HardChemistryHigh School (11–12)Organic Compounds with Functional Groups › Alcohols, Phenols and EthersNumerical
Dehydration of an alcohol by concentrated sulphuric acid takes a hydrogen from a carbon next to the…
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Dehydration of an alcohol by concentrated sulphuric acid takes a hydrogen from a carbon next to the OH carbon. Count the different alkenes that can form directly in this way, counting cis and trans forms (E and Z) as different. Do this for 3-methylpentan-2-ol and for 2-methylbutan-2-ol. Then count the structural isomers (ignoring E and Z) that form from 3-methylpentan-2-ol.
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. Two alcohols.
Step 1. 3-Methylpentan-2-ol, . The OH carbon is C2. Its neighbours are C1 (a methyl group) and C3. Loss of H from C1 gives 3-methylpent-1-ene. Loss of H from C3 gives 3-methylpent-2-ene, which has E and Z forms. That is alkenes, from 2 structures.
Step 2. 2-Methylbutan-2-ol, . The OH carbon has two equivalent methyl neighbours and one CH. A methyl gives 2-methylbut-1-ene. The CH gives 2-methylbut-2-ene, which has two identical groups on one carbon, so it has no E and Z forms. That is 2 alkenes.
Check. By Saytzeff's rule the main products are 3-methylpent-2-ene and 2-methylbut-2-ene, the more substituted alkenes.
Answer: 3, 2 and 2.
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