Alcohols And Phenols

Classification (Alcohols & Phenols)

Alcohols and phenols are organic compounds characterized by the presence of a hydroxyl (-OH) group. Their classification depends on the nature of the carbon atom to which the hydroxyl group is attached and the number of hydroxyl groups present.

Alcohols — Mono, Di, Tri Or Polyhydric Alcohols

Definition: Alcohols are organic compounds in which the hydroxyl (-OH) group is attached to an $sp^3$ hybridized carbon atom.

Classification Based on the Number of -OH Groups:

Monohydric Alcohols: Contain one hydroxyl group (e.g., Methanol $CH_3OH$, Ethanol $C_2H_5OH$).
Dihydric Alcohols (Diols or Glycols): Contain two hydroxyl groups (e.g., Ethane-1,2-diol $CH_2(OH)CH_2(OH)$).
Trihydric Alcohols (Triols): Contain three hydroxyl groups (e.g., Propane-1,2,3-triol, Glycerol $CH_2(OH)CH(OH)CH_2(OH)$).
Polyhydric Alcohols: Contain more than three hydroxyl groups (e.g., Carbohydrates like glucose $C_6H_{12}O_6$).

Classification Based on the Carbon Attached to -OH Group: This is the same classification as for haloalkanes.

Primary (1°) Alcohols: The -OH group is attached to a primary carbon atom (a carbon attached to only one other carbon atom).

Example: Ethanol ($CH_3CH_2OH$)

Secondary (2°) Alcohols: The -OH group is attached to a secondary carbon atom (a carbon attached to two other carbon atoms).

Example: Propan-2-ol ($CH_3CH(OH)CH_3$)

Tertiary (3°) Alcohols: The -OH group is attached to a tertiary carbon atom (a carbon attached to three other carbon atoms).

Example: 2-Methylpropan-2-ol ($(CH_3)_3COH$)

Phenols — Mono, Di And Trihydric Phenols

Definition: Phenols are organic compounds in which the hydroxyl (-OH) group is directly attached to an $sp^2$ hybridized carbon atom of an aromatic ring.

Classification Based on the Number of -OH Groups:

Monohydric Phenols: Contain one hydroxyl group attached to the benzene ring (e.g., Phenol $C_6H_5OH$, Cresol $C_6H_4(CH_3)OH$).
Dihydric Phenols (Phenols): Contain two hydroxyl groups attached to the benzene ring (e.g., Catechol (1,2-dihydroxybenzene), Resorcinol (1,3-dihydroxybenzene), Hydroquinone (1,4-dihydroxybenzene)).
Trihydric Phenols: Contain three hydroxyl groups attached to the benzene ring (e.g., Pyrogallol (1,2,3-trihydroxybenzene), Phloroglucinol (1,3,5-trihydroxybenzene)).

Distinction from Phenols: It's important to distinguish phenols from alcohols. In alcohols, the -OH group is attached to an $sp^3$ carbon, while in phenols, it's attached directly to an $sp^2$ carbon of an aromatic ring. This difference significantly impacts their properties, particularly acidity.

Nomenclature (Alcohols & Phenols)

The IUPAC system provides a systematic way to name alcohols and phenols.

Nomenclature of Alcohols:

Parent Chain: Identify the longest continuous carbon chain containing the hydroxyl group.
Numbering: Number the chain starting from the end nearest to the hydroxyl group, giving it the lowest possible number.
Suffix: Replace the '-e' ending of the parent alkane name with '-ol'. Indicate the position of the hydroxyl group by the number of the carbon atom to which it is attached.
Substituents: Name and number other substituents as prefixes in alphabetical order.

Examples:

$CH_3OH$: Methanol
$CH_3CH_2OH$: Ethanol
$CH_3CH_2CH_2OH$: Propan-1-ol
$CH_3CH(OH)CH_3$: Propan-2-ol
$CH_3CH(CH_3)CH(OH)CH_3$: 4-Methylpentan-2-ol
Ethane-1,2-diol (Ethylene glycol)
Propane-1,2,3-triol (Glycerol)

Nomenclature of Phenols:

Parent Name: The simplest phenol ($C_6H_5OH$) is named phenol.
Substituted Phenols: Number the benzene ring starting from the carbon attached to the hydroxyl group as C1. Other substituents are given the lowest possible numbers.
Common Names: Some substituted phenols have widely accepted common names that are also used in IUPAC nomenclature (e.g., Cresols for methylphenols, Xylenes for dimethylphenols).
Prefixes: For di- and trihydric phenols, numbering is used to indicate the positions of the hydroxyl groups (e.g., Benzene-1,2-diol for Catechol).

Examples:

$C_6H_5OH$: Phenol
$o$-Cresol (2-methylphenol)
$m$-Cresol (3-methylphenol)
$p$-Cresol (4-methylphenol)
Benzene-1,2-diol (Catechol)
Benzene-1,3-diol (Resorcinol)
Benzene-1,4-diol (Hydroquinone)
Pyrogallol (Benzene-1,2,3-triol)

Structures Of Functional Groups (Alcohols & Phenols)

The structure of the hydroxyl (-OH) group and its attachment to the carbon framework determine the classification and properties of alcohols and phenols.

Structure of Alcohol Functional Group:

The hydroxyl group (-OH) consists of an oxygen atom covalently bonded to a hydrogen atom.
The oxygen atom is also covalently bonded to a carbon atom.
Bond Angle: The $H-O-C$ bond angle is typically around 108.9° (similar to water but slightly affected by the alkyl group).
Polarity: The $O-H$ bond is highly polar due to the large electronegativity difference between oxygen and hydrogen, leading to partial negative charge on oxygen ($\delta^-$) and partial positive charge on hydrogen ($\delta^+$). The $C-O$ bond is also polar.
Hydrogen Bonding: The polar nature of the $O-H$ bond allows alcohols to form intermolecular hydrogen bonds, which significantly influence their physical properties (high boiling points, solubility in water).
Attachment to Carbon:

Primary Alcohol: -OH attached to a primary ($1^\circ$) carbon (e.g., Ethanol: $CH_3-CH_2-OH$).
Secondary Alcohol: -OH attached to a secondary ($2^\circ$) carbon (e.g., Propan-2-ol: $CH_3-CH(OH)-CH_3$).
Tertiary Alcohol: -OH attached to a tertiary ($3^\circ$) carbon (e.g., 2-methylpropan-2-ol: $(CH_3)_3C-OH$).

Structure of Phenol Functional Group:

The hydroxyl (-OH) group is directly attached to an $sp^2$ hybridized carbon atom of an aromatic ring (like benzene).
Bond Angle: The $H-O-C_{aryl}$ bond angle is slightly different from that in alcohols, typically around 109°.
Polarity: The $O-H$ bond is highly polar, and the $C_{aryl}-O$ bond is also polar, though less so than $C_{alkyl}-O$ due to resonance.
Resonance: The lone pairs on the oxygen atom of the -OH group can participate in resonance with the aromatic ring, increasing electron density in the ring and making the $O-H$ bond slightly weaker and more acidic than in alcohols.
Hydrogen Bonding: Phenols can also form intermolecular hydrogen bonds, but generally weaker than in alcohols due to the electron-withdrawing effect of the aromatic ring on the oxygen atom.

Difference from Alcohols: The attachment of the -OH group to an $sp^2$ hybridized carbon in phenols (compared to $sp^3$ in alcohols) results in significant differences in their acidity, reactivity, and physical properties.

Alcohols And Phenols

Alcohols and phenols are important classes of organic compounds with diverse applications.

Preparation Of Alcohols

1. From Alkenes:

Acid-Catalysed Hydration: Addition of water across the double bond in the presence of an acid catalyst ($H^+$). Follows Markovnikov's rule (hydrogen adds to the carbon with more hydrogens).

$CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$

Hydroboration-Oxidation: Anti-Markovnikov addition of water across the double bond, yielding primary alcohols from terminal alkenes.

$3RCH=CH_2 + (BH_3)_2 \rightarrow (RCH_2CH_2)_3B \xrightarrow{H_2O_2, OH^-} 3RCH_2CH_2OH$

2. From Carbonyl Compounds:

Reduction of Aldehydes and Ketones: Using reducing agents like $LiAlH_4$, $NaBH_4$, or catalytic hydrogenation ($H_2$/Ni, Pt, Pd). Aldehydes form primary alcohols, and ketones form secondary alcohols.

$RCHO \xrightarrow{[H]} RCH_2OH$ (Primary)

$R_2CO \xrightarrow{[H]} R_2CHOH$ (Secondary)

Reduction of Carboxylic Acids and Esters: Requires stronger reducing agents like $LiAlH_4$.

$RCOOH \xrightarrow{LiAlH_4} RCH_2OH$

$RCOOR' \xrightarrow{LiAlH_4} ROH + R'OH$

3. From Haloalkanes:

Reaction with Aqueous $NaOH$ or $KOH$: Nucleophilic substitution ($S_N2$ favored for primary and secondary, $S_N1$ for tertiary).

$R-X + OH^- \rightarrow R-OH + X^-$

Reaction with moist silver oxide:

$2R-X + Ag_2O + H_2O \rightarrow 2R-OH + 2AgX$

4. From Grignard Reagents: Reaction of Grignard reagents with aldehydes and ketones, followed by hydrolysis.

With Formaldehyde: Grignard reagent + $HCHO$ $\rightarrow$ Primary alcohol
With Other Aldehydes: Grignard reagent + $RCHO$ $\rightarrow$ Secondary alcohol
With Ketones: Grignard reagent + $R_2CO$ $\rightarrow$ Tertiary alcohol
With Esters: Grignard reagent + $RCOOR'$ $\rightarrow$ Tertiary alcohol (after reaction with two moles of Grignard reagent)

Preparation Of Phenols

1. From Halogenated Hydrocarbons (Aryl Halides):

Nucleophilic Aromatic Substitution: Requires harsh conditions. Reaction of chlorobenzene with $NaOH$ at high temperature and pressure.

$C_6H_5Cl + 2NaOH \xrightarrow{623K, 300atm} C_6H_5ONa + NaCl + H_2O$

$C_6H_5ONa + H^+ \rightarrow C_6H_5OH$

2. From Diazonium Salts: Warming the diazonium salt solution decomposes it to phenol and nitrogen gas.

$[C_6H_5N_2]^+Cl^- + H_2O \xrightarrow{warm} C_6H_5OH + N_2(g) + HCl$

3. From Benzene Sulfonic Acid: Fusion of sodium salt of benzene sulfonic acid with sodium hydroxide ($NaOH$).

$C_6H_5SO_3Na + 2NaOH \xrightarrow{fusion} C_6H_5ONa + Na_2SO_3 + H_2O$

$C_6H_5ONa + H^+ \rightarrow C_6H_5OH$

4. Cumene Process: Industrial production of phenol involves oxidation of cumene (isopropylbenzene) to cumene hydroperoxide, followed by acid-catalyzed decomposition.

$C_6H_5CH(CH_3)_2 \xrightarrow{O_2} C_6H_5C(OOH)(CH_3)_2 \xrightarrow{H^+ \ catalyst} C_6H_5OH + CH_3COCH_3$

Physical Properties

Alcohols:

State: Lower alcohols (methanol, ethanol) are colorless liquids with characteristic odors. Higher alcohols are solids.
Boiling Points: Relatively high due to intermolecular hydrogen bonding. Boiling points increase with molecular weight and increase in branching (primary > secondary > tertiary).
Solubility in Water: Lower alcohols (up to C12) are soluble in water due to their ability to form hydrogen bonds with water. Solubility decreases as the size of the nonpolar alkyl chain increases. Higher alcohols are insoluble.

Phenols:

State: Phenol is a colorless crystalline solid with a characteristic odor. Higher phenols are solids.
Boiling Points: Higher than alcohols of comparable molecular weight due to stronger intermolecular forces (more effective resonance in the ring and hydrogen bonding).
Solubility in Water: Phenol is sparingly soluble in water due to the large nonpolar part (benzene ring) opposing the polarity of the -OH group and hydrogen bonding. It is more soluble in hot water. Soluble in organic solvents.

Chemical Reactions

1. Reactions involving cleavage of O—H bond (Acidity):

Alcohols: They are very weak acids. Their acidity is generally lower than water.

React with highly electropositive metals (Na, K) to liberate $H_2$.

$2R-OH + 2Na \rightarrow 2R-ONa + H_2$

Acidity order: Primary > Secondary > Tertiary (due to +I effect of alkyl groups destabilizing the alkoxide ion).

Phenols: They are acidic due to the resonance stabilization of the phenoxide ion. Acidity is stronger than alcohols but weaker than mineral acids.

React with strong bases like $NaOH$ to form sodium phenoxide.

$C_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O$

Do not react with weak bases like $NaHCO_3$.
Acidity order: Phenol > $p$-nitrophenol > $m$-nitrophenol > $p$-cresol > Phenol itself > $m$-cresol > $p$-methoxyphenol. Electron-withdrawing groups increase acidity, while electron-donating groups decrease it.

2. Reactions involving cleavage of C—O bond:

Reaction with Hydrogen Halides (HX):

Alcohols react with $HX$ to form alkyl halides. Reactivity order: Tertiary > Secondary > Primary. Mechanism is $S_N1$ for 3°/2° and $S_N2$ for 1°.
Phenols do not react with $HX$ because the $C_{aryl}-O$ bond is stronger and resonance stabilized. They react with $PCl_5$, $PCl_3$, $SOCl_2$.

Reaction with Phosphorus Halides and Thionyl Chloride: Similar to reactions with $HX$.
Dehydration: Elimination of water.

Intramolecular dehydration of alcohols gives alkenes (at 440°C with $H_2SO_4$) or ethers (at 413°C with $H_2SO_4$).
Intermolecular dehydration of alcohols gives ethers.

Oxidation: Primary alcohols are oxidized to aldehydes and then to carboxylic acids. Secondary alcohols are oxidized to ketones. Tertiary alcohols are resistant to oxidation under mild conditions but are oxidized by strong oxidizing agents with the breaking of $C-C$ bonds.

Primary Alcohol $\xrightarrow{[O]}$ Aldehyde $\xrightarrow{[O]}$ Carboxylic Acid
Secondary Alcohol $\xrightarrow{[O]}$ Ketone
Tertiary Alcohol $\xrightarrow{strong \ oxidizing \ agent}$ Mixture of smaller fragments

Special Reactions of Phenols:

Electrophilic Substitution: Phenols undergo electrophilic substitution reactions more readily than benzene due to the activating and ortho, para-directing nature of the -OH group.

Examples: Nitration, halogenation, sulfonation, Friedel-Crafts reactions.

Bromination: Phenol reacts readily with bromine water ($Br_2$/$H_2O$) to give 2,4,6-tribromophenol precipitate.
Nitration: Phenol reacts with dilute $HNO_3$ to give ortho- and para-nitrophenols. With concentrated $HNO_3$, it gives 2,4,6-trinitrophenol (Picric acid).
Coupling Reaction: Phenols react with diazonium salts in alkaline medium to form azo compounds (dyeing).
Kolbe's Reaction: Reaction of sodium phenoxide with $CO_2$ under pressure to form sodium salicylate.
Reimer-Tiemann Reaction: Reaction of phenol with chloroform ($CHCl_3$) in the presence of alkali to form salicylaldehyde.

Properties Of Ethanol

Physical Properties: Colorless liquid, characteristic smell, volatile, flammable, soluble in water, has a sweetish taste. Soluble in water due to hydrogen bonding.

Chemical Properties:

Acidity: Reacts with active metals ($Na$) to give sodium ethoxide and $H_2$.

$2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2$

Dehydration:

Intramolecular dehydration gives ethene (at 440K, conc. $H_2SO_4$).
Intermolecular dehydration gives diethyl ether (at 413K, conc. $H_2SO_4$).

Oxidation:

Mild oxidation (e.g., $PCC$, heated $CuO$) gives ethanal.
Strong oxidation (e.g., acidified $K_2Cr_2O_7$ or $KMnO_4$) gives ethanoic acid.

Reaction with $Na$ to form Sodium Ethoxide: $2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2$
Esterification: Reacts with carboxylic acids in the presence of acid catalyst to form esters.

$CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$

Combustion: Burns with a blue flame to form $CO_2$ and $H_2O$.

$C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$

Industrial Uses: Solvent, beverages, fuel, source of other organic compounds.