Class 12 Chemistry
Chapter 11 – Alcohols, Phenols and Ethers
Chapter Notes
· Alcohols are formed when a hydrogen atom in a hydrocarbon is replaced by –OH group.
· Phenols are formed when a hydrogen atom attached directly to an aromatic compound is replaced by –OH group.
· An alcohol contains one or more hydroxyl (OH) group(s) directly attached to carbon atom(s), of an aliphatic system (CH3OH).
· A phenol contains –OH group(s) directly attached to carbon atom(s) of an aromatic system (C6H5OH).
· Phenol is also known as carbolic acid.
· The substitution of a hydrogen atom in a hydrocarbon by an alkoxy or aryloxy group (R–O/Ar–O) yields another class of compounds known as ‘ethers’, for example, CH3OCH3 (dimethyl ether). R and Ar represent alkyl and aryl groups, respectively.
· Ethers can also be visualised as compounds formed by substituting the hydrogen atom of hydroxyl group of an alcohol or phenol by an alkyl or aryl group.
Classification (Alcohols and Phenols)
Mono, Di, Tri or Polyhydric Compounds
(i) Monohydric alcohol compounds containing Csp3 - OH bond
(a) Alkylic alcohols
(b) Allylic alcohols
(c) Benzylic alcohols
(ii) Monohydric alcohol compounds containing Csp2 - OH bond
(a) Vinylic alcohol
(b) Phenols
Classification (Ethers)
(i) Simple or symmetrical
When the alkyl or aryl groups attached to the oxygen atom are the same.
E.g., Diethyl ether, C2H5OC2H5
(ii) Mixed or unsymmetrical
When the two groups are different. E.g., C2H5OCH3 and C2H5OC6H5
Nomenclature
(a) Alcohols
Common name of an alcohol includes the word “alcohol”. E.g., methyl alcohol, CH3OH.
According to IUPAC system, the name of an alcohol is derived by substituting ‘e’ of alkane with the suffix ‘ol’.
The number of –OH groups is indicated by adding the multiplicative prefix, di, tri, etc., before ‘ol’. The positions of –OH groups are indicated by appropriate locants e.g., HO–CH2–CH2–OH is named as ethane–1, 2-diol.
Cyclic alcohols are named using the prefix cyclo and considering the —OH group attached to C–1.
(b) Phenols
The simplest hydroxy derivative of benzene is phenol. It is its common name and also an accepted IUPAC name.
The terms ortho (1,2- disubstituted), meta (1,3-disubstituted) and para (1,4-disubstituted) are often used in the common names.
Dihydroxy derivatives of benzene are known as 1, 2-, 1, 3- and 1, 4-benzenediol.
(c) Ethers
Common names of ethers are derived from the names of alkyl/aryl groups written as separate words in alphabetical order and adding the word ‘ether’ at the end. For example, CH3OC2H5 is ethylmethyl ether.
The larger (R) group is chosen as the parent hydrocarbon.
E.g.,
2-Ethoxypropane 1-Ethoxy-2-nitrocyclohexane
Structures of Functional Groups
In alcohols, the oxygen of the –OH group is attached to carbon by a sigma (σ) bond formed by the overlap of a sp3 hybridised orbital of carbon with a sp3 hybridised orbital of oxygen.
Due to the repulsion between the unshared electron pairs of oxygen, the bond angle C – O – H in alcohols is slightly less than the tetrahedral angle (109°-28’).
In phenols, the –OH group is attached to sp2 hybridised carbon of an aromatic ring.
The carbon– oxygen bond length (136 pm) in phenol is slightly less than that in methanol. This is due to
(i) partial double bond character on account of the conjugation of unshared electron pair of oxygen with the aromatic ring.
(ii) sp2 hybridised state of carbon to which oxygen is attached.
In ethers, the four electron pairs, i.e., the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (–R) groups. The C–O bond length (141 pm) is almost the same as in alcohols.
Preparation of Alcohols
1. From alkenes
(i) By acid catalysed hydration
Mechanism (3 steps)
Step 1: Protonation of alkene to form carbocation by electrophilic attack of H3O+.
Step 2: Nucleophilic attack of water on carbocation.
Step 3: Deprotonation to form an alcohol.
(ii) By hydroboration–oxidation [ with Diborane (BH3)2 ]
It is believed that in this reaction, the alcohol is formed by the addition of water to the alkene as per anti-Markovnikov’s rule.
In this reaction, alcohol is obtained in excellent yield.
2. From carbonyl compounds
(i) By reduction of aldehydes and ketones
(ii) By reduction of carboxylic acids and esters
3. From Grignard reagents
Step-wise reaction:
Overall reactions for different aldehydes and ketones
This reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones.
Preparation of Phenols
1. From haloarenes
2. From benzenesulphonic acid
3. From diazonium salts
4. From cumene
Cumene Cumene Phenol Acetone
(isopropylbenzene) hydroperoxide .
Physical Properties
Boiling Points
The boiling points of alcohols and phenols increase with increase in the number of carbon atoms (due to increase in van der Waals forces).
In alcohols, the boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area).
The –OH group in alcohols and phenols is involved in intermolecular hydrogen bonding as shown below:
Due to intermolecular hydrogen bonding, boiling points of alcohols and phenols are higher in comparison to other classes of compounds, namely hydrocarbons, ethers, haloalkanes and haloarenes of comparable molecular masses.
For example, ethanol and propane have comparable molecular masses but ethanol has much higher boiling point than that of propane.
Solubility
Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules.
The solubility decreases with increase in size of alkyl/aryl (hydrophobic) groups.
Chemical Reactions involving Alcohols and Phenols
Alcohols react both as nucleophiles and electrophiles.
Alcohols as nucleophiles
The bond between O–H is broken when alcohols react as nucleophiles.
Protonated alcohols as electrophiles
The bond between C–O is broken when they react as electrophiles. Protonated alcohols react in this manner.
(a) Reactions involving cleavage of O–H bond
1. Acidity of alcohols and phenols
(i) Reaction with metals
Alcohols and phenols react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen.
In addition to this, phenols react with aqueous sodium hydroxide to form sodium phenoxides.
The above reactions show that alcohols and phenols are acidic in nature. In fact, alcohols and phenols are Brönsted acids i.e., they can donate a proton to a stronger base (B:).
(ii) Acidity of alcohols
The acidic character of alcohols is due to the polar nature of
O–H bond. An electron-releasing group
(–CH3, –C2H5) increases
electron density on oxygen tending to decrease the polarity of O–H bond.
This decreases the acid strength. For this reason, the acid strength of
alcohols decreases in the following order:
Alcohols are, however, weaker acids than water. This can be illustrated by the reaction of water with an alkoxide.
This reaction shows that water is a better proton donor (i.e., stronger acid) than alcohol.
Also, in the above reaction, we note that an alkoxide ion is a better proton acceptor than hydroxide ion, which suggests that alkoxides are stronger bases (e.g., sodium ethoxide is a stronger base than sodium hydroxide).
Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors.
(iii) Acidity of phenols
The hydroxyl group, in phenol is directly attached to the sp2 hybridised carbon of benzene ring which acts as an electron withdrawing group. Due to this, the charge distribution in phenol molecule, as depicted in its resonance structures, causes the oxygen of –OH group to be positive.
The reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water.
A compound in which hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group is attached to an alkyl group.
The ionisation of an alcohol and a phenol takes place as follows:
Higher the s-character, higher the electronegativity.
Due to the higher electronegativity of sp2 hybridised carbon of phenol to which –OH is attached, electron density decreases on oxygen. This increases the polarity of O–H bond and results in an increase in ionisation of phenols than that of alcohols.
Stabilities of alkoxide and phenoxide ions:
In alkoxide ion, the negative charge is localised on oxygen while in phenoxide ion, the charge is delocalised.
The delocalisation of negative charge (structures I-V shown above) makes phenoxide ion more stable and favours the ionisation of phenol.
Although there is also charge delocalisation in phenol, its resonance structures have charge separation due to which the phenol molecule is less stable than phenoxide ion.
In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion.
On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol.
2. Esterification
Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters.
3. Acetylation:
The introduction of acetyl (CH3CO) group in alcohols or phenols is known as acetylation.
Acetylation of salicylic acid produces aspirin.
(b) Reactions involving cleavage of carbon – oxygen (C–O) bond in alcohols
Cleavage of C–O bond takes place primarily in alcohols. Phenols show this type of reaction only with zinc.
1. Reaction with hydrogen halides
Alcohols react with hydrogen halides to form alkyl halides.
ROH + HX à R–X + H2O
The difference in reactivity of primary, secondary and tertiary alcohols with HCl distinguishes them from one another. This test is known as Lucas test.
Lucas test:
In this method, the alcohol is treated with Lucas reagent (a mixture of conc. HCl and anhydrous ZnCl2). The alcohol gets converted into alkyl halides.
As alcohols are soluble in Lucas reagent while their halides are insoluble, the formation of alkyl halide is indicated by the appearance of turbidity in the reaction mixture.
As the reactivity of alcohols with halogen acids is in the order tertiary > secondary > primary, the time required for the appearance of turbidity will be different for primary, secondary and tertiary alcohols which helps to distinguish them from one another.
In the case of tertiary alcohols, turbidity is produced immediately at room temperature. Secondary alcohols give turbidity in few minutes while primary alcohols do not produce appreciable turbidity at room temperature, but give turbidity only on heating.
2. Reaction with phosphorus trihalides
Alcohols are converted to alkyl halides by reaction with phosphorus trihalides.
3. Dehydration
Removal of a molecule of water is known as dehydration.
Alcohols undergo dehydration to form alkenes on treating with a protic acid e.g., concentrated H2SO4 or H3PO4, or catalysts such as anhydrous zinc chloride or alumina.
Ethanol undergoes dehydration by heating it with concentrated H2SO4 at 443 K.
Secondary and tertiary alcohols are dehydrated under milder conditions. For example
The relative ease of dehydration of alcohols follows the following order:
Tertiary > Secondary > Primary
Mechanism
Step 1: Formation of protonated alcohol.
Step 2: Formation of carbocation: It is the slowest step and hence, the rate determining step of the reaction.
Step 3: Formation of ethene by elimination of a proton.
The acid used in step 1 is released in step 3.
To drive the equilibrium to the right, ethene is removed as it is formed.
4. Oxidation
Oxidation of alcohols involves the formation of a carbon-oxygen double bond with cleavage of an O-H and C-H bonds.
Such a cleavage and formation of bonds occur in oxidation reactions. These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule. Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid.
Strong oxidising agents such as acidified potassium permanganate are used for getting carboxylic acids from alcohols directly. CrO3 in anhydrous medium is used as the oxidising agent for the isolation of aldehydes.
A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl.
Secondary alcohols are oxidised to ketones by chromic anhydride (CrO3).
Tertiary alcohols do not undergo oxidation reaction. Under strong reaction conditions such as strong oxidising agents (KMnO4) and elevated temperatures, cleavage of various C-C bonds takes place and a mixture of carboxylic acids containing lesser number of carbon atoms is formed.
When the vapours of a primary or a secondary alcohol are passed over heated copper at 573 K, dehydrogenation takes place and an aldehyde or a ketone is formed while tertiary alcohols undergo dehydration.
Biological oxidation of methanol and ethanol in the body produces the corresponding aldehyde followed by the acid. At times the alcoholics, by mistake, drink ethanol, mixed with methanol also called denatured alcohol. In the body, methanol is oxidized first to methanal and then to methanoic acid, which may cause blindness and death. A methanol poisoned patient is treated by giving intravenous infusions of diluted ethanol. The enzyme responsible for oxidation of aldehyde (HCHO) to acid is swamped allowing time for kidneys to excrete methanol.
(c) Reactions of phenols
Following reactions are shown by phenols only.
1. Electrophilic aromatic substitution
In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions. The –OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to the resonance effect caused by –OH group.
Common electrophilic aromatic substitution reactions taking place in phenol are as follows:
(i) Nitration
With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols.
The ortho and para isomers can be separated by steam distillation.
o-Nitrophenol is steam volatile due to intramolecular hydrogen bonding while p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules.
With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol. The product is commonly known as picric acid. The yield of the reaction product is poor.
Nowadays picric acid is prepared by treating phenol first with concentrated sulphuric acid which converts it to phenol-2,4-disulphonic acid, and then with concentrated nitric acid to get 2,4,6-trinitrophenol.
(ii) Halogenation
On treating phenol with bromine, different reaction products are formed under different experimental conditions.
(a) When the reaction is carried out in solvents of low polarity such as CHCl3 or CS2 and at low temperature, monobromophenols are formed.
The usual halogenation of benzene takes place in the presence of a Lewis acid, such as FeBr3, which polarises the halogen molecule. In case of phenol, the polarisation of bromine molecule takes place even in the absence of Lewis acid. It is due to the highly activating effect of –OH group attached to the benzene ring.
(b) When phenol is treated with bromine water, 2,4,6-tribromophenol is formed as white precipitate.
2. Kolbe’s reaction
Phenoxide ion generated by treating phenol with sodium hydroxide is even more reactive than phenol towards electrophilic aromatic substitution. Hence, it undergoes electrophilic substitution with carbon dioxide, a weak electrophile. Ortho hydroxybenzoic acid is formed as the main reaction product.
3. Reimer-Tiemann reaction
On treating phenol with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of benzene ring. This reaction is known as Reimer - Tiemann reaction.
The intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde.
4. Reaction of phenol with zinc dust
Phenol is converted to benzene on heating with zinc dust. (Very important conversion reaction).
5. Oxidation
Oxidation of phenol with chromic acid produces a conjugated diketone known as benzoquinone. In the presence of air, phenols are slowly oxidised to dark coloured mixtures containing quinones.
Some Commercially Important Alcohols
1. Methanol
Methanol, CH3OH, also known as ‘wood spirit’, was produced by destructive distillation of wood. Today, most of the methanol is produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO – Cr2O3 catalyst.
Methanol is a colourless liquid and boils at 337 K. It is highly poisonous in nature. Ingestion of even small quantities of methanol can cause blindness and large quantities causes even death. Methanol is used as a solvent in paints, varnishes and chiefly for making formaldehyde.
2. Ethanol
Ethanol, C2H5OH, is obtained commercially by fermentation, the oldest method is from sugars. The sugar in molasses, sugarcane or fruits such as grapes is converted to glucose and fructose, (both of which have the formula C6H12O6), in the presence of an enzyme, invertase. Glucose and fructose undergo fermentation in the presence of another enzyme, zymase, which is found in yeast.
In wine making, grapes are the source of sugars and yeast. As grapes ripen, the quantity of sugar increases and yeast grows on the outer skin. When grapes are crushed, sugar and the enzyme come in contact and fermentation starts. Fermentation takes place in anaerobic conditions i.e. in absence of air. Carbon dioxide is released during fermentation.
The action of zymase is inhibited once the percentage of alcohol formed exceeds 14 percent. If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks.
Ethanol is a colourless liquid with boiling point 351 K. It is used as a solvent in paint industry and in the preparation of a number of carbon compounds. The commercial alcohol is made unfit for drinking by mixing in it some copper sulphate (to give it a colour) and pyridine (a foul-smelling liquid). It is known as denaturation of alcohol.
Nowadays, large quantities of ethanol are obtained by hydration of ethane.
Ethers
Preparation of Ethers
1. By dehydration of alcohols
Alcohols undergo dehydration in the presence of protic acids (H2SO4, H3PO4). The formation of the reaction product, alkene or ether depends on the reaction conditions. For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K.
At 413 K, ethoxyethane is the main product.
The formation of ether is a nucleophilic bimolecular reaction (SN2) involving the attack of alcohol molecule on a protonated alcohol, as indicated below:
Acidic dehydration of alcohols, to give an alkene is also associated with substitution reaction to give an ether.
The method is suitable for the preparation of ethers having primary alkyl groups only. The alkyl group should be unhindered and the temperature be kept low. Otherwise the reaction favours the formation of alkene. The reaction follows SN1 pathway when the alcohol is secondary or tertiary about which you will learn in higher classes.
However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed.
Bimolecular dehydration is not appropriate for the preparation of ethyl methyl ether because to produce ethyl methyl ether, both ethyl alcohol and methyl alcohol will be required. These two will also produce dimethyl ether and diethyl ether in addition of ethyl methyl ether.
2. Williamson synthesis
It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers.
In this method, an alkyl halide is allowed to react with sodium alkoxide.
Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method.
The reaction involves SN2 attack of an alkoxide ion on primary alkyl halide.
Better results are obtained if the alkyl halide is primary.
In case of secondary and tertiary alkyl halides, elimination competes over substitution.
If a tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed. For example, the reaction of CH3ONa with (CH3)3C–Br gives exclusively 2-methylpropene.
It is because alkoxides are not only nucleophiles but strong bases as well. They react with alkyl halides leading to elimination reactions.
Phenols are also converted to ethers by this method. In this, phenol is used as the phenoxide moiety.
Physical Properties of Ethers
The C-O bonds in ethers are polar and thus, ethers have a net dipole moment. The weak polarity of ethers does not appreciably affect their boiling points which are comparable to those of the alkanes of comparable molecular masses but are much lower than the boiling points of alcohols as shown in the following cases:
The large difference in boiling points of alcohols and ethers is due to the presence of hydrogen bonding in alcohols.
The miscibility of ethers with water resembles those of alcohols of the same molecular mass. Both ethoxyethane and butan-1-ol are miscible to almost the same extent i.e., 7.5 and 9 g per 100 mL water, respectively while pentane is essentially immiscible with water. This is due to the fact that just like alcohols, oxygen of ether can also form hydrogen bonds with water molecule as shown:
Chemical Reactions involving Ethers
1. Cleavage of C–O bond in ethers
Ethers are the least reactive of the functional groups. The cleavage of C-O bond in ethers takes place under drastic conditions with excess of hydrogen halides. The reaction of dialkyl ether gives two alkyl halide molecules.
Alkyl aryl ethers are cleaved at the alkyl-oxygen bond due to the more stable aryl-oxygen bond. The reaction yields phenol and alkyl halide.
Ethers with two different alkyl groups are also cleaved in the same manner.
The order of reactivity of hydrogen halides is as follows:
HI > HBr > HCl.
The cleavage of ethers takes place with concentrated HI or HBr at high temperature.
Mechanism:
The reaction of an ether with concentrated HI starts with protonation of ether molecule.
Step 1:
The reaction takes place with HBr or HI because these reagents are sufficiently acidic.
Step 2:
Iodide is a good nucleophile. It attacks the least substituted carbon of the oxonium ion formed in step 1 and displaces an alcohol molecule by SN2 mechanism.
Thus, in the cleavage of mixed ethers with two different alkyl groups, the alcohol and alkyl iodide formed, depend on the nature of alkyl groups. When primary or secondary alkyl groups are present, it is the lower alkyl group that forms alkyl iodide (SN2 reaction).
When HI is in excess and the reaction is carried out at high temperature, ethanol reacts with another molecule of HI and is converted to ethyl iodide.
Step 3:
However, when one of the alkyl group is a tertiary group, the halide formed is a tertiary halide.
It is because in step 2 of the reaction, the departure of leaving group (HO–CH3) creates a more stable carbocation [(CH3)3C+], and the reaction follows SN1 mechanism.
In case of anisole, methylphenyl oxonium
ion, is formed by protonation of ether. The bond between
O–CH3 is weaker than the bond between O–C6H5
because the carbon of phenyl group is sp2 hybridised and there is a
partial double bond character.
Therefore, the attack by I– ion breaks O–CH3 bond to form CH3I. Phenols do not react further to give halides because the sp2 hybridised carbon of phenol cannot undergo nucleophilic substitution reaction needed for conversion to the halide.
2. Electrophilic substitution in ethers
The alkoxy group (-OR) is ortho, para directing and activates the aromatic ring towards electrophilic substitution in the same way as in phenol.
(i) Halogenation
Phenylalkyl ethers undergo usual halogenation in the benzene ring, e.g., anisole undergoes bromination with bromine in ethanoic acid even in the absence of iron (III) bromide catalyst. It is due to the activation of benzene ring by the methoxy group. Para isomer is obtained in 90% yield.
(ii) Friedel-Crafts reaction
Anisole undergoes Friedel-Crafts reaction, i.e., the alkyl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous aluminium chloride (a Lewis acid) as catalyst.
(iii) Nitration
Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole.
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