the other structures. Problem . Write resonance structures of CH =CH–CHO. Indicate relative stability of the contributing structures.
The two structures are less important contributors as they involve charge separation. Additionally, structure I contains a carbon atom with an incomplete octe t. . .
Resonance Effect The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two π -bonds or between a π -bond and lone pair of electrons present on an adjacent atom’. The effect is transmitted through the chain. There are two types of resonance or mesomeric effect designated as R or M effect. (i) Positive Resonance Effect (+R effect) In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system.
This electron displacement makes certain positions in the molecule of high electron densities. This effect in aniline is shown as : Stability: I > II > III [I: Most stable, more number of covalent bonds, each carbon and oxygen atom has an octet and no separation of opposite charge II: negative charge on more electronegative atom and positive charge on more electropositive atom; III: does not contribute as oxygen has positive charge and carbon has negative charge, hence least stable]. Problem . Explain why the following two structures, I and II cannot be the major contributors to the real structure of CH COOCH .
(ii) Negative Resonance Effect (- R effect) This effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugated system. For example in nitrobenzene this electron displacement can be depicted as : The atoms or substituent groups, which represent +R or –R electron displacement effects are as follows : +R effect: – halogen, –OH, –OR, –OCOR, –NH , –NHR, –NR , –NHCOR, – R effect: – COOH, –CHO, >C=O, – CN, –NO The presence of alternate single and double bonds in an open chain or cyclic system is termed as a conjugated system. These systems often show abnormal behaviour. The examples are , - butadiene,