaction of the Grignard reagent with ethylene oxide (H2COCH2).
Primary alcohols react with metals less violently than water. This results in the production of metal alkoxides. The acidity of primary alcohol is much greater than secondary alcohols.
The order of acidity of alcohols is given below:
Methyl > Primary alcohols > Secondary alcohols > Tertiary alcohols
The order of acidity of alcohols is not the result of the polar effect of alkyl groups but is due to the effect of the solvent.
The oxidation of primary alcohols is different from secondary alcohols. Primary alcohols with two α-hydrogens are first oxidized to aldehyde (RCHO). The aldehyde has one α-hydrogen remaining which is further oxidized to carboxylic acid (RCOOH). The oxidation of primary alcohols to aldehyde required a strong oxidizing agent while the oxidation of the aldehyde to carboxylic acid carried out under milder oxidizing agents. That’s why the final product of oxidation of primary alcohols is a carboxylic acid.
Usually, an acidic or basic aqueous solution of potassium permanganate (KMnO4) is used as an oxidizing agent for primary alcohols to carboxylic acid. The only aldehydes can be obtained from the oxidation of primary alcohols by using pyridinium chlorochromate (PPC) in dichloromethane. This avoids further oxidation of primary alcohols to carboxylic acid.
What is secondary alcohol?
When the hydroxyl group (OH) is directly attached to the second carbon atom in the alcohol molecule is called secondary alcohol and the carbon atom is known as secondary carbon. They have more stability as compared to primary alcohols.
Like primary alcohols, the secondary alcohols can be produced by the reaction of the Grignard reagent. The difference in the production of secondary alcohols is the use of aldehyde (RCHO) instead of formaldehyde (HCHO) in the case of primary alcohols.
Esterification of secondary alcohols does not give good yields in comparison with primary alcohols. It is due to the steric hindrance of the bulky groups in the secondary alcohols. This slow down the rate of esterification as well as shift the equilibrium to the left side of the reaction.
Secondary alcohols have only one α-hydrogen which is oxidized to give ketone (RCOR). The oxidizing agent which is used for this purpose is chromic acid (H2CrO4) which is formed by the reaction of potassium dichromate (K2Cr2O7) with sulfuric acid or the addition of chromic oxide (CrO3) with sulfuric acid. Chromic acid is used for the oxidation of secondary alcohols due to the stability of the final product (Ketones). Chromic acid in the aqueous acetone is known as jones’s reagent.
What are different types of alcohol?
There are three types of alcohol based on the position of the hydroxyl group (OH).
- Primary alcohol
- Secondary alcohol
- Tertiary alcohol
How can you tell the difference between primary secondary and tertiary alcohols?
Primary alcohols have a hydroxyl group (OH) attached to the first carbon atom while in the case of tertiary alcohols, the hydroxyl group (OH) is attached to the third carbon atom.
How do you know if alcohol is primary or secondary?
These are some tests through which we can find whether alcohols are primary or secondary.
- Locus test
- Victor Meyer’s test
Which is tertiary alcohol?
Tertiary alcohol has (OH) group attached to the third carbon atom. This type of alcohol is called tertiary alcohol and the carbon atom is known as the tertiary carbon atom.
Why are secondary alcohols more soluble than primary alcohols in H2O?
Because they are more stable carbocation as compared to primary alcohols. They form carbocation easily in the water that why they are more soluble in water.
Is the boiling point of primary alcohol higher than secondary and tertiary alcohol Why
Yes, The OH group of the primary alcohols is more exposed as compared to secondary and tertiary alcohols. More hydrogen bonds are formed with primary alcohols which means it has high intermolecular forces. That’s why more energy is required to break the bonds.
- Organic chemistry by M.Younas, (Ph.D. Cambridge), University of Punjab
- Organic chemistry by Graham Solomons, Craig Fryhle, Scott Synder (11 edition)
- Primary vs secondary alcohols (Pediaa.com)
- Types of alcohols (Byjus.com)