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How to draw curved arrows?

Posted: Sat Mar 14, 2015 2:17 pm
by Christopher Phung 3G
I'm a little lost when it comes to drawing the curved arrows in a reaction mechanism.
I was working on Question 6 from the 2014 Final and I was confused with where to begin the curved arrow and where to end it.
For example, for one of the steps, I drew a curved arrow going from a bond to an atom, but the correct way to draw it was to draw the curved arrow from the bond to another bond connected to the aforementioned atom.
How do I know where to begin and where to end the arrows?

Re: How to draw curved arrows?

Posted: Sat Mar 14, 2015 5:47 pm
by Victoria Liang 3L
The tail of the arrow is from the the source of the electron and the arrow goes to where the election goes. For example in step 1 of q6, the electron source is the bond between N and H. The electrons go to the N and leaves the H with no electrons.

However, I am confused about step 3. The arrows show that H and OH is detached from the molecule. However, there is no arrow showing the bonding of H and OH to form HOH in the products. Why is this?

Re: How to draw curved arrows?

Posted: Sat Mar 14, 2015 8:03 pm
by Chem_Mod
Could you please post an image?

Re: How to draw curved arrows?

Posted: Sat Mar 14, 2015 8:35 pm
by LeilaBushweller1F
The arrow is going from one bond to another bond because that bond is breaking, and those electrons are being used to form a double bond between oxygen and carbon. You can figure this out by looking at the product of step 3: the R and NH3 do not change during step 3, but the two C-OH bonds change. We can see in the products that C forms a double bond with O. In addition, one OH group is completely detached, and a hydrogen has also been separated from the remaining oxygen. The electrons from one the O-H bond are used to form a C=O bond (and the Hydrogen is detached), and the electrons in the C-OH bond go to the hydroxide (so there in no longer a bond).

As for the bonding of H and OH, I think it is just assumed that HOH forms, since water is constantly losing and gaining hydrogens (alternating between HOH, OH, and H).

Re: How to draw curved arrows?

Posted: Sun Mar 15, 2015 1:14 am
by Christopher Phung 3G
Instead of drawing a curved arrow from one bond (one that's being broken) to another bond (to make a pi bond), would I be wrong to draw two curved arrows - one going from the bond that's being broken to the atom, and a lone pair from the atom to the other bond to make a pi bond?
I just don't feel right about taking electrons from one bond and putting it into another if there is an electronegative atom like O linking the two bonds. Wouldn't electrons go to the oxygen first, and then to make the pi bond?

Re: How to draw curved arrows?

Posted: Sun Mar 15, 2015 1:19 am
by Christopher Phung 3G
Here is the picture on the problem that I'm stuck on.

See that there is a curved arrow from the hydrogen bond to the C-O bond to make a C=O bond.
Would I be wrong to draw the curved arrow from the hydrogen bond to the C, and then another curved arrow from the C to the C-O bond to make the C=O bond?

Re: How to draw curved arrows?

Posted: Sun Mar 15, 2015 2:30 am
by Chem_Mod
Leila is correct, the electrons in O--H are being used to create C=O pi bond so you draw the arrow as such. Arrows are just a notation telling you where the electrons start and where they finally end up. They should be simple, there should not be two arrows to accomplisih something when one would suffice.