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The halogen atoms in an alkyl halide are electron withdrawing while the alkyl groups have electron donating tendencies. If the electronegative atom (missing an electron, thus having a positive charge) is then joined to a chain of atoms , typically carbon , the positive charge is relayed to the other atoms in the chain.
Electron donating groups are generally ortho/para directors for electrophilic aromatic substitutions, while electron withdrawing groups (except the halogens) are generally meta directors. The selectivities observed with EDGs and EWGs were first described in 1892 and have been known as the Crum Brown–Gibson rule.
Electron-withdrawing groups exert an "inductive" or "electron-pulling" effect on covalent bonds. The strength of the electron-withdrawing group is inversely proportional to the pKa of the carboxylic acid. [2] The inductive effect is cumulative: trichloroacetic acid is 1000x stronger than chloroacetic acid.
The captodative effect is the stabilization of radicals by a synergistic effect of an electron-withdrawing substituent and an electron-donating substituent. [2] [3] The name originates as the electron-withdrawing group (EWG) is sometimes called the "captor" group, whilst the electron-donating group (EDG) is the "dative" substituent. [3]
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.
With respect to acidity, a common trend to note is that, inductively, an electron-withdrawing substituent in the vicinity of an acidic proton will lower the pKa (i.e. increase the acidity) and, correspondingly, an electron-donating substituent will raise the pKa. [7] The reorganization of charge due to field effects will have the same result.
Electron-withdrawing groups are known to slow down the rate of the reaction, with electron donating groups increasing the rate of the reaction. This is consistent with initial attack of the phosphorus reagent on the alkyl halide as the rate-determining step of the reaction. The reaction proceeds smoothly when the R group is aliphatic.
The molecular orbitals of electron withdrawing groups at C7 overlap with the HOMO Walsh orbitals of the cyclopropane ring causing a shortening of the C1-C6 bond. In the case of electron donating groups, orbital overlap is again possible now in the LUMO, resulting in an increase in antibonding character destabilizing the norcaradiene tautomer ...