Alcohols, Phenols, and Ethers

• Alcohols, phenols, and ethers form the foundation for detergents, antiseptics, and fragrances.

Key Concepts

• Replacing one or more hydrogen atoms in a hydrocarbon with another atom or group results in a new compound with different properties.
• Alcohols and phenols result from replacing a hydrogen atom in a hydrocarbon (aliphatic and aromatic, respectively) with a hydroxyl (-OH) group.
• Ethers are formed by substituting a hydrogen atom in a hydrocarbon with an alkoxy (R-O) or aryloxy (Ar-O) group.

Alcohols, Phenols, and Ethers Described

• Alcohol: Contains one or more hydroxyl (OH) groups directly bonded to a carbon atom in an aliphatic system (e.g., CH3OH).
• Phenol: Contains one or more –OH groups directly bonded to a carbon atom of an aromatic system (e.g., C6H5OH).
• Ether: A compound in which a hydrogen atom in a hydrocarbon is replaced by an alkoxy or aryloxy group (R-O/Ar-O), as seen in dimethyl ether (CH3OCH3).

Nomenclature Objectives

• Naming alcohols, phenols, and ethers requires an understanding of the IUPAC nomenclature system.
• Reactions involved in alcohol preparation should include those from alkenes, aldehydes, ketones, and carboxylic acids.
• Reactions for phenol preparation involve haloarenes, benzene sulfonic acids, diazonium salts, and cumene.
• Reactions applicable to ether preparation can proceed from alcohols, alkyl halides, and sodium alkoxides/aryloxides.
• Physical properties of alcohols, phenols, and ethers can be correlated to their structures.

Classification of Alcohols and Phenols

• Can be classified as mono-, di-, tri-, or polyhydric.
• The classification depends on the number of hydroxyl groups in their structures.

Monohydric Alcohol Classification

• Can be further classified on the basis on the hybridisation of the carbon atom to which the hydroxyl group is attached.

Compounds containing Csp3-OH bond

• The -OH group is attached to an sp3 hybridised carbon atom of an alkyl group.
• They can be further classified as:
  • Primary, secondary, and tertiary alcohols
    • In these three types of alcohols, the –OH group is attached to primary, secondary and tertiary carbon atom, respectively
  • Allylic alcohols
    • The —OH group is attached to a sp3 hybridised carbon adjacent to the carbon-carbon double bond, that is to an allylic carbon
  • Benzylic alcohols
    • The —OH group is attached to a sp3-hybridised carbon atom next to an aromatic ring
    • Allylic and benzylic alcohols may be primary, secondary, or tertiary
  • Compounds containing Csp2-OH bond bond
    • These alcohols contain -OH group bonded to a carbon-carbon double bond, i.e., to a vinylic carbon or to an aryl carbon.
    • These alcohols are also known as vinylic alcohols

Ethers

• Classified as simple or symmetrical if the alkyl or aryl groups attached to the oxygen atom are the same.
• Classified as mixed or unsymmetrical if the two groups are different.
  • Diethyl ether, C2H5OC2H5, is a symmetrical ether.
  • C2H5OCH3 and C2H5OC6H5 are unsymmetrical ethers.

Alcohol Nomenclature

• Common name for alcohols is derived from the alkyl group and the word alcohol

Alcohol IUPAC Nomenclature

• Name derived from alkane, replacing 'e' with 'ol'
• Substituent positions indicated by numerals, with numbering starting nearest to the hydroxyl group.
• For polyhydric alcohols, retain the 'e' of alkane, adding 'ol' and indicate number of -OH groups with prefixes like di, tri, etc.

Cyclic Alcohols

• Named using the prefix cyclo, with the –OH group attached to C-1.

Phenols

• Simplest hydroxy derivative of benzene
• Common name and accepted IUPAC name is phenol itself
• In substituted compounds, use ortho (1,2-), meta (1,3-), and para (1,4-) prefixes

Ethers

• Common names are derived from alkyl/aryl groups, alphabetized and followed by "ether."

Ether IUPAC Names

• Regarded as hydrocarbon derivatives with H replaced by -OR or -OAr.
• The larger (R) group is the parent hydrocarbon.
• Prefix "di" is added if both alkyl groups are the same (e.g., diethyl ether).

Functional Group Structures of Alcohols, Phenols and Ethers

• In alcohols, the oxygen of the -OH group is attached to carbon by a sigma (σ) bond, formed by overlapping a sp³ hybridised orbital of carbon and a sp³ hybridised orbital of oxygen.

Bond Angle in Alcohols

• Slightly less than the tetrahedral angle (109°-28') due to repulsion between unshared electron pairs on oxygen.

Bond Angle in Phenols

• The –OH group is attached to sp² hybridised carbon of an aromatic ring.
• Carbon-oxygen bond length is 136 pm
• Shorter than methanol because partial double bond character from conjugation of oxygen's unshared electron pair with the aromatic ring and sp² hybridisation of carbon.

Bond Angle in Ethers

• Four electron pairs around oxygen are arranged tetrahedrally.
• Bond angle is slightly greater than tetrahedral due to repulsion between bulky (-R) groups.
• The C-O bond length is 141 pm, almost the same as in alcohols.

Preparation of Alcohols

From Alkenes

• By acid catalysed hydration
  • Alkenes react with water in the presence of acid
  • With unsymmetrical alkenes, the reaction follows Markovnikov's rule.
• Mechanism involves
  • Protonation of alkene, forming carbocation via electrophilic attack of H3O+.
  • Nucleophilic attack of water on carbocation intermediate.
  • Deprotonation to yield an alcohol product and regenerate the acid catalyst

From borane-oxidation

• Diborane reacts with alkenes to form trialkyl boranes
• Addition follows anti-Markovnikov's rule, with boron attaching to the less substituted carbon

From Carbonyl Compounds

• Catalytic hydrogenation reduces aldehydes/ketones to corresponding alcohols.
• Aldehydes yield primary and ketones yield secondary alcohols
• Reduction of carboxylic acids and esters
  • Carboxylic acids reduced to primary alcohols with lithium aluminum hydride (LiAlH4), though expensive.
• Acids commercially converted to esters, reduced via catalytic hydrogenation

From Grignard Reagents

• Alcohols produced from aldehydes and ketones, via Grignard reagents
• Nucleophilic addition of Grignard reagent to carbonyl group
• Hydrolysis of the adduct yields an alcohol

Preparation of Phenols

• Preparation starts from benzene derivatives

From Haloarenes

• Chlorobenzene fused with NaOH at 623K and 320 atm
• Acidification of resulting sodium phenoxide yields a phenol

From Benzenesulfonic Acid

• Sulfonation with oleum, molten NaOH converts acid to sodium phenoxide
• Acidification yields a phenol

From Diazonium Salts

• Primary aromatic amine treated with nitrous acid (NaNO2 + HCl) at 273-278 K creates diazonium salt
• Hydrolysis by warming water or treating with dilute acids yields a phenol

From Cumene

• Cumene (isopropylbenzene) oxidised with air to cumene hydroperoxide
• Converted to phenol and acetone with dilute acid; acetone produced as a byproduct

Physical Properties of Alcohols and Phenols

• Properties are chiefly due to the –OH group.
• Alkyl and aryl groups affect the modifications

Boiling Points

Increase with number of carbon atoms owing to increased van der Waals forces

Alcohol Boiling Points

Decrease with branching, reducing van der Waals forces with surface area

Hydrogen Bonding

• OH group involved in intermolecular hydrogen bonding

Solubility

• Alcohols and phenols have higher boiling points than other compounds because of hydrogen bonding.
• Solubility in water decreases with larger alkyl/aryl groups (hydrophobic). Lower mass alcohols are miscible.

Chemical Behavior of Alcohols and Phenols

Chemical Reactions

• React as both nucleophiles and electrophiles, with bond breakage occurring at the O–H or C–O bond.

Acidity of Alcohols and Phenols

• Reaction with metals yields alkoxides/phenoxides and hydrogen.
• Phenols react with aqueous sodium hydroxide to form sodium phenoxides.

Acidity of Alcohols

• Electron-releasing groups on carbon decrease acidity due to increased electron density on oxygen
• Acidity Increases in the order primary > secondary > tertiary

Bronsted Bases

• Alcohols are weaker acids than water, and act as Bronsted bases due to unshared electron pairs on oxygen

Acidity of Phenols

• Metals and sodium hydroxide shows acidity
• Electron-withdrawing benzene ring of phenol results in charge distribution
• Oxygen of -OH group to be positive. Phenols are stronger acids than alcohols and water.

Resonance Structures

• Exhibit charge delocalization, making phenoxide ions more stable and favouring ionisation
• Greater acidity in substituted phenols is found with electron-withdrawing groups (especially at ortho and para positions)
• Electron-releasing groups reduce acidity.

Esterification

• Alcohols and phenols react with carboxylic acids, acid chlorides, and acid anhydrides to produce esters
• Reactions with carboxylic acids and acid anhydrides is catalyzed by conc. sulfuric acid, removing water shifts equilibrium to ester product. Acid chlorides use pyridine to neutralize HCl. Acetylation, introducing CH3CO, produces aspirin from salicylic acid.

Reactions Involving Carbon-Oxygen Bond Cleavage in Alcohols

Alcohol Reactions

• These only take place in alcohols

Phenol Reactions

• These only take place with zinc

Reaction with Hydrogen Halides

• Alcohols react to yield alkyl halides

Reactivity

Distinguishes 3 classes of alcohols

Reaction with Phosphorus Trihalides

Alcohols converted to alkyl bromides

Dehydration

• Alcohols undergo dehydration and form alkenes with protic acids, e.g concentrated sulfuric acid or catalysts, at high temperatures

Ethanol Dehydration

Occurs at 443K, creating ethene

Alcohol Oxidation

• Alcohols form carbon-oxygen double bond by oxidation
• O-H and C-H bonds are cleaved.
• Can result in aldehydes, ketones or carboxylic acids

Strong Oxidizing Agents

Acidified potassium permanganate can be used to produce carboxylic acids directly

Anhydrous medium

• Used for CRO3

Pyridinium Chlorochromate (PCC)

Is used for oxidation of primary alcohols

Secondary Alcohols

Chromic anhydride oxidizes to ketones

Tertiary Alcohols

Undergo dehydration instead of oxidation

Reactions involving Phenols

Electrophilic Aromatic Substitution

• –OH group activates benzene ring and directs to ortho/para positions
• Typical reactions include nitration and halogenation

Nitration

• Dilute nitric acid at low temperature yields ortho and para nitrophenols
• Steam distillation separates ortho isomer (intramolecular H-bonding makes volatile)

Concentrated Nitric Acid

• Concentrated converts phenol converts to 2,4,6-trinitrophenol
• Product is known as picric acid, despite poor yields

Halogenation

• Bromine at low temperatures in CHCl3 or CS2 yields monobromophenols
• Bromine water yields 2,4,6-tribromophenol as a white precipitate

Kolbe Reaction

Phenoxide ion (with NaOH) is more reactive than phenol towards electrophiles for ortho-hydroxybenzoic acid

Reimer-Tiemann Reaction

• Phenol and chloroform, with sodium hydroxide, introduces –CHO group at ortho position. Intermediate benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde.

Reaction with Zinc Dust

Phenol is converted into Benzene upon heating

Oxidation

• Chromic acid produces conjugated diketone (benzoquinone). In air, undergoes oxidation into dark-coloured mixtures from quinones

Industrial Production of Alcohols

Methanol

• Catalytic hydrogenation of carbon monoxide:

Ethanol

• Fermentation of sugars, catalyzed by invertase and zymase enzymes
• Ethanol limited to 14% mixture
• Oxidation will occurs with oxygen in air

Preparation of Ethers

Alcohol Dehydration

• Requires protic acids (H2SO4, H3PO4) and temperature control
• Dehydration gives alkenes or ethers, depending on conditions

Bimolecular Reactions

• Ether formation is more likely to result from primary alcohols

Williamson Ether Synthesis

• Alkyl halide reacts with sodium alkoxide Symmetrical and unsymmetrical ethers are prepared. Secondary or tertiary alkyl groups also prepared. Elimination reactions can occur with secondary.Phenols also converted into eithers

Physical Properties Of Ethers

Bonding

C-O polar, Ethers have net dipole momomenmt, but is weak

Boiling points

Comparable to alkanes, boiling points are lower than respective alcohols

Miscibility

Water solubility is similiar to alcohols

Chemical Properties of Ethers

• C-O cleavage requires drastic conditions and excess hydrogen halides
• Dialkyl ethers yield alkyl halides. More stable aryl-oxygen bond in alkyl aryl ethers favors cleavage at alkyl-oxygen bond

Electrophilic

The aromatic ring is activated by and directs aromatic ring

1. Halogenation: Phenylakyl Ethers undergoes isusal halogenation, group ortho to reaction medium
2. Friedel Crafts: alkyl and acyl
3. Nitration: Anizole reacts with mix and concentrate and yields products