Why is active sec-pentyl group called so?

My teacher as well as [Wikipedia — Pentyl group](https://en.wikipedia.org/wiki/Pentyl_group) use the following names for the “pentyl” groups:





|  | Group name | Structure with attachment point |
| --- | --- | --- |
| **1** | active *sec*-pentyl | [active sec-pentyl](https://i.stack.imgur.com/S1fT0.png) |
| **2** | *sec*-pentyl | [sec-pentyl](https://i.stack.imgur.com/ngrqH.png) |
| **3** | active pentyl | [active pentyl](https://i.stack.imgur.com/an6Ir.png) |



If a group is attached to the second carbon of pentyl, then it is called “active *sec*-pentyl” (**1**). If it is attached to the third carbon, then it is called “*sec*-pentyl” (**2**). In case **3** it is called “active pentyl”.


What is the difference between them, and why are they called so?




I, as orthocresol♦ questioned in the comments section, don't know where these terms came from. Thus, any attempt to rationalize these terms would be a speculation. For clarity, it could be argued that each of the structures of these alkyl radicals are drawn in such a way that each of them is attached to a functional group such as hydroxyl or halide or etc. For sake of an argument, let's assume the functional group is hydroxyl, thus the compound from the first radical would be 2-pentanol (or pent-2-ol). Thus, let's try to give a meaningful rationalization to the terminology in the question:


1. Why it is called 'active sec pentyl' (case 1): If you carefully look at the alkyl radical in case 1, you would realize it is a secondary radical so that it is a *sec*-pentyl group. Thus, the compound is **optically active** alcohol (in our case), pent-2-ol, which can exists in both (*R*)- and (*S*)-forms. Thus, I assume 'active' part comes to indicate optically active compounds.
2. Why it is called 'sec pentyl' (case 2): If you carefully look at the alkyl radical in case 2, you would realize it is also a secondary radical so that it is also a *sec*-pentyl group. Thus, the relevant compound in our case is pent-3-ol, which can exists in only one form, because it is **not optically active**. Thus, I assume 'active' part is dropped from 'active sec pentyl' to indicate the radical forms only the non-optically active compounds, yet it is a 'sec pentyl.'


The compound related to "case 3' is 3-methylbutan-1-ol, which is **optically active** so that it has been given the terminology, 'active pentyl.'


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