Alcohols, Phenols and EthersClass 12 Chemistry Notes

Introduction to Alcohols, Phenols and Ethers

In organic chemistry, when one or more hydrogen atoms in a hydrocarbon are replaced by another atom or group, a new compound with entirely different properties is formed. Alcohols, Phenols, and Ethers are important classes of such compounds.

• Alcohols are formed when a hydrogen atom in an aliphatic hydrocarbon (like methane or ethane) is replaced by a hydroxyl (–OH) group. A simple example is methanol, $CH_3OH$.
• Phenols are formed when a hydrogen atom in an aromatic hydrocarbon (like benzene) is replaced by a hydroxyl (–OH) group. The simplest example is phenol, $C_6H_5OH$.
• Ethers are formed when a hydrogen atom in a hydrocarbon is replaced by an alkoxy (R-O) or aryloxy (Ar-O) group. A common example is dimethyl ether, $CH_3OCH_3$.

These compounds are essential in our daily lives. Ethanol is the spirit used for polishing furniture, detergents are derived from alcohols, antiseptics from phenols, and fragrances from ethers. Even the paper we write on and the cotton we wear are made of compounds containing –OH groups.

Classification

Classifying these compounds helps in studying them systematically.

Alcohols

Alcohols are classified in two main ways:

1. Based on the number of hydroxyl (–OH) groups:

• Monohydric alcohols contain one –OH group (e.g., ethanol, $C_2H_5OH$).
• Dihydric alcohols contain two –OH groups (e.g., ethylene glycol, $HO-CH_2-CH_2-OH$).
• Trihydric alcohols contain three –OH groups (e.g., glycerol, $HO-CH_2-CH(OH)-CH_2-OH$).
• Polyhydric alcohols contain many –OH groups.

2. Based on the hybridization of the carbon atom attached to the –OH group:

• Compounds containing a $C_{sp^3}-OH$ bond: The –OH group is attached to a carbon atom with single bonds (an $sp^3$ hybridized carbon).

  • Primary ($1^\circ$), Secondary ($2^\circ$), and Tertiary ($3^\circ$) Alcohols: This classification depends on whether the –OH group is attached to a primary, secondary, or tertiary carbon atom.
    • Primary (1°): The carbon with the –OH group is attached to only one other carbon atom (e.g., Ethanol).
    • Secondary (2°): The carbon with the –OH group is attached to two other carbon atoms (e.g., Propan-2-ol).
    • Tertiary (3°): The carbon with the –OH group is attached to three other carbon atoms (e.g., 2-Methylpropan-2-ol).
  • Allylic Alcohols: The –OH group is attached to an $sp^3$ carbon atom that is next to a carbon-carbon double bond ($C=C$). These can also be primary, secondary, or tertiary.
  • Benzylic Alcohols: The –OH group is attached to an $sp^3$ carbon atom that is next to an aromatic ring. These can also be primary, secondary, or tertiary.

• Compounds containing a $C_{sp^2}-OH$ bond: The –OH group is bonded directly to a carbon atom of a double bond.

  • Vinylic Alcohols: The –OH group is attached to a carbon atom of a $C=C$ double bond (e.g., $CH_2=CH-OH$).
  • Phenols are also in this category, as the -OH group is attached to an $sp^2$ carbon of the benzene ring.

Phenols

Phenols are classified based on the number of –OH groups attached to the aromatic ring.

• Monohydric phenols have one –OH group (e.g., Phenol).
• Dihydric phenols have two –OH groups (e.g., Catechol).
• Trihydric phenols have three –OH groups.

Ethers

Ethers are classified based on the nature of the alkyl or aryl groups attached to the oxygen atom.

• Simple or Symmetrical Ethers: The two groups attached to the oxygen are identical (e.g., Diethyl ether, $C_2H_5OC_2H_5$).
• Mixed or Unsymmetrical Ethers: The two groups attached to the oxygen are different (e.g., Ethyl methyl ether, $C_2H_5OCH_3$).

Nomenclature

Alcohols

• Common Name: Named by taking the name of the alkyl group and adding the word "alcohol". For example, $CH_3OH$ is methyl alcohol.
• IUPAC Name: Derived from the parent alkane. The '-e' at the end of the alkane name is replaced with '-ol'.
  1. Find the longest carbon chain that contains the –OH group.
  2. Number the chain starting from the end closer to the –OH group.
  3. Indicate the position of the –OH group and any substituents with numbers.
  • Example: $CH_3-CH_2-CH_2-OH$ is Propan-1-ol.
  • Example: $CH_3-CH(OH)-CH_3$ is Propan-2-ol.
• Polyhydric Alcohols: For compounds with more than one –OH group, the '-e' of the alkane is kept, and suffixes like '-diol' or '-triol' are added.
  • Example: $HO-CH_2-CH_2-OH$ is Ethane-1,2-diol.
• Cyclic Alcohols: The prefix 'cyclo' is used, and the carbon atom bearing the –OH group is numbered as C-1.
  • Example: Cyclohexanol.

Phenols

• The simplest hydroxy derivative of benzene is phenol, which is its common name and also its accepted IUPAC name.
• For substituted phenols, the terms ortho (o-) for 1,2-disubstituted, meta (m-) for 1,3-disubstituted, and para (p-) for 1,4-disubstituted are used in common names.
• In the IUPAC system, substituted phenols are named as derivatives of phenol, with the carbon attached to the -OH group as C-1.
  • Example: The compound with a $-CH_3$ group at position 2 is called o-Cresol (common name) or 2-Methylphenol (IUPAC name).
• Dihydroxy derivatives of benzene have specific common names:
  • Benzene-1,2-diol is Catechol.
  • Benzene-1,3-diol is Resorcinol.
  • Benzene-1,4-diol is Hydroquinone or quinol.

Ethers

• Common Name: The two alkyl/aryl groups are named alphabetically, followed by the word "ether". If the groups are the same, the prefix 'di-' is used.
  • Example: $C_2H_5OC_2H_5$ is diethyl ether.
  • Example: $CH_3OC_2H_5$ is ethyl methyl ether.
• IUPAC Name: Ethers are named as alkoxyalkanes. The smaller alkyl group and the oxygen atom are treated as an alkoxy substituent. The larger alkyl group forms the parent alkane.
  • Example: $CH_3OCH_2CH_3$ is Methoxyethane. (The $-OCH_3$ group is methoxy, and the parent chain is ethane).
  • Example: $C_6H_5OCH_3$ is Methoxybenzene, commonly known as Anisole.

Solution

(i) 4-Chloro-2,3-dimethylpentan-1-ol (ii) 2-Ethoxypropane (iii) 2,6-Dimethylphenol (iv) 1-Ethoxy-2-nitrocyclohexane

Structures of Functional Groups

• Alcohols: The oxygen atom in the –OH group is $sp^3$ hybridized. It forms a sigma ($\sigma$) bond with an $sp^3$ hybridized carbon. The C-O-H bond angle is slightly less than the ideal tetrahedral angle ($109.5^\circ$) due to repulsion between the lone pairs of electrons on the oxygen atom.
• Phenols: The oxygen atom is attached to an $sp^2$ hybridized carbon of the aromatic ring. The C–O bond length (136 pm) is shorter than in alcohols. This is because of:
  1. Partial double bond character due to resonance (conjugation) of an oxygen lone pair with the aromatic ring.
  2. The $sp^2$ hybridized carbon is more electronegative, pulling the oxygen closer.
• Ethers: The oxygen atom is $sp^3$ hybridized, and the geometry is roughly tetrahedral. The R-O-R bond angle is slightly larger than the tetrahedral angle because the bulky alkyl/aryl (–R) groups repel each other. The C–O bond length (141 pm) is similar to that in alcohols.

Preparation of Alcohols

1. From Alkenes

(i) By Acid-Catalysed Hydration Alkenes react with water in the presence of an acid catalyst (like dilute $H_2SO_4$) to form alcohols.

• For unsymmetrical alkenes, the addition follows Markovnikov's rule, which states that the hydrogen atom adds to the carbon with more hydrogen atoms, and the –OH group adds to the carbon with fewer hydrogen atoms.

Mechanism: This reaction occurs in three steps:

1. Protonation of Alkene: The alkene accepts a proton ($H^+$) from $H_3O^+$ to form a stable carbocation.
2. Nucleophilic Attack of Water: A water molecule acts as a nucleophile and attacks the positively charged carbocation.
3. Deprotonation: Another water molecule removes a proton from the intermediate to form the final alcohol.

(ii) By Hydroboration–Oxidation This is a two-step process that results in the anti-Markovnikov addition of water across the double bond.

1. Hydroboration: Diborane ($(BH_3)_2$) adds to the alkene to form a trialkyl borane. The boron atom attaches to the $sp^2$ carbon with more hydrogen atoms.
2. Oxidation: The trialkyl borane is then oxidized by hydrogen peroxide ($H_2O_2$) in the presence of aqueous sodium hydroxide (NaOH) to yield an alcohol.

2. From Carbonyl Compounds

(i) By Reduction of Aldehydes and Ketones Aldehydes and ketones can be reduced to alcohols.

• Aldehydes are reduced to primary alcohols.
• Ketones are reduced to secondary alcohols.

Reducing Agents:

• Catalytic Hydrogenation: Addition of hydrogen gas ($H_2$) in the presence of a metal catalyst like Platinum (Pt), Palladium (Pd), or Nickel (Ni).
• Chemical Reagents: Sodium borohydride ($NaBH_4$) or lithium aluminium hydride ($LiAlH_4$).

(ii) By Reduction of Carboxylic Acids and Esters

• Carboxylic acids are reduced to primary alcohols using a strong reducing agent like lithium aluminium hydride ($LiAlH_4$).
• $LiAlH_4$ is expensive, so commercially, acids are first converted to esters, which are then reduced to alcohols via catalytic hydrogenation.

3. From Grignard Reagents

The reaction of a Grignard reagent (R-MgX) with an aldehyde or ketone is a very useful way to form alcohols.

1. The Grignard reagent acts as a nucleophile and adds to the carbonyl carbon.
2. This forms an intermediate adduct, which upon hydrolysis (reaction with water) yields an alcohol.

• Reaction with methanal (formaldehyde) gives a primary alcohol.
• Reaction with any other aldehyde gives a secondary alcohol.
• Reaction with a ketone gives a tertiary alcohol.

Solution

(a) Butanal is an aldehyde. Its reduction will yield a primary alcohol, butan-1-ol. Structure: $CH_3-CH_2-CH_2-CH_2-OH$ IUPAC Name: Butan-1-ol

(b) Hydration of propene follows Markovnikov's rule. The –OH group adds to the middle carbon. Structure: $CH_3-CH(OH)-CH_3$ IUPAC Name: Propan-2-ol

(c) Propanone is a ketone. Reaction with a Grignard reagent gives a tertiary alcohol. Structure: $(CH_3)_3C-OH$ IUPAC Name: 2-Methylpropan-2-ol

Preparation of Phenols

Phenol, also known as carbolic acid, is produced synthetically on a large scale.

1. From Haloarenes (Dow's Process)

Chlorobenzene is heated with aqueous sodium hydroxide (NaOH) at high temperature (623 K) and pressure (320 atm). This produces sodium phenoxide, which is then acidified to yield phenol. $C_6H_5Cl \xrightarrow[{(ii) H^+}]{\text{(i) NaOH, 623 K, 320 atm}} C_6H_5OH$

2. From Benzenesulphonic Acid

Benzene is treated with oleum (fuming sulphuric acid) to form benzenesulphonic acid. This is then heated with molten NaOH to form sodium phenoxide, which upon acidification gives phenol. $C_6H_6 \xrightarrow{\text{Oleum}} C_6H_5SO_3H \xrightarrow[{(ii) H^+}]{\text{(i) NaOH, heat}} C_6H_5OH$

3. From Diazonium Salts

An aromatic primary amine (like aniline) is treated with nitrous acid ($NaNO_2 + HCl$) at low temperatures (273-278 K) to form a diazonium salt. Warming this salt with water or dilute acid hydrolyzes it to phenol. $C_6H_5NH_2 \xrightarrow{NaNO_2 + HCl, 273-278 K} C_6H_5N_2^+Cl^- \xrightarrow{H_2O, \text{warm}} C_6H_5OH + N_2 + HCl$

4. From Cumene (Industrial Method)

This is the most common method for manufacturing phenol worldwide.

1. Cumene (isopropylbenzene) is oxidized in the presence of air to form cumene hydroperoxide.
2. The hydroperoxide is then treated with dilute acid, which converts it into phenol and acetone.

Physical Properties of Alcohols and Phenols

Boiling Points

• Alcohols and phenols have significantly higher boiling points than hydrocarbons, ethers, and haloalkanes of comparable molecular masses.
• This is due to the presence of intermolecular hydrogen bonding between the –OH groups of adjacent molecules. A large amount of energy is required to break these bonds.
• As the number of carbon atoms increases, the boiling point increases due to stronger van der Waals forces.
• Among isomeric alcohols, boiling points decrease with increased branching because branching reduces the surface area, weakening the van der Waals forces.

Solubility

• Lower molecular mass alcohols and phenols are soluble in water.
• Their solubility is due to their ability to form hydrogen bonds with water molecules.
• As the size of the alkyl or aryl group (the hydrophobic or "water-hating" part) increases, the solubility in water decreases.

Chemical Reactions of Alcohols and Phenols

Alcohols are versatile and can react as both nucleophiles and electrophiles.

• As Nucleophiles: When the O–H bond breaks, the alcohol donates an electron pair.
• As Electrophiles: When the C–O bond breaks (after protonation), the alcohol accepts an electron pair.

Reactions Involving Cleavage of O–H Bond

1. Acidity of Alcohols and Phenols

• Reaction with Metals: Alcohols and phenols are weakly acidic. They react with active metals like sodium (Na), potassium (K), and aluminium (Al) to release hydrogen gas and form alkoxides or phenoxides.
• Acidity of Alcohols: The acidic character is due to the polar O-H bond. Electron-releasing alkyl groups (like $-CH_3$, $-C_2H_5$) increase the electron density on the oxygen, making the O-H bond less polar and reducing the acid strength.
  • Order of Acidity: Primary ($1^\circ$) > Secondary ($2^\circ$) > Tertiary ($3^\circ$)
• Alcohols vs. Water: Alcohols are generally weaker acids than water.
• Acidity of Phenols: Phenols are much stronger acids than alcohols and even water. This increased acidity is due to two main reasons:
  1. The $sp^2$ hybridized carbon of the benzene ring is electron-withdrawing, which increases the polarity of the O-H bond.
  2. The phenoxide ion, formed after losing a proton, is highly stabilized by resonance. The negative charge is delocalized over the entire benzene ring, making the ion very stable and favoring the release of the proton.
• Effect of Substituents on Phenol Acidity:
  • Electron-withdrawing groups (like $-NO_2$) increase the acidity by further stabilizing the phenoxide ion, especially when at the ortho and para positions.
  • Electron-releasing groups (like $-CH_3$) decrease the acidity by destabilizing the phenoxide ion.

2. Esterification

Alcohols and phenols react with carboxylic acids, acid chlorides, or acid anhydrides to form esters.

• The reaction with a carboxylic acid is reversible and is catalyzed by a small amount of concentrated sulphuric acid.
• The introduction of an acetyl group ($CH_3CO-$) is called acetylation. For example, the acetylation of salicylic acid produces aspirin.

Reactions Involving Cleavage of C–O Bond in Alcohols

These reactions occur mainly with alcohols. Phenols only show this type of reaction with zinc dust.

1. Reaction with Hydrogen Halides

Alcohols react with hydrogen halides (HX) to form alkyl halides (R-X).

• The reactivity of alcohols is $3^\circ > 2^\circ > 1^\circ$.
• Lucas Test: This difference in reactivity is used to distinguish between primary, secondary, and tertiary alcohols. The Lucas reagent (conc. HCl and anhydrous $ZnCl_2$) causes turbidity (cloudiness) when the insoluble alkyl halide is formed.
  • Tertiary alcohols: Immediate turbidity.
  • Secondary alcohols: Turbidity after a few minutes.
  • Primary alcohols: No turbidity at room temperature.

2. Dehydration

Alcohols lose a molecule of water when heated with a protic acid (like conc. $H_2SO_4$) or a catalyst (like alumina) to form alkenes.

• The ease of dehydration follows the order: Tertiary ($3^\circ$) > Secondary ($2^\circ$) > Primary ($1^\circ$). This is because the reaction proceeds via a carbocation intermediate, and tertiary carbocations are the most stable.

3. Oxidation

Oxidation of alcohols involves the cleavage of both an O-H and a C-H bond to form a carbon-oxygen double bond.

• Primary Alcohols:
  • Oxidized to aldehydes using mild oxidizing agents like pyridinium chlorochromate (PCC).
  • Oxidized to carboxylic acids using strong oxidizing agents like acidified potassium permanganate ($KMnO_4$).
• Secondary Alcohols: Oxidized to ketones using agents like chromic anhydride ($CrO_3$).
• Tertiary Alcohols: Do not undergo oxidation under normal conditions. Strong conditions cause cleavage of C-C bonds.
• Catalytic Dehydrogenation: When alcohol vapors are passed over heated copper at 573 K:
  • Primary alcohols give aldehydes.
  • Secondary alcohols give ketones.
  • Tertiary alcohols undergo dehydration to form alkenes.

Reactions of Phenols

1. Electrophilic Aromatic Substitution

The –OH group is a powerful activating group and directs incoming electrophiles to the ortho and para positions. This is because the lone pairs on the oxygen atom increase the electron density at these positions through resonance.

• Nitration:
  • With dilute nitric acid at low temperature, a mixture of o-nitrophenol and p-nitrophenol is formed. These can be separated by steam distillation because o-nitrophenol has intramolecular hydrogen bonding, making it more volatile.
  • With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol, commonly known as picric acid.
• Halogenation:
  • With bromine in a low polarity solvent (like $CS_2$), a mixture of o-bromophenol and p-bromophenol is formed.
  • With bromine water, phenol reacts to form a white precipitate of 2,4,6-tribromophenol.

2. Kolbe's Reaction

Phenoxide ion (formed by treating phenol with NaOH) is highly reactive. It reacts with carbon dioxide ($CO_2$, a weak electrophile) to form ortho-hydroxybenzoic acid (salicylic acid).

3. Reimer-Tiemann Reaction

When phenol is treated with chloroform ($CHCl_3$) in the presence of aqueous NaOH, an aldehyde group (–CHO) is introduced at the ortho position, forming salicylaldehyde.

4. Reaction with Zinc Dust

Heating phenol with zinc dust reduces it to benzene.

5. Oxidation

Oxidation of phenol with chromic acid produces a conjugated diketone called benzoquinone.

Some Commercially Important Alcohols

1. Methanol ($CH_3OH$)

• Also known as wood spirit as it was once produced by the destructive distillation of wood.
• Production: Now made by the catalytic hydrogenation of carbon monoxide (CO) at high pressure and temperature. $CO + 2H_2 \xrightarrow{ZnO-Cr_2O_3 \text{ catalyst}} CH_3OH$
• Properties: A colorless, highly poisonous liquid. Ingesting even small amounts can cause blindness, and larger amounts can be fatal.
• Uses: Used as a solvent for paints and varnishes, and for making formaldehyde.

2. Ethanol ($C_2H_5OH$)

• Production:
  • Fermentation: The oldest method, where sugars from sources like molasses or grapes are converted to ethanol by enzymes (invertase and zymase) found in yeast.
  • Hydration of Ethene: The modern industrial method.
• Properties: A colorless liquid used widely as a solvent and in the preparation of many organic compounds.
• Denaturation of Alcohol: To make commercial alcohol unfit for drinking, it is mixed with substances like copper sulphate (to give it color) and pyridine (a foul-smelling liquid).

Preparation of Ethers

1. By Dehydration of Alcohols

When alcohols are heated with a protic acid (like $H_2SO_4$), the product can be either an alkene or an ether, depending on the conditions.

• At 443 K: Ethanol dehydrates to ethene (elimination is favored).
• At 413 K: Ethanol reacts to form ethoxyethane (substitution is favored).

2. Williamson Synthesis

This is a very important laboratory method for preparing both symmetrical and unsymmetrical ethers.

• Reaction: An alkyl halide is reacted with a sodium alkoxide. $R-X + R'-ONa \rightarrow R-O-R' + NaX$
• Mechanism: The reaction proceeds via an $S_N2$ mechanism, where the alkoxide ion attacks the alkyl halide.
• Limitation: The alkyl halide must be primary. If a secondary or tertiary alkyl halide is used, elimination becomes the major reaction, and an alkene is formed instead of an ether. This is because alkoxides are strong bases as well as strong nucleophiles. To prepare an ether with a tertiary group, the tertiary group must be part of the alkoxide, and the alkyl halide must be primary.

Solution

(i) The major product is 2-methylprop-1-ene. Here, sodium ethoxide ($C_2H_5ONa$) is a strong base, and it reacts with the tertiary alkyl halide ($(CH_3)_3C-Cl$) via an elimination reaction, not substitution.

(ii) To prepare t-butyl ethyl ether, the alkyl halide must be primary and the alkoxide must be tertiary. Suitable Reaction: $(CH_3)_3C-ONa + C_2H_5-Cl \rightarrow (CH_3)_3C-O-C_2H_5 + NaCl$ (Sodium t-butoxide + Ethyl chloride $\rightarrow$ t-Butyl ethyl ether)

Physical Properties of Ethers

• Boiling Points: Ethers have much lower boiling points than alcohols of comparable molecular mass because they cannot form intermolecular hydrogen bonds with each other. Their boiling points are similar to those of alkanes of similar mass.
• Solubility: Ethers are slightly soluble in water. The oxygen atom in an ether can form hydrogen bonds with water molecules, but the lack of an –OH group makes their solubility much lower than that of alcohols.

Chemical Reactions of Ethers

Ethers are generally unreactive. The C-O bond can only be cleaved under drastic conditions.

1. Cleavage of C–O Bond in Ethers

Ethers react with strong hydrogen halides (HI or HBr) at high temperatures to cleave the C-O bond.

• Dialkyl Ethers: The reaction produces an alkyl halide and an alcohol. If excess HX is used, the alcohol formed will also be converted to an alkyl halide. $R-O-R' + HX \rightarrow R-X + R'-OH$ $R'-OH + HX \rightarrow R'-X + H_2O$
• Alkyl Aryl Ethers: The cleavage occurs at the alkyl-oxygen bond, producing a phenol and an alkyl halide. This is because the aryl-oxygen bond is stronger due to partial double bond character.
• Reactivity of HX: $HI > HBr > HCl$.
• Mechanism:
  • If the alkyl groups are primary or secondary, the reaction follows an $S_N2$ mechanism. The halide ion attacks the less sterically hindered alkyl group.
  • If one of the alkyl groups is tertiary, the reaction follows an $S_N1$ mechanism. The products are a tertiary alkyl halide and an alcohol, because the tertiary carbocation intermediate is very stable.

2. Electrophilic Substitution in Aromatic Ethers

The alkoxy group (–OR) is an activating group and is ortho, para-directing for electrophilic substitution on the aromatic ring.

• Halogenation: Anisole (methoxybenzene) undergoes bromination even without a Lewis acid catalyst to give mainly p-bromoanisole.
• Friedel-Crafts Reaction: Anisole reacts with alkyl halides (alkylation) or acyl halides (acylation) in the presence of a Lewis acid catalyst ($AlCl_3$) to give ortho and para substituted products. The para isomer is usually the major product.
• Nitration: Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole.