Alcohols: Structure, Properties, Synthesis, and Reactions

Alcohols – Chemistry of the –OH Group

Introduction to Alcohols

Alcohols are organic compounds characterized by the presence of a hydroxyl group (–OH) attached to a saturated carbon atom. They play crucial roles in biological systems, industrial processes, and synthetic organic chemistry.

• General Structure: R–OH, where R is an alkyl or substituted alkyl group.

• Examples: Ethanol (found in alcoholic beverages), methanol, and complex natural products such as azaspiracidone A and madindoline A.

Alcohols – Nomenclature

Systematic Naming of Alcohols

The IUPAC system is used to name alcohols, prioritizing the longest carbon chain containing the –OH group and assigning the lowest possible number to the carbon bearing the hydroxyl group.

• Primary (1°) Alcohol: –OH attached to a carbon bonded to one other carbon (e.g., 1-butanol).

• Secondary (2°) Alcohol: –OH attached to a carbon bonded to two other carbons (e.g., 2-butanol).

• Tertiary (3°) Alcohol: –OH attached to a carbon bonded to three other carbons (e.g., 2-methyl-2-propanol).

• Cycloalcohols: Named by numbering the ring to give the –OH group the lowest possible number (e.g., cyclopentanol).

Example: (R)-2-chloro-3-phenyl-1-propanol demonstrates the use of locants and stereochemistry in naming.

Alcohols – Solubility and Acidity

Solubility of Alcohols in Water

Alcohols are generally soluble in water due to hydrogen bonding, but solubility decreases as the hydrophobic alkyl chain length increases.

Alcohol	Solubility in Water (at 25°C)
methyl	miscible
ethyl	miscible
n-propyl	miscible
isopropyl	miscible
n-butyl	9.1%
isobutyl	7.9%
n-pentyl	2.7%
cyclohexyl	0.96%
n-hexyl	0.60%
phenol	8.3%
butane-1,4-diol	miscible

Acidity of Alcohols

• Relative Acidity: Alcohols are less acidic than water but more acidic than alkanes and ethers. Phenol is significantly more acidic due to resonance stabilization of its conjugate base.

• pKa Values: Ethanol (pKa ≈ 16), tert-butanol (pKa ≈ 18), phenol (pKa ≈ 10).

• Deprotonation: Alcohols can be deprotonated by strong bases (e.g., NaH) to form alkoxides.

Equation:

$\ce{R-OH + Na -> R-O^- Na^+ + 1/2 H2}$

Alcohols – Preparation Methods

Hydration of Alkenes

• Acid-Catalyzed Hydration: Addition of water across a double bond using dilute $\ce{H2SO4}$.

• Oxymercuration-Demercuration: Markovnikov addition of water using $\ce{Hg(OAc)2}$ and $\ce{NaBH4}$.

• Hydroboration-Oxidation: Anti-Markovnikov addition using $\ce{BH3}$, followed by oxidation with $\ce{H2O2}$ and $\ce{NaOH}$.

Reduction of Carbonyl Compounds

• Hydrogenation: Reduction of aldehydes and ketones with $\ce{H2}$ and a metal catalyst (Pt, Pd, or Ni).

• Sodium Borohydride ($\ce{NaBH4}$): Reduces aldehydes and ketones to primary and secondary alcohols, respectively.

• Lithium Aluminum Hydride ($\ce{LiAlH4}$): A stronger reducing agent that reduces esters, carboxylic acids, and amides to alcohols.

Equation:

$\ce{RCHO + NaBH4 + H2O -> RCH2OH}$

$\ce{RCOOR' + 2 LiAlH4 + 2 H2O -> RCH2OH + R'OH}$

Preparation of Vicinal Diols

• Anti Dihydroxylation: Epoxidation followed by acid-catalyzed hydrolysis.

• Syn Dihydroxylation: Use of $\ce{OsO4}$ or $\ce{KMnO4}$ to add two hydroxyl groups to the same side of an alkene.

Grignard Reactions

• Grignard Reagents: Organomagnesium halides ($\ce{R-MgX}$) react with carbonyl compounds to form alcohols after hydrolysis.

• Solvents: Diethyl ether ($\ce{Et2O}$) or tetrahydrofuran (THF) are required to stabilize the Grignard reagent.

• Reactivity: Grignard reagents react with formaldehyde to give primary alcohols, with aldehydes to give secondary alcohols, and with ketones or esters to give tertiary alcohols.

Equation:

$\ce{R-MgX + R'CHO -> R'CH(OH)R}$

Alcohols – Protecting Groups

Use of Protecting Groups

Alcohols can interfere with certain reactions, so they are often temporarily converted to protecting groups such as trimethylsilyl ethers (TMS) to prevent unwanted side reactions. The protecting group can be removed (deprotected) after the desired transformation.

• Protection: $\ce{ROH + TMSCl -> ROTMS}$

• Deprotection: $\ce{ROTMS + TBAF -> ROH}$

Alcohols – Reactions and Transformations

Conversion to Good Leaving Groups

• Formation of Tosylates/Mesylates: Alcohols react with tosyl chloride ($\ce{TsCl}$) or mesyl chloride ($\ce{MsCl}$) to form tosylates/mesylates, which are excellent leaving groups for substitution or elimination reactions.

• Reaction with Thionyl Chloride ($\ce{SOCl2}$): Converts alcohols to alkyl chlorides via a good leaving group intermediate.

• Reaction with Phosphorus Tribromide ($\ce{PBr3}$): Converts alcohols to alkyl bromides.

Dehydration to Alkenes

• Acid-Catalyzed Dehydration: Alcohols can be dehydrated to alkenes using strong acids (e.g., $\ce{H2SO4}$) or reagents like $\ce{POCl3}$ in pyridine.

Oxidation of Alcohols

• Primary Alcohols: Oxidized to aldehydes (PCC, Swern) or further to carboxylic acids (Jones reagent, $\ce{KMnO4}$).

• Secondary Alcohols: Oxidized to ketones.

• Tertiary Alcohols: Generally resistant to oxidation under mild conditions.

Common Oxidizing Agents: PCC (pyridinium chlorochromate), Jones reagent ($\ce{CrO3}$ in acid), Swern oxidation (DMSO, oxalyl chloride, base).

Equation:

$\ce{RCH2OH + PCC -> RCHO}$

$\ce{RCH2OH + [O] -> RCOOH}$

Alcohols – Synthesis Strategies and Concept Map

Overview of Alcohol Synthesis

Alcohols can be synthesized from a variety of starting materials, including alkenes, alkynes, carbonyl compounds, and alkyl halides, using the methods described above. The choice of method depends on the desired product and functional group compatibility.

• From Alkenes: Hydration, hydroboration-oxidation, dihydroxylation.

• From Alkynes: Hydration followed by tautomerization.

• From Carbonyls: Reduction (NaBH4, LiAlH4), Grignard addition.

• From Alkyl Halides: Nucleophilic substitution (SN2, SN1).

Concept Map

The concept map summarizes the interconversion of alcohols with other functional groups, including oxidation, reduction, substitution, and elimination reactions.

Summary Table: Alcohol Reactions and Transformations

Transformation	Reagent/Condition	Product
Oxidation (1° alcohol)	PCC, Swern	Aldehyde
Oxidation (1° alcohol)	Jones, KMnO4	Carboxylic acid
Oxidation (2° alcohol)	PCC, Jones	Ketone
Reduction (carbonyl)	NaBH4, LiAlH4	Alcohol
Grignard addition	R-MgX, H2O	Alcohol
Dehydration	H2SO4, POCl3/pyridine	Alkene
Substitution	SOCl2, PBr3, TsCl	Alkyl halide or tosylate

Additional info: These notes integrate content from multiple slides, expanding on mechanisms, definitions, and synthetic strategies for alcohols as covered in a standard college-level Organic Chemistry course (Ch. 17: Alcohols, Phenols, and Organometallic Compounds).