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According to general organic chemistry the order of stability is as follows:

aromaticity > resonance > hyperconjugation > inductive effect

Since primary allylic carbocation shows resonance as well as hyperconjugation, moreover resonance is greater than any other effect then why is primary allylic carbocation less stable than tertiary carbocation.

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  • $\begingroup$ I am not quite sure about the numbers myself. I can believe that the relative energy of a tertiary carbocation is higher than the relative energy of a primary allylic cation (therefore making the tertiary cation less stable). You also need to consider that in a chemical reaction, stability does not only depend on the energy, but also on the reaction speed. Since primary allylic cation is sterically unhindered, it may react faster, making it less stable. Again, it is only a first guess, I do not have numbers for this. $\endgroup$
    – user23638
    Commented Mar 1, 2017 at 11:33
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    $\begingroup$ Because simple concepts only get you so far. $\endgroup$
    – Jan
    Commented Mar 1, 2017 at 12:35

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The decreasing order of stablity of carbocations is:

aromatic (tropylium, cyclopropenyl) > benzyl > allyl > tertiary > secondary > primary

Allylic carbocations are resonance stabilized whereas tertiary alkyl groups are stabilized by hyperconjugation (and inductive effect). The hyperconjugation (and inductive effect), in general, is quite weaker than resonance (and hyperconjugation effect). The only exceptions to the previous statement I am aware of are halogens (inductive effect is stronger than resonance).

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  • $\begingroup$ I would put aromatic ring ions (tropylium, cyclopropenyl) first before benzyl. $\endgroup$ Commented Apr 3, 2017 at 21:11
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Hyperconjugation effect on a 3 degree carbocation often overpowers the resonance effect in a primary-allyl carbocation.

If we consider the following compounds, enter image description here And, enter image description here

Solvolysis (a reaction which occurs via SN1 mechanism, causing formation of an intermediate carbocation) will occur at a faster rate in the second compound, even though the carbocation intermediate in the first compound will be resonance-stabilized.

Again, this is because 3 degree carbocations are very stable due to Hyperconjugation effect.

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