An electron-withdrawing group ensures effective delocalization of negative charge through resonance or inductive effect. I’m Mubashir Abid, 22 years old. The relative acid-strengths of some phenols are as follows: Post Comments ♥. Phenols are acidic due to the formation of stable phenoxide ion in aqueous solution. The acidity of carboxylic acids further depends on the nature of substituent alkyl or aryl group attached to the carboxyl group. Hi there! As expected, the higher the electronegativity of the substituent the greater the increase in acidity (F > Cl > Br > I), and the closer the substituent is to the carboxyl group the greater is its effect (isomers in the 3rd row). ). One the other hand, no resonance is possible in alkoxide ions (. In general, resonance effects are more powerful than inductive effects. Electronegative substituents increase acidity by inductive electron withdrawal. This charge delocalisation is a stabilizing factor in the phenoxide ion. Because the inductive effect depends on electronegativity, fluorine substituents have a more pronounced pKa-lowered effect than chlorine substituents. The phenoxide ion is stable due to resonance: Notice that the negative charge is spread throughout benzene ring, and thereby effectively dispersed.
This is indicated in order of acidity constants: Acidity of phenols with substituents - Genuine chemistry - The ultimate resource for Chemistry.
Atom In addition, the inductive takes place through covalent bonds, and its influence decreases markedly with distance – thus a chlorine two carbons away from a carboxylic acid group … ( I hope you enjoy your visit to my website. For example, phenols itself gives phenoxide ion on dissociation. Much love!
Phenols are considerably more acidic than alcohols however less so than carboxylic acids or even carbonic acid.