Carboxylic acids preparing, reactions, physical properties

Physical properties of carboxylic acids

• Carboxylic group is a polar group. The -OH group can make H bonds. Therefore aliphatic carboxylic acids show similar solubility pattern like alcohols.
• First carboxylic acids (five carbon atoms or less) dissolve in water very well. Carboxylic acids with more than five carbon atoms are not soluble in water. Aromatic carboxylic acids such as benzoic acid also do not dissolve in water.
• Boiling points of carboxylic acids are more higher than comparable alcohols boiling points. Carboxylic acids exist as dimers because two carboxylic molecules are bonded through two H bonds
  
• All aromatic carboxylic acids are in solid state at room temperature.

Carboxylic acid IUPAC nomenclature

IUPAC names of carboxylic acids ends with oic acid. The carbon atom of carboxylic acid group holds number 1. Therefore in the nomenclature, the number of carboxylic group is neglected. Other all IUPAC nomenclature rules are applied for carboxylic nomenclature.

Preparing carboxylic acids by alcohols

Alcohols can be oxidized into carboxylic acid using acidic strong oxidizing agents such as KMnO₄, K₂CrO₄, K₂Cr₂O₇.

Ethanol and acidic potassium permanganate

Ethanoic acid is given as the product. Purple colour of potassium permanganate is changed into colourless or pink colour.

Preparing carboxylic acids by aldehydes

Aldehydes can be oxidized into carboxylic acid using weak oxidizing agent or acidic strong oxidizing agents Ammonical silver nitrate( [Ag(NH₃]NO₃ ), Fehling's solution( includesCu(I) ions ), H₂O₂ are weak oxidizing agents which can oxidize aldehydes into carboxylic acids.

Ethanal and Ammonical silver nitrate reaction give ethanoic acid

Ag⁺ ions are reduced into Ag. This Ag metal can be identified as a silver mirror.

Ethanal and Fehling's solution reaction give ethanoic acid

Cu²⁺ ions are reduced into red colour Cu₂O.

Ozonolysis of alkenes

When strong oxidizing agents are in the presence with some alkenes, carboxylic acids can be given as products. This reaction is defined as ozonolysis. H⁺/ KMnO₄ , H⁺/ K₂CrO₄ or O₃ can be used as reagents.

The double bond of alkene is braked to give products. According to the structure of alkene compounds mixture of carboxylic acids or ketones or CO₂ can be given.

2-butene ozonolysis

2-butene ozonolysis gives ethanoic acid.

2-pentene ozonolysis

2-pentene ozonolysis gives mixture of ethanoic acid and propanoic acid.

What happens when ozonolysis of 3-methyl-2-pentene?

A mixture of carboxylic acid and ketone is given.

Reaction of CO₂ and grignard reagent

When Grignard reagent, CO₂ and H₂O react, carboxylic acids are given. The number of carbon atoms in the grignard reagent is increased by one when final product is given.

methyl magnesium chloride, CO₂ and water reaction

ethanoic acid given.

ethyl magnesium bromide, CO₂ and water reaction

propanoic acid is given.

phenyl magnesium bromide, CO₂ and water reaction

benzoic acid is given.

CO₂, grignard reagent and H₂O reaction mechanism

C atom in the CO₂ is polarized as positive. Alkyl group in the grignard reagent has negative charge.

step 1: C atom in the CO₂ is attacked by :CH₃^- in the grignard reagent.

step 2: A H atom in the water molecule is attacked by O^-.

Nitrile hydrolysis

From hydrolysis of aryl or alkyl cyanide, carboxylic acids are prepared. Any acid or acidic substance can catalyse this reaction.

Reactions of carboxylic acids

Carboxylic acid and metals reaction

Carboxylic acids react with metals such as sodium, potassium and give hydrogen(H₂) and salt. Alcohols also react with Na and K. But carboxylic acids react more quickly than alcohols.

Carboxylic acids and sodium hydroxide reaction

Carboxylic acids react with NaOH and give R-COO^-Na⁺. This is a reaction of weak acid and a strong alkali.As an example, ethanoic acid(CH₃COOH) reacts with NaOH_(aq) and give sodium ethanoate and water as products.

Sodium carbonate or sodium bicarbonate reaction

Carboxylic acid reacts with Na₂CO₃ or NaHCO₃ and emits CO₂ gas. This reaction can be used to identify phenol and carboxylic acids. Phenol does not react with sodium carbonate or sodium bicarbonate to emit a gas or gases..

With inorganic acid chlorides SOCl₂, PCl₃, PCl₅

Carboxylic acids react with inorganic acid chlorides such as SOCl₂, PCl₃, PCl₅ and give carboxylic acid chlorides.

ethanoic acid and SOCl₂

ethanoic acid and PCl₃

ethanoic acid and PCl₅

Esterification

Carboxylic acid react with alcohol when heating and in the presence concentrated sulfuric acid(catalyst) and give esters. This reaction is a reversible one.

Carboxylation with soda lime

Every carboxylic acids (aliphatic and aromatic carboxylic acids) are subjected to carboxylation. Hydrocarbons are given as products. CaO_(s) / Ca(OH)_2(s) and NaOH_(s) mixture is used as the reagent in carboxylation.

acetic acid carboxylation

benzoic acid carboxylation

With ammonia

Carboxylic acids react with ammonia gas(NH₃) or ammonia hydroxide solution (NH₄OH) to produce ammonium salt of carboxylic acid. These ammonium salts are soluble in water and they show ionic characteristics.

3) with NaOH at room temperature. When ammonium salts of carboxylic acid are heated, it is dehydrated and gives amide compounds.

ethanoic acid and ammonia gas reaction

Ammonium salts of carboxylic acid and NaOH reaction

Ammonium salts of carboxylic acid heating

Carboxylic acids and lithium aluminum hydride reaction

Carboxylic acids are not reduced easily. But using a strong reducing reagent such as lithium aluminum hydride (LiAlH₄) can be used to reduce carboxylic acid to a primary alcohol.

ethanoic acid and LiAlH₄

Synthesis acid anhydride

1. When carboxylic acid heat with diphosporous pentaoxide(P₂O₅) you can get acid anhydride.
2. Add Na to the carboxylic acid. It will give sodium salt of carboxylic acid. Next PCl₅ is added to another carboxylic acid solution. It will give carboxylic acid halide. Mix both sodium salt of carboxylic acid and acid halide. Then acid anhydride forms as the product by eliminating NaCl.
   
   

Acidity of carboxylic acids

1. Phenol reacts with NaOH. But alcohols do not reacts with NaOH. Therefore we can say acidity of phenol is higher than alcohols.
2. carboxylic acid react with both sodium carbonate(Na₂CO₃) and sodium bicarbonate(NaHCO₃).
3. But phenol does not react with both sodium carbonate(Na₂CO₃) and sodium bicarbonate(NaHCO₃).
4. So we see acidity of carboxylic acids are higher than phenol.

Generally we can say acidity of alcohols, phenols and carboxylic acids are

Acidity of aromatic carboxylic acids and aliphatic carboxylic acids

All aromatic carboxylic acids are more acidic than aliphatic carboxylic acids.

Example: benzoic acid is more acidic than acetic acid.

ethanoic acid in aqueous solution

Ethanoic acid partially dissociate in the water and form ethanoate ion (acetate, CH₃COO^-) and H₃O⁺.

benzoic acid in aqueous solution

benzoic acid partially dissociate in the water and form benzoate ion. and H₃O⁺.

In the benzoate ion, negative charge overlap with benzene ring. Therefore negative charge decreases. Stability of carbanion is high.

But Alkyl groups repel electrons. Therefore negative charge on oxygen atom increases. Hence Stability of carbanion is less than benzoate ion.

Therefore equilibrium point of benzoic acid trends to the right. Thus acidity of benzoic acid is higher than ethanoic acid.

Acidity of halogen substituted carboxylic acids

Halogens are more electro negative than carbon. Carboxylic acids form carboxylate ions in the aqueous solution. If this carboxylate ion is more stable, the acidity increases. In halogens, F is the most electro negative atom.

P_ka values of carboxylic acids

Comparing P_ka values of acids are easy to compare strength of acids. When P_ka is decreased, acidic strength is increased.

Acidity of carboxylic acids are less than inorganic acids (mineral acids, H₂SO₄, HNO₃).