2009 dat destroyer organic chemistry question number 47 ?

[dallaspage]

Can you explain DAT 2009 destroyer question number 47,please?
I checked the answer, I cannot understand.

In the B, Which carbon has an sp3 carbon, why does it lacks p-orbital?
In the C, Why does it have to be anti-aromatic. I cannot understand,not 4n+2 pi electrons,but 4n pi electrons.
In the D, Which carbon has an sp3 carbon, why does not have an empty p-orbital?

And also in the A, Why does it has to be aromatic?
And what is the difference between Anti-aromatic and non-aromatic?

Please answer my questions.

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[Kneecoal]

i'll try to explain choice by choice...

A) every carbon bond is a double bond - also known as conjugated. conjugation is a requirement for aromaticity, which we'll find out in then next choice. note that when a carbon is involved in a double bond it is sp2 hybridized. each double bond counts as 2 pi electrons, so right there you have 8. then count each individual electron showing with the N --> 10 pi electrons. this fits the 4n + 2 rule where n = 2.

B) count up the total number of pi electrons, and you'll see there are 6, which fits the 4n + 2 rule. however, the molecule isn't completely conjugated, as the top carbon has no double bonds to any other carbon - also known as sp3, and so can't be aromatic.

C) Right away, you see that the number of pi electrons doesn't fit the 4n + 2 rule, but fits the 4n rule, and so is known as anti-aromatic.

D) Combination of choices B and C - has an sp3 carbon (no double bond), and doesn't fill the 4n + 2 rule - but fits the 4n.

 

[Murl]

1. The molecule must have a cyclic continuous overlap of P orbitals. Meaning it has to be conjugated, having a double bond every other bond and be cyclic, like benzene.

2. The molecule must be planar, meaning all carbons involved in the ring must be sp2 hybridized which means they have an empty p orbital. Sp2 hybridized means that there are only 3 "things" connected to that carbon, but there are exceptions like furan and pyridine where they put a lone pair in an empty P orbital for aromaticity. This is why pyridine is a non-nucleophilic base that is used in tosylation etc.

3. If the above criteria are met the number of Pi electrons(double bonds) must equal 4n+2, where n can be any positive number and zero that is not a decimal. Look at benzene for an example, it has 6 pi electrons, so 6=4n+2, n=1 so it is aromatic. This is called Huckel's rule if you want to read about it.

Now if the above criteria are met #'s one and two, and the number of pi electrons equals 4n then the molecule is said to be antiaromatic, meaning it is less stable than it's non cyclic counterpart.

I hope this helps let me know if you have any questions.