[aqua05]

In sodium benzoate the two carbon oxygen bonds are of the same length, whereas
in methyl benzoate these lengths are different.
Suggest why this is the case, illustrating your answer with appropriate
diagrams.

Can someone please explain to me the answer to this question

[EierVonSatan]

Recall that the C-C bond lengths in benzene are all the same due to resonance of the pi system in the ring and not double - single - double etc.

Similar effect here with the benzoate ion:

[aqua05]

(Original post by EierVonSatan)
Recall that the C-C bond lengths in benzene are all the same due to resonance of the pi system in the ring and not double - single - double etc.

Similar effect here with the benzoate ion:

??

I'm sorry but i fail to understand
What is resonance of the pi system? I know that benzene is a hybrid which means it neither has single bonds, neither does it have double bonds

[EierVonSatan]

(Original post by aqua05)
??

I'm sorry but i fail to understand
What is resonance of the pi system? I know that benzene is a hybrid which means it neither has single bonds, neither does it have double bonds

Okay, so you know that benzene doesn't have alternating single and double bonds. This is because the electrons are able to move around the ring and do not stay in the same place, that's why we draw a circle instead of double and single bonds

The COO^- group is just the same draw one 'state' (like in the left of the diagram above)and it has a C=O bond and a C-O^- bond. Like in benzene these electrons are free to move and so can switch (to the other state such as the right structure on the diagram). So there are no double and single bond - just a uniform C-O bond inbetween double and single.

[aqua05]

(Original post by EierVonSatan)
Okay, so you know that benzene doesn't have alternating single and double bonds. This is because the electrons are able to move around the ring and do not stay in the same place, that's why we draw a circle instead of double and single bonds

The COO^- group is just the same draw one 'state' (like in the left of the diagram above)and it has a C=O bond and a C-O^- bond. Like in benzene these electrons are free to move and so can switch (to the other state such as the right structure on the diagram). So there are no double and single bond - just a uniform C-O bond inbetween double and single.

Woww:-) actually understood it.,

Just one thing.. methy benzoate cannot change its state so has alternating lengths.right??

And in the exam how should i answer it? I mean I can't write about benzene ring & stuff:s

[EierVonSatan]

(Original post by aqua05)
Woww:-) actually understood it.,

Just one thing.. methy benzoate cannot change its state so has alternating lengths.right??

The electrons in the ester aren't free to move because the two electrons (which can move in the benzoate ion) are being shared with the carbon.

And in the exam how should i answer it? I mean I can't write about benzene ring & stuff:s

If you draw the diagram above (it asks you to use diagrams) then say the electrons are free to move between the two forms - creating an average bond length.

[aqua05]

(Original post by EierVonSatan)
The electrons in the ester aren't free to move because the two electrons (which can move in the benzoate ion) are being shared with the carbon.



If you draw the diagram above (it asks you to use diagrams) then say the electrons are free to move between the two forms - creating an average bond length.

thanks so much for your help!
Got it