Study Notes on Alcohols and Phenols
ALCOHOLS AND PHENOLS
PROPERTIES OF ALCOHOLS AND PHENOLS
1. Physical Properties
Alcohols contain an –OH group connected to a saturated carbon (sp³).
Phenols contain an –OH group connected to a benzene ring.
The structure is similar to the structure of water.
Alcohols and phenols have higher boiling points than similar alkanes or alkyl halides.
2. Hydrogen Bonding
A positively charged hydrogen atom from the –OH group of one alcohol molecule is attracted to a lone pair of electrons on the negatively polarized oxygen atom of another alcohol molecule.
This attraction produces a force that holds the two atoms together.
The intermolecular attraction is present in solution but not in the gas phase, thus contributing to the elevated boiling point of the solution.
3. Acidity/Basicity
Basicity
Alcohols are classified as weak Brønsted bases.
They can be protonated by strong acids to yield oxonium ions (ROH₂⁺).
Alkoxides are bases used as reagents in organic chemistry.
Electron-withdrawing groups can enhance the acidity of an alcohol by stabilizing the conjugate base (alkoxide).
Acidity
Alcohols exhibit acidity comparable to water.
They can transfer a proton to water to a very small extent, producing hydronium ions (H₃O⁺) and alkoxide ions (RO⁻).
Steric effects are significant, as the structural arrangement can influence acidity.
Detailed Acidity Characteristics:
Simple alcohols with alkyl groups generally exhibit weaker acidity.
A strong base is required to form alkoxides, commonly using reagents such as sodium hydride (NaH), sodium amide (NaNH₂), or Grignard reagents (RMgX).
4. Phenol Acidity
Phenols (pKa ~ 10) are significantly more acidic than alcohols (pKa ~ 16) due to resonance stabilization of the phenoxide ion.
Phenols react with sodium hydroxide solutions to form soluble salts, which are soluble in dilute aqueous solutions.
A phenolic component can be separated from an organic solution by extraction into a basic aqueous solution and can be isolated after the addition of acid to the solution.
Table 17-1 Acidity Constants of Some Alcohols and Phenols
(CH₃)₃COH: pKa = 18 (Weaker acid)
CH₃CH₂OH: pKa = 16
H₂O: pKa = 15.74
CH₃OH: pKa = 15.54
CF₃CH₂OH: pKa = 12.43
p-Aminophenol: pKa = 10.46
CH₃SH: pKa = 10.3
p-Methylphenol: pKa = 10.17
Phenol: pKa = 9.89
p-Chlorophenol: pKa = 9.38 (Stronger acid)
p-Nitrophenol: pKa = 7.15
USES OF ALCOHOLS AND PHENOLS
Methanol
Prepared historically by heating wood in the absence of air, hence referred to as wood alcohol.
Approximately 13 billion gallons of methanol are manufactured today through the catalytic reduction of carbon monoxide with hydrogen gas.
It is toxic to humans, causing blindness in small doses (around 15 mL) and death in larger quantities (100–250 mL).
Industrial Uses: Solvent and starting material for the production of formaldehyde (CH₂O) and acetic acid (CH₃CO₂H).
Ethanol
Produced from the fermentation of grains and sugars.
Approximately 18 billion gallons of ethanol are currently manufactured primarily by fermenting corn, barley, sorghum, etc. for automobile fuel.
Industrial Uses: Serves as a solvent or chemical intermediate largely obtained by acid-catalyzed hydration of ethylene at high temperatures.
Phenol
Commonly found in nature, serving as a general disinfectant found in coal tar; methyl salicylate is a flavoring agent in oil of wintergreen, and urushiols are the allergenic constituents of poison oak and poison ivy.
Industrial applications include intermediates in the synthesis of diverse products, such as adhesives and antiseptics.
NAMING ALCOHOLS AND PHENOLS
Alcohols
Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of organic groups bonded to the carbon bearing the hydroxyl group.
Simple alcohols are named by the IUPAC system as derivatives of the parent alkane, with the suffix '-ol' added.
Rules for Naming Simple Alcohols
Select the longest carbon chain containing the hydroxyl group and derive the parent name by replacing the -e ending of the corresponding alkane with -ol.
Number the alkane chain starting from the end nearer the hydroxyl group.
Number the substituents according to their positions on the chain. Write the name, listing the substituents in alphabetical order while indicating the carbon position to which the C–OH group is bonded.
Naming Phenols
Phenols are named according to the rules for aromatic compounds. The suffix -phenol is used as the parent name instead of -benzene.
Examples of Alcohols
Benzyl alcohol (phenylmethanol), Allyl alcohol (2-propen-1-ol), tert-Butyl alcohol (2-methyl-2-propanol), Ethylene glycol (1,2-ethanediol).
Structural Framework of Alcohols
In alcohols, the oxygen of the –OH group is attached to carbon via a sigma (σ) bond formed through the overlap of a sp³ hybridized orbital from carbon with a sp³ hybridized orbital from oxygen.
The bond angle in alcohols is slightly less than the tetrahedral angle (109°-28′) due to the repulsion between the unshared electron pairs present on oxygen.
Structural Framework of Phenols
In phenols, the –OH group is attached to an sp² hybridized carbon of the aromatic ring.
The carbon–oxygen bond length in phenol is slightly less than that in methanol (136 pm) due to:
(i) Partial double bond character as a result of the conjugation of the unshared electron pair of oxygen with the aromatic ring.
(ii) The sp² hybridized state of the carbon to which oxygen is attached.
PREPARATION OF ALCOHOLS
1. From Alkenes
A. Acid-Catalyzed Hydration
Alkenes react with water in the presence of an acid catalyst to form alcohols.
For unsymmetrical alkenes, the addition reaction follows Markovnikov’s rule.
B. Hydroboration Reaction
Diborane (BH₃) reacts with alkenes to produce trialkyl boranes as addition products.
These trialkyl boranes are oxidized to alcohols using hydrogen peroxide in the presence of aqueous sodium hydroxide.
Boron attaches to the sp² carbon carrying a greater number of hydrogen atoms.
C. Oxymercuration-Demercuration
The reaction involves the addition of mercury acetate (Hg(OAc)₂) to an alkene followed by hydrolysis.
A high yield of alcohol (92%) can be obtained from a carbocation intermediate.
D. Syn Hydroxylation
Treatment of an alkene with osmium tetroxide (OsO₄) in the presence of sodium hydrosulfite yields a cis-diol.
2. From Carbonyl Compounds
A. By Reduction of Aldehydes and Ketones
Aldehydes and ketones can be converted to corresponding alcohols through hydrogen addition in the presence of catalysts like platinum, palladium, or nickel.
Aldehydes yield primary alcohols, while ketones yield secondary alcohols.
B. By Reduction of Carboxylic Acids and Esters
Carboxylic acids can be reduced to primary alcohols using lithium aluminum hydride (LiAlH₄) or by converting the acids to esters which are subsequently reduced using hydrogen in the presence of catalysts.
3. From Grignard Reagents
Grignard reagents react with aldehydes and ketones to produce alcohols through nucleophilic addition.
4. General Preparation Process of Alcohols
Grignard reagents can also be synthesized from alkyl halides which react with magnesium.
PREPARATION OF PHENOLS
From Haloarenes
Fusion of chlorobenzene with NaOH at high temperatures and pressures yields phenol through acidification.
From Benzenesulfonic Acid
Converting benzene to benzenesulfonic acid followed by sodium phenoxide formation yields phenol upon acidification.
From Diazonium Salts
Hydrolysis of diazonium salts results in the formation of phenols.
From Cumene
The oxidation of cumene in the presence of air yields phenol.
REACTIVITY OF ALCOHOLS AND PHENOLS
Alcohols react as nucleophiles and electrophiles. The bond between O–H breaks when acting as nucleophiles, while the C–O bond breaks when acting as electrophiles.
REACTION OF ALCOHOLS AND PHENOLS
Reactions Involving Cleavage of O–H - Acidity of Alcohols and Phenols
Active metals like sodium and potassium react with alcohols to yield corresponding metal alkoxides.
Alcohols demonstrate weak acidity compared to water, and the acidic character depends on the polarity of the O–H bond.
Reactions Involving Cleavage of Carbon-Oxygen (C–O) -
Alcohols can react with hydrogen halides to form alkyl halides. The general equation is: ROH + HX → R–X + H₂O.
Esterification
Alcohols react with carboxylic acids or anhydrides to form esters, with the reaction being reversible.
Removal of water drives the reaction forward, and the use of acid chlorides leads to a more efficient ester formation process.
Dehydration
Alcohols can undergo dehydration reactions with protic acids to form alkenes or ethers.
When treated with phosphorous oxychloride (POCl₃) in pyridine, dehydration proceeds through an E2 mechanism.
Oxidation
Primary alcohols are oxidized to aldehydes and then to carboxylic acids; secondary alcohols yield ketones while tertiary alcohols resist oxidation.
Electrophilic Aromatic Substitution Reactivity of Phenols
Phenols can undergo electrophilic substitution via nitration, halogenation, Kolbe’s reaction, Reimer-Tiemann reaction, and oxidation.
STRUCTURE DETERMINATION OF ALCOHOLS AND PHENOLS
1. Infrared (IR) Spectroscopy
Alcohols have strong C–O stretching absorption near 1050 cm⁻¹ and characteristic O–H absorption at 3300 to 3600 cm⁻¹.
Phenols have a broad IR absorption at 3500 cm⁻¹ due to –OH group presence.
2. Mass Spectroscopy
Fragmentation patterns include alpha cleavage and dehydration pathways in mass spectrometry.
Events create neutral radicals and resonance-stabilized oxygen-containing cations.
PROBLEM SET
I. Grignard Reactions to Prepare Alcohols
2-Methyl-2-propanol
1-Methylcyclohexanol
3-Methyl-3-pentanol
2-Phenyl-2-butanol
Benzyl alcohol
4-Methyl-1-pentanol