One of the most important things to know for success in organic chemistry is to know how all the different series of compounds are related, and how a member of one homologous series can be converted to the corresponding member of any of the other series. The relationships are set out on the chart below.
There are two “trios” of compounds, and a sub-relationship between them:
- The alkanes, alkenes, and alkynes
- The alcohols, aldehydes, and carboxylic acids
- The relationship between the alkenes and alcohols (hydration/dehydration) forms the “bridge” between these two series.
Alcohols are central to the whole scheme:
- Dehydration of alcohols yields alkenes, and, in reverse, hydration of alkenes yields alcohols.
- One of the most important reactions of (primary) alcohols is that on oxidation they form aldehydes – which, on further oxidation form carboxylic acids. Thus, alcohols, aldehydes and acids are three closely-related series of compounds. (Note: oxidation of secondary alcohols yield ketones.)
The oxidising agent normally used for this purpose is acidified sodium dichromate solution. This reacts as follows, effectively to provide oxygen atoms for oxidation:
Na2Cr2O7 + 4H2SO4 —» Na2SO4 + 4H2O + 3[O] + Cr2(S04)
Or, ironically: Cr2O72– + 8H+ —» 2Cr3+ + 4H2O + 3[O]
Dichromate ions, Cr2O2– (and therefore sodium dichromate) are orange, and chromium ions, Cr3+ (and therefore Cr2(SO4)3 are green. Therefore as the reaction proceeds, the colour changes form orange to green.
Types of Reaction and Tests to be known: