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The +M effect, also known as the positive mesomeric effect, occurs when the substituent is an electron donating group. The group must have one of two things: a lone pair of electrons, or a negative charge. In the +M effect, the pi electrons are transferred from the group towards the conjugate system, increasing the density of the system.
However, another effect that plays a role is the +M effect which adds electron density back into the benzene ring (thus having the opposite effect of the -I effect but by a different mechanism). This is called the mesomeric effect (hence +M) and the result for fluorine is that the +M effect approximately cancels out the -I effect.
Contributing structures of the carbonate ion. In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or forms, [1] also variously known as resonance structures or canonical structures) into a resonance hybrid (or hybrid structure) in valence bond theory.
This effect is depicted in scheme 3, where, in a para substituted arene 1a, one resonance structure 1b is a quinoid with positive charge on the X substituent, releasing electrons and thus destabilizing the Y substituent. This destabilizing effect is not possible when X has a meta orientation. Scheme 3. Hammett Inductive Mesomeric Effects
A completely uncharged structure cannot be written and mesoionic compounds cannot be represented satisfactorily by any one mesomeric structure. [1] Mesoionic compounds are a subclass of betaines. [1] Examples are sydnones and sydnone imines (e.g. the stimulant mesocarb), münchnones, [1] [2] and mesoionic carbenes.
The Vietnamese Wikipedia initially went online in November 2002, with a front page and an article about the Internet Society.The project received little attention and did not begin to receive significant contributions until it was "restarted" in October 2003 [3] and the newer, Unicode-capable MediaWiki software was installed soon after.
This effect was described in 1935 by John W. Baker and W. S. Nathan. [ 2 ] [ 3 ] [ 4 ] They examined the chemical kinetics for the reaction of pyridine with benzyl bromide to form a pyridinium salt, and a series of benzyl bromides having different alkyl groups as substituents at the para position.
Anisole undergoes electrophilic aromatic substitution reaction at a faster speed than benzene, which in turn reacts more quickly than nitrobenzene.The methoxy group is an ortho/para directing group, which means that electrophilic substitution preferentially occurs at these three sites.