Acidity and Basicity

Acidity and basicity are fundamental concepts in chemistry that describe the ability of substances to donate or accept protons (H⁺) or electrons. These properties help to understand the mechanism of different chemical reactions, particularly in acid-base chemistry.

1.0Bronsted-Lowry Concept of Acids and Bases

According to this concept, Acidity refers to the ability of a substance to donate a proton
and Basicity The ability of a substance to accept a proton. For example-

• Acid: A substance that donates a proton (H⁺) in a chemical reaction.

Example: HCl (hydrochloric acid) donates H⁺ to water.

• Base: A substance that accepts a proton (H⁺) in a chemical reaction.

Example: NH₃ (ammonia) accepts H⁺ to form NH₄⁺.

2.0Lewis Concept of Acids and Bases

According to this concept, Acid is a substance that can accept a pair of electrons.

• Example: BF₃ (boron trifluoride) accepts electron pairs.

A base is a substance that can donate a pair of electrons.

• Example: NH₃ (ammonia) donates a pair of electrons to form a bond with BF₃.

Acidity: The ability of a substance to accept electrons.
Basicity: The ability of a substance to donate electrons.

3.0Relative Acidic Strength of Hydrocarbons

• Hydrocarbons generally exhibit very low acidity because they lack groups that can donate protons easily. However, their relative acidic strength is as follows:

                                 Alkynes > Alkenes > Alkanes

• Alkynes are more acidic due to the higher s-character (50%) in the sp-hybridized carbon, which makes the hydrogen more acidic. Alkanes are the least acidic.

4.0Acidic Strength of Phenols

• Phenols (C₆H₅OH) are more acidic than alcohols due to the resonance stabilization of the phenoxide ion (C₆H₅O⁻) after deprotonation.

• The electron-withdrawing groups on the benzene ring (like -NO₂) increase the acidic strength of phenols, whereas electron-donating groups (like -CH₃) decrease it.

5.0Acidic Strength of Carboxylic Acids

• Carboxylic acids (R-COOH) are acidic due to the resonance stabilization of the carboxylate ion (R-COO⁻) formed after losing a proton.
• Electron-withdrawing groups (like -Cl, -NO₂) enhance the acidic strength by stabilizing the negative charge on the carboxylate ion.
• Electron-donating groups (like -CH₃) reduce acidic strength by destabilizing the carboxylate ion.

6.0Ortho Effect on Acidity

• The Ortho Effect in carboxylic acids refers to the phenomenon where a substituent at the ortho position relative to the carboxyl group increases the acidity of the acid. This happens because steric hindrance prevents resonance between the carboxyl group and the benzene ring, leading to increased acidity.
• Example: Ortho-substituted benzoic acids are generally more acidic than para- or meta-substituted benzoic acids.

7.0Basic Strength of Aliphatic Amines

• Aliphatic amines (R-NH₂) are generally more basic than aromatic amines. The +I effect of alkyl groups increases the electron density on nitrogen, making it more available to accept a proton.
• • Example: Ethylamine (CH₃CH₂NH₂) is more basic than ammonia (NH₃) due to the electron-donating ethyl group.

8.0Basic Strength of Aromatic Amines (Anilines)

• Aromatic amines like aniline (C₆H₅NH₂) are less basic than aliphatic amines. The lone pair of electrons on the nitrogen in aniline is delocalized into the benzene ring via resonance, reducing its availability for protonation.
• • Example: Aniline is less basic than methylamine due to the resonance effect in the benzene ring.

9.0Ortho Effect on Basicity (Aromatic Amines)

The Ortho Effect in aromatic amines refers to the observation that ortho-substituted anilines are generally less basic than aniline itself. This is due to steric hindrance and electronic effects caused by the ortho substituent, which reduces the availability of the nitrogen lone pair for protonation.

For more clarification lets understand this example- 

Ex. Basicity order in given structure will be

I (Aniline) > IV (Para-Chloroaniline) > III (Meta-Chloroaniline) > II (Ortho-Chloroaniline)