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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.
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
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.
It is often said the resonance stability of phenol makes it a stronger acid than that of aliphatic alcohols such as ethanol (pK a = 10 vs. 16–18). However, a significant contribution is the greater electronegativity of the sp 2 alpha carbon in phenol compared to the sp 3 alpha carbon in aliphatic alcohols. [7]
Hyperconjugation can be used to rationalize a variety of chemical phenomena, including the anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations and substituted carbon centred radicals, and the thermodynamic Zaitsev's rule for alkene stability.
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Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also many herbicides and pharmaceutical drugs. Phenol is a component in liquid–liquid phenol–chloroform extraction technique used in molecular biology for obtaining nucleic acids from tissues or cell culture samples.