Victor- Meyer’s method and Lucas test are two famous methods for the distinction of Primary (10) , Secondary (20) and Tertiary(30) alcohols. Lucas test can be done by using two chemicals only ,i.e. ZnCl2 and HCl . So it is an easiest method to distinguish different types of alcohols.
Contents
What are Primary (10) , Secondary (20) and Tertiary(30) alcohols ?
- Alcohol in which the carbon atom bonded with –OH group is further bonded with one or non other carbon atom is called primary alcohol. Eg. methanol, ethanol, etc.
- Alcohol in which the carbon atom bonded with –OH group is further bonded with two other carbon atoms is called secondary alcohol. Eg. propan-2-ol.
- Alcohol in which the carbon atom bonded with –OH group is further bonded with three other carbon atoms is called tertiary alcohol. Eg. 2-methyl propan-2-ol.
Methods for the distinction of primary (10) , secondary (20) and tertiary(30) alcohols :
Victor Meyer’s method to distinguish alcohols :
In Victor Meyer’s method, alcohol is first treated with P and I2 to get iodoalkane , which is then treated with AgNO2 ( silver nitrate) to get nitroalkane. The nitroalkane thus obtained is treated with nitrous acid ( a mixture of NaNO2 and dil. HCl) and the resulting solution is finally made alkaline with KOH and the colour is observed. If the colour obtained is :
Red Colour = Primary alcohol
Blue Colour = Secondary alcohol
No colour = Tertiary alcohol.
Note : Don’t be confused on third step i.e. reaction of nitroalkane with HNO2 ( HONO). Just try to remove water molecule as by-product.
Q. How would you distinguish ethanol and propan-2- ol using Victor Meyer’s method?
→ Ethanol is a primary alcohol while propan-2-ol is a secondary alcohol.
In Victor Meyer’s method, alcohol is first treated with P and I2 to get iodoalkane , which is then treated with AgNO2 ( silver nitrate) to get nitroalkane. The nitroalkane thus obtained is treated withnitrous acid ( a mixture of NaNO2 and dil. HCl) and the resulting solution is finally made alkaline with KOH and the colour is observed. If the colour obtained is :
Red Colour = Primary alcohol
Blue Colour = Secondary alcohol
Hence, ethanol and propan-2-ol can be distinguished by observing colour in Victor- Meyer’s method as ethanol gives red colour while propan-2-ol gives blue colour .
Lucas test for the distinction of alcohols:
The Lucas reagent is an equimolar mixture of ZnCl2 and HCl.
In this method, few drops of Lucas reagent is added to the given sample of alcohol in a test tube and the speed of reaction(reactivity of alcohol) is observed.
This is done by measuring the time taken for the clear solution of alcohol to turn turbid.
- Tertiary Alcohol : The solution turns turbid and forms an oily layer immediately. Example: 2-hydroxy-2-methyl propane.
- Secondary alcohol : The solution turns turbid and forms an oily layer in three to five minutes (varies based on the solubility). Example: propan-2-ol.
- Primary alcohol : The solution remains colourless unless it is subjected to heat. The solution forms an oily layer when heated. Example: ethanol.
i.e Reactivity of alcohols : 30 alcohol > 20 alcohol > 10 alcohol
REFERENCES :
- Bahl, B.S., A., Advanced Organic Chemistry, S. Chand and company Ltd, New Delhi, 1992.
- Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
- Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
- https://www.toppr.com/ask/question/in-victor-mayers-test-which-compound-do-we-get-forprimary-alcohol/
- https://socratic.org/questions/5a1cf75bb72cff4a73dd4d2a
- https://www.topperlearning.com/answer/give-two-tests-to-distinguish-between-primary-secondary-tertiary-alcohols/ehj6m244
- https://pubs.acs.org/doi/pdf/10.1021/ed074p424
