NEET Chapter Page - Alcohols, Phenols and Ethers

When a hydrogen atom in an aliphatic or aromatic hydrocarbon is replaced by a –OH group, alcohols and phenols are produced. The substitution of an alkoxy or aryloxy group (R–O/Ar–O) for a hydrogen atom in a hydrocarbon produces another class of molecules known as ethers, such as CH3OCH3 (dimethyl ether).

The structure, classification, nomenclature, preparation, physical and chemical properties, and notable examples are the key subjects covered in the chapter Alcohol, Phenols, and Ethers. This will aid in recalling key ideas, parallels, and remarkable distinctions.

Important Topics for Alcohols, Phenols, and Ethers

• Types of Alcohols

• Reactions of Alcohols

• Phenol

• Uses of Phenol

• Preparation of Phenol

• Properties of Ether

Important Concepts in Alcohols, Phenols, and Ethers

Alcohols

• When a hydrogen atom bonded to an aliphatic carbon atom is replaced by a hydroxyl group i.e. -OH group, it is known as alcohol. Therefore alcohol is an aliphatic carbon chain with a hydroxyl group instead of a hydrogen atom attached to the carbon. 

• 'Alcohol' is appended to the name of the alkyl group, for example, methyl alcohol, ethyl alcohol, and so on.

• IUPAC – The suffix 'ol' is added to the name of an alkane, for example, methanol (CH3OH), ethanol (C2H5OH), and so on.

Classification of Alcohols

• Monohydric, dihydric, and polyhydric, alcohols are classified depending on the number of -OH groups present.
  Examples of Monohydric Alcohols: Methanol, Ethanol, and Isopropanol.
  Examples of Dihydric Alcohols: 1, 2-Ethanediol, 1, 3-Propanediol.
  Examples of Polyhydric Alcohols: Glycol and Glycerol.

• Depending on the type of carbon atom (sp3) connected to -OH, it can be classified as primary, secondary, or tertiary.

• Allylic – The OH group is connected to the allylic carbon (sp3), for example, CH2=CH-CH-OH.

• Benzylic - an aromatic ring with an OH group linked to C (sp3).

• CH2=CH-OH has a vinylic – OH group connected to C (sp2) with C=C.

Structure of Alcohols

• The sp3 hybridised orbitals of C and O have a bonding.

• The bond angle between C and O is 108.9°, which is less than tetrahedral (109°-28').

• It is caused by the repulsion of Oxygen electron pairs.

• Methanol has a C-O bond length of 142 pm.

• Alkenes (from alkenes):

a. Acid-catalysed hydration - Markovnikov's rule-based H2O addition.

b. Hydroboration-oxidation - Reaction with diborane to produce trialkyl boranes, which are then oxidised with H2O2 and NaOH(aq).

The eventual result is the polar opposite of Markonikov's rule.

• Compounds derived from carbonyls:

a. By catalytic hydrogenation in the presence of a Ni, Pd, or Pt metal catalyst, or by LiAlH4 or NaBH4 reduction of aldehyde and ketones.
Aldehydes → primary alcohols

Ketones → secondary alcohols

b. Through carboxylic acid and ester reduction

• Grignard Reagents (from Grignard Reagents):

• The grignard reagent is used to react aldehydes and ketones (R-Mg-X). The Grignard reagent is added nucleophilically and then hydrolyzed.
  HCHO + RMgX → RCH2OMgX + H2O → RCH2OH + Mg(OH)X

Properties of Alcohols

• Increases when the number of carbon atoms increases due to increased Van der Waals forces.

• Due to the decrease in surface area and Van der Waals forces, it decreases as branching increases.

• Because of intermolecular hydrogen bonding, the boiling point is higher than that of other compounds such as alkanes, ethers, haloalkanes, and so on.

• The creation of hydrogen bonds with H2O is responsible for solubility.

• As the size of the alkyl group grows larger, solubility decreases (hydrophobic).

• Bronsted acids are strong acids that can give electrons to a strong base.

• The polarity of the O-H bond causes acidity. The acidity of alcohol decreases as electron-releasing alkyl groups release electrons, hence the order of acidity is:

1° > 2° > 3°

• Alcohols are a better proton donor or stronger acid than water.

• Due to unshared electron pairs on oxygen, they also behave as proton acceptors or Bronsted bases.

Reactions of Alcohols

• Alcohol reacts as a nucleophile in the following reactions (O-H bond cleavage):

a. Metals react with alkoxides and H2 to create the corresponding alkoxides and H2.

2R-O-H + 2Na → 2R-O-Na + H2

b. Esterification: Alcohols create esters when they react with carboxylic acids, acid anhydrides, and acid chlorides.

• Alcohol interacts as an electrophile (C-O bond cleavage) in the following reactions:

• a. Alkyl halides are formed by reacting with HX. The Lucas test, which distinguishes between 1°, 2°, and 3° alcohols, is based on this. Because 3° alcohols easily form halides, turbidity is formed almost immediately.

• b. Alkyl halides are formed by reacting with PX3.

• c. Dehydrate to create alkene in the presence of protic acid, e.g., ethanol reacts with H2SO4 at 443 K to form ethylene.

• Dehydration of 2° and 3° alcohols occurs under more benign conditions. The following is a list of the stages of dehydration:
  3° > 2° > 1°

• d. Aldehyde and ketone formation via oxidation or dehydrogenation.

• Aldehydes are formed from primary alcohols. Carboxylic acid is formed directly by strong oxidising agents (KMnO4). Aldehydes are made with CrO3 and PCC (pyrimidine chlorochromate). When secondary alcohols are oxidised by CrO3, they produce ketones. Tertiary alcohols are unaffected by oxidation. Dissociation of the C-C bond occurs when KMnO4 is applied at a higher temperature, resulting in a variety of carboxylic acids with fewer carbon atoms.

• e. When alcohols are heated with Cu at 573 K, they dehydrate. Aldehyde is the first level of alcohol. Ketone is a type of alcohol that is made up of two parts alcohol and one part ketone.

1° alcohol → Aldehyde

2° alcohol → Ketone

3° alcohol → Alkene (Dehydration)

Phenols

• The -OH (hydroxyl) group replaces the hydrogen in an aromatic hydrocarbon.

• Phenol (C6H5OH) is the most basic. It is the common name, and it is also recognised by the International Union of Scientific Organisations (IUPAC).

• The OH group's position is indicated by the letters o (ortho), m (meta), p (para), or by counting the cyclic carbons. For example, 2-Methylphenol is o-Cresol, while Benzene-1,2-diol is Catechol.

Classification of Phenols

• Depending on how many -OH groups there are, There are three types of hydrocarbons: monohydric, dihydric, and polyhydric.

• Catechol, resorcinol, hydroquinone and pyrogallol are some of the polyhydric phenols. 

Structure of Phenol

• The sp2 hybridised orbitals of C in the aromatic ring and O form a bond.

• The angle of Bonding: In Phenol, the C-O-H bond angle is 109°.

• Length of Bond: Phenol has a C-O bond length of 136 pm.

• Due to sp2 hybridization of Carbon and conjugation of pi electrons of the aromatic ring, it has a lower molecular weight than Methanol.

• Chlorobenzene is converted to sodium phenoxide by reacting with NaOH, which is subsequently converted to phenol by reacting with acid.

C6H5Cl + NaOH → C6H5ONa + HCl → C6H5OH.

• Sulphonation of benzene with oleum is the initial stage. The resulting benzene sulphonic acid is heated with molten NaOH to generate sodium phenoxide, which is subsequently acidified to yield phenol.

• When aniline (C6H5NH2) interacts with NaNO2 + HCl, benzene diazonium chloride (C6H5N2Cl) is formed, which when hydrolyzed yields phenol.

• Cumene (isopropylbenzene) is oxidised to cumene hydroperoxide, which is then treated with dilute acid to produce phenol. The reaction produces acetone as a by-product.

Properties of Phenols

• Boiling Point: Because of intermolecular hydrogen bonding, the boiling point of arenes, ethers, haloarenes, and other compounds is higher.

• The creation of hydrogen bonds with H2O is responsible for solubility. As the size of the aryl group grows larger, solubility declines (hydrophobic).

• Bronsted acids are strong acids that can give electrons to a strong base.

• The electron-withdrawing group is the benzene ring connected to the OH group. The oxygen of OH is positive because its electron pairs are conjugated with the double bond of the benzene ring.

• Water and alcohols are weaker acids than phenols.

• This can be explained by the fact that the phenoxide ion is more stable and the OH bond is more polar.

• The substituted phenols with electron-withdrawing groups (-NO2) are more acidic, whereas those with electron giving groups (alkyl) are less acidic.

• Acidity in ascending order:
  Nitrophenol > Phenol > Cresol > Ethanol.

Phenolic Reactions

• Nucleophilic reactions using phenol (O-H bond cleavage):
  a. Phenol becomes sodium phenoxide when it interacts with metal or aqueous NaOH.

b. Esterification: Phenols create esters when they react with carboxylic acids, acid anhydrides, and acid chlorides.

Salicylic acid + Acetic anhydride → Aspirin (Acetylsalicylic acid).

• Cleavage of the C-O bond in reactions: Benzene is formed when phenol combines with Zn dust.
  C6H5OH + Zn → C6H6 + ZnO

• Electrophilic aromatic substitution occurs in phenol at the ortho (o) and para (p) positions.

• a. Dilution nitration At 298 K, HNO3 produces a mixture of o- and p-Nitrophenols. Because of intermolecular hydrogen bonding, p-Nitrophenol is less volatile. Intramolecular hydrogen bonding is observed in o-Nitrophenol. Picric acid is formed by nitration with concentrated HNO3 (2,4,6-trinitrophenol)

b. In the presence of CS2 or CHCl3, monosubstituted phenols are produced. When 2,4,6-tribromophenol is treated with bromine water, it forms a white precipitate.

• When phenol interacts with NaOH, sodium phenoxide is formed. When sodium phenoxide reacts with CO2 and undergoes electrophilic substitution, salicylic acid (2-Hydroxybenzoic acid) is produced. This is known as Kolbe's Reaction.

• In the Reimer-Tiemann Reaction in phenol, the -CHO group is linked to the o- position.
  C6H5OH + CHCl3 + aq. NaOH → C6H5CHO (Salicylaldehyde).

• Benzoquinone is formed when phenol is oxidised in the presence of Na2Cr2O7 and H2SO4.

Ethers

• The -OR/-OAr (Alkoxy or Aryloxy) group replaces the hydrogen in a hydrocarbon in an ether. 

• Ethyl Methyl ether, Diethyl ether, and so on. Common Name - the word 'ether' follows the names of the alkyl groups in alphabetical order, e.g. Ethyl Methyl ether, Diethyl ether, and so on are examples of simple ether.

• Named as an alkoxy or aryloxy hydrocarbon derivative by IUPAC. The parent hydrocarbon is the bulkier group, such as methoxymethane, methoxybenzene, and so on.

Classification of Ethers

• Symmetrical Ether (Simple) – O is connected to the identical groups (alkyl/aryl).

• Unsymmetrical Ether (Mixed) - O is attached to multiple groups.

Structure of Ether

• The angle of the C-O-C bond is 111.7 degrees (Methoxymethane).

• Due to repulsion between the two R (bulky) groups, it is more than tetrahedral.

• The C-O bond length is 141 pm, which is approximately identical to that of alcohols.

• The nucleophilic bimolecular reaction forms ether when primary alcohols are treated with protic acids (H2SO4, H3PO4). This reaction is temperature-dependent; alkene is the main product at 443 K, but ether is a major result at 413 K.

• Elimination reaction competes with SN1 when alcohol is 2° or 3°, resulting in alkene as the main product.

• When sodium alkoxide reacts with an alkyl halide, ether is produced.

• When using a 1° alkyl halide, SN² is favoured and ether is generated as a major product; however, when using a 2° or 3° alkyl halide, elimination proceeds and alkene is formed as the major product. This process can also be used to convert phenol to ether.

Properties of Ether

• Boiling Point: Because alcohols have intermolecular hydrogen bonding, they have less intermolecular hydrogen bonding.

• Because of the creation of a hydrogen bond with water and O of ether, its miscibility is comparable to that of alcohol and greater than that of an alkane of the same molecular mass.

• Ethers are less acidic than ketones and aldehydes, but more acidic than hydrocarbons.

• Lewis or Bronsted bases can be used with Ethers. After protonation by strong acids, they create the oxonium ion.

Reactions of Ether

• In ethers, there is no O-H bond.

• Cleavage of the C-O bond in reactions: Ethers, on the other hand, are less reactive. Cleavage of the C-O bond happens at extreme circumstances. When dialkyl ethers react with HX, they produce two alkyl halides. Because the aryl-oxygen link is more stable, alkyl aryl ethers react with HX to generate phenol and alkyl halide. The reactivity order of HX is HI > HBr > HCl.

• Substitution of Electrophilic Compounds: Electrophilic aromatic substitution occurs at the ortho (o) and para (p) positions in aryl ethers.

• Friedel's reaction is as follows: At the o and p positions, halogenation and nitration occur. In the presence of AlCl3 (anhydrous), anisole combines with alkyl or acyl halides to produce o and p substituted products.

Solved Examples from Chapter

Example 1: Which of the following options has an increasing order of boiling points (1) CH3-CH2-CH2-CH2-OH, (2) CH3CH(OH)CH3 (3) (CH3)3COH?

(a) (1) > (2) > (3)

(b) (2) < (1) < (3)

(c) (3) < (2) < (1)

(d) (2) < (3) < (1)

Solution: 

• The features of alcohol's boiling point are as follows:

- Increases when the number of C atoms increases due to increased Van der Waals forces

- Decreases when branching increases due to reduced surface area and Van der Waals forces.

• From the given characteristics it is clear that the boiling point decreases from primary to tertiary alcohol. 

• The given compounds are:
  (1) CH3-CH,-CH2-CH2-OH → Primary Alcohol
  (2) CH3CH(OH)CH3 → Secondary Alcohol
  (3) (CH3)3COH → Tertiary Alcohol 

• Hence, the answer to the given question is, (a) (1) > (2) > (3).

Key Point to Remember: It is a good practice to always remember the structural and physical properties of organic compounds. In the case of alcohol, there are always trends with respect to different forms of the same alcoholic group. Using that information one can solve these types of questions easily. 

Example 2: Arrange the following in increasing order of their acidic character:

Ethanol, Phenol, Water 

(a) Ethanol < Water < Phenol

(b) Ethanol > Water < Phenol

(c) Ethanol < Water > Phenol

(d) Ethanol > Water > Phenol

Solution: 

• Following are the solubility characteristics of ethanol and phenol:
  Alcohol: The creation of hydrogen bonds with H2O causes solubility. As the size of the alkyl group grows larger, solubility decreases (hydrophobic).
  Phenol: The creation of hydrogen bonds with H2O causes solubility. As the size of the aryl group grows larger, solubility declines (hydrophobic).

• Now, the acidic character depends on the release of H+ ions in the solution. Therefore, the compound having high hydrogen (of the hydroxyl group) releasing capability will be more acidic in nature. 

• Thus, from the above observations, phenol can easily release the hydrogen ion from the hydroxyl group as the negative charge is in resonance with the benzene ring. 

• On the other hand, the hydrogen releasing ability of aliphatic alcohol will be low as compared to the other two given compounds. 

• Therefore, the answer is (a) Ethanol < Water < Phenol.

Key Point to Remember: It is a good practice to always remember the structural and physical properties of organic compounds. Solubility is the basic property of any compound and hence, a lot is explained as to why a compound is soluble in water or insoluble from the structural point of view. Hence using the structural information and basis of the given organic compound these types of questions are solvable.

Solved Questions from The Previous Year Question Papers

Question 1: Anisole on cleavage with HI gives:

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Solution: 

• The given reaction is as follows:

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• From the above reaction it is clear that the correct answer is option C. 

Question 2: In sodium hydrogen carbonate, which of the following will not dissolve?

(1) 2,4,6 - Trinitrophenol

(2) Benzoic Acid

(3) o-Nitrophenol

(4) Benzenesulfonic Acid

Solution: 

• 2,4,6-Trinitrophenol, Benzoic Acid, and Benzene Sulphonic Acid are soluble in NaHCO3. 

• If the given acid is more acidic than H2CO3, this reaction can proceed in the forward direction. 

• H2CO3 is more acidic than o-nitrophenol. 

• As a result, sodium hydrogen carbonate does not dissolve it.

• Hence the correct answer is (3) o-Nitrophenol.

Question 3: An unknown alcohol is treated with the “Lucas reagent'' to determine whether the alcohol is primary, secondary or tertiary. Which alcohol reacts fastest and by what mechanism:-

(1) Secondary alcohol by SN1

(2) Tertiary alcohol by SN1

(3) Secondary alcohol by SN2

(4) Tertiary alcohol by SN2

Solution: 

• To distinguish between primary, secondary, and tertiary alcohols, the Lucas test is used. 

• The variation in reactivity of the three types of alcohols with hydrogen halides provides the basis for this. 

• Because tertiary carbocation is extremely stable, the reaction occurs through carbocation formation. 

• The reaction then continues with tertiary alcohol via SN1.

• As a result, option (2) is the correct response.

Practice Questions

Question 1: The difference between n-propyl alcohol and iso-propyl alcohol is given by

(a) PCl5

(b) Reduction

(c) Ozonolysis

(d) Oxidation with Potassium Dichromate

Answer: (d) Oxidation with Potassium Dichromate

Question 2: When phenol is treated with diethyl sulphate in the presence of NaOH, it produces

(i) Phenate

(ii) Anisole

(iii) Diphenyl Ether

(iv) Diethyl Ether

Answer: (iv) Diethyl Ether

Question 3: The correct order of acidic strength of the following compounds is, (I) Phenol, (II) p-Cresol, (III) Nitrophenol, (IV) p-Nitrophenol

(i) (III) > (II) > (I) > (IV)

(ii) (IV) > (III) > (I) > (II)

(iii) (II) > (IV) > (I) > (III)

(iv) (I) > (II) > (IV) > (III)

Answer: (ii) (IV) > (III) > (I) > (II)

Conclusion

The primary issues discussed in the chapter Alcohol, Phenols, and Ethers are structure, classification, nomenclature, preparation, physical and chemical properties, and notable instances. This will help you remember essential ideas, parallels, and significant differences.