Alcohols and Phenols: Structure, Properties, Preparation, and Reactions

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Chapter 17: Alcohols and Phenols

Introduction to Alcohols and Phenols

Alcohols and phenols are organic compounds containing hydroxyl (-OH) groups. Alcohols have the -OH group attached to a saturated (sp3) carbon, while phenols have the -OH group attached to a benzene ring (aromatic system).

Alcohols: R-OH, pKa ≈ 16–18
Phenols: Ar-OH, pKa ≈ 10
Enol and Keto Tautomerism: Alcohols can exist in equilibrium with their keto forms, especially in carbonyl chemistry.

Classification and Related Functional Groups

Alcohols are classified based on the carbon to which the hydroxyl group is attached:

Primary (1°) alcohol: -OH on a carbon attached to one other carbon
Secondary (2°) alcohol: -OH on a carbon attached to two other carbons
Tertiary (3°) alcohol: -OH on a carbon attached to three other carbons

Related functional groups include water, ethers, peroxides, thiols, thioethers, and disulfides.

Functional Group	General Formula
Alcohol	R-OH
Ether	R-O-R'
Peroxide	R-O-O-R'
Thiols	R-SH
Thioether	R-S-R'
Disulfide	R-S-S-R'

17.1 Nomenclature of Alcohols and Phenols

Alcohols are named similarly to alkanes, replacing the '-e' with '-ol'. The carbon chain is numbered so the hydroxyl group gets the lowest possible number. Substituents are listed alphabetically. For phenols, benzene is the parent name and the -OH group is assigned position 1.

Examples: 2-methyl-2-pentanol, 3-phenyl-2-butanol, 3,4-dimethylphenol
Non-systematic names are common for simple alcohols (e.g., benzyl alcohol, glycerol, ethylene glycol).

17.2 Properties: Hydrogen Bonding and Boiling Points

The oxygen atom in alcohols and phenols is sp3 hybridized, similar to water. Alcohols and phenols can form hydrogen bonds, leading to higher boiling points than comparable alkanes and alkyl halides.

Hydrogen Bonding: Non-covalent interaction between a hydrogen atom (δ+) and a lone pair on O or N in a polar covalent bond.
Boiling Points: Alcohols and phenols have much higher boiling points due to hydrogen bonding.

Compound	Boiling Point (°C)
H2O	100
CH3OH	65
CH3CH2OH	78
CH3CH2CH2OH	97
CH3CH2CH2CH2OH	118
C6H5OH	182

17.3 Acidity and Basicity of Alcohols and Phenols

Alcohols are weak Brønsted acids and bases. The nature of the R group and the environment around the oxygen atom influence acidity and basicity.

Acidity: Methanol is more acidic than 1° alcohol > 2° alcohol > 3° alcohol, due to solvation effects.
Solvation: The alkoxide ion is stabilized by hydrogen bonding with water; steric hindrance reduces solvation for more substituted alcohols.
Electronic Effects: Electron-withdrawing groups increase acidity by stabilizing the conjugate base; electron-donating groups decrease acidity.
Phenols: More acidic than aliphatic alcohols due to resonance stabilization of the phenoxide ion by the benzene ring.

Alcohol	pKa
CH3OH	15.5
CH3CH2OH	16
CH3CH(OH)CH3	17
Phenol	10

17.4 Preparation of Alcohols

Alcohols can be prepared by several methods, including hydration of alkenes, hydroboration-oxidation, and syn addition of -OH groups.

Markovnikov Addition: Hydration of alkenes with acid yields alcohols at the more substituted carbon.
Anti-Markovnikov Addition: Hydroboration-oxidation yields alcohols at the less substituted carbon.
Other Methods: Oxymercuration, dihydroxylation, and other alkene reactions.

17.5 Alcohols from Reduction of Carbonyl Compounds

Alcohols can be synthesized by reducing carbonyl compounds (aldehydes, ketones, carboxylic acids, esters) using hydride reagents.

Sodium Borohydride (NaBH4): Reduces aldehydes to 1° alcohols and ketones to 2° alcohols.
Lithium Aluminum Hydride (LiAlH4): Reduces aldehydes, ketones, carboxylic acids, and esters to alcohols.

General Reduction Equation:

17.6 Alcohols from Reaction of Carbonyl Compounds with Grignard Reagents

Grignard reagents (R-MgX) are organomagnesium compounds that react with carbonyl compounds to form alcohols.

Preparation: Alkyl halide + Mg in ether/THF → R-MgX
Reactions: Grignard reagents react with aldehydes, ketones, and esters to give 1°, 2°, and 3° alcohols, respectively.
Limitations: Grignard reagents are strong bases and react with acidic functional groups (e.g., -COOH, -OH, -SH, -NH2).

17.7 Some Reactions of Alcohols

Reactions Involving the C-O Bond

Dehydration to Alkenes: E1 mechanism (3° > 2° > 1°), requires strong acid (H2SO4), follows Zaitsev's rule.
Dehydration with POCl3: E2 mechanism, requires anti-periplanar conformation, mild conditions with base (pyridine).
Conversion to Alkyl Halides: Substitution with HX (SN1 mechanism), SOCl2 (thionyl chloride), PBr3 (phosphorus tribromide) via SN2 mechanism.

Reactions Involving the O-H Bond

Conversion to Tosylates: Reaction with p-toluenesulfonyl chloride (TsCl) in pyridine forms tosylates, which are good leaving groups for further substitution or elimination reactions.

17.8 Oxidation of Alcohols

Alcohols can be oxidized to aldehydes, ketones, or carboxylic acids depending on their structure and the oxidizing agent used.

Primary Alcohols: Oxidized to aldehydes (PCC) or carboxylic acids (chromic acid, H2CrO4).
Secondary Alcohols: Oxidized to ketones.
Tertiary Alcohols: Generally resistant to oxidation.

Oxidation Equations:

(PCC)

(H2CrO4)

17.9 Protection of Alcohols

Hydroxyl groups are weakly acidic and can interfere with reactions. Protecting groups temporarily mask the -OH group to prevent unwanted reactions, and can be removed (deprotection) after the desired transformation.

Protecting Group: A functional group added to make the alcohol compatible with reaction conditions.
Deprotection: Removal of the protecting group to restore the original alcohol.

Additional info: These notes cover the structure, nomenclature, physical and chemical properties, preparation, and reactions of alcohols and phenols, suitable for a General Chemistry or introductory Organic Chemistry course.