Search results
Results from the WOW.Com Content Network
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.
Monochloroacetic acid (pK a =2.82), though, is stronger than formic acid, due to the electron-withdrawing effect of chlorine promoting ionization. In benzoic acid, the carbon atoms which are present in the ring are sp 2 hybridised. As a result, benzoic acid (pK a =4.20) is a stronger acid than cyclohexanecarboxylic acid (pK a =4.87).
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.
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.
An electron withdrawing group (EWG) will have the opposite effect on the nucleophilicity of the ring. The EWG removes electron density from a π system, making it less reactive in this type of reaction, [ 2 ] [ 3 ] and therefore called deactivating groups .
For meta-directing groups (electron withdrawing group or EWG), σ meta and σ para are more positive than σ’. (The superscript, c, in table denotes data from Hammett, 1940. [11] [page needed]) For ortho-para directing groups (electron donating group or EDG), σ’ more positive than σ meta and σ para.
Electron withdrawing groups (which can stabilize the molecule by increasing charge distribution) or electron donating groups (which destabilize by decreasing charge distribution) present on a molecule also determine its pK a. The solvent used can also assist in the stabilization of the negative charge on a conjugated base.
They are strong electron-withdrawing groups that increase the acidity of alpha-protons on the aromatic ring. In the presence of two DMGs, metalation often occurs ortho to the stronger directing group, though mixed products are also observed. A number of heterocycles that contain acidic protons can also undergo ortho-metalation.