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So I was comparing the $+M$ effect of $\ce{-CH^-_2}$, $\ce{-NH^-}$ and $\ce{-O^-}$. Answer was given as:

$$ \ce{-CH^-2 > -NH^- > -O^-}$$

So, I attach them with ethene and made their respective resonating structures as: $\require{enclose}$

  1. $\ce{\underset{(a)}{CH2=CH-\overset{\ominus}{C}H2} <-> \underset{(b)}{\overset{\ominus}{C}H2-CH=CH2}}$

  2. $\ce{\underset{(a')}{CH2=CH-\overset{\ominus}{N}H} <-> \underset{(b')}{\overset{\ominus}{C}H2-CH=NH}}$

  3. $\ce{\underset{(a'')}{CH2=CH-\overset{\ominus}{O}} <-> \underset{(b'')}{\overset{\ominus}{C}H2-CH=O}}$

So my doubt was whether $b$, $b'$ and $b''$ have same contribution to their respective resonance hybrid or it's different ?

Or it's that they have same relative contribution but order of $+M$ is determined by inductive effect in resonating structures or inductive effect determines resonating structure contribution ?

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    $\begingroup$ For future reference: bodies of questions, answers, and comments on ChemSE (on occasion, equally sibling sites on stackexchange.com, too) equally understand mathjax and mhchem. Especially mhchem offers an easier input and better rendering of chemistry-related content (e.g. simple formulae in reaction equations) and is presented e.g. here. Note: because browsers do not universally understand this addition, do not use them in the title of a question / answer. $\endgroup$ – Buttonwood Mar 24 at 15:16
  • $\begingroup$ Both the structures a' and b' are incorrect. Nitrogen does not have d orbitals and cannot expand its octet. $\endgroup$ – Aditya Roychowdhury Mar 25 at 2:27
  • $\begingroup$ The $\ce{-NH2}$ group with a negative charge indicates pentavalent nitrogen. So I corrected it. $\endgroup$ – Mathew Mahindaratne Mar 25 at 18:01
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For $b$, you have equivalent resonating structures as the structures are identical.

The structures $a'$ and $b'$ are not correct as nitrogen cannot expand it's octet.

For $a''$ and $b''$, the first structure $(a'')$ is more stable as there is negative charge localised on electronegative atom as compared to $b''$ where it is localised on carbon.

For the second question, I think inductive effect is completely different than the mesomeric effect. For determining the order of the strength of mesomeric effect, you can simply compare their electronegativity orders, more is the electronegativity, lesser is the strength to donate electrons.

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  • $\begingroup$ Thanks ! I got what you said but can you comment on whether we can use inductive effect for comparing contributions of resonating structures or just the rules like "full octet,more pi bond ,-ve charge on more EN atom,...." are valid ? $\endgroup$ – Yash Agrawal Mar 24 at 15:06
  • $\begingroup$ Why should inductive effect be considered for resonating structures? It has to do with stability, the one which is more stable will have more contribution in resonating structures. Inductive effect is an entirely different thing than stability. $\endgroup$ – Light Yagami Mar 24 at 15:17
  • $\begingroup$ But in cases when we compare stability of carbocation and carbanion, We also use inductive effect to determine stability order of them , So, I was applying the same logic in resonating structures. Is it wrong to do so ? $\endgroup$ – Yash Agrawal Mar 24 at 15:22
  • $\begingroup$ While comparing the stability among carbocations and carbanions, we use inductive effect because it gives the idea of electron density on that species, electron deficient species would get stabilsed if it is attached to ERG, similarly carbanions will get stabilised if they are attached to EWG. When comparing "resonating structures", we have to chose which among them is more stable, for which we have different parameters like seeing if their octet is complete, no. of $\pi$ bonds, charge separation, negative charge on EN atom and so on... inductive effect is not relevant here. $\endgroup$ – Light Yagami Mar 24 at 15:28
  • $\begingroup$ So how we'll be comparing -M effect of $\ce{-COH}$ and $\ce{-COR}$ ? $\endgroup$ – Yash Agrawal Mar 24 at 15:38

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