Ncert Solutions Chemistry Class 12th

11.80 

The preparation of ether by acid dehydration of primary alcohol involves the nucleophilic addition of alcohol molecule to the protonated alcohol molecule as shown below: -

However, under these conditions secondary and tertiary alcohols forms alkenes rather than ethers. The reason for this being that due to stearic hindrance, nucleophilic attack by the alcohol molecule on the protonated alcohol molecule does not take place. Instead protonated 2⁰ and 3⁰ alcohols lose a molecule of water to form stable carbocations. The stable carbocations so formed prefers to lose a proton to form alkenes instead of forming ethers by undergoing nucleophilic attack by another alcohol molecule. This could be seen clearly from the following reactions : -

11.77 

During the reaction, an alkoxide ion is formed which is then added to an alkyl halide to form the ether via S_N2 mechanism.

In the primary step, the nucleophile is formed (O^- ) which will the approach to the alkyl halide and after the transition stage, the substitution takes place.

During the reaction, an alkoxide ion is formed which is then added to an alkyl halide to form the ether via S_N2 mechanism.

During the reaction, an alkoxide ion is formed which is then added to an alkyl halide to form the ether via S_N2 mechanism.

During the reaction, an alkoxide ion is formed which is then added to an alkyl halide to form the ether via S_N2 mechanism.

11.74 

1. Acidified KMnO₄ (potassium permanganate)

Potassium permanganate is a strong oxidant and is able to react with many functional groups. Here KMnO₄ will readily react with primary carbon (where hydrogen is attached) and transforms that to acid.

2. PCC (Pyridinium Chlorochromate)

This is actually a milder version of chromic acid. This works as a sort of elimination reaction. The formation of aldehyde occurs because of the action chromium (a good leaving group) which will be replaced when the C-H bond is broken.

3. Bromine water

Bromine water is actually an aqueous form of bromine. Here in aqueous solution, phenol ionizes to form phenoxide ion due to the presence of negative charge, the oxygen of phenoxide ion donates electron on benzene ring to a large extent, as a result, the ring gets highly activated and hence tri-substituion occurs.

4. Acidified KMnO₄ (potassium permanganate)

Potassium permanganate is a strong oxidant and oxidizes benzyl alcohol to benzoic acid. The compound is used due to its high oxidation power and the reaction proceeds due to the presence of hydrogen attached to the carbon group (Benzylic position)

5. Sulphuric acid or concentrated Phosphoric acid

Alcohols are amphoteric. So, the lone pair of oxygen atoms makes the -OH group weakly basic. Thus, in the presence of a strong acid, R—OH acts as a base and protonates into the very acidic alkyloxonium ion ⁺OH2.This basic characteristic of alcohol necessarily helps in conversion to propene when dehydrated with a strong acid.

6. L_iAlH₄ (lithium aluminum hydride) or NaBH₄

Both compounds are best-reducing agents containing 4 Hydrogen atoms each. In reaction with a ketone, the double bond is reduced and hydrogen atoms are substituted to form alcohol.

11.73

1. The conversion of Propene to propane-2-ol takes place according to markovnikoff rule. The positive part of H2O that is H+ goes to the carbon which has more hydrogen and the negative part that is OH-goes to carbon that has less number of carbons

2. NaOH act as a base in the conversion of benzyl chloride to benzyl On hydrolysis removal of NaCl takes place and OH is inserted in place of Cl.

3. Ethyl magnesium chloride (Grignard reagent) attacks on the carbon of the (The partial positive and negative charge is because of the electronegativity difference). After the formation of the addition product, hydrolysis takes place which further results in the formation of propane-1-ol and Mg (OH)Cl as the by product.

4. Attack of methyl magnesium bromide (Grignard reagent) on carbonyl carbon results in the formation of adduct, which has partial charges due to electronegativity differences. The adduct on hydrolysis yields 2-methylpropan2-ol.

11.71 

Kolbe's Reaction: it is a carboxylation chemical reaction that proceeds by heating sodium phenoxide (the sodium salt of phenol)with carbon dioxide under pressure (100 atm,125°C), then treating the product with a sulphuric acid. The final product is salicylic acid (the precursor to aspirin).

The reaction is given as:

The mechanism is given below:

Reimer-Tiemann reaction: The Reimer Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols, with the simplest example being the conversion

of phenol to salicylaldehyde.

When phenol is treated at 340K with chloroform and alkali, it forms salicylaldehyde.

Williamson ether synthesis: It is an organic reaction forming ether from an organohalide and a deprotonated alcohol (alkoxide). Typically it involves the reaction of an alkoxide ion with primary alkyl halide via S_N2 reaction.

During this reaction, the main bonds broken is the C-Br bond and the new bonds formed are “C- O” bond.

Unsymmetrical ether: It is an ether in the molecule of which the two ligands on the ether group are different.

Eg.