I'm starting the very basics of hydrocarbons and I'm having a hard time at understanding when to use the common/trivial name prefixes (sec-, iso-, neo-, etc.).

One particular problem is the difference between iso and sec. I seem to get a grasp of when iso is used. But I'm having a difficult time with sec.

It's clear with butane, for example:

  • secondary butyl group: $\ce{CH3-C(\bond{...})H-CH2-CH3}$

  • iso-butyl group: $\ce{CH3-C(CH3)H-CH2\bond{...}}$

However, when it comes to pentane, how can I find the secondary pentyl group? The isopentyl one is clear: $\ce{CH3-C(CH3)H-CH2-CH2\bond{...}}$. But with secondary pentyl group (of normal pentane, $\ce{CH3-CH2-CH2-CH2-CH3}$), which one is the secondary? Does the common naming fail here and we need the IUPAC system or am I missing something?

If someone can sum up when to use the prefixes as well, I'd be very grateful.


1 Answer 1


See, let me explain iso means a type of isomer of the existing carbon chain for eg when you say iso pentane the srtucture would be CH3--C(CH3)H--CH2--CH3 as you can see from here that in the normal pentane all the carbon atoms come in a straight long chain without branching. iso- i.e. isomer having same molecular formula and different structural formula. Here the structure that i have made is an isomeric form of pentane.

And when we talk about sec it means secondary carbon atoms carbon atom joined with two other caron atom eg CH3-CH2-CH3 | The carbon in the center is a sec carbon at since it is attached with 2 other carbon atom if it was attached with only one carbon atom like the first C atom then it would be called a primary carbon atom. Again if you look at the first structure i.e. iso pentane you would notice that the 2nd carbon is attached with 3other carbon atoms which is called a tertiary carbon atom.

Again on coming to neo pentane if one carbon is surrounded by 4 other ch3 it becomes neo pentane again different structure of the same molecule. Iso,neo are sort of convention created to differentiate between different isomer of the same molecule. Draw the structure and identify each Carbon and the number of carbon they are attached with slowly and steadily you would come to know. I have tried to explain this but i would suggest you to first read from a chemistry book or use any reference or ask a teacher or you may even use youtube to know about it and then again read this for better understanding.

Happy learning

  • 2
    $\begingroup$ The prefix iso- does hold a very specific meaning, even though it is no longer recommended and its usage is basically deprecated. Similarly the sec- prefix has a specific meaning and does not mean secondary carbon. $\endgroup$ Nov 23, 2019 at 15:06

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