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  • Which carbocation is more stable, the ethyl carbocation or n-propyl carbocation?
  • What is the effect of alkyl group length on the stability of a carbocation?
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    $\begingroup$ Your first question, which carbocation is more stable, ethyl or n-propyl, has already been answered (see here). $\endgroup$
    – ron
    Commented Mar 8, 2016 at 19:20
  • $\begingroup$ @ron actually i couldn't completely understand the answer (quite a high level for me).But what i found from your answer is ethyl cation is an exception right?.But i have a doubt,is it correct that only methyl group can show inductive effect (can -CH2- group show inductive effect?)?Also would you post a simple answer about the dependence on length of alkyl group. $\endgroup$ Commented Mar 9, 2016 at 15:46
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    $\begingroup$ Yes $\ce{-CH2 -}$ can show an inductive effect, all alkyl groups can. $\endgroup$
    – ron
    Commented Mar 9, 2016 at 15:52
  • $\begingroup$ @ron thanks.Would you please explain me the dependence of +i effect on length?Is there any dependency?Am I wrong about this concept? $\endgroup$ Commented Mar 9, 2016 at 15:56
  • $\begingroup$ There is likely a measurable difference between methyl and ethyl due to the hyperconjugation reasons mentioned in the answer below. Beyond that I would expect very small differences (for example between n-propyl and ethyl). $\endgroup$
    – ron
    Commented Mar 9, 2016 at 16:15

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Carbocation stability is widely known to increase going from primary -> secondary -> tertiary.

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In the absence of other factors (such as lone pair donation etc.), the major contribution to the greater stability of tertiary carbocations vs primary is due to hyperconjugation between the vacant p-orbital (carbocation) and the C-H sigma bonding MO's.

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Based on hyper conjugation alone, you can predict that length of the alkyl chain will have no effect on the stability by itself, only the number of neighbouring C-H bonds with which hyper conjugation can take place.

In specific regard to ethyl vs n-propyl cation, neither are considered stable.

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  • $\begingroup$ Then why is there a variation in the acidity of primary aliphatic acids say between hexanoic acid and butanoic acid ?though the ka value difference is small still what is the reason for the differencce? $\endgroup$ Commented Feb 7, 2016 at 23:00

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