How To Predict Acidities and Basicities of Compounds

Acidity
An acid is a molecule or ion capable of donating a hydrogen ( proton or hydrogen ion ), or, alternatively, capable of forming a covalent bond with an electron pair. This property of a molecule to donate proton is referred to as its acidity.
PREDICTING TRENDS IN ACIDITIES
Remember the following points when predicting acidities of compounds:

• Acids donate a proton. That is, they lose a proton.
• Acidity ( acid - strength ) depends on the ease with which they lose a proton.
• Aliphatic carboxylic acids are acidic and lose a proton readily. This is because the carboxylate ion formed by ionization or reaction with a base is stabilized by resonance.
• The acidity of substituted aliphatic carboxylic acids depends on the inductive effect and position of the substituents. Electron - releasing alkyl groups decreases the acidity. Electron - withdrawing substituents ( F, Cl, CN, COOH ) increases the acidity.
• Phenol is acidic. This is because the phenoxide ion formed by ionization or reaction with a base is stabilized by resonance.
• The acidity of phenols depends on the nature and position of the substituent. Electron - releasing alkyl groups decrease the acidity of phenols. Electron - withdrawing substituents ( Cl, CN ) increases the acidity of phenols.
• Phenols are much more acidic than alcohols. This is because alcohols give alkoxide ions on the loss of a proton. The alkoxide ions so formed are not resonance stabilized.
• p-Nitrophenol is almost 1000 times more acidic than phenol.
• Picric acid ( 2,4,6-Trinitrophenol ) is as strong as HCl.
• Alcohols are weaker acids than water.
• Thiols are more acidic than alcohols.
• Aromatic carboxylic acids are acidic and lose a proton readily. This is because the carboxylate ion formed by ionization or reaction with a base is stabilized by resonance.
• The acidity of substituted benzoic acids depends on the nature and position of the substituent. Electron - releasing substituents ( CH3, OCH3 ) decreases the acidity of a benzoic acid. Electron - withdrawing substituents increase the acidity of a benzoic acid.
• Almost all ortho substituents increase the acidity of a benzoic acid.
• The acidity of a compound may be expressed quantitatively in terms of its acidity constant, pKa. Stronger acids will have smaller pKa. Weaker acids will have larger pKa.

      Basicity

The ability of a chemical species to accept or attract proton is referred to as its basicity. Basicity of a molecule is normally expressed by the acidity of the conjugated acid ( conjugate acid - base pair ).

PREDICTING TRENDS IN BASICITIES

Remember the following points when predicting basicities of compounds:

• Amines are basic because they have a lone pair ( unshared pair ) of electrons on nitrogen. This lone pair of electrons is available for formation of a new bond with a proton or Lewis acids.
• Basicity ( base - strength ) depends on the degree of availability of the lone pair of electrons.
• Any factor which increases the availability of the lone pair of electrons on nitrogen for protonation, increase the basicity.
• Electron - releasing groups on the nitrogen atom of amines increase the basicity an electron - withdrawing groups decrease the basicity.
• Aliphatic amines are stronger bases than ammonia.
• Aromatic amines are less basic than aliphatic amines. This is because the lone pair of electrons on the nitrogen in aromatic amines is less available due to delocalization. 
• Electron - releasing groups increase the basicity of aromatic amines. Electron - withdrawing groups on the aromatic ring decrease the basicity.
• The basicity of a compound may be expressed quantitatively in terms of its basicity constant, pKb. Stronger bases will have larger pKb. Weaker bases will have smaller pKb.