Alcohol Reactions in Organic Chemistry and SynthesisThe alcoholic functional group is the –OH or hydroxyl group attached to a carbon atom. Alcohol contains one or more hydroxyl groups directly attached to the carbon atom of an aliphatic chain. When the hydroxyl is attached to a carbon atom in an aromatic system, it is known as phenol.
Alcohols are acidic in nature and can donate their hydrogen from the hydroxyl group. The O-H bond is polar, and thus the hydrogen can be released giving oxygen a negative charge. The electron releasing group attached to oxygen increases charge density and makes it less stable. Since the R-group is an electron releasing group the order of acidity is as followsPrimary alcohol > Secondary alcohol> Tertiary alcoholHowever, alcohols are weaker acids compared to water as water is a better proton donor than acid, and alkoxide ion is a better proton acceptor due to the lone pair of electrons on oxygen. Phenols are comparatively more acidic than alcohols because the negative charge on oxygen gets delocalized due to resonance.
Alcohol undergoes dehydration on heating in an acidic medium to give alkene and water. The order of ease of alcohol dehydration reaction mechanism is
Tertiary alcohol> secondary alcohol> primary alcohol
The lone pair of oxygen first attracts the protons present in acidic medium, resulting in the formation of protonated alcohol. The oxygen attached to the two hydrogen atom leaves to form water and leaves a carbocation. Elimination of proton occurs to give alkene.
Primary alcohols are oxidized with mild reducing agents to give aldehydes. If oxidized further it gives carboxylic acid. Secondary alcohols are oxidized to give ketones. Pyridinium chlorochromate (PCC) and chromium trioxide are used to oxidize alcohols. Tertiary alcohols do not get oxidized. If treated with strong oxidizing agents at high temperatures, cleavage of C-C bonds occur to give carboxylic acids. These carboxylic acids have with lesser number of carbon atoms than the alcohol substrate.The following reactions occur when alcohol vapors are passed over heated copper.
Both alcohols and phenols give esters when they react with carboxylic acids, acid chlorides, and acid anhydrides. The reaction with carboxylic acid and acid anhydrides is one in acidic medium, but the reaction with acid chlorides is done in an alkaline medium to neutralize the HCl formed as the product to shift the reaction towards right and increase the yield.
Alcohols produce ethers in an acidic medium in high temperatures. It follows the SN2 mechanism.
Reactions involving phenols
The hydroxyl group activates the ring towards electrophilic aromatic substitution. The lone pair of electrons on oxygen get delocalized due to resonance and increase electron density at ortho and para positions. So the incoming electrophilic group attaches at ortho or para.
When phenol reacts with dilute nitric acid then ortho and para derivatives of nitrophenol are formed. Para nitrophenol is less volatile due to intramolecular hydrogen bonding, while the ortho derivative has mostly intramolecular hydrogen bonding.
Phenol on reacting with concentrated nitric acid gives 2,4,6-trinitrophenol.
Insolvents of low polarity like CHCl3 or CS2
Phenol reacts with bromine at low temperatures to form ortho or para monobromophenol.
In aqueous medium
Phenol reacts with bromine to form a white precipitate of 2,4,6-tribromophenol.
Phenol is treated with chloroform and sodium chloride to give a substituted Benzal chloride intermediate. This is hydrolyzed to give salicylaldehyde.
With reducing zinc dust
Phenol is reduced to benzene on heating with zinc dust.
Phenol is treated with sodium hydroxide to convert into sodium phenoxide, which activates the ring even more strongly to electrophilic aromatic substitution. Thus it can undergo substitution even with carbon dioxide which is a weak electrophile.
Oxidation to quinones
Phenol reacts with chromic acid which is a powerful oxidizing agent to produce benzoquinone, which is a conjugated diketone.
When exposed to air phenol gets slowly oxidized to a dark quinone containing a mixture.
Alcohol formation reactions
1)This is acid-catalyzed hydration done in the presence of acid as a catalyst. Water is added to alkene; addition reaction takes place which follows Markovnikov’s rule.
>C=C< +H2O >CH-COH<
2)An alkene is added to diborane to give trialkyl boranes which are oxidized by hydrogen peroxide to give alcohol. This follows anti-Markovnikov’s rule.
3) Aldehydes are reduced by reducing agents like LiAlH4 and NaBH4 to give primary alcohols. Primary alcohols can also be synthesized from aldehydes by adding hydrogen in the presence of catalysts like finely divided palladium, nickel, or platinum.
RCHO + H2 RCH2OH
4) Ketones are reduced by reducing agents like LiAlH4 and NaBH4 to give secondary alcohols. Secondary alcohols can also be synthesized from ketones by adding hydrogen in the presence of catalysts like finely divided metals like palladium, nickel, or platinum.
RCOR’ + NaBH4 RCHOHR’ + other products
5) Strong reducing agents like LiAlH4 can be used to reduce carboxylic acids into alcohols. We can get high yields from this.
6) Esters undergo catalytic hydrogenation to give alcohol. The carboxylic acid may be converted to ester first, and then the hydrogenation can be done.
7) Grignard reagent and water can be used to create alcohol. Methanol is used to produce primary alcohol while other aldehydes are used to create secondary alcohols. Ketones produce tertiary alcohols.
Reactions used to prepare phenols
1) NaOH is added to chlorobenzene and heated to a very high temperature under high pressure to phenoxide ion, which is acidified to phenol.
2) Benzene is converted to sulphonic benzene acid using oleum, which is then converted to phenol by heating with NaOH and then acidifying.
3) Aniline is treated with NaNO2 and HCl at 0 to 5 degrees celsius converting it to benzene diazonium chloride, which is then slightly warmed with water to give phenol.
4) Cumene is oxidized to cumene peroxide in the presence of air, which is then acidified to convert to phenol.