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Chapter 7 Alcohols, Phenols And Ethers
Classification
Alcohols—Mono, Di, Tri Or Polyhydric Alcohols
Alcohols are classified based on the number of hydroxyl (-OH) groups:
- Monohydric: One -OH group (e.g., methanol $CH_3OH$).
- Dihydric (Glycols): Two -OH groups (e.g., ethane-1,2-diol).
- Trihydric: Three -OH groups (e.g., propane-1,2,3-triol or glycerol).
- Polyhydric: More than three -OH groups.
Further classification of monohydric alcohols is based on the hybridization of the carbon atom bearing the -OH group:
- sp³ hybridized Carbon: Alkyl halides (-OH attached to primary, secondary, or tertiary carbon), Allylic halides (-OH on sp³ carbon adjacent to C=C), Benzylic halides (-OH on sp³ carbon attached to an aromatic ring).
- sp² hybridized Carbon: Vinylic alcohols (-OH attached to C=C carbon) and Phenols (-OH attached directly to an aromatic ring carbon).
Phenols—Mono, Di And Trihydric Phenols
Phenols are classified based on the number of hydroxyl groups attached to the aromatic ring:
- Monohydric: One -OH group (e.g., phenol, cresol).
- Dihydric: Two -OH groups (e.g., catechol, resorcinol, hydroquinone).
- Trihydric: Three -OH groups (e.g., pyrogallol).
Ethers
Ethers contain an alkoxy ($R-O-$) or aryloxy ($Ar-O-$) group. They are classified as:
- Simple/Symmetrical: Both groups attached to oxygen are identical (e.g., diethyl ether $C_2H_5OC_2H_5$).
- Mixed/Unsymmetrical: The groups attached to oxygen are different (e.g., ethyl methyl ether $CH_3OCH_2CH_3$, methyl phenyl ether $C_6H_5OCH_3$).
Nomenclature
Alcohols: IUPAC names are derived from the parent alkane by replacing '-e' with '-ol'. Numbering starts from the end nearest to the -OH group. For polyhydric alcohols, locants and prefixes (di-, tri-) are used before '-ol' (e.g., ethane-1,2-diol).
Phenols: Phenol is the IUPAC name for $C_6H_5OH$. Substituted phenols use numerical locants (1,2-; 1,3-; 1,4-) or ortho-, meta-, para- prefixes for common names.
Ethers: IUPAC names are derived by considering one alkyl/aryl group as an alkoxy/aryloxy substituent on the parent hydrocarbon (e.g., methoxyethane for $CH_3OCH_2CH_3$). Common names list the alkyl/aryl groups alphabetically followed by 'ether'.
Structures Of Functional Groups
The hydroxyl (-OH) group in alcohols involves an $sp^3$ hybridized oxygen atom bonded to an $sp^3$ hybridized carbon atom of an alkyl group. The C-O bond length is about 143 pm, and the H-O-C bond angle is slightly less than the tetrahedral angle due to lone pair repulsion on oxygen. In phenols, the -OH group is attached to an $sp^2$ hybridized carbon of an aromatic ring, resulting in a shorter C-O bond (136 pm) due to conjugation and partial double bond character. Ethers have an oxygen atom bonded to two alkyl or aryl groups, with $sp^3$ hybridization and a bond angle slightly larger than tetrahedral due to steric repulsion between the groups.
Alcohols And Phenols
Alcohols and phenols exhibit reactions involving the cleavage of O-H bonds (showing acidic character) and C-O bonds (reactivity as electrophiles, especially after protonation).
Preparation Of Alcohols
- From Alkenes:
- Acid-catalyzed hydration: Addition of water follows Markovnikov's rule.
- Hydroboration-oxidation: Anti-Markovnikov addition of water.
- From Carbonyl Compounds:
- Reduction of aldehydes and ketones using reducing agents like $LiAlH_4$, $NaBH_4$, or catalytic hydrogenation yields primary and secondary alcohols, respectively.
- Reduction of carboxylic acids and esters yields primary alcohols, often via esterification followed by catalytic hydrogenation or $LiAlH_4$ reduction.
- From Grignard Reagents: Reaction with aldehydes and ketones followed by hydrolysis yields primary (from methanal), secondary (from other aldehydes), or tertiary (from ketones) alcohols.
Preparation Of Phenols
- From Haloarenes: Fusion of chlorobenzene with NaOH at high temperature and pressure, followed by acidification.
- From Benzenesulfonic Acid: Sulfonation of benzene, followed by fusion with NaOH and acidification.
- From Diazonium Salts: Hydrolysis of diazonium salts (formed from primary aromatic amines) yields phenols.
- From Cumene: Industrial production involves oxidation of cumene to cumene hydroperoxide, followed by acid-catalyzed rearrangement to phenol and acetone.
Physical Properties
Alcohols and phenols have higher boiling points than hydrocarbons, ethers, and haloalkanes of similar molecular mass due to intermolecular hydrogen bonding. Solubility in water is significant for lower alcohols (due to H-bonding with water) but decreases with increasing alkyl/aryl group size. Phenols are less soluble than lower alcohols.
Chemical Reactions
- Reactions involving O-H bond cleavage (Acidity):
- Alcohols react with active metals (Na, Al) to form alkoxides/aluminates and $H_2$. They are weak acids.
- Phenols are more acidic than alcohols due to the stabilization of the phenoxide ion by resonance. Electron-withdrawing groups (-NO₂) increase acidity, while electron-donating groups (-CH₃) decrease it.
- Reactions involving C-O bond cleavage (in Alcohols):
- Reaction with HX (3° > 2° > 1°) and phosphorus halides ($PBr_3, PI_3$) yields alkyl halides.
- Dehydration with concentrated $H_2SO_4$ or $P_2O_5$ yields alkenes (tertiary > secondary > primary) or ethers (from primary alcohols at lower temps).
- Oxidation: Primary alcohols yield aldehydes (with mild oxidants like PCC) or carboxylic acids (strong oxidants like $KMnO_4$, $K_2Cr_2O_7$). Secondary alcohols yield ketones. Tertiary alcohols are resistant to oxidation under mild conditions.
- Reactions of Phenols:
- Electrophilic Aromatic Substitution: The -OH group is activating and ortho-, para- directing due to resonance. Reactions include nitration (dilute $HNO_3$ gives o-/p-nitrophenols; conc. $HNO_3$ gives picric acid), halogenation (bromine water gives 2,4,6-tribromophenol), Kolbe's reaction (with $CO_2$ under pressure yields salicylic acid), and Reimer-Tiemann reaction (with $CHCl_3$ and base yields salicylaldehyde). Phenols react with zinc dust to form benzene.
Some Commercially Important Alcohols
Methanol ($CH_3OH$): Known as 'wood spirit', produced industrially by catalytic hydrogenation of CO. It's a poisonous solvent used in paints and formaldehyde production.
Ethanol ($C_2H_5OH$): Commercially produced by fermentation of sugars (from molasses, grapes) using yeast ($Saccharomyces cerevisiae$). It acts as a solvent and starting material for other compounds. Denatured alcohol (made unfit for drinking) is used industrially.
Ethers
Ethers contain an alkoxy ($R-O-R'$) or aryloxy ($Ar-O-R'$) group. They are classified as simple (R=R') or mixed (R≠R').
Preparation Of Ethers
- Dehydration of Alcohols: Primary alcohols dehydrate to form ethers at lower temperatures (~413 K) in the presence of acids ($H_2SO_4$). Higher temperatures (~443 K) favor alkene formation. This method is less suitable for secondary and tertiary alcohols due to elimination.
- Williamson Synthesis: Reaction of sodium alkoxide/phenoxide with a primary alkyl halide via $S_N2$ mechanism. This is suitable for preparing both symmetrical and unsymmetrical ethers. Using secondary or tertiary alkyl halides leads to elimination as a major competing reaction.
Physical Properties
Ethers have lower boiling points than alcohols of similar molecular mass because they cannot form intermolecular hydrogen bonds. Their solubility in water is moderate, similar to lower alcohols, due to hydrogen bonding between ether oxygen and water molecules.
Chemical Reactions
- Cleavage of C-O Bond: Ethers react with strong acids like HI or HBr, especially at high temperatures. Cleavage occurs at the alkyl-oxygen bond if the alkyl group is primary or secondary (via $S_N2$). If a tertiary alkyl group is present, cleavage occurs at the tertiary alkyl-oxygen bond via $S_N1$ due to carbocation stability. Aryl ethers cleave at the alkyl-oxygen bond, yielding phenols and alkyl halides.
- Electrophilic Substitution: The alkoxy group (-OR) is activating and ortho-, para- directing in electrophilic aromatic substitution reactions (like halogenation, Friedel-Crafts alkylation/acylation, nitration) due to resonance stabilization, similar to phenols.
Intext Questions
Question 7.1. Classify the following as primary, secondary and tertiary alcohols:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Answer:
Question 7.2. Identify allylic alcohols in the above examples.
Answer:
Question 7.3. Name the following compounds according to IUPAC system.
(i)
(ii)
(iii)
(iv)
(v)
Answer:
Question 7.4. Show how are the following alcohols prepared by the reaction of a suitable Grignard reagent on methanal ?
(i)
(ii)
Answer:
Question 7.5. Write structures of the products of the following reactions:
(i)
(ii)
(iii)
Answer:
Question 7.6. Give structures of the products you would expect when each of the following alcohol reacts with (a) $HCl –ZnCl_2$ (b) HBr and (c) $SOCl_2$.
(i) Butan-1-ol
(ii) 2-Methylbutan-2-ol
Answer:
Question 7.7. Predict the major product of acid catalysed dehydration of
(i) 1-methylcyclohexanol and
(ii) butan-1-ol
Answer:
Question 7.8. Ortho and para nitrophenols are more acidic than phenol. Draw the resonance structures of the corresponding phenoxide ions.
Answer:
Question 7.9. Write the equations involved in the following reactions:
(i) Reimer - Tiemann reaction
(ii) Kolbe’s reaction
Answer:
Question 7.10. Write the reactions of Williamson synthesis of 2-ethoxy-3-methylpentane starting from ethanol and 3-methylpentan-2-ol.
Answer:
Question 7.11. Which of the following is an appropriate set of reactants for the preparation of 1-methoxy-4-nitrobenzene and why?
(i)
(ii)
Answer:
Question 7.12. Predict the products of the following reactions:
(i)
(ii)
(iii)
(iv)
Answer:
Exercises
Question 7.1. Write IUPAC names of the following compounds:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
(viii)
(ix)
(x)
(xi)
(xii)
Answer:
Question 7.2. Write structures of the compounds whose IUPAC names are as follows:
(i) 2-Methylbutan-2-ol
(ii) 1-Phenylpropan-2-ol
(iii) 3,5-Dimethylhexane –1, 3, 5-triol
(iv) 2,3 – Diethylphenol
(v) 1 – Ethoxypropane
(vi) 2-Ethoxy-3-methylpentane
(vii) Cyclohexylmethanol
(viii) 3-Cyclohexylpentan-3-ol
(ix) Cyclopent-3-en-1-ol
(x) 4-Chloro-3-ethylbutan-1-ol.
Answer:
Question 7.3. (i) Draw the structures of all isomeric alcohols of molecular formula $C_5H_{12}O$ and give their IUPAC names.
(ii) Classify the isomers of alcohols in question 7.3 (i) as primary, secondary and tertiary alcohols.
Answer:
Question 7.4. Explain why propanol has higher boiling point than that of the hydrocarbon, butane?
Answer:
Question 7.5. Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.
Answer:
Question 7.6. What is meant by hydroboration-oxidation reaction? Illustrate it with an example.
Answer:
Question 7.7. Give the structures and IUPAC names of monohydric phenols of molecular formula, $C_7H_8O$.
Answer:
Question 7.8. While separating a mixture of ortho and para nitrophenols by steam distillation, name the isomer which will be steam volatile. Give reason.
Answer:
Question 7.9. Give the equations of reactions for the preparation of phenol from cumene.
Answer:
Question 7.10. Write chemical reaction for the preparation of phenol from chlorobenzene.
Answer:
Question 7.11. Write the mechanism of hydration of ethene to yield ethanol.
Answer:
Question 7.12. You are given benzene, conc. $H_2SO_4$ and NaOH. Write the equations for the preparation of phenol using these reagents.
Answer:
Question 7.13. Show how will you synthesise:
(i) 1-phenylethanol from a suitable alkene.
(ii) cyclohexylmethanol using an alkyl halide by an $S_N2$ reaction.
(iii) pentan-1-ol using a suitable alkyl halide?
Answer:
Question 7.14. Give two reactions that show the acidic nature of phenol. Compare acidity of phenol with that of ethanol.
Answer:
Question 7.15. Explain why is ortho nitrophenol more acidic than ortho methoxyphenol ?
Answer:
Question 7.16. Explain how does the –OH group attached to a carbon of benzene ring activate it towards electrophilic substitution?
Answer:
Question 7.17. Give equations of the following reactions:
(i) Oxidation of propan-1-ol with alkaline $KMnO_4$ solution.
(ii) Bromine in $CS_2$ with phenol.
(iii) Dilute $HNO_3$ with phenol.
(iv) Treating phenol wih chloroform in presence of aqueous NaOH.
Answer:
Question 7.18. Explain the following with an example.
(i) Kolbe’s reaction.
(ii) Reimer-Tiemann reaction.
(iii) Williamson ether synthesis.
(iv) Unsymmetrical ether.
Answer:
Question 7.19. Write the mechanism of acid dehydration of ethanol to yield ethene.
Answer:
Question 7.20. How are the following conversions carried out?
(i) Propene $\rightarrow$ Propan-2-ol.
(ii) Benzyl chloride $\rightarrow$ Benzyl alcohol.
(iii) Ethyl magnesium chloride $\rightarrow$ Propan-1-ol.
(iv) Methyl magnesium bromide $\rightarrow$ 2-Methylpropan-2-ol.
Answer:
Question 7.21. Name the reagents used in the following reactions:
(i) Oxidation of a primary alcohol to carboxylic acid.
(ii) Oxidation of a primary alcohol to aldehyde.
(iii) Bromination of phenol to 2,4,6-tribromophenol.
(iv) Benzyl alcohol to benzoic acid.
(v) Dehydration of propan-2-ol to propene.
(vi) Butan-2-one to butan-2-ol.
Answer:
Question 7.22. Give reason for the higher boiling point of ethanol in comparison to methoxymethane.
Answer:
Question 7.23. Give IUPAC names of the following ethers:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Answer:
Question 7.24. Write the names of reagents and equations for the preparation of the following ethers by Williamson’s synthesis:
(i) 1-Propoxypropane
(ii) Ethoxybenzene
(iii) 2-Methoxy-2-methylpropane
(iv) 1-Methoxyethane
Answer:
Question 7.25. Illustrate with examples the limitations of Williamson synthesis for the preparation of certain types of ethers.
Answer:
Question 7.26. How is 1-propoxypropane synthesised from propan-1-ol? Write mechanism of this reaction.
Answer:
Question 7.27. Preparation of ethers by acid dehydration of secondary or tertiary alcohols is not a suitable method. Give reason.
Answer:
Question 7.28. Write the equation of the reaction of hydrogen iodide with:
(i) 1-propoxypropane
(ii) methoxybenzene and
(iii) benzyl ethyl ether.
Answer:
Question 7.29. Explain the fact that in aryl alkyl ethers (i) the alkoxy group activates the benzene ring towards electrophilic substitution and (ii) it directs the incoming substituents to ortho and para positions in benzene ring.
Answer:
Question 7.30. Write the mechanism of the reaction of HI with methoxymethane.
Answer:
Question 7.31. Write equations of the following reactions:
(i) Friedel-Crafts reaction – alkylation of anisole.
(ii) Nitration of anisole.
(iii) Bromination of anisole in ethanoic acid medium.
(iv) Friedel-Craft’s acetylation of anisole.
Answer:
Question 7.32. Show how would you synthesise the following alcohols from appropriate alkenes?
(i)
(ii)
(iii)
(iv)
Answer:
Question 7.33. When 3-methylbutan-2-ol is treated with HBr, the following reaction takes place:
Give a mechanism for this reaction.
(Hint : The secondary carbocation formed in step II rearranges to a more stable tertiary carbocation by a hydride ion shift from 3rd carbon atom.)
Answer: