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I read somewhere that the structure in the following picture is a Lewis structure of ethyl acetate.

enter image description here

However, I don't understand why:

  • The $\ce{C}$ has a positive charge, but only three bonds
  • The $\ce{C}$ does not satisfy the octet rule.
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    $\begingroup$ The gist is that no bond is purely ionic or purely covalent. Those are two extremes like pure black and pure white. You drawing shows how the C=O double bond can have some ionic character. $\endgroup$ – MaxW Mar 17 '16 at 15:07
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The two rules you cite are guidelines for the 'best' Lewis structure. However, in the example provided, all electrons are accounted for and all atoms are connected making it a 'valid' Lewis structure.

In the context of resonance forms, this is likely a minor contributor.

However the electrophilic center in ethyl acetate is the carbon you marked as positive, this form has some descriptive and predictive power in discussing reactions of ethyl acetate. In general, for $\ce{R2C=O}$ molecules (esters, aldehydes, ketones, etc), The carbocation (sextet) carbon has some relevance.

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