Thursday, January 8, 2009

Carboxylic Acids - Conceptual Questions - Answers

1. What are carboxylic acids?
Carboxylic acids are the compounds containing the carboxyl group in their molecules.

The carboxyl group is made up of carbonyl, -C=O and hodroxyl, -OH group.

2.. What is the common system of nomenclature of carboxylic acids?

The common names of carboxylic acids are based on their source of origin. Formic acid was first obtained from red ants (Latin formica means red ants) and it was named from that. Acetic acid was obtained from vinegar (Latin aceum means vinegar) and so got that name.

In the common system the position of substituents is indicated by the Greek letters α, β, γ, ō.

α, β, γ, ō Carbon atoms

The carbon atom next to the carboxyl carbon is assigned the letter α. The carbon next to α-carbon is the β-carbon. The carbon next to β-carbon is the γ-carbon. The carbon next to γ-carbon is the ō Carbon.

ō-γ-β-α carbons

3. What is the IUPAC system of nomenclature of carboxylic acids?
According to IUPAC system, the name of the monocarboxylic acid is derived by changing the final 'e' from the name of the corresponding hydrocarbon with 'oic' and adding the word acid.

Formic acid - Methanoic acid
Acetic acid - Ethanoic acid
n-Butyric acid - Butanoic acid
Isobutyric acid - 2-Mehtylpropanoic acid

Dicarboxylic acids

Oxalic acid - Ethanedioic acid
Malonic acid - Propanedioic acid

The position of substituents is indicated by the following rules.

1. The longest chain of carbon atoms containing the carboxylic group(-COOH) is selected.
2. The numbering of carbons starts from the carboxylic acid group and the carbon of carboxyl group is given number 1.
3. The position of the substituents is indicated by the number of carbon atom to which they are attached.

4. What is the IUPAC system of nomenclature of carboxylic acids if more than two acid groups are present?

If an unbrached chain is directly bonded to more than two like functional groups, the organic compound is named as a derivative of parent alkane which does not include the carbon atoms of the functional groups. These are named by the use of suffix such as tricarboxylic acid

For example: Pentane-1,3,5-tricarboxylic acid

But if three COOH groups are not directly linked to the unbranched chain (say one COOH is linked to an alkyl group on the branched chain), the two like groups are considered in the parent chain and are named by using the suffix di before the name of the functional group. The third group (part of the side chain) is considered as a substituent group.

Example: (3-Carboxymethyl) heptane -1,7-dioic acid

5. What is the IUPAC system of nomenclature of aromatic carboxylic acids?

The simplest aromatic carboxylic acid is benzoic acid.

The IUPAC names of substituted aromatic carboxylic acids are derived by prefixing the name of the substituent to the name of the parent acid i.e., benzoic acid and the position is indicated by an Arabic numeral with the carbon atom carrying the –COOH group being numbered as 1.

6. What is the IUPAC system of nomenclature of dicarboxylic acids?
These are named as alkanedioic acids.

Dicarboxylic acids

Common name – IUPAC name

Oxalic acid - Ethanedioic acid
Malonic acid - Propanedioic acid

7. Explain the methods of preparation of carboxylic acids.

a. From oxidation of primary alcohols
Primary alcohols are oxidized with potassium permanganate and potassium dichromate to aldehydes which on further oxidation give carboxylic acids.

b. from oxidation of aldehydes and ketones
Aldehydes are easily oxidized to carboxylic acids with mild oxidizing agents like Tollen;s reagent.

c. from hydrolysis of nitriles(cyanides)
The nitriles are hydrolysed in dilute aqueous acidic or alkaline medium.

The alkyl cyanides used for the purpose are prepared from corresponding alkyl halides or alcohols.

The acid produced contains one more carbon atom than the original alkyl halide or alcohol. Thus, this method is a useful method for the preparation of carboxylic acids containing one more carbon atom than the starting alkyl halide or alcohol.

d. from Grignard reagents

The reaction is carried out by bubbling CO2 through the etheral solution of suitable Grignard reagent.

e. by hydrolysis of esters

Hydrolysis of esters with mineral acids or alkalies gives carboxylic acids

f. carboxylation of alkenes

Heating alkenes with CO and steam under pressure with phosporic acid at 673 K. This reaction is called Koch reaction.

g. from trihalogen derivatives of hydrocarbons

Hydrolysis of 1,1,1,-trihalogen derivatives of alkanes with acqueous KOH.

h. preparation of aromatic acids from alkyl benzenes
the alkyl side chain of benzene ring can be easily oxidized to carboxylic group with alkaline KMnO4, chromic anhydride or conc. HNO3.

8. Explain the physical properties of carboxylic acids.
a. Physical state and smell

The first three members are colourless liquids and have pungent smell. The next six members are oily liquids with a faint unpleasant odour.

Still higher acids are colourless waxy solids.

Benzoic acids and its homologues are colourless solids.

b. Boiling points

They have higher boiling points than the corresponding alcohols of comparable molecular masses.

Carboxylic acids have higher boiling points due to the presence of intramolecular hydrogen bonding. Due to the hydrogen bonding, carboxylic acids exist as dimers.

c. Melting point

In the case of first ten carboxylic acids, the melting points of acids containing even number of carbon atoms is higher than the next lower and higher member containing odd number of carbon atoms.

The melting and boiling points of aromatic acids are usually higher than those of aliphatic acids of comparable molecular masses.

d. Solubility in water

The first four members of aliphatic carboxylic acids are very soluble in water. The solubility in water decreases gradually with rise in molecular mass. All are soluble in alcohol or ether.

Benzoic acid is sparingly soluble in cold water but is soluble in hot water, alcohol and ether.

9. Chemical properties or reactions of carboxylic acids

1. Acidic character
a. How do you express strength of carboxylic acids?
The strength of acids can be expressed in terms of dissociation constant Ka or Ph number of PKa number which is pKa = -log Ka

b. what is effect of substituents on strength of carboxylic acids?
i.. Effect of electron releasing substituents,
Electron releasing substituents: Alkyl is an electron releasing group. If the H atom of formic acid (HCOOH) is replaced by CH3 group to form acetic acid (CH3COOH) the alkyl group will tend to increase the electron density on the oxygen atom of the O-H bond. This increase will make removal H+ ion difficult in comparison to formic acid.

Acetic acid is a weaker acid in comparison to formic acid.

The electron release effect is called +I effect. As +I effect increases, acidic strength will go down. As more alkyl groups are there +I effect increases
CH3 is less than C2H5 is less than (CH3)2CH

Therefore acidic property is stronger or more for CH3COOH.

Acidic strength is in the following order
acidic strength of HCOOH>CH3COOH>CH3CH2COOH>(CH3)2CHCOOH

ii. Effect of electron withdrawing substituents

Electron withdrawing substituents: Substituents like halogens tend to withdraw the electron charge. Halogens are electron attracting atoms(-I inductive effect). They withdraw the electrons from the carbon to which they are attached and this effect is transmitted throughout the chain. The increases positive charge on O atom in the O-H bond and dissociation of H+ ion or proton takes place more easily.

Hence chloroacetic acid is stronger acid than acetic acid.

c. compare relative acidic strength of carboxylic acids and alcohols.
d. compare relative acidic strength of carboxylic acids and phenols

e. What are the reactions that show acidic charater of carboxylic acids?
(i). Action with blue litmus
all carboxylic acids turn blue litmus red.

(ii). Reaction with metals: liberation of hydrogen
Carboxylic acids react with active metals such as Na, K, Ca, Mg, Zn, etc., to form their salts with the liberation of hydrogen.

(iii). Action with alkalies: formation of salts
Carboxylic acids neutralize alkalies forming salts and water.

(iv). Action with carbonates and bicarbonates: evolving carbon dioxide
Carboxylic acids decompose carbonates and bicarbonates evolving carbon dioxide with brisk effervescence.

2. Formation of acid chlorides

3. Formation of esters

When carboxylic acids are heated with alcohols in the presence of concentrated sulphuric acid, esters are formed.
a. Mechanism of esterification

4. Formation of amides

Carboxylic acids react with ammonia to form ammonium salts.
Acetic acid + Ammonia gives Ammonium acetate.
Ammonium acetate on heating gives acetamide plus water.

5.Formation of acid anhydrides

Carboxylic acids on heating in the presence of a strong dehydrating agent such as phosphorous pentoxide form acid anhydrides.

6. Decarboxylation of carboxylic acids
- electrolytic decarboxylation

Salts of carboxylic acids get decarboxylated – lose carbon dioxide in some reactions.

a. Sodium or potassium salts of carboxylic acids on heating with soda lime give alkanes.
b. Electrolysis of acqueous solutions of sodium or potassium salts of carboxylic acids gives alkanes due to decarboxylation.
c. When calcium salts of monocarboxylic acids (fatty acids) are heated aldehydes or ketones are formed.

7a. Partial reduction to alcohols

a. Partial reduction: Carboxylic acids on reduction with lithium aluminium hydride or with hydrogen in the presence of copper chromite are reduced to alcohols.

b. Complete reduction to alkanes: Carboxylic acids on reduction with HI and red P give alkanes.

8. Action of bromide on silver salt of the acid – Hunsdiecker reaction

The silver saltsof the carboxylic acid on treatment with Br2 in the presence of CCl4 give alkyl halides having one carbon atom less than the parent acid.

9. Halogenation

Carboxylic acids react with chlorine on bromine in the presence of a small amount of phosphorus to give halogenated compounds. The reaction is called Hell Volhard-Zelinksy reaction.
10 Ring substitution in aromatic acids – bromination, sulphonation, nitration

In substitution reactions with aromatic carboxylic acids, carboxyl group is an electron withdrawing group and therefore it favours meta position for the substituent.

Hence benzoinc acid + bromine gives 3-Bromobenzoic acid

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