Functional Group Interconversion

Beyond classifying reactions by mechanism, organic synthesis in practice is very often about functional group interconversion -- taking a molecule that already carries one functional group and converting it, using a suitable reagent, into a related molecule carrying a different functional group, while keeping the rest of the carbon skeleton (the 'R' group) intact.

Worked example -- the carboxylic acid group. The carboxylic acid group ($-COOH$), present in organic acids, can be transformed into several different functional groups depending purely on which reagent is chosen:

• Treatment with $LiAlH_4$ (a strong reducing agent) converts $-COOH$ to $-CH_2OH$ (a primary alcohol).
• Treatment with $NH_3$ converts $-COOH$ to $-CONH_2$ (an amide).
• Treatment with $SOCl_2$ converts $-COOH$ to $-COCl$ (an acid chloride).

The bigger picture -- a network of interconversions. Many such functional groups are linked to each other, forming a whole network of possible transformations rather than a single one-way path:

• Alcohols and their oxidation products. A primary alcohol ($R$-$CH_2OH$) is oxidised by acidic $K_2Cr_2O_7$ to an aldehyde ($R$-$CHO$), and the aldehyde can be reduced straight back to the primary alcohol with $LiAlH_4$. In exactly the same way, a secondary alcohol ($R_2$-$CHOH$) is oxidised by acidic $K_2Cr_2O_7$ to a ketone ($R$-$CO$-$R$), reducible back to the secondary alcohol with $LiAlH_4$.
• Nitriles, amines, amides and carboxylic acids. A nitrile ($R$-$C\equiv N$) is reduced by $LiAlH_4$/[H] straight to a primary (1$^\circ$) amine ($R$-$NH_2$); the same primary amine can also be reached from an amide ($R$-$CO$-$NH_2$) by $LiAlH_4$/ether reduction. The nitrile can alternatively be hydrolysed directly to the amide with $H_2O/H^+$, and the amide can be hydrolysed further, again with $H_2O/H^+$, all the way to a carboxylic acid ($R$-$COOH$) -- so nitrile, amide and carboxylic acid form a linked hydrolysis sequence, with the amine reachable as a branch via reduction. …