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

Alcohols and Phenols

Functional Group Overview and Occurrence in Nature

Alcohols and phenols are organic compounds characterized by the presence of a hydroxyl group (-OH). Alcohols have the hydroxyl group attached to a saturated carbon atom, while phenols have it directly attached to an aromatic benzene ring. These functional groups are prevalent in many natural compounds, including cholesterol, dopamine, and capsaicin.

• Alcohols: Hydroxyl group attached to an sp3 carbon (e.g., ethanol, cyclopentanol).

• Phenols: Hydroxyl group attached directly to a benzene ring (e.g., phenol, dopamine).

• Occurrence: Hydroxyl groups are found in steroids, neurotransmitters, flavor compounds, and more.

• Example: Cholesterol contains a hydroxyl group essential for its biological function.

Nomenclature and Classification

Nomenclature of Alcohols

Alcohols are named using IUPAC rules with specific modifications for the hydroxyl group.

1. Select the longest carbon chain containing the hydroxyl group.

2. Replace the -e ending of the corresponding alkane with -ol.

3. Number the chain from the end closest to the hydroxyl group.

4. List substituents in alphabetical order and indicate their positions.

• The parent chain must include the carbon bearing the -OH, even if it is shorter than another possible chain.

• Assign the lowest possible number to the carbon with the -OH group.

• Locant for the -OH group can be placed before the parent name or just before the -ol suffix (e.g., 3-pentanol or pentan-3-ol).

• For cyclic alcohols, the -OH group is always at carbon 1.

• Common names are frequently used for simple alcohols (e.g., isopropyl alcohol, benzyl alcohol).

Alcohol Classification

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

• Primary (1°) Alcohol: -OH attached to a carbon bonded to one other carbon.

• Secondary (2°) Alcohol: -OH attached to a carbon bonded to two other carbons.

• Tertiary (3°) Alcohol: -OH attached to a carbon bonded to three other carbons.

Phenol Nomenclature

When an -OH group is attached to a benzene ring, the parent name is phenol. Substituents are named according to their relative position from the hydroxyl group (ortho-, meta-, para- or numerical locants).

• Example: 4-chloro-2-nitrophenol

Physical Properties

Boiling Points and Hydrogen Bonding

The hydroxyl group significantly increases the boiling point of alcohols due to hydrogen bonding.

• Alcohols have much higher boiling points than analogous alkanes or alkyl halides.

• Hydrogen bonding occurs between the oxygen of one molecule and the hydrogen of another.

• Example: Ethanol (bp = 78°C) vs. ethane (bp = -89°C) and chloroethane (bp = 12°C).

Solubility in Water

• Alcohols with three or fewer carbon atoms are miscible in water due to the hydrophilic nature of the -OH group.

• Alcohols with longer carbon chains are less soluble due to the hydrophobic alkyl region.

Antibacterial Properties

The effectiveness of alcohols as antibacterial agents depends on the balance between hydrophobic and hydrophilic regions.

• Alcohols need some water solubility to travel through aqueous media.

• A significant hydrophobic region is required to penetrate microbial membranes.

• Hexylresorcinol is used as an antibacterial and antifungal agent due to its optimal balance.

Acid/Base Chemistry of Alcohols and Phenols

Acidity

Alcohols and phenols are weak Brønsted acids. Their acidity is influenced by substituents and molecular structure.

• Alcohols: $pK_a \approx 15$

• Phenols: More acidic than alcohols due to resonance stabilization of the conjugate base.

• The conjugate base of an alcohol is called an alkoxide ($RO^-$).

• Strong bases (e.g., NaH) are required to deprotonate alcohols.

• Electron-withdrawing substituents increase acidity via inductive effects.

Name	Formula	pKa
Water	HO–H	15.7
Methanol	CH3OH	15.5
Ethanol	CH3CH2OH	16
t-Butyl alcohol	(CH3)3COH	18
2,2,2-Trichloroethanol	CCl3CH2OH	12.2
Phenol	C6H5OH	10
p-Nitrophenol		7.2
Picric acid		0.25

Basicity

• Alcohols can act as weak Brønsted bases and be protonated by strong acids.

• The conjugate acid of an alcohol is called an oxonium ion ($ROH_2^+$).

Preparation of Alcohols

From Alkyl Halides (Substitution)

Alcohols can be synthesized from alkyl halides via nucleophilic substitution reactions.

• Primary alkyl halides: $S_N2$ mechanism predominates.

• Tertiary alkyl halides: $S_N1$ mechanism predominates.

• General reaction: $R-X \rightarrow R-OH$

From Alkenes (Addition)

Alcohols can be synthesized from alkenes via addition reactions, such as acid-catalyzed hydration.

• Markovnikov addition: The -OH group adds to the more substituted carbon.

• General reaction: $R_2C=CR_2 + H_2O \xrightarrow{H^+} R_2C(OH)-CR_2$

Reactions of Alcohols

Dehydration of Alcohols

Alcohols can undergo elimination reactions to form alkenes under acidic conditions.

• 2° and 3° alcohols favor the $E1$ mechanism under strong acid and heat.

• Regioselectivity: Favors formation of the more substituted alkene (Zaitsev's Rule).

• Equation: $R_2CH-CH_2OH \xrightarrow{H_2SO_4,\ heat} R_2C=CH_2 + H_2O$

Conversion of Alcohols into Alkyl Halides

Alcohols can be converted into alkyl halides using acids or reagents such as thionyl chloride and phosphorous tribromide.

• With HBr: The -OH group is protonated, forming water as a leaving group, followed by substitution.

• Thionyl chloride ($SOCl_2$) and phosphorous tribromide ($PBr_3$) convert alcohols to alkyl halides via $S_N2$ mechanism (works for 1° and 2° alcohols).

Thionyl Chloride Mechanism

• Alcohol reacts with $SOCl_2$ to form a chlorosulfite ester intermediate.

• $S_N2$ reaction leads to alkyl chloride, $SO_2$, and $HCl$ as products.

• Pyridine is often added to neutralize $HCl$.

Phosphorous Tribromide Mechanism

• Alcohol reacts with $PBr_3$ to form alkyl bromide and $HOPBr_2$.

• Improves leaving group ability for substitution.

Oxidation of Alcohols

Alcohols are oxidized to carbonyl compounds. The product depends on the type of alcohol and the oxidizing agent.

• Primary alcohols: Can be oxidized to aldehydes, then to carboxylic acids.

• Secondary alcohols: Oxidized to ketones.

• Tertiary alcohols: Generally do not undergo oxidation (no α-hydrogen).

Common Oxidizing Agents

• Chromic acid (Jones reagent): Formed from $CrO_3$ or $Na_2Cr_2O_7$ in aqueous acid. Strong oxidant, exhaustively oxidizes primary alcohols to carboxylic acids and secondary alcohols to ketones.

• Pyridinium chlorochromate (PCC): Milder oxidant, oxidizes primary alcohols to aldehydes and secondary alcohols to ketones, but does not oxidize aldehydes further.

Jones Oxidation

• Primary alcohols: $RCH_2OH \xrightarrow{Jones} RCOOH$

• Secondary alcohols: $R_2CHOH \xrightarrow{Jones} R_2CO$

• Aldehydes: $RCHO \xrightarrow{Jones} RCOOH$

Aldehydes from Primary Alcohols

• PCC allows selective oxidation of primary alcohols to aldehydes.

• Equation: $RCH_2OH \xrightarrow{PCC} RCHO$

Summary Reaction Map (Alcohols)

The following summarizes the main transformations involving alcohols:

• Preparation: Alkyl halide substitution ($S_N1$, $S_N2$), alkene addition (hydration).

• Reactions: Dehydration (elimination), conversion to alkyl halides ($SOCl_2$, $PBr_3$), oxidation (Jones, PCC).

Additional info:

• Alcohols and phenols are central to organic synthesis and biochemistry, serving as intermediates and functional groups in many reactions.

• Understanding their reactivity and transformations is essential for advanced study in organic chemistry.