Structure and Synthesis of Alcohols – Study Notes

Structure and Synthesis of Alcohols

Introduction to Alcohols

Alcohols are a fundamental class of organic compounds characterized by the presence of one or more hydroxyl (–OH) groups attached to a saturated carbon atom. Their general formula is R–OH, where R represents an alkyl or aryl group. Alcohols are widely found in nature and serve as important intermediates in organic synthesis.

• Examples: Ethanol (CH3CH2OH), Methanol (CH3OH), Isopropanol (CH3CHOHCH3), 4-methylcyclohexanol, menthol, cholesterol.

• The hydroxyl group is the functional group responsible for the characteristic properties of alcohols.

Structure and Classification of Alcohols

The oxygen atom in alcohols is sp3 hybridized, resulting in a tetrahedral geometry around the oxygen. The bond angle in water is 104.5°, while in methanol it is slightly larger (108.9°) due to the presence of the methyl group.

• Carbinol carbon: The carbon atom bonded directly to the –OH group.

• Alcohols are classified based on the number of carbon atoms attached to the carbinol carbon:

  • Primary (1°) alcohol: Carbinol carbon attached to one other carbon (e.g., CH3CH2OH).

  • Secondary (2°) alcohol: Carbinol carbon attached to two other carbons (e.g., CH3CHOHCH3).

  • Tertiary (3°) alcohol: Carbinol carbon attached to three other carbons (e.g., (CH3)3COH).

  • Aromatic alcohols (Phenols): –OH group attached to a benzene ring.

Table: Types of Alcohols

Type	General Structure	Examples
Primary (1°)	RCH2OH	Ethanol, 2-methylpropan-1-ol, benzyl alcohol
Secondary (2°)	R2CHOH	Butan-2-ol, cyclohexanol
Tertiary (3°)	R3COH	2-methyl-2-propanol (tert-butanol), 1-methylcyclohexanol
Phenols	ArOH	Phenol, 3-methylphenol, 4-hydroxyphenol

IUPAC Nomenclature of Alcohols

Alcohols are named systematically according to IUPAC rules:

• Identify the longest carbon chain containing the carbon bonded to the –OH group.

• Drop the -e from the parent alkane name and add -ol (e.g., ethane → ethanol).

• Number the chain to give the –OH group the lowest possible number.

• Name and number all substituents, listing them in alphabetical order.

Functional Group Priority in Nomenclature

Priority (Highest to Lowest)
Acids
Esters
Aldehydes
Ketones
Alcohols
Amines
Alkenes, Alkynes
Alkanes
Ethers
Halides

Common Names of Alcohols

• Derived from the common name of the alkyl group followed by the word alcohol (e.g., methyl alcohol, isopropyl alcohol).

• Useful mainly for small alkyl groups.

• Alcohols can be viewed as water molecules with one hydrogen replaced by an alkyl group.

Naming Diols and Glycols

• Diols: Compounds with two –OH groups. The suffix -diol is used, and both positions are numbered (e.g., hexane-1,6-diol).

• Glycols: 1,2-diols (vicinal diols) are called glycols. Common names are based on the alkene from which they are derived (e.g., ethylene glycol for ethane-1,2-diol).

Naming Phenols

• Phenols are named as derivatives of the parent compound phenol.

• The –OH group is assumed to be on carbon 1.

• Substituents are named and numbered accordingly (e.g., 3-chlorophenol, 4-methylphenol, 2-bromophenol).

Physical Properties of Alcohols

Alcohols exhibit unique physical properties due to their ability to form hydrogen bonds.

• Alcohols with up to 11 or 12 carbon atoms are typically liquids at room temperature.

• Methanol and ethanol are volatile liquids with fruity odors; higher alcohols are more viscous and may be solids if highly branched.

• Boiling points are higher than those of ethers and alkanes of similar molecular weight due to hydrogen bonding.

Table: Physical Properties of Some Alcohols

IUPAC Name	Common Name	Formula	mp (°C)	bp (°C)	Density (g/mL)
methanol	wood alcohol	CH3OH	-98	65	0.79
ethanol	grain alcohol	CH3CH2OH	-114	78	0.79
propan-1-ol	n-propyl alcohol	CH3CH2CH2OH	-127	97	0.80
propan-2-ol	isopropyl alcohol	(CH3)2CHOH	-89	82	0.79
butan-1-ol	n-butyl alcohol	CH3CH2CH2CH2OH	-90	118	0.81
butan-2-ol	sec-butyl alcohol	CH3CH(OH)CH2CH3	-115	100	0.81

Boiling Points of Alcohols

• Alcohols have higher boiling points than ethers and alkanes with the same number of carbons due to strong hydrogen bonding.

• The stronger the intermolecular forces, the more energy is required to break them, resulting in higher boiling points.

Example: H-bonding in ethanol leads to a boiling point of 78°C, compared to 35°C for dimethyl ether (same molecular weight).

Solubility in Water

• Alcohols and water can form hydrogen bonds, making lower alcohols miscible with water.

• As the alkyl chain length increases, the nonpolar character increases, decreasing solubility in water.

Table: Solubility of Alcohols in Water (at 25°C)

Alcohol	Solubility in Water
methanol	miscible
ethanol	miscible
propan-1-ol	miscible
butan-1-ol	7.9%
pentan-1-ol	2.7%
hexan-1-ol	0.6%

Commercially Important Alcohols

• Methanol: Also known as wood alcohol, produced from synthesis gas (CO + H2), used as a solvent and fuel, toxic in small amounts.

• Ethanol: Produced by fermentation of sugars or hydration of ethene, used in beverages, solvents, and as a fuel additive (gasohol).

• Propan-2-ol (Isopropyl alcohol): Produced by hydration of propylene, commonly used as rubbing alcohol (70% solution), toxicity similar to methanol.

Acidity of Alcohols and Phenols

• Alcohols have pKa values in the range 15.5–18.0 (water: 15.7).

• Acidity decreases as substitution on the alkyl group increases (tertiary < secondary < primary).

• Electron-withdrawing groups (e.g., halogens) increase acidity.

• Phenol is much more acidic than cyclohexanol due to resonance stabilization of the phenoxide ion.

Table: pKa Values of Alcohols and Phenols

Compound	Structure	pKa
methanol	CH3OH	15.5
ethanol	CH3CH2OH	15.9
isopropanol	(CH3)2CHOH	17.1
tert-butanol	(CH3)3COH	18.0
phenol	C6H5OH	10.0
cyclohexanol	C6H11OH	16.0

Formation of Alkoxide and Phenoxide Ions

• Alkoxide ions (RO–): Strong bases and nucleophiles, formed by reaction of alcohols with sodium or potassium metal.

• Example: $2\ \mathrm{CH_3CH_2OH} + 2\ \mathrm{Na} \rightarrow 2\ \mathrm{CH_3CH_2O^-Na^+} + \mathrm{H_2}\uparrow$

• More hindered alcohols react faster with potassium than sodium.

• Sodium hydride (NaH) can also deprotonate alcohols.

• Phenoxide ions: Formed by deprotonation of phenols with mild bases (e.g., NaOH). Phenols are more acidic due to resonance stabilization of the negative charge over the aromatic ring.

• Alcohols (aliphatic) do not react with NaOH to form alkoxides under normal conditions.

Resonance Stabilization of Phenoxide Ion

• The negative charge on the oxygen atom in the phenoxide ion is delocalized over the ortho and para positions of the aromatic ring, resulting in four resonance structures.

• This resonance stabilization makes phenol much more acidic than cyclohexanol.

Synthesis of Alcohols

Alcohols can be synthesized by several methods:

• Nucleophilic substitution (SN2): Reaction of alkyl halides with hydroxide ion (OH–).

• Addition of water to alkenes:

  • Direct hydration (acid-catalyzed)

  • Oxymercuration-demercuration

  • Hydroboration-oxidation

• Synthesis of vicinal diols:

  • Syn hydroxylation: Osmium tetroxide (OsO4), hydrogen peroxide (H2O2), or cold, dilute potassium permanganate (KMnO4).

  • Anti hydroxylation: Peroxyacids (e.g., mCPBA) followed by hydrolysis.

Organometallic Reagents in Alcohol Synthesis

Organometallic compounds contain covalent bonds between carbon and metal atoms (e.g., Grignard reagents, organolithiums). They are powerful nucleophiles used to form new carbon–carbon bonds.

• Grignard reagents (R–MgX): Prepared by reacting alkyl or aryl halides with magnesium in dry ether.

• Organolithium reagents (R–Li): Prepared by reacting alkyl halides with lithium metal; do not require ether as solvent.

• Sodium acetylides: Terminal alkynes treated with NaNH2 to form sodium acetylides, which are strong nucleophiles.

Reactions of Organometallic Compounds

• Grignard and organolithium reagents add to carbonyl groups (C=O) to form alcohols after protonation.

• Primary alcohols: Addition to formaldehyde.

• Secondary alcohols: Addition to other aldehydes.

• Tertiary alcohols: Addition to ketones.

General equation for Grignard addition to a carbonyl:

$\mathrm{R'MgX} + \mathrm{R_2C=O} \xrightarrow{\text{ether}} \mathrm{R_2C(OMgX)R'} \xrightarrow{\text{H_2O}} \mathrm{R_2C(OH)R'}$

Grignard Reactions with Acid Chlorides and Esters

• Two equivalents of Grignard reagent react with acid chlorides or esters to give tertiary alcohols with two identical alkyl groups.

• The first equivalent forms a ketone intermediate, which reacts with the second equivalent to form the alcohol.

Mechanism with Acid Chloride

• Grignard attacks the carbonyl carbon, expelling chloride to form a ketone.

• The ketone reacts with a second equivalent of Grignard to form a tertiary alkoxide, which is protonated to yield the alcohol.

Mechanism with Esters

• Grignard attacks the carbonyl carbon, expelling the alkoxide to form a ketone.

• The ketone reacts with a second equivalent of Grignard to form a tertiary alkoxide, which is protonated to yield the alcohol.

Addition to Ethylene Oxide

• Grignard and organolithium reagents open epoxides (oxiranes) to form alcohols, extending the carbon chain by two carbons.

Limitations of Organometallics

• Grignard and organolithium reagents are strong bases and react with acidic protons (e.g., –OH, –NH, –SH), so these groups must be absent or protected during reactions.

• They react as nucleophiles with polar multiple bonds (e.g., C=O, C≡N, C=N).

Reduction of Carbonyl Compounds to Alcohols

• Hydride reagents (NaBH4, LiAlH4) add hydride (H–) to carbonyl groups, reducing them to alcohols.

• Reduction of aldehydes yields primary alcohols; reduction of ketones yields secondary alcohols.

• NaBH4 is selective for aldehydes and ketones; LiAlH4 is stronger and reduces esters and carboxylic acids as well.

General equation:

$\mathrm{R_2C=O} + \mathrm{H^-} \rightarrow \mathrm{R_2CHO^-} \xrightarrow{\text{H^+}} \mathrm{R_2CHOH}$

Catalytic Hydrogenation

• Ketones and aldehydes can be reduced to alcohols by catalytic hydrogenation (H2, Raney nickel catalyst).

• This method is less selective and may reduce other unsaturated bonds.

Thiols (Mercaptans)

• Thiols are sulfur analogues of alcohols (R–SH), with the –SH group called a mercapto group.

• Named by adding the suffix -thiol to the alkane name (e.g., methanethiol).

• Thiols are more acidic than alcohols due to the larger size and polarizability of sulfur.

• Thiols are synthesized by SN2 reactions of alkyl halides with HS–.

• Thiols are easily oxidized to disulfides (R–S–S–R) and, under vigorous conditions, to sulfonic acids (R–SO3H).

Summary Table: Key Reactions and Properties

Reaction/Property	Reagents/Conditions	Product
Alcohol synthesis (SN2)	Alkyl halide + OH–	Alcohol
Hydration of alkene	H2O/H+, or oxymercuration, or hydroboration-oxidation	Alcohol
Grignard addition to carbonyl	R–MgX, ether, H2O	Alcohol (1°, 2°, or 3°)
Reduction of carbonyl	NaBH4 or LiAlH4	Alcohol
Thiols oxidation	O2, mild/strong oxidants	Disulfide or sulfonic acid

Additional info: These notes are based on lecture slides and textbook content for Chapter 10 of a standard Organic Chemistry course, focusing on the structure, nomenclature, physical properties, acidity, and synthesis of alcohols and related compounds (thiols, phenols). Mechanistic details and example problems are included to reinforce key concepts.