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My teacher as well as Wikipedia — Pentyl group use the following names for the “pentyl” groups:

Group name Structure with attachment point
1 active sec-pentyl active sec-pentyl
2 sec-pentyl sec-pentyl
3 active pentyl active pentyl

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?

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    $\begingroup$ Where are these terms from? $\endgroup$
    – orthocresol
    Aug 2 at 21:05
  • $\begingroup$ @orthocresol Really, you never heard these!! Am I reading wrong things, because when I search sec pentyl chloride on google, i get results. chem.libretexts.org/Bookshelves/Organic_Chemistry/… here it says we have sec butyl but sec pentyl doesn't exist, but i didn't got reason. And i have read these names as common names. I am literally confused. $\endgroup$
    – Arpit Raj Choudhary
    Aug 3 at 1:09
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    $\begingroup$ Oh, sec-pentyl is fine, even if uncommon (the usefulness of the sec- prefix probably stops at butyl, because for pentyl onwards there are plenty of possible isomeric candidates which can lay claim to the 'sec' description). But 'active', that's a new one. $\endgroup$
    – orthocresol
    Aug 3 at 1:15
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    $\begingroup$ @orthocresol Wikipedia considers sec-pentyl as 1-methylbutyl and active pentyl as 2-methylbutyl. Are they referring to "active" as in optically active? $\endgroup$
    – Nilay Ghosh
    Aug 3 at 1:37
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    $\begingroup$ @NilayGhosh I see the answer has taken a similar approach at least... $\endgroup$
    – orthocresol
    Aug 3 at 2:08

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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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