Carboxylic acids preparing, reactions, physical properties

Carboxylic group includes a carbonyl group and a hydroxyl group. Carboxylic acids are weak acids which dissociate partially in the water. Carboxylic acids can be prepared by oxidizing alcohols, aldehydes etc. There are aliphatic and aromatic carboxylic acids in the world.


Active group of carboxylic acids

Active group of carboxylic acid is -COOH.

Active group of carboxylic acids

Occurrence of carboxylic acids

We can see carboxylic acids are in natural sources.

  • Formic acid (HCOOH) contains in the ant venom.
  • Acetic acid (CH3COOH) contains in the vinegar.

Examples and general names of carboxylic acids

general names of carboxylic acids

Formic acid

Formic acid(HCOOH) is the simplest carboxylic acid compound.

formic acid

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
    carboxylic acid dimers
  • 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.

iupac nomenclature of carboxylic acids


Carboxylic acids aqueous solution

Carboxylic acids dissociates partially in the water. Therefore those solutions are weak acids in aqueous solutions. Ka values of carboxylic acids are usually in 10-4 - 10-8 mol dm-3.


Preparing carboxylic acids

Carboxylic acids can be prepared by oxidizing alcohols and aldehydes.


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.

Ethanol and acidic potassium permanganate


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 Ammonical silver nitrate reaction give ethanoic acid

Ethanal and Fehling's solution reaction give ethanoic acid

Cu2+ ions are reduced into red colour Cu2O.

Ethanal and Fehling's solution reaction give ethanoic acid



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-butene ozonolysis gives ethanoic acid

2-pentene ozonolysis

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

2-pentene ozonolysis gives ethanoic acid

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

A mixture of carboxylic acid and ketone is given.

ozonolysis of 3-methyl-2-pentene



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.

methyl-magnesium-chloride, CO2 and water reaction


ethyl magnesium bromide, CO2 and water reaction

propanoic acid is given.

ethyl magnesium bromide, CO2 and water reaction

phenyl magnesium bromide, CO2 and water reaction

benzoic acid is given.

phenyl magnesium bromide, CO2 and water reaction

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-.

CO2-grignard-H2O-reaction-mechanism



Nitrile hydrolysis

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

Nitrile hydrolysis gives carboxylic acids


Reactions of carboxylic acids

Carboxylic compounds show acidic characteristics. Therefore carboxylic acids can react with Na, NaOH, Na2CO3, NaHCO3 as a acid. There are three main types of reactions of carboxylic acids.

  1. Breaking -OH bond in -COOH group
  2. Breaking C-O bond in -COOH group
  3. Breaking C=O bond in -COOH group


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.

carboxylic-acids-and-NaOH


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..

ethanoic-and-Na2CO3-and-NaHCO3

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 and SOCl2

ethanoic acid and PCl3

ethanoic and PCl3

ethanoic acid and PCl5

ethanoic 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.

esterification - carboxylic acid and alcohols reaction

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

acetic acid carboxylation

benzoic 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

ethanoic and ammonia reaction

Ammonium salts of carboxylic acid and NaOH reaction

Ammonium salts of carboxylic acid and NaOH reaction

Ammonium salts of carboxylic acid heating

ammonium acetate heating give amides


Carboxylic acids and lithium aluminum hydride reaction

Carboxylic acids are not reduced easily. But a strong oxidizing reagent, lithium aluminum hydride(LiAlH4) can be used to reduce carboxylic acid. Reducing carboxylic acids by LiAlH4 gives alcohols.


ethanoic acid and LiAlH4

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.
    Synthesis acid anhydride from carboxylic acids

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 alcohols, phenols and carboxylic acids

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+.



Acidity of aromatic carboxylic acids and aliphatic carboxylic acids

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.

Acidity of halogen substituted carboxylic acids

Pka values of carboxylic acids

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

Pka values of carboxylic acids

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




Formic acid (HCOOH)

Formic acid has the lowest molecular mass in carboxylic acids range. Formic acid has special reducing agent. When oxidizing agents are in the presence, formic acid is converted to CO2 and H2O. The H atom which is attached to the acidic C atom is the reason for this reducing characteristics.


Oxidizing formic acid

Formic acid can be oxidized using following oxidizing agents.

  1. Strong oxidizing agents
  2. With mercuric chlorides
  3. With tollen's reagent
  4. Fehlingh's reagent

Formic acid and strong oxidizing agents

CO2 and H2O are given by the reaction of HCHO and strong oxidizing agents.

HCHO and H+ / KMnO4

HCHO and H+ KMnO4

HCHO and H+ / K2CrO4

HCHO and H+ K2CrO4

HCHO and H+ / K2Cr2O7

HCHO and H+  K2Cr2O7

HCHO and Hg2Cl2 reaction

HCHO reacts with Hg2Cl2 and give CO2, H2O and Hg2Cl2 white precipitate.

HCHO and Hg2Cl2

HCHO and tollen's reagent reaction

HCHO reacts with tollen's reagent and give CO2, H2O and Ag(silver mirror).

HCHO and tollen's reagent reaction

HCHO and fehlingh's reagent reaction

HCHO reacts with fehlingh's reagent and give CO2, H2O and Cu2O(red precipitate).

HCHO and fehlingh's reagent reaction

None of other carboxylic acids are oxidized and give CO2 and H2O like HCHO acid.




Questions and Answers of carboxylic acids


How to separate alcohols and carboxylic acids

Add Na2CO3 to both solutions

  • CO2 gas is emitted from carboxylic acid.
  • No any change or emission of gas in alcohols.


What is the most acidic carboxylic acid?

Formic acid is the most acidic carboxylic acid because formic acid has reducing characteristics.



Related tutorials for carboxylic acids

Why carboxylic acids are more acidic than alcohols? Prepare butanoic acid Prepare ethanoic acid Nomenclature, IUPAC naming of carboxylic acids Carboxylic acid chloride hydrolysis and mechanism Acids and alkalis Phenol preparing, reactions and physical properties Na2CO3 , NaHCO3