Unit 7 Alcohols, Phenols and Ethers Full Study Notes

Overview of Alcohols, Phenols, and Ethers

• Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon (aliphatic and aromatic respectively) is replaced by an $-OH$ group.

• Alcohols: Contain one or more hydroxyl ($-OH$) group(s) directly attached to carbon atom(s) of an aliphatic system (e.g., $CH_3OH$).

• Phenols: Contain $-OH$ group(s) directly attached to carbon atom(s) of an aromatic system (e.g., $C_6H_5OH$).

• Ethers: Formed by the substitution of a hydrogen atom in a hydrocarbon by an alkoxy ($R-O$) or aryloxy ($Ar-O$) group. They can also be visualized as compounds where the hydrogen atom of the hydroxyl group of an alcohol or phenol is replaced by an alkyl or aryl group.

• Applications:

  • Alcohols are basic for detergents.

  • Phenols are used in antiseptics.

  • Ethers are used in fragrances.

  • Ethanol (spirit) is used for polishing furniture.

  • Sugar, cotton (fabrics), and paper (writing) are all compounds containing $-OH$ groups.

Classification of Alcohols and Phenols

• Compounds are classified as mono-, di-, tri-, or polyhydric depending on whether they contain one, two, three, or many hydroxyl groups.

• Monohydric Alcohols are further classified based on the hybridization of the carbon atom to which the $-OH$ group is attached:

1. Compounds containing $C_{sp^3}-OH$ bonds

• Primary ($1^{\circ}$), Secondary ($2^{\circ}$), and Tertiary ($3^{\circ}$) Alcohols: The $-OH$ group is attached to a primary, secondary, or tertiary carbon atom respectively.

• Allylic Alcohols: The $-OH$ group is attached to an $sp^3$ hybridized carbon adjacent to a carbon-carbon double bond (an allylic carbon).

• Benzylic Alcohols: The $-OH$ group is attached to an $sp^3$ hybridized carbon atom next to an aromatic ring.

• Note: Allylic and benzylic alcohols can themselves be primary, secondary, or tertiary.

2. Compounds containing $C_{sp^2}-OH$ bonds

• Vinylic Alcohols: The $-OH$ group is bonded to a carbon-carbon double bond (a vinylic carbon). Example: $CH_2=CH-OH$.

• Phenols: The $-OH$ group is bonded directly to an aryl carbon.

Classification of Ethers

• Simple or Symmetrical Ethers: Alkyl or aryl groups attached to the oxygen atom are the same (e.g., Diethyl ether, $C_2H_5OC_2H_5$).

• Mixed or Unsymmetrical Ethers: The two groups attached to the oxygen are different (e.g., $C_2H_5OCH_3$ and $C_2H_5OC_6H_5$).

Nomenclature

Alcohols

• Common Names: Derived from the common name of the alkyl group followed by the word "alcohol" (e.g., $CH_3OH$ is methyl alcohol).

• IUPAC Names: Derived from the parent alkane by substituting the suffix '-e' with '-ol'.

• Rules:

  • Number the longest carbon chain starting from the end nearest the $-OH$ group.

  • For polyhydric alcohols, keep the '-e' of the alkane name and add 'diol', 'triol', etc.

  • Example: $HO-CH_2-CH_2-OH$ is ethane-1,2-diol.

  • Cyclic alcohols use the prefix 'cyclo-' with the $-OH$ group assigned to $C-1$.

Phenols

• Phenol is both the common and accepted IUPAC name for the simplest hydroxy derivative of benzene.

• Substituted phenols use prefixes: ortho ($1,2$-disubstituted), meta ($1,3$-disubstituted), and para ($1,4$-disubstituted) in common names.

• Dihydroxy derivatives are named as benzenediols (e.g., benzene-1,2-diol).

Ethers

• Common Names: Names of alkyl/aryl groups are written in alphabetical order followed by the word 'ether'.

• IUPAC Names: Ethers are named as alkoxyalkanes. The larger alkyl group is chosen as the parent hydrocarbon.

• Example: $CH_3OCH_3$ is methoxymethane; $C_6H_5OCH_3$ is methoxybenzene (Anisole).

Structures of Functional Groups

• Methanol: The oxygen atom is attached to carbon by a sigma bond formed by the overlap of $sp^3$ orbitals of both atoms. The $C-O-H$ bond angle is roughly $108.9^{\circ}$, slightly less than the tetrahedral angle ($109^{\circ}28\text{'}$) due to repulsion between unshared electron pairs.

• Phenol: The $-OH$ group is attached to an $sp^2$ hybridized carbon. The $C-O$ bond length is $136\,pm$, shorter than in methanol ($142\,pm$) due to partial double bond character from conjugation and the $sp^2$ hybridized state of carbon.

• Methoxymethane (Ether): The four electron pairs on oxygen are in a tetrahedral arrangement. Structural bond angle ($C-O-C$) is $111.7^{\circ}$, slightly greater than tetrahedral due to repulsive interactions between bulky alkyl groups. The $C-O$ bond length is $141\,pm$.

Preparation of Alcohols

1. From Alkenes

• Acid Catalyzed Hydration: Alkenes react with water in the presence of acid. Unsymmetrical alkenes follow Markovnikov’s rule.

  • Mechanism:

    1. Protonation of alkene to form carbocation via $H_3O^+$.

    2. Nucleophilic attack of water on the carbocation.

    3. Deprotonation to form the alcohol.

• Hydroboration-Oxidation: Diborane ($(BH_3)_2$) reacts with alkenes to form trialkyl boranes, which are then oxidized by hydrogen peroxide ($H_2O_2$) in aqueous sodium hydroxide ($NaOH$).

  • Result: Addition of water in a manner opposite to Markovnikov’s rule.

  • Yield is excellent. Reported by H.C. Brown in 1959 (Nobel Prize 1979).

2. From Carbonyl Compounds

• Reduction of Aldehydes and Ketones: Catalytic hydrogenation using $Pt$, $Pd$, or $Ni$, or chemical reduction using $NaBH_4$ or $LiAlH_4$.

  • Aldehydes yield primary alcohols.

  • Ketones yield secondary alcohols.

• Reduction of Carboxylic Acids and Esters:

  • Carboxylic acids are reduced to primary alcohols by $LiAlH_4$ (an expensive reagent).

  • Commercial method: Convert acids to esters, then reduce via catalytic hydrogenation.

3. From Grignard Reagents ($RMgX$)

• Nucleophilic addition of Grignard reagent to a carbonyl group forms an adduct, which is hydrolyzed to an alcohol.

• Yields:

  • Methanal ($HCHO$) + $RMgX \rightarrow$ Primary Alcohol.

  • Other Aldehydes + $RMgX \rightarrow$ Secondary Alcohol.

  • Ketones + $RMgX \rightarrow$ Tertiary Alcohol.

Preparation of Phenols

1. From Haloarenes: Chlorobenzene fused with $NaOH$ at $623\,K$ and $320\,atm$, followed by acidification.

2. From Benzene Sulphonic Acid: Benzene is sulfonated with oleum, heated with molten $NaOH$ to form sodium phenoxide, then acidified.

3. From Diazonium Salts: Aniline treated with $NaNO_2 + HCl$ ($273\text{-}278\,K$) forms benzene diazonium chloride. Hydrolysis by warming with water or dilute acid produces phenol.

4. From Cumene (Isopropylbenzene): Cumene is oxidized by air to cumene hydroperoxide. Treatment with dilute acid yields phenol and acetone (a valuable byproduct).

Physical Properties of Alcohols and Phenols

• Boiling Points:

  • Increase with increasing carbon atoms (increase in van der Waals forces).

  • Decrease with increased branching (decrease in surface area/van der Waals forces).

  • Alcohols/Phenols have significantly higher boiling points than hydrocarbons, ethers, or haloalkanes of similar mass due to intermolecular hydrogen bonding.

• Solubility:

  • Soluble in water due to ability to form hydrogen bonds with water molecules.

  • Solubility decreases as the size of the hydrophobic alkyl/aryl group increases.

Chemical Reactions of Alcohols and Phenols

Alcohols are versatile, acting as both nucleophiles (O-H bond breaks) and electrophiles (C-O bond breaks in protonated alcohols).

A. Reactions involving O-H bond cleavage

1. Acidity:

   • Alcohols/Phenols react with active metals ($Na$, $K$, $Al$) to form alkoxides/phenoxides and release $H_2$.

   • Alcohols are weaker acids than water (verified by alkoxide reacting with water).

   • Acid strength order: Primary > Secondary > Tertiary (due to electron-releasing effect of alkyl groups increasing electron density on oxygen).

   • Phenols are more acidic than alcohols because the phenoxide ion is stabilized by resonance (delocalization of negative charge).

   • Electron-withdrawing groups (e.g., nitro groups) increase phenol acidity, especially at ortho/para positions. Electron-releasing groups (e.g., alkyl) decrease it.

2. Esterification: Alcohols/Phenols react with carboxylic acids, acid chlorides, or acid anhydrides.

   • Acetylation: Introduction of acetyl ($CH_3CO$) group. Acetylation of salicylic acid produces Aspirin (analgesic, antipyretic, anti-inflammatory).

B. Reactions involving C-O bond cleavage (Alcohols Only)

1. Reaction with Hydrogen Halides: $ROH + HX \rightarrow R-X + H_2O$.

   • Lucas Test: Conc. $HCl$ and $ZnCl_2$. Tertiary alcohols show immediate turbidity; secondary alcohols show it in $5$ minutes; primary alcohols show no turbidity at room temperature.

2. Reaction with Phosphorus Trihalides: $ROH + PBr_3 \rightarrow R-Br$.

3. Dehydration: Formation of alkenes using protic acids ($H_2SO_4$, $H_3PO_4$).

   • Ease of dehydration: Tertiary > Secondary > Primary.

   • Mechanism (Ethanol to Ethene): Protonation $\rightarrow$ Carbocation formation (slowest step) $\rightarrow$ Elimination of proton.

4. Oxidation: Loss of dihydrogen (dehydrogenation).

   • Primary alcohols $\rightarrow$ Aldehydes (using $CrO_3$ or PCC) $\rightarrow$ Carboxylic acids (using $KMnO_4$).

   • Secondary alcohols $\rightarrow$ Ketones (using $CrO_3$).

   • Tertiary alcohols: Resistant to oxidation; under strong conditions, they undergo C-C bond cleavage.

   • Passing vapors over heated copper ($573\,K$):

     • Primary $\rightarrow$ Aldehyde.

     • Secondary $\rightarrow$ Ketone.

     • Tertiary $\rightarrow$ Alkene (dehydration).

C. Specific Reactions of Phenol

1. Electrophilic Aromatic Substitution:

   • Nitration: Dilute $HNO_3$ yields ortho and para nitrophenols (separated by steam distillation: ortho is more volatile due to intramolecular H-bonding). Conc. $HNO_3$ yields Picric acid ($2,4,6$-trinitrophenol).

   • Halogenation: Phenol + $Br_2$ in solvent ($CS_2$) yields mono-bromophenols. Phenol + Bromine water yields a white precipitate of $2,4,6$-tribromophenol.

2. Kolbe’s Reaction: Phenoxide ion + $CO_2$ (followed by acidification) yields ortho-hydroxybenzoic acid (Salicylic acid).

3. Reimer-Tiemann Reaction: Phenol + $CHCl_3$ + $NaOH$ introduces a $-CHO$ group at the ortho position, resulting in Salicylaldehyde.

4. Reduction with Zinc Dust: Phenol + $Zn \rightarrow$ Benzene + $ZnO$.

5. Oxidation: Oxidation with chromic acid yields Benzoquinone.

Commercially Important Alcohols

1. Methanol ($CH_3OH$): Known as 'wood spirit'. Produced by catalytic hydrogenation of carbon monoxide ($CO + 2H_2 \xrightarrow{ZnO-Cr_2O_3} CH_3OH$). Highly poisonous; causes blindness and death.

2. Ethanol (C_2_H_5OH): Obtained by fermentation of sugars (molasses/grapes) using invertase and zymase.

   • Fermentation is anaerobic; releases $CO_2$.

   • Denaturation: Ethanol made unfit for drinking by adding copper sulphate and pyridine.

Ethers: Preparation and Reactions

Preparation

1. Dehydration of Alcohols: Ethanol at $413\,K$ with $H_2SO_4$ yields ethoxyethane. (At $443\,K$, it yields ethene).

2. Williamson Synthesis: Reaction of alkyl halide with sodium alkoxide ($R-X + R'-ONa \rightarrow R-O-R' + NaX$).

   • This is an $S_N2$ reaction.

   • Primary alkyl halides give best results. Tertiary halides yield alkenes exclusively due to elimination.

Physical Properties

• Ethers have a net dipole moment but boiling points are comparable to alkanes of similar mass (much lower than alcohols due to lack of H-bonding).

• Solubility in water is comparable to alcohols of the same mass due to H-bonding with water.

Chemical Reactions

1. Cleavage of C-O bond: Occurs under drastic conditions with excess $HX$.

   • Order of reactivity: HI > HBr > HCl.

   • With mixed ethers ($R-O-R'$):

     • If groups are $1^{\circ}$ or $2^{\circ}$, halide forms with the smaller alkyl group ($S_N2$).

     • If one group is $3^{\circ}$, a tertiary halide forms ($S_N1$).

2. Electrophilic Substitution: Alkoxy group ($-OR$) is ortho, para directing.

   • Bromination: Anisole + $Br_2$ (in ethanoic acid) $\rightarrow$ para-bromoanisole ($90\%$ yield).

   • Friedel-Crafts: Anisole reacts with alkyl/acyl halides using anhydrous $AlCl_3$.

   • Nitration: Anisole + conc. $H_2SO_4$ / $HNO_3 \rightarrow$ mixture of ortho/para nitroanisole.

Questions & Discussion

• Q: Why is ortho nitrophenol steam volatile?

• A: It is due to intramolecular hydrogen bonding. Para-nitrophenol is less volatile because of intermolecular hydrogen bonding which causes molecular association.

• Q: Why is benzene diazonium chloride used to make phenol?

• A: It is formed from aniline and readily hydrolyzes to phenol when warmed with water.

• Q: What happens if an alcoholic drinks methanol?

• A: Methanol is oxidized to methanal then methanoic acid in the body, causing blindness or death. It is treated with intravenous diluted ethanol to swamp the enzymes and allow kidneys to excrete methanol.

• Q: Limitations of Williamson synthesis?

• A: If a tertiary alkyl halide is used, the alkoxide (acting as a strong base) causes elimination rather than substitution, forming an alkene instead of an ether.