Alcohol Phenol And Ethers

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(i) In aryl alkyl ethers the +R effect of the alkoxy group leads to an increase in the electron density of the benzene ring as they push electrons into the ring making the benzene ring activated towards electrophilic substitution reactions. This could be understood more clearly from the following resonating structures : -

(ii) It could be clearly seen from the above resonating structures that the electron density increases more at the ortho and para positions as compared to the meta positions. Hence, we can conclude that the alkoxy group directs the incoming substituents to ortho and para positions in the benzene ring.

For example –

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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 : -

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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.

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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.

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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.