The Classification of Alcohols as 1°, 2° or 3°
Alcohols are the homologous series with the functional group R-OH. They have important uses as solvents, fuels, and of course ethanol in drinks.
Alcohols, in particular Ethanol (CH3CH2OH), are manufactured industrially in two ways.
Below is a comparison between the two processes:
| Fermentation | Direct Hydration | |
|---|---|---|
| Equation for the reaction | C6H12O6 → 2C2H5OH + 2CO2 This reaction uses yeast and its enzymes as a catalyst |
C2H4O(g) + H2O(g) → C2H5OH(g) |
| Rate of reaction | Slow Temperatures of about 35°C have to be used so the yeast isn't killed. |
Fast High temperatures (300°C), and high pressures (6.5x10-3kPa) can be used as the phosphoric acid catalyst is not affected by the extreme conditions. |
| Quality of the product | Impure The yeast cells used in the reaction, along with their byproducts contaminate the ethanol |
Pure |
| Raw Material | Sugar (glucose) This is a renewable as the sugar beet and cane can be grown readily. This makes it more favourable in the long term. |
Ethene This comes from the distillation and cracking of crude oil, so is a finite resource. |
| Type of Process | Batch | Continuous |
At the moment in industry, Direct Hydration is the preferred method. This is because although the equiptment is expensive, the manpower needed to run it is not. The reaction is also faster and gives a purer product. Unfortunately though oil is a finite resource, so fermention is a more environmentally sound method of ethanol production.
Alchols can be classified depending on the number of alkykl groups are attached to the hydroxyl carbon atom.
The Classification of Alcohols as 1°, 2° or 3°
The position of the hydroxyl group has a large effect on the chemistry on the compound. this can be used to tell 1°, 2° and 3° alcohols apart.
The first test that can be done is heating the alcohol with acidified Potassium Dichromate(VI) (H+/Cr2O72+). If the alcohol is 1° or 2°, there will be a reaction and there will be a colour change from orange to green. There will be no reaction if the alcohol is 3°.
This happens because the orange dichromate(VI) ions acts as oxidising agents, and in oxidising the alcohol are reduced to green chromium(III) ions. The 1° and 2° alcohols are easily oxidised, unlike the 3° alcohols, which is why there is no reaction with these.
The reactions of Dichromate(VI) ions with 1°, 2° and 3° alcohols
The 1° and 2° alcohols can then be distinguished by smell. For example if you were trying to tell ethanol and propan-2-ol apart, upon oxidation, the ethanol would smell of vinegar and the propan-2-ol would smell of nail varnish remover. This is because the ethanol is oxidised to ethanoic acid, the main constituent of vinegar, and the propan-2-ol is oxidised to propanone which is the main ingrdient of nail varnish remover.