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just a simple question from me today...

I was looking at the structure of Norepinephrine for a school project and was curious about the diagrams on the Wikipedia page for it.

noradrenaline/norepinephrine structure

I was wondering why hexagons are used in this diagram and what the purpose of the three lines on the inside of the hexagon mean.

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    $\begingroup$ Snipergirl has a nice detailed answer, but the short answer is - that's what the structure is. This molecule and many other contain six carbon atoms in a ring with that bonding pattern. The parent compound is benzene. $\endgroup$ – Ben Norris Aug 8 '13 at 11:34
  • $\begingroup$ Here is some more information on skeletal org chem diagrams: ivy-rose.co.uk/Chemistry/Organic/… $\endgroup$ – Tomcat Aug 8 '13 at 11:42
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Noradrenaline/norepinephrine is an aromatic organic compound containing a benzene ring (the hexagon). Essentially, the vertices in the diagram are carbon atoms and the number of lines represents the covalent bond order; 1 line (eg connecting an OH to the benzene ring) means a single covalent bond. Any 'spare' bonds are where a C-H bond is. So it can be rewritten like this:

Redrawn Noradrenaline with all bonds, atoms and lone pairs

HOWEVER, when it comes to a benzene ring (the hexagon), you do not simply have alternating single and double bonds. In fact you have what is known as a 'resonance structure', where the electrons/bond is shared around the entire ring. One way of thinking of it is that there are 1.5 bonds between each carbon atom in the ring. Another way of thinking of it is that pi-orbitals from each carbon atom merge to form a ring combined orbital structure, so instead of existing in that figure-8 type area around one carbon nucleus, those 6 electrons can be in a donut shaped area above and below the carbon nuclei:

benzenes

This is why you often see a hexagon with a circle inside it to represent a benzene ring as it is less misleading than the alternate single and double bonds. In that case, you would redraw the noradrenaline diagram like this:

Noradrenaline redrawn in aromatic form

The advantage, though, of having the alternate double and single bonds is in drawing diagrams when working out reactions.

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    $\begingroup$ The reason most chemists don't draw aromatic rings with a circle is that it makes it a right pain when you do reaction mechanisms with curly arrows! The circle is a better representation conceptually, but the double bonds being drawn inside the hexagon are more useful for mechanisms. $\endgroup$ – Nick Aug 8 '13 at 14:22
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    $\begingroup$ Good point, i'll adjust the post! $\endgroup$ – Tomcat Aug 8 '13 at 16:00

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