A student gets four alcohols labelled A to D: A is 2-methylpropan-2-ol, B is butan-2-ol, C is…
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A student gets four alcohols labelled A to D: A is 2-methylpropan-2-ol, B is butan-2-ol, C is butan-1-ol and D is 2-methylbutan-2-ol. She shakes each with the Lucas reagent at room temperature and watches for 10 minutes. Find how many of the four give a cloudy mixture at once, how many stay clear for the whole time, and how many give a cloudy mixture after about five minutes.
Need a reminder? The key ideaShow
Lucas test: tertiary at once, secondary in about five minutes, primary clear at room temperature. Dehydration follows Saytzeff's rule. Esterification is an equilibrium with . Primary alcohols oxidise to aldehydes and acids, secondary alcohols to ketones, and tertiary alcohols are not oxidised.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Four alcohols.
Step 1. Classes. A has three carbon neighbours on the OH carbon: tertiary. B has two: secondary. C has one: primary. D has three: tertiary.
Step 2. Lucas results. Tertiary alcohols give a cloudy mixture at once: A and D, so 2. The secondary alcohol B gives it after about five minutes: 1. The primary alcohol C stays clear: 1.
Check. , so every alcohol is placed.
Answer: 2 at once, 1 clear and 1 after about five minutes.
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