Chemistry note for CTEVT diploma level : Organic Chemistry(V)

Contents

Chapter: 8 – Carbonyl Compounds
- General methods of preparation :
- Chemical properties of aldehydes and ketones :
Chapter – 9 : Carboxylic acid
- General methods of preparation :
- Chemical properties of carboxylic acids :
Chaper- 10 : Ethers (R – O – R )

Chapter: 8 – Carbonyl Compounds

Compounds containing carbonyl group.

Nomenclature of Aldehydes and Ketones : eg.

Formula	Common name	IUPAC name
HCHO	Formaldehyde	Methanal
CH₃CHO	Acetaldehyde	Ethanal
CH₃COCH₃	Acetone(dimethyl ketone)	Propanone
CH₃CH₂COCH₃	Ethyl methyl ketone	Butanone

General methods of preparation :

1. By oxidation of alcohols : Alcohols on controlled oxidation give aldehydes and ketones. Primary alcohols give aldehydes while secondary alcohols give ketones. Acidified K₂Cr₂O₇ or KMnO₄ is used as an oxidizing agent. Eg.

2. By catalytic dehydrogenation of alcohols : When vapour of alcohol is passed over heated copper at 300⁰C, it gets dehydrogenated. Primary alcohols give aldehydes while secondary alcohols give ketones.

3. By ozonolysis of alkenes : Alkene reacts with ozone to give ozonide. On warming ozonide with Zn in water, it breaks down to give two molecules of carbonyl compounds (aldehyde or ketone).

Chemical properties of aldehydes and ketones :

1. Addition of sodium bisulphate : Aldehydes and ketones react with sodium bisulphate solution to form bisulphate addition products. Eg.

2. Addition followed by elimination of water ( Addition of ammonia derivatives) : Aldehydes and ketones react with number of ammonia derivatives such as hydroxylamine (NH₂OH), hydrazine (NH₂NH₂), phenylhydrazine (C₆H₅NHNH₂), etc. in weekly acidic medium to form compounds containing C = N group.

i. Reaction with hydroxylamine : Aldehydes and ketones react with hydroxylamine to form oximes. Eg.

ii. Reaction with hydrazine : Aldehydes and ketones react with hydrazine to form hydrazone.

iii. Reaction with phenylhydrazine : Aldehydes and ketones react with phenylhydrazine to form phenylhydrazone.

iv. Reaction with 2,4-dinitrophenylhydrazine(2,4-DNP test) : Aldehydes and ketones react with 2,4-dinitrophenylhydrazine(2,4-DNP) to form yellow, orange or red ppt. of 2,4- dinitrophenylhydrazone.

DNP and silver mirror test

3. Reaction with Tollen’s reagent (silver mirror test) :

Tollen’s reagent is an ammonical solution of silver nitrate. It is prepared by adding solution of NH₄OH to AgNO₃ solution till the precipitate once formed (i.e. Ag₂O) gets dissolved.

Aldehydes on heating with Tollen’s reagent reduces the reagent to metallic silver.

This test is known as silver mirror test. Eg.

4. Reaction of formaldehyde(methanal) with ammonia :

Formaldehyde reacts with ammonia to form hexamethylenetetramine which is commonly known as urotropine. It is used as medicine to treat urinary infections.

6HCHO + 4NH₃ → (CH₂)₆N₄ + 6H₂O

Structure of urotropine:

Chapter – 9 : Carboxylic acid

Examples of carboxylic acids :

Compound	Common name	IUPAC name
HCOOH	Formic acid	Methanoic acid
CH₃COOH	Acetic acid	Ethanoic acid
	Oxalic acid	Ethanedioic acid

General methods of preparation :

1. By the oxidation of primary alcohols and aldehydes :

2. From alkyl benzene :

Aromatic carboxylic acids are obtained by the oxidation of alkyl benzene such as toluene. Eg.

Acidity of carboxylic acids :

Due to presence of polar O – H group, carboxylic acids ionize to giveproton and hence behave as acids.

Effect of substituents on acidic strength of carboxylic acids :

1. Effect of electron donating substituent :

Q) Why is methanoic acid stronger than ethanoic acid?

Positive inductive effect (+I effect) of electron donating(releasing) groups like alkyl groups( CH₃ – , CH₃CH₂ -, etc.) increases the electron density on O – H bond(group). It makes the release of H⁺ ion difficult. Therefore, the acidic nature decreases with the increase in + I effect. Hence, methanoic acid (formic acid) is stronger acid than ethanoic acid (acetic acid).

2. Effect of electron withdrawing substituent :

Q) Why is chloroacetic acid stronger than acetic acid?

Negative inductive effect (- I effect) of electron withdrawing groups like halogens, – NO₂, -CN, etc. decreases the electron density on O – H bond (group). It makes the release of H⁺ ion easier. Therefore, the acidic nature increases with the increase in – I effect. Hence, chloroacetic acid is stronger acid than acetic acid.

Chemical properties of carboxylic acids :

1. Reaction with base and basic salt ( acidic nature) : Eg.

2. Reaction with ammonia(Formation of amide) : Carboxylic acids react with ammonia to form ammonium salt which on heating give amides. Eg.

3. Reaction with PCl₅, PCl₃ or SOCl₂ ( formation of acid chloride) : Carboxylic acids react with phosphorus pentachloride(PCl5), phosphorus trichloride (PCl₃) or thionyl chloride (SOCl₂) to form acid chloride. Eg.

4. Reaction with alcohols ( Formation of ester) :

Carboxylic acids react with alcohols in the presence of conc. H₂SO₄ or dry HCl to form esters. This reaction is called esterification reaction. Eg.

5. Formation of acid anhydrides (Dehydration) : Carboxylic acids on heating in the presence of dehydrating agent like P₂O₅ form acid anhydrides. Eg.

Chaper- 10 : Ethers (R – O – R )

Examples of ether:

Compounds	Common name	IUPAC name
CH₃ – O – CH₃	Dimethyl ether	methoxymethane
CH₃ – O – CH₂CH₃	Ethyl methyl ether	methoxyethane
CH₃CH₂ – O – CH₂CH₃	Diethyl ether	ethoxyethane

Laboratory preparation of diethyl ether :

When an excess of ethyl alcohol is heated with conc. H₂SO₄at 140⁰C, diethyl ether is obtained.

Procedure : A mixture of ethyl alcohol and conc. H₂SO₄ in the ratio of 1:1 by volume is taken in the distillation flask. The flask is then fitted with a dropping funnel containing alcohol. The flask is heated on a sand bath at 140⁰C. ethanol is added at nearly the same rate that of the distillation so that the ether formed is continuously received in the receiver kept cold in the ice cold water.

Purification : The distillate thus obtained contains ether, ethyl alcohol, water and sulphurous acid. The acid is removed by washing with KOH or NaOH solution. The solution is then stirred with anhydrous CaCl2 to remove alcohol. Finally it is redistilled to obtain almost pure ether.

Chemical properties of ether :

1. Reaction involving the ethereal oxygen (Addition reaction) : (Formation of peroxide) :

Q) It is dangerous to boil sample of ether stored for a long time, why?

→ Due to presence of lone pair of electrons on the ethereal oxygen, ether when comes in contact with atmospheric oxygen in the presence of sunlight, it reacts with oxygen to form ether peroxide. Ether peroxide is highly unstable and explodes violently on heating causing serious accidents. Therefore, it is dangerous to boil the sample of ether stored for a long time.

2. Reactions involving the cleavage of C – O bond ( Fission reaction) :

i. Reactions with halogen acids(HX) : When ethers are heated with conc. halogen acids( specially HI and HBr), the C – O bond is cleaved and gives alcohol and alkyl halide.