I came across this question in DAT Destroyer 2009 (Q 64). I have the question and the answer attached. Can someone please provide an explanation.
Thanks
Thanks
OChem SN1 reaction
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Sn1 reactions proceed via a two step mechanism. Step 1 forms a carbocation intermediate, step 2 involves a Nu. attack on that intermediate. The first step is the "rate determining" step, the speed of the overall Sn1 reaction will be determined by how fast that step proceeds. The easier it is to form the carbocation (the most stable intermediate), the faster the reaction will proceed.
What I'm not sure about is why B is considered "horrible." As far as I can tell it's also aromatic.
Ok I THINK I got it. Note I'm commenting on the carbocation intermediates formed by the compounds in the questions.
A. Fair, 2nd degree carbocation
B. I think it's aromatic, considering it has 6 pi electrons (and is planar), but you can't draw a resonance structure for it. Keeping that + charge on an sp2 carbon doesn't sound too stable.
C. Apparently anti aromatic, I'm not sure how to identify an "anti" aromatic compound - just considering it a 2nd degree carbocat would do for the question, though.
D. Fair, 2nd degree carbocation
E. A good 2nd degree carbocation, is aromatic, and is resonance stable.
Ok I THINK I got it. Note I'm commenting on the carbocation intermediates formed by the compounds in the questions.
A. Fair, 2nd degree carbocation
B. I think it's aromatic, considering it has 6 pi electrons (and is planar), but you can't draw a resonance structure for it. Keeping that + charge on an sp2 carbon doesn't sound too stable.
C. Apparently anti aromatic, I'm not sure how to identify an "anti" aromatic compound - just considering it a 2nd degree carbocat would do for the question, though.
D. Fair, 2nd degree carbocation
E. A good 2nd degree carbocation, is aromatic, and is resonance stable.
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Basically I think the question wants you to identify between aromatic vs nonaromatic vs anti-aromatic.
So you should know how to identify each.
Nonaromatic = the norm, Aromatic like benzene = more stable, anti-aromatic = less stable.
Look at the carbocations...
So A is a normal ring secondary carbocation.
B is a benzene. A benzene wiht a + charge = bad. Remember benzene does not even react with Br2 it needs a catalyst like FeBr3.
C is antiaromatic because you have 4n pi electrons (4 pi electrons). But you still satisfy all the other rules of aromaticity.
D is non-aromatic because the bottom carbon is sp3 hybridized. But it has resonance so it is more stable than A (I think).
E is aromatic. It has 4n+2 pi electrons (6 here). All rings atoms are sp2 hybridized. It is planar and cyclic.
So E is most stable.
Google the rules of determining aromaticity vs non vs anti. Or watch chad's videos.
So you should know how to identify each.
Nonaromatic = the norm, Aromatic like benzene = more stable, anti-aromatic = less stable.
Look at the carbocations...
So A is a normal ring secondary carbocation.
B is a benzene. A benzene wiht a + charge = bad. Remember benzene does not even react with Br2 it needs a catalyst like FeBr3.
C is antiaromatic because you have 4n pi electrons (4 pi electrons). But you still satisfy all the other rules of aromaticity.
D is non-aromatic because the bottom carbon is sp3 hybridized. But it has resonance so it is more stable than A (I think).
E is aromatic. It has 4n+2 pi electrons (6 here). All rings atoms are sp2 hybridized. It is planar and cyclic.
So E is most stable.
Google the rules of determining aromaticity vs non vs anti. Or watch chad's videos.
