How to tell if aromatic compound is planar?

[Chrono]

Hey guys,

How do you tell if an aromatic compound is planar or not planar? For example how would you tell in the compound below.

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[2PacClone23]

Yeah I wanna know this too. Good question. There seems to be at least 2 questions in the ochem section regarding aromaticity.

 

[bigstix808]

Hey guys,

How do you tell if an aromatic compound is planar or not planar? For example how would you tell in the compound below.

just do lots of VESPR problems.

break it down by section and guess the geometry. if you did this with this problem you would see that the carbons connecting the "two" rings only have "3" bonds visible. well you know that carbon has 4 bonds so hydrogen will make up that last one. now look at how you can keep the 2x bond, the single bond, and the hydrogen the furthest distance apart...and if done so you can see that the only way to do that is to bend the ring structure thus making it not planar.

i know this is long winded...but pretty straight forward if you look at part of a molecule and then see how they can work with the rest of it.

 

[dennisDDS]

Usually anything <7 Carbons is planar when you have anything more it starts twisting and turning..

 

[Orgolover]

Fair point. But beware of napthalene monkey-face. It is 10 carbons, looks similar to the structure above but ISSSSS aromatic. . .

 

[sixkiller]

Fair point. But beware of napthalene monkey-face. It is 10 carbons, looks similar to the structure above but ISSSSS aromatic. . .

May as well add anthracene and quinoline to the mix......

 

[sixkiller]

There is no way to know for sure in an absolute sense. By the way, the term AROMATIC implies that the molecule is planar because it's part of the definition.

The only way to know for sure that it's planar or non-planar is to know that the steric and angle strain overcomes the stability attained by aromaticity (hence planar). In other words, the stability attained by adopting a non-planar conformation is greater than the stability attained by adopting a planar conformation (thus being aromatic).

At the end of the day, it's a quantitative matter.....

Of course, knowing that there is some steric strain makes it a tad suspicious. Thus, if you were given bicyclic conjugated molecules with a huckel number.....you'd probably go with something along those lines if asked which of the following is aromatic.