Saved Bookmarks
| 1. |
Discuss the general chemical properties of carboxylic acids. |
|
Answer» Solution :Chemical properties of carboxylic acids. Carboxylic acids give the following reactions : (A) Reactions due to replaceable hydrogen 1. Acidic character. Carboxylic acids ionise in water to give hydrogen ion (`H^(+)`) which are responsible for their acidic character.  2. Action with alkalies and carbonates. Carboxylic acids neutralise alkalies forming salts and decompose carbonates (or bicarbonates) evolving carbon dioxide with effervescence. `underset("Acetic acid")(CH_(3)COOH)+NaOH to underset("Sodium acetate")(CH_(3)COONa)+H_(2)O` `underset("Acetic acid")(2CH_(3)COOH)+Na_(2)CO_(3) to 2CH_(3)COONa+CO_(2)+H_(2)O` 3. Action with metals. Carboxylic acids react with strongly electropositive metals such as Na, Mg, Ca, Zn, ETC. liberating hydrogen gas. `2CH_(3)COOH+2Na to underset("Sodium acetate")(2CH_(3)COONa)+H_(2)` `2CH_(3)COOH+Zn to (CH_(3)COO)_(2)Z +H_(2)`  (B) Reactions due to -OH group of carboxylic acids 4. Action with phosphorus halides. Carboxylic acids react with phosphorus trichloride or phosphorus pentachloride to give acid chlorides as : `3CH_(3)COOH+PCl_(3) to underset("Acetyl chloride")(3CH_(3)COCl)+H_(3)PO_(3)` `CH_(3)COOH+PCl_(5) to underset("Acetyl chloride")(CH_(3)COCl)+POCl_(3)+HCl` 5. Action with thionyl chloride. Carboxylic acids react with thionyl chloride to form acid halides. `CH_(3)COOH +SOCl_(2) to underset("Acetyl chloride")(CH_(3)COCl) +SO_(2) + HCI` 6. Esterification. Carboxylic acids react with alcohols in the presence of an acid catalyst (conc. `H_(2)SO_(4)`) to form esters  7. Formation of anhydrides. Carboxylic acids on treatment with strong dehydrating agents such as phosphorus pentaoxide form acid anhydrides by the elimination of water molecule as:  (C) Reactions due to carboxylic group 8. DECARBOXYLATION. Carboxylic acids get decarboxylated.i.e. lose carbon dioxide when their sodium salts are heated with soda lime (NaOH + CaO). The condition of decarboxylation of carboxylic acids also depend upon their structures. `CH_(3)-overset(O)overset(||)C-ONa + NaOH overset("HEAT")to CH_(4) + Na_(2)CO_(3)` When two carboxylic groups are attached to the same carbon atom, decarboxylation takes place simply on heating.  Alkali METAL salts of carboxylic acids undergo decarboxylation by electrolysis. This method is known as Kolbe.s electrolysis. `3CH_(3)COOH to 2CH_(3)COO^(-)+2K^(+)` At anode: `2CH_(3)COO^(-) underset("Unstable")(2CH_(3)COO) to underset("Ethane")(CH_(3)-CH_(3))+2CO_(2)` At cathode: `2K^(+) overset(+2e)to 2K overset(H_(2)O)to 2KOH + H_(2) uarr` 9. Reduction. Carboxylic acids on .reduction with lithium aluminium hydride (`LiAIH_(4)`) in ether are reduced to alcohols. `LiAIH_(4)` is a mild reducing agent. `RCOOH overset(LiAIH_(4))to underset("alcohol")(RCH_(2)OH)` `underset("Acetic acid ")(CH_(3)COOH)+4[H] underset("ether")overset(LiAIH_(4))to underset("Ethyl alcohol")(CH_(3)CH_(2)OH)+H_(2)O` However, carboxylic acids on drastic reduction with concentrated hydriodic acid and red phosphorus under high pressure yield hydrocarbons as: `CH_(3)COOH + 6HI overset("Red P")to underset("Ethane")(CH_(3)CH_(3)) + 2H_(2)O + 3I_(2)` 10. Ring substitution in aromatic acids. Carboxyl group in benzoic acid is an electron withdrawing group and therefore, it is meta directing group. Some common electrophilic substitution reactions of benzoic acid are : 
|
|