compare stabilities.

which one is more stable?

16 Answers

51
Subhomoy Bakshi ·

i vote for cycloheptan-2,4,6-trieneyl cation [1]

1
Hodge Conjecture ·

well i don't know the answer...

but when two cycloproply groups are present then it is more stable

than cycloheptan-2,4,6-trieneyl cation ( tropolium ion).

51
Subhomoy Bakshi ·

cycloheptan-2,4,6-trieneyl cation

here it is more stable because of the aromaticity of the ring...

also we can call the conjugation warrior to explain stability...

1
Hodge Conjecture ·

its right that tropolium ion is stable due to aromaticity , but even more

stable than aromatic cation are cyclo propyl methyl carbocations, and if

there are two cyclopropyl group then definitely it will be more stable,

but when only one group is present it is not easy to say....

the main difficulty here is that the reason for stability of cyclo propyl

methyl carbocations is not very clear ( it is becoz of some sort of

interaction b/w bent orbitals and vaccant p orbitals)

don't know more than this:)

106
Asish Mahapatra ·

sincere advice : even organic chemists dont know the answer properly, hence it is not wise to discuss this question.

Main ideas are:
reason for stability of tropylium cation (aromaticity)
and cyclopropyl methyl cation

comparision is not required (here)

23
qwerty ·

1st one is more stable

1
Hodge Conjecture ·

do provide any reason / evidence for your answer!

1
harrys ·

1st is more stable becoz in 1st resonance exist but not in 2nd and also 3 membered ring is unstable

31
Pritish Chakraborty ·

I read about this one in Solomons. The tropylium substrate has an ionisation constant of the order 1030. This observation was explained by the fact that the tropylium cation is extremely stable due to its aromaticity. If you need a solid explanation you will have to look at stuff like orbital overlaps and all...if I find that page in Solomons I'll scan it and put it up here.

106
Asish Mahapatra ·

@pritish: what abt the second carbocation ?? I think t\it has comparable stability compared totropylium cation

31
Pritish Chakraborty ·

@ashish : Does aromaticity compare to relief of ring strain?

1
scintillating dev ·

In structure 2 as (horribly) drawn above, in the 3 membered ring, inspite of being sp2 hybridised, the the bond is bent and makes an angle of 60°( instead of 120°).
Hence the p- orbitals remain almost in the same plane as the p orbitals of CH2+.
Thus there is a transfer of elctrons to the vacant p- orbital of CH2+ and hence the structure is highly stable.

In structure 1, the structure is stabilized by aromaticity. but between the 2
strucs, struc1 is more stable(probably) as it is an exception and they wouldnt have given the comparison otherwise!!!

1
MONISUR RAHMAN ·

Generally Dancing Resonance are more stable than aromatic compounds but only Cylopropylethyl cation is less stable than Tropilium cation..This is an exception

1
scintillating dev ·

am i correct????

23
qwerty ·

yes scintillating , and that is the reason why the second structure is more stable than benzyl carbocation, (CH3)3C+ ,allylic, etc

1
scintillating dev ·

thanks a lot, qwerty!!!

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