Question

Why is an alkyl group denoted by $R$ ?

Answer 1

An alkyl group is a group that was formed by the removal of one hydrogen atom from alkanes. Intermediates are the molecules or groups that were formed in between the reactions. Radicals are one of the types of intermediates known as the detached groups or molecules containing odd electrons and these were represented by $R$ in olden days, later replaced by alkyl groups which were formed by the detaching of one hydrogen atom.

Complete Step By Step Answer:
Alkanes are saturated hydrocarbons in which the carbon and hydrogen atoms are involved in single bonds. The general molecular formula of alkanes is ${C_n}{H_{2n + 2}}$ , where $n$ is the number of carbon atoms.
When one hydrogen atoms are removed from the alkane, then the group formed can be known as alkyl group which has the general molecular formula of ${C_n}{H_{2n + 1}}$ , where $n$ is the number of carbon atoms. Alkyl groups can only be prepared synthetically but not available naturally.
Radicals were the intermediates that were formed from the elimination of electrons. These are the compounds consisting of an odd electron. Previously, radicals were represented by $R$ as radicals are the representation of a portion or part of a molecule in the complete molecule. But later alkyl groups were also represented by $R$ as alkyl groups are also a part of the complete molecule.

Note:
Many functional groups like carboxylic acids, ethers, esters have the representation as $R$ , in which an alkyl group is a part of these all molecules. Carboxylic acids are represented by $RCOOH$ , ethers by $RO{R'}$ and esters by $RCOO{R'}$ . In all these molecules $R$ and ${R'}$ represent different alkyl groups.