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Alcohols Phenols and Ethers
These are the organic compounds in which –OH group is directly attached to the carbon. These are hydroxyl derivatives of alkanes, monoalkyl derivatives of water. Their general formula is CnHn+1OH or CnH2n+2O.
Classification of Alcohols
Alcohols and phenols may be classified as mono-, di-, tri- or polyhydric compounds depending on whether they contain one, two, three or many hydroxyl groups respectively in their structures.
Compounds Containing Csp3 – OH Bond
In this class of alcohols, the –OH group is attached to an sp3 hybridised carbon atom of an alkyl group. They are further classified.
(a) Primary, secondary and tertiary alcohols:
In these types of alcohols, the –OH group is attached primary secondary and tertiary carbon atom, respectively as depicted below.
(b) Allylic alcohols: In these alcohols, the –OH group is attached to an sp3 hybridised carbon next to the carbon-carbon double bond, i.e. to an allylic carbon. For example
(c) Benzylic alcohols: In these alcohols, the –OH group is attached to an sp3 –hybridized carbon atom next to an aromatic ring.
Allylic and benzylic alcohols may be primary, secondary or tertiary.
Compounds Containing Csp2–OH Bond
These alcohols contain –OH group bonded to a carbon-carbon double i.e., to a vinylic carbon or to an aryl carbon and are known as vinylic alcohols. Eg: CH2=CH-OH
PREPARATION OF ALCOHOLS
- From Alkenes
(i) By Acid-Catalyzed Hydration
Alkenes react with water in the presence of an acid as a catalyst to form alcohols. In the case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markonikov’s rule.
(ii) By Hydroboration-Oxidation
Diborane (BH3)2 reacts with alkenes to give trialkyl boranes as addition product which upon subsequent oxidation with H2O2. Addition of borane at double bond takes place according to Anti-Markonikov Rule.
- From Carbonyl Compounds
(i) By Reduction of Carbonyl Compounds
(ii) By Reduction of Acid and its Derivatives
- From Grignard Reagents
a. Reaction with Carbonyl Compounds
b. Reaction with Acetaldehyde
c. Reaction with Ketone
PHYSICAL PROPERTIES OF ALCOHOLS
(a) The lower alcohols are colourless liquids with a distinct smell and burning taste. The higher members are colourless, odourless waxy solids.
(b) The lower alcohols are readily soluble in water and the solubility decreases with the increase in molecular weight.
The solubility of alcohols in water can be explained due to the formation of hydrogen bond between the highly polarized –OH groups present both in alcohols and water.
(c) Boiling points of alcohols increase with the increase in the number of C-atoms (increase in van der Waals’ forces). Amongst isomeric alcohols, the boiling points decrease with the increase of branching in carbon chain because of the decrease in van der Waals’ forces with a decrease in surface area.
CHEMICAL PROPERTIES OF ALCOHOLS
- Reaction with Active Metals-Acidic Character
Alcohols are weakly acidic in nature due to which when they react with group one alkali metals they liberate hydrogen gas and form alkoxides.
2R – O – H + 2Na → 2R – O– Na+ + H2↑
The acidic order of alcohols is MeOH > 1 o > 2 o > 3o. This acidic nature of alcohol is due to the presence of polar O-H bond.
- Esterification/Reaction with Carboxylic Acid
The reaction of an alcohol with a carboxylic acid in presence of the sulphuric acid gives an ester. In this reaction sulphuric acid react as a protonating agent as well as a dehydrating agent.
Note: The above reduction is laboratory method of ester preparation.
- Reaction with Acid Derivatives
When alcohols are treated with acid derivatives, hydrogen or hydroxyl group is substituted by an acyl group.
- Action of Halogen Acids
Alcohol reacts with HX to give RX. Reactivity order of ROH is 1°>2°>3°. Hence primary alcohols react in presence of a catalyst (If X is Cl Lucas reagent and if X is Br small amount of H2SO4), but secondary and tertiary alcohols can react in absence of a catalyst. However, when alcohol reacts with HI/Red P they reduced in hydrocarbon.
C2H5OH + HCl —anhy. ZnCl2→ C2H5Cl + H2O
The reactivity of halogens is in the order: HI > HBr > HCl
Reaction with H2SO4
(i) CH3-CH2-OH + H2SO4 (excess) -140oC→ CH3-CH2-O-CH2-CH3
(ii) CH3-CH2-OH + H2SO4 (excess) -160oC→ CH2= CH2
Action of Phosphorus Halides (PX5 and PX3)
Phosphorous halide reacts with alcohols to form corresponding haloalkanes.
For Example: C2H5OH + PCl → C2H5Cl + HCl + POCl3
Alcohols undergo dehydration (removal of a molecule of water) to form alkenes on treating with acid e.g., concentrated H2SO4 or H3PO4 or catalysts such as anhydrous zinc chloride or alumina.
Mechanism of Dehydration:
The relative ease of dehydration, i.e., 3º > 2º > 1º, of alcohols follows the order of stability of carbonium ions.
Oxidation of Alcohols
(a) Oxidation: Oxidation of alcohols involves the formation of carbon-oxygen double bond with cleavage of O–H and C–H bond.
These are also called dehydrogenation reactions since it involves loss of hydrogen from the alcohol molecule. The oxidation of alcohols can be carried out with a variety of reagents such as neutral, acidic or alkaline KMnO4, acidified K2Cr2O7 or dil. HNO3. The ease of oxidations and nature of the products, however, depends upon the type of alcohol used.
(i) Primary Alcohols are easily oxidized first to aldehydes and then to acids, both containing the same number of carbon atoms as the original alcohol.
The oxidation can, however, be stopped at the aldehyde stage if Cr(VI) reagent such as Collin’s reagent (CrO3.2C5H5N, chromium trioxide-pyridine complex), Corey’s reagent or pyridinium chlorochromate (PCC, CrO3.C5H5N.HCl or C5H5NH + CrO3Cl–) pyridinium dichromate [PDC, (C5H5NH)22+ Cr2O72–] in anhydrous medium (i.e., CH2Cl2) are used as the oxidizing agents.
(ii) Secondary Alcohols are easily oxidized to ketones with the same number of carbon atoms. However, ketones resist further oxidation but in some conditions, they are oxidized to carboxylic acids containing a lesser number of carbon atoms than the original alcohol.
This oxidation can be stopped at the ketone stage by using chromic anhydride (CrO3).
(iii) Tertiary Alcohols are resistance to oxidation in neutral or alkaline KMnO4 solution but are readily oxidized in acidic solution (K2Cr2O7/H2SO4 or KMnO4/H2SO4) to a mixture of a ketone, and an acid each containing lesser number of carbon atoms than the original alcohol. The oxidation presumably occurs via alkenes formed through dehydration of alcohols under acidic conditions.
When OH group is attached at benzene ring, the compound is known as phenol.
Nomenclature of Phenols
METHODS OF PREPARATION OF PHENOLS
2. From Benzenesulphonic Acid
3. From Diazonium Salts
When diazonium salts react with water vapour it gives phenol.
- From Cumene
When cumene (isopropylbenzene) is oxidized in the presence of air and acid, it gives phenol and acetone.
PHYSICAL PROPERTIES OF PHENOLS
(a) Pure phenols are generally colourless solids or liquids.
(b) The solubility of phenols in water can be explained due to the formation of hydrogen bond between the highly polarized –OH groups present both in alcohols and water. Solubility decreases with the increase in the size of the alkyl/aryl groups.
(c) Boiling points of alcohols increase with the increase in the number of C-atoms (increase in van der Waals’ forces). Amongst isomeric alcohols, the boiling points decrease with the increase of branching in carbon chain because of the decrease in van der Waals’ forces with the decrease in surface area.
Acidity of Phenols
(i) Phenol exists as a resonance hybrid of the following structures.
Due to resonance oxygen atom of the –OH group acquires & positive charge (see structures III to V) and hence attract electron pair of the O–H bond leading to the release of a hydrogen atom as a proton.
Since resonance is not possible in alcohols (due to the absence of conjugation of the lone pair of electron of oxygen with a double bond), the hydrogen atom is more firmly linked to the oxygen atom and hence alcohols are neutral in nature.
(ii) Once the phenoxide ion is formed, it stabilizes itself by resonance, actually, phenol acid ion is more stable than the parent phenol.
Comparison of acidity of phenols and carbonic acid
Relative acidity of the various common compounds.
RCOOH > H2CO3 > C6H5OH > HOH > ROH
Carboxylic acid Carbonic acid Phenol Water Alcohols.
CHEMICAL PROPERTIES OF PHENOLS
(a) When phenol reacts with dilute nitric acid at low temperature (290 K), give a mixture of ortho and para nitrophenols.
(b) When phenols react with concentrated nitric acid, it gives 2, 4, 6-trinitrophenol.
(a) When the reactions carried out in solvents of low polarity such as CHCl3 or CS2 and at low temperature, monobromophenols are formed.
(b) When phenol is treated with bromine water 2, 4, 6-tribromophenol is formed as a white precipitate.
Phenoxide ion produced by treating phenol with NaOH is more reactive than phenol towards electrophilic substitution. Hence, it undergoes electrophilic substitution with weak electrophile CO2.
On treating phenol with chloroform in the presence of sodium hydroxide, a–CHO group is introduced at the ortho position of the benzene ring. This reaction is known as Reimer – Tiemann reaction. The intermediate substituted benzal chloride is hydrolyzed in the presence of alkali to produce salicylaldehyde.
The mechanism of the Reimer – Tiemann reaction is believed to involve the formation of dichlorocarbene.
NaOH + CHCl3 → :CCl2 + NaCl+ H2O
Phenols with blocked p-positions give cyclohexadienones containing the dichloromethyl group.
In the Reimer-Tiemann reaction, the ortho-isomer predominates, but if one of the o-position is occupied the aldehyde group tend to go to the p-positions; e.g. guaiacol forms vanillin.
Reaction of Phenol with Zinc Dust
When phenol is heated with zinc dust, it gives benzene.
Oxidation of phenol with chromic acid produces a conjugate diketone known as benzoquinone. In the presence of an oxidizing agent, phenols are slowly oxidized to dark coloured moisture containing benzoquinone.
DISTINCTION BETWEEN ALCOHOL AND PHENOLS
(a) Phenols turns blue litmus red but alcohols do not.
(b) Phenols neutralize base, while alcohols do not.
(c) Phenols give violet colour with FeCl3, while alcohols do not.
The substitution of a hydrogen atom in a hydrocarbon (aliphatic/aromatic) by an alkoxy(OR)/aryloxy (OAr) group yields ethers.
METHODS OF PREPARATION OF ETHERS
(a) Williamson’ Synthesis: Heating of alkyl halide with sodium or potassium alkoxide gives ether. This is a good method for preparation of simple as well as mixed either.
R-X + Na-O-R’ → R-O-R’ + NaX
This method is not applicable to tert-alkyl halides because the alkoxide ions being both powerful nucleophiles and bases could bring dehydrogenation of the tertiary alkyl halides to form alkenes.
The reactivity of primary (1º) alkyl halide is in the order CH3– > CH3 – CH2– > CH3 – CH2 – CH2– and the tendency of the alkyl halide to undergo elimination is 3º > 2º > 1º. Hence for better yield, the alkyl halide should be primary and the alkoxide should be secondary or tertiary.
(b) Heating excess of alcohols with conc. H2SO4 e.g.,
2C2H5OH –conc. H2SO4/1400C--> C2H5-O-C2H5 + H2O
Recall that 2° and 3° alcohols under the above conditions give alkenes as the main product. Moreover, this method is limited only for the preparation of simple ethers.
PHYSICAL PROPERTIES OF ETHERS
(d) The B.P of ether is much lower than isomeric alcohols due to the absence of intermolecular H-bonding.
(e) Ethers are slightly polar with some net dipole. (e.g. 1.18 D for diethyl ether.)This is due to a bend structure with a bond angle of 1100 which causes because of repulsion between bulky alkyl groups.
CHEMICAL PROPERTIES OF ETHERS
Ethers are less reactive of the functional group. They do not react with active metals like Na, strong base like NaOH, reducing or oxidizing agents.
(1) Reactions involving cleavage of C — 0 bond :
R – O – R + HX → ROH + RX
For a given ether, the reactivity of hydrogen halides follows the order: HCl < HBr < HI (strongest acid). In case of alkyl aryl ethers, reaction with HX results in the cleavage of R — O bond due to more stable Ar — O bond as it has a double bond character.
In case of mixed ethers with different R groups, the product depends on the nature of R groups:
- When 1° or 2° R groups are present, the smaller R group forms alkyl halide.
CH3OCH2CH3 + HI —> CH3I + CH3CH2OH
- When one of the R group is 3. Ft group, then tertiary alkyl halide is formed because 3° carbocation is more stable.
(CH3)3—C— OCH3 + HI —> (CH3)3—C— I + CH3OH
(2) Ring substitution in aromatic ethers :
Alkoxy group is ortho and para directing and it directs the incoming groups to ortho and para
position. It activates the aromatic ring towards electrophilic substitution reaction.
(ii) Friedel craft reaction:
This article has tried to highlight all the important parts of Alcohols Phenols and Ethers in the form of short notes for class 12 students in order to understand the basic concepts of the chapter. The notes on Alcohols Phenols and Ethers have not only been prepared for class 12 but also for the different competitive exams such as iit jee, neet, etc.