How do you distinguish between primary and secondary alcohols?

2 Answers
Sirash
Apr 25, 2016

By victor mayer method,
Treat the alcohol with the following reagents in order-
1.P/I2
2.AgNO2
3.HNO2
4.KOH

Explanation:

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Ernest Z.
Dec 30, 2016

I would use the Lucas test to distinguish between a primary and a secondary alcohol.

Explanation:

The Lucas test differentiates between primary and secondary alcohols.

It is based on the difference in reactivity of the alcohols with #"HCl"# in an #"S"_"N"1# reaction:

#"ROH + HCl" → "RCl" + "H"_2"O"#

It works because secondary carbocations are more stable and form faster than primary carbocations.

The Lucas reagent is an equimolar mixture of #"ZnCl"_2# and #"HCl"#.

You add a few drops of your alcohol to the reagent in a test tube

A secondary alcohol will react within 3 min to 5 min to form the alkyl halide, which is insoluble and forms an oily layer.

#underbrace("CH"_3"CH"_2"CH"_2"CH(OH)CH"_3)_color(red)("pentan-2-ol") + "HCl" stackrelcolor(blue)("ZnCl"_2, "HCl"color(white)(mm))(→) underbrace("CH"_3"CH"_2"CH"_2"CHClCH"_3)_color(red)("2-chloropentane") + "H"_2"O"#

A primary alcohol does not noticeably react with Lucas reagent at room temperature.

#underbrace("CH"_3"CH"_2"CH"_2"CH"_2"CH"_2"OH")_color(red)("pentan-1-ol") + "HCl" stackrelcolor(blue)("ZnCl"_2, "HCl"color(white)(mm))(→) "No reaction"#

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