How do I decide the electromeric effect on pent-2-ene?

There are two possible case
**π electrons shifting to C2**
In this case, nucleophile attacks C3
It is stabilized by +I effect of two ethyl groups
**π electrons shifting to C3**
In this case, nucleophile attacks C2
It is stabilized by +I effect of a methyl group and a propyl group.


How do I decide which one is more stable, and ultimately, which one is favoured?




Here what you should consider first is **hyperconjugation**, rather than **inductive effect** (a [brief explanation by me](https://chemistry.stackexchange.com/questions/131101/odd-looking-carbocation-stability-order/131103#131103)).


You're right that $\ce{Nu-}$ will either attack on $\ce{C-3}$ or $\ce{C-2}$, and hence two carbocations would be formed. Now, we'll analyse the stability of these carbocations on the basis of hyperconjugation which is as follows,


* carbocation at $\ce{C-2}$ will have a total of 5 **HC** structures
* carbocation at $\ce{C-3}$ will have a total of 4 **HC** structures


Based on the above difference, it is easy to conclude that $\ce{Nu-}$ will attack on $\ce{C-2}$ carbon. Whereas based on inductive effect, those carbocations are nearly equally stable.




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Note: HC is short for "hyperconjugative structures".


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