Alcohols, Phenols, and Ethers Lecture Notes

Introduction to Alcohols, Phenols, and Ethers

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

• Alcohols are the hydroxyl derivatives of hydrocarbons in which one or more hydrogen atoms are replaced by a corresponding number of hydroxyl ($-OH$) groups.

  • Examples include:

    • Methyl alcohol: $\text{H}_3\text{C - OH}$

    • Ethyl alcohol: $\text{H}_5\text{C}_2\text{ - OH}$

• Phenols are aromatic hydroxyl compounds in which one or more hydroxyl groups are directly attached to the aromatic nucleus (i.e., the Benzene ring).

  • Equation: $\text{C}_6\text{H}_5\text{-H} + \text{OH} \rightarrow \text{C}_6\text{H}_5\text{-OH}$

• Ethers are formed when a hydrogen atom in a hydrocarbon is replaced by an alkoxy or aryloxy group ($R-O/Ar-O$).

Classification and Nomenclature of Alcohols

• Alcohols are classified based on the number of hydroxyl groups they contain:

  • Monohydric Alcohols: Contain only one $-OH$ group. Example: Methyl alcohol ($\text{CH}_3\text{ - OH}$).

  • Dihydric Alcohols: Contain two $-OH$ groups. Example: Ethylene glycol (Ethane-1,2-diol) with the structure $\text{CH}_2(\text{OH})-\text{CH}_2(\text{OH})$.

  • Trihydric Alcohols: Contain three $-OH$ groups. Example: Glycerol/Glycerin (Propane-1,2,3-triol) with the structure $\text{CH}_2(\text{OH})-\text{CH}(\text{OH})-\text{CH}_2(\text{OH})$.

  • Polyhydric Alcohols: Contain four or more $-OH$ groups. Example: Sorbitol (Hexane-1,2,3,4,5,6-hexaol) with the structure $\text{CH}_2(\text{OH})-(\text{CHOH})_4-\text{CH}_2(\text{OH})$.

• Alcohols are also classified according to the hybridization of the carbon atom to which the hydroxyl group is attached:

  • Compounds containing $C_{sp^3}-OH$ bond:

    1. Alkyl Alcohols: The $-OH$ group is attached to an alkyl group. These are subdivided into:

       • Primary ($1^{\circ}$) Alcohol: The hydroxyl group is attached to a primary carbon atom. Example: Ethanol ($\text{H}_3\text{C-CH}_2\text{-OH}$).

       • Secondary ($2^{\circ}$) Alcohol: The hydroxyl group is attached to a secondary carbon atom ($2^{\circ}$ carbon). Example: Isopropyl alcohol (Propan-2-ol) with the structure $\text{H}_3\text{C-CH(OH)-CH}_3$.

       • Tertiary ($3^{\circ}$) Alcohol: The hydroxyl group is attached to a tertiary carbon atom ($3^{\circ}$ carbon). Example: Tert-butyl alcohol (2-methylpropan-2-ol) with the structure $\text{C}(\text{CH}_3)_3\text{OH}$.

    2. Allylic Alcohols: The hydroxyl group is attached to a $sp^3$ hybridized carbon atom next to a carbon-carbon double bond (an allylic carbon).

       • Primary Allylic: Prop-2-en-1-ol ($\text{H}_2\text{C=CH-CH}_2\text{-OH}$).

       • Secondary Allylic: But-3-en-2-ol ($\text{H}_2\text{C=CH-CH(OH)-CH}_3$).

       • Tertiary Allylic: 2-Methylbut-3-en-2-ol ($\text{H}_2\text{C=CH-C(OH)(CH}_3)_2$).

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

       • Primary Benzylic: Benzyl alcohol (Phenyl methanol).

       • Secondary Benzylic: 1-Phenylethanol.

       • Tertiary Benzylic: 2-Phenylpropan-2-ol.

  • Compounds containing $C_{sp^2}-OH$ bond:

    • Vinylic Alcohols: The $-OH$ group is attached to a vinylic carbon ($C=C$). Example: Vinyl alcohol ($\text{H}_2\text{C=CH-OH}$).

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

Classification and Nomenclature of Phenols

• Monohydric Phenols: Contain one $-OH$ group.

  • Phenol (Carbolic acid).

  1. 1-Naphthol ($\alpha$-Naphthol).

  2. 2-Methylphenol (o-Cresol).

  3. 3-Methylphenol (m-Cresol).

  4. 4-Methylphenol (p-Cresol).

  5. 2-Hydroxybenzaldehyde (Salicylaldehyde).

  6. 2-Hydroxybenzoic acid (Salicylic acid).

  7. 4-Hydroxyethyl benzoate (Paracetamol).

• Dihydric Phenols: Contain two $-OH$ groups.

  1. Catechol (Benzene-1,2-diol).

  2. Resorcinol (Benzene-1,3-diol).

  3. Hydroquinone/Quinol (Benzene-1,4-diol).

• Trihydric Phenols: Contain three $-OH$ groups.

  1. Pyrogallol (Benzene-1,2,3-triol).

  2. Phloroglucinol (Benzene-1,3,5-triol).

  3. Benzene-1,2,4-triol.

Methods of Preparation of Phenols

• Dow's Process:

  • Chlorobenzene (Phenyl Chloride) reacts with $6-8\% \, \text{NaOH}$ at $613\,K$ and $300\,atm$ pressure.

  • Reaction: $\text{C}_6\text{H}_5\text{Cl} + 2\text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{ONa} + \text{NaCl} + \text{H}_2\text{O}$.

  • The intermediate is Sodium phenoxide (Phenate), which is then treated with dilute $\text{HCl}$ to produce Phenol: $\text{C}_6\text{H}_5\text{ONa} + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{NaCl}$.

• From Diazonium Salts:

  • Aniline reacts with $\text{NaNO}_2$ and $\text{HCl}$ (to form Nitrous acid $\text{HNO}_2$) at $273-278\,K$ ($0^{\circ}\text{C}-5^{\circ}\text{C}$) to form Benzene diazonium chloride.

  • Benzene diazonium chloride is hydrolyzed with warm water ($\text{H}_2\text{O}/\text{H}^+$) to produce Phenol, $\text{HCl}$, and $\text{N}_2$ gas.

• From Cumene (Isopropyl benzene):

  • This is the first commercial process for phenol production.

  • Cumene is oxidized by air ($\text{O}_2$) in the presence of Cobalt naphthenate at $423\,K$ in an alkaline medium (distilled water and $\text{Na}_2\text{CO}_3$) to form Cumene hydroperoxide.

  • Cumene hydroperoxide is treated with dilute $\text{H}_2\text{SO}_4$ and distilled to yield Phenol and Acetone ($\text{H}_3\text{C-CO-CH}_3$).

• From Benzene Sulphonic Acid:

  • Benzene sulphonic acid is neutralized with $\text{NaOH}$ to form the Sodium salt of benzene sulphonic acid ($\text{C}_6\text{H}_5\text{SO}_3\text{Na}$).

  • This salt is fused with solid $\text{NaOH}$ to form Sodium phenoxide: $\text{C}_6\text{H}_5\text{SO}_3\text{Na} + 2\text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{ONa} + \text{Na}_2\text{SO}_3 + \text{H}_2\text{O}$.

  • Acidification with dilute $\text{HCl}$ produces Phenol.

Methods of Preparation of Alcohols

• Hydration of Alkenes (Commercial/Industrial Method):

  • Alkenes react with cold concentrated sulphuric acid to form alkyl hydrogen sulphates, which are then hydrolyzed with water to form alcohols.

  • Example with Ethene: $\text{H}_2\text{C=CH}_2 + \text{H-OSO}_3\text{H} \rightarrow \text{H}_3\text{C-CH}_2\text{OSO}_3\text{H} \xrightarrow{\text{H}_2\text{O}} \text{H}_3\text{C-CH}_2\text{OH} + \text{H}_2\text{SO}_4$. Ethanol produced this way is commonly called "Grain alcohol".

  • For higher alkenes, the reaction follows Markovnikov's Rule (M. Rule): the negative part of the addendum goes to the carbon with fewer hydrogen atoms.

  • Propene yields Isopropyl alcohol (Propan-2-ol, a $2^{\circ}$ alcohol).

  • Isobutylene yields tert-butyl alcohol (2-methylpropan-2-ol, a $3^{\circ}$ alcohol).

  • Mechanism of Hydration:

    1. Protonation of alkene to form a carbocation by electrophilic attack of $\text{H}_3\text{O}^+$.

    2. Nucleophilic attack of water on the carbocation.

    3. Deprotonation to form the alcohol.

• Hydroboration-Oxidation:

  • Alkenes react with Diborane ($\text{B}_2\text{H}_6$) to form trialkylboranes.

  • Trialkylboranes are then oxidized by hydrogen peroxide ($\text{H}_2\text{O}_2$) in an aqueous sodium hydroxide solution.

  • This reaction produces Anti-Markovnikov’s product.

  • Example: Propene yields n-propyl alcohol (propan-1-ol), which is a primary alcohol. This provides an excellent yield of primary alcohols.

• Reduction of Carbonyl Compounds:

  1. Catalytic Hydrogenation: Using $\text{H}_2$ and Raney Ni (porous nickel providing a surface for hydrogen) at $413\,K$.

     • Formaldehyde ($\text{H-CHO}$) $\rightarrow$ Methanol ($1^{\circ}$).

     • Acetaldehyde ($\text{CH}_3\text{CHO}$) $\rightarrow$ Ethanol ($1^{\circ}$).

     • Acetone ($\text{CH}_3\text{COCH}_3$) $\rightarrow$ Propan-2-ol ($2^{\circ}$).

  2. Using Nascent Hydrogen ($2\text{[H]}$): Sodium amalgam ($\text{Na-Hg}$) with $\text{H}_2\text{O}$, or metal hydrides like $\text{NaBH}_4$ (Sodium borohydride) or $\text{LiAlH}_4$ (Lithium aluminium hydride).

     • $3^{\circ}$ alcohols cannot be prepared by reduction.

     • Aldehydes give $1^{\circ}$ alcohols; Ketones give $2^{\circ}$ alcohols.

     • $\text{LiAlH}_4$ is an expensive reagent used for special chemicals. Carboxylic acids are reduced to alcohols by first converting them to esters, followed by catalytic reduction.

• From Grignard Reagent ($RMgX$):

  • Reaction involves the nucleophilic attack of the Grignard reagent on the carbonyl group ($C=O$) followed by hydrolysis ($\text{H}_2\text{O}/\text{H}^+$).

  • Formaldehyde + $RMgX \rightarrow 1^{\circ}$ alcohol with more carbon atoms.

  • Other Aldehydes + $RMgX \rightarrow 2^{\circ}$ alcohol.

  • Ketones + $RMgX \rightarrow 3^{\circ}$ alcohol.

  • Example: Acetaldehyde + $\text{CH}_3\text{MgBr} \rightarrow$ Isopropyl alcohol.

Physical Properties of Alcohols and Phenols

• Boiling Points:

  • Alcohols have higher boiling points than corresponding alkanes, alkyl halides, aldehydes, ketones, and ethers due to the presence of intermolecular hydrogen bonding.

  • Boiling points increase with the increase in molecular weight.

  • Among isomeric alcohols, branched-chain alcohols have lower boiling points due to a smaller surface area and weaker Van der Waals forces.

  • Phenols have higher boiling points than hydrocarbons because of hydrogen bonding. Carboxylic acids have higher boiling points than phenols.

• Solubility:

  • Lower members of alcohols are soluble in water due to hydrogen bonding with water molecules.

  • Solubility decreases as the molecular weight (alkyl chain length) increases because the non-polar hydrophobic part grows larger.

• Physical State and Odor:

  • Lower members are colorless liquids with a distinctive smell.

  • Higher members are almost colorless solids.

  • Phenols have a distinctive odor.

  • Methanol is a colorless, highly poisonous liquid (boils at $337\,K$).

  • Ethanol is a colorless liquid (boils at $351\,K$).

Chemical Properties and Reactions

• Acidity of Alcohols and Phenols:

  • Alcohols act as Bronsted acids (donating a proton) and Lewis bases (lone pair on oxygen).

  • Acidity order: 1^{\circ} > 2^{\circ} > 3^{\circ}. This is because alkyl groups are electron-donating ($+I$ effect), which increases electron density on oxygen and hinders the release of the hydrogen ion.

  • Phenol vs. Alcohol: Phenol is more acidic than alcohol. This is because the Phenoxide ion (conjugate base) is stabilized by resonance, while the Alkoxide ion (conjugate base of alcohol) has no resonance.

  • Alcohols are neutral to litmus.

• Reaction with Metals:

  • Alcohols react with active metals like Sodium ($\text{Na}$) or Aluminium ($\text{Al}$) to release hydrogen gas.

  • $2\text{R-OH} + 2\text{Na} \rightarrow 2\text{R-ONa} + \text{H}_2 \uparrow$ (Sodium Alkoxide).

  • $6\text{R-OH} + 2\text{Al} \rightarrow 2(\text{R-O})_3\text{Al} + 3\text{H}_2 \uparrow$ (Aluminium Alkoxide).

• Esterification:

  • Alcohols react with carboxylic acids in the presence of concentrated $\text{H}_2\text{SO}_4$ (dehydrating agent) to form esters.

  • Reaction: $\text{R-COOH} + \text{R’-OH} \rightleftharpoons \text{R-COOR’} + \text{H}_2\text{O}$.

  • The reaction involves the breaking of the acyl-oxygen linkage ($C-O$ bond in the acid).

  • Mechanism:

    1. Protonation of the carbonyl oxygen of the acid to form a carbocation.

    2. Nucleophilic attack by the alcohol molecule.

    3. Proton transfer and loss of a water molecule to form the ester.

• Dehydration of Alcohols:

  • Alcohols undergo dehydration (removal of water) to form alkenes when heated with concentrated $\text{H}_2\text{SO}_4$ or $\text{Al}_2\text{O}_3$.

  • Primary alcohols ($1^{\circ}$): Require $95\% \, \text{H}_2\text{SO}_4$ and $443\,K$. (Example: Ethanol to Ethene).

  • Secondary alcohols ($2^{\circ}$): Require $60\% \, \text{H}_2\text{SO}_4$ and $373\,K$. (Example: Propan-2-ol to Propene).

  • Tertiary alcohols ($3^{\circ}$): Require $20\% \, \text{H}_2\text{SO}_4$ and $363\,K$. (Example: Tert-butyl alcohol to Isobutylene).

  • Order of ease of dehydration: 3^{\circ} > 2^{\circ} > 1^{\circ}.

  • Mechanism (Ethanol to Ethene):

    1. Formation of protonated alcohol.

    2. Formation of carbocation (Slow, Rate Determining Step).

    3. Formation of ethene by deprotonation.

• Oxidation of Alcohols:

  • Carried out with oxidizing agents like $\text{K}_2\text{Cr}_2\text{O}_7$ (acidified), Pyridinium chlorochromate (PCC), or Pyridinium dichromate (PDC).

  • Mild agents like PCC convert primary alcohols into aldehydes without further oxidation into carboxylic acids.

  • Primary Alcohols: Oxidize to Aldehyde, then to Carboxylic Acid (same number of carbon atoms).

  • Secondary Alcohols: Oxidize to Ketone (same number of carbon atoms). Further vigorous oxidation gives carboxylic acids with fewer carbon atoms.

  • Tertiary Alcohols: Difficult to oxidize. Under vigorous conditions, they break down into a mixture of ketones and carboxylic acids with fewer carbon atoms.

• Dehydrogenation using Copper ($\text{Cu}$) at $573\,K$ ($300^{\circ}\text{C}$):

  • Primary alcohols $\rightarrow$ Aldehydes.

  • Secondary alcohols $\rightarrow$ Ketones.

  • Tertiary alcohols $\rightarrow$ Alkenes (Mechanism involving free radicals, yields Isobutylene from tert-butyl alcohol).

Distinguishing and Identification Tests

• Lucas Test:

  • Reagent: Mixture of concentrated $\text{HCl}$ and anhydrous $\text{ZnCl}_2$.

  • Tertiary ($3^{\circ}$) alcohol: Immediate appearance of turbidity.

  • Secondary ($2^{\circ}$) alcohol: Turbidity appears within 5 minutes.

  • Primary ($1^{\circ}$) alcohol: Does not produce turbidity at room temperature.

Ethers and Preparation Details

• Continuous Etherification Process:

  • Primary alcohols yield ethers when heated with concentrated $\text{H}_2\text{SO}_4$ at a controlled temperature of $413\,K$.

  • Reaction: $2\text{R-OH} \xrightarrow{\text{conc. } \text{H}_2\text{SO}_4, 413\,K} \text{R-O-R} + \text{H}_2\text{O}$.

  • Example: Ethanol heated to $413\,K$ yields diethyl ether (ethoxyethane).

  • If the temperature rises to $443\,K$, the alcohol undergoes dehydration to form an alkene instead.

• Mechanism of Etherification:

  1. Formation of protonated alcohol ($\text{H}_3\text{C-CH}_2\text{-O}^+\text{H}_2$).

  2. Nucleophilic attack by a second alcohol molecule on the protonated alcohol (Slow step).

  3. Deprotonation to give the ether.

• Limitations of this Method:

  1. Only simple (symmetrical) ethers can be prepared effectively.

  2. If a mixture of two different alcohols is used, a mixture of three different ethers is obtained, which is difficult to separate.

  3. Method only works for primary alcohols. Secondary and tertiary alcohols yield alkenes due to elimination reactions.

Commercial Applications and Notes

• Methanol (Wood Spirit):

  • Historically produced by destructive distillation of wood.

  • Produced industrially by catalytic hydrogenation of Carbon Monoxide: $\text{CO} + 2\text{H}_2 \xrightarrow{\text{ZnO-Cr}_2\text{O}_3, 573-673\,K, 200-300\,atm} \text{CH}_3\text{OH}$.

  • Used as a solvent in paints and varnishes.

• Ethanol:

  • Obtained commercially by fermentation of sucrose (molasses).

  • Invertase enzyme converts Sucrose to Glucose and Fructose.

  • Zymase enzyme converts Glucose/Fructose to Ethanol and $\text{CO}_2$.

  • Used as a solvent in the paint industry and for preparing carbon compounds.

• Formalin: A $40\%$ aqueous solution of Formaldehyde used to preserve dead bodies.

Nomenclature Examples and Special Cases

• Acetophenone: Structure is $\text{Ph-CO-CH}_3$.

• Benzophenone: Structure is $\text{Ph-CO-Ph}$.

• Acetamide: $\text{H}_3\text{C-C}(\text{O})\text{NH}_2$.

• Acetaldehyde: $\text{H}_3\text{C-CHO}$.

• Acetone: $\text{H}_3\text{C-CO-CH}_3$.

• 3-Chlorobutan-1-ol and Pentan-2,3-diol are examples of IUPAC naming rules.

• 4-Methylhexan-3-ol and 4-Ethylheptan-4-ol follow the longest chain rule.