How would you distinguish between primary, secondary, and tertiary alcohols?

2 Answers
Dec 29, 2017

This is pretty simple nomenclature.

Here is propan-1-ol,

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Here is 2-methylpropan-2-ol,

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The numbers indicate which carbon on the parent chain the functional group or substituent is on. Both of these have an alcohol group and the second has a methyl group on the second carbon.

Dec 29, 2017

I would use the Lucas test.

Explanation:

The Lucas test differentiates between primary, secondary, and tertiary 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, and tertiary carbocations are so stable that the reaction takes place almost immediately.

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

You shake a few drops of your alcohol with the reagent in a test tube.

A tertiary alcohol reacts almost immediately to form the alkyl halide, which is insoluble and forms an oily layer.

#underbrace(("CH"_3)_3"COH")_color(red)("2-methylpropan-2-ol") + "HCl" stackrelcolor(blue)("ZnCl"_2color(white)(mm))(→) underbrace(("CH"_3)_3"CCl")_color(red)("2-chloro-2-methylpropane") + "H"_2"O"#

A secondary alcohol reacts within 3 min to 5 min.

#underbrace("CH"_3"CH(OH)CH"_3)_color(red)("propan-2-ol") + "HCl" stackrelcolor(blue)("ZnCl"_2color(white)(mm))(→) underbrace("CH"_3"CHClCH"_3)_color(red)("2-chloropropane") + "H"_2"O"#

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

#underbrace("CH"_3"CH"_2"CH"_2"OH")_color(red)("propan-1-ol") + "HCl" stackrelcolor(blue)("ZnCl"_2color(white)(mm))(→) "No reaction"#