aromatic vs.antiaromatic

[KiriKat]

How can you distinguish whether molecules are aromatic or antiaromatic?

 

for example: (a) is antiaromatic whereas (e) is aromatic

(refer to attach file)

 

 

Edited October 30, 2012 by KiriKat

[CarbonCopy]

The molecue should follow huckel rule that is the should be 4N+2 pi electrons. i.e., count the number of double bonds and equate that number to 4N+2, where N is some integer. If we get a N as 1,2,3... then it is aromatic else not.

Also, there should be double bonds,or a charge in conjugation ( alternating )

a) is anti-aromatic because it has 4 pi electrons, if we take 4N+2 = 4 we don't get N a 1,2,3... but we get it as 0.5 instead.

b)is aromatic because there are 6 pi electrons and there are alternanting double bonds and a +ve charge.

c) is aromatic. it may seem as though it has 4 pi electrons, but remember oxygen has a lone pair so it becomes 6.

d)is not aromatic. nitrogen has lone pair of electrons, that makes the number of electrons 6 but, these are not alternating as the nitrogen lone pair is just next to a double bond

e)aromatic same as b)

f)aromatic. 4 pi elect + 2 on nitrigen.

g) is not aromatic. it may appear as though it has 6 electrons but it has 8 due to negative charge.

h) is not aromatic. has only 4 elec.

i)is aromatic. has 6 electrons, and present in conjugation.

j)is not aromatic

k) is aromatic because 6 electrons

l) Is aromatic. 4 electrons and 2 in sulphur as lone pairs

 

between the pictures of j) and k) there is a negative charge. because the picture is small I can't make out if that charge belongs to j) or k). I assumed it belongs to k) and gave the answer. correct me if that is wrong.

Edited October 31, 2012 by CarbonCopy

[KiriKat]

this helps a lot, thank you so much!

[CarbonCopy]

Ur welcome! If you need any more help just ask.