Important Notes For NEET Chemistry - Alcohols, Phenols and Ethers

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Chemistry

Alcohols, Phenols and Ethers

NEET exams

The hydrogen of an aliphatic hydrocarbon gets replaced by -OH (hydroxyl) group

The hydrogen of an aromatic hydrocarbon gets replaced by -OH (hydroxyl) group

The hydrogen of a hydrocarbon gets replaced by -OR/-OAr (Alkoxy or Aryloxy) group

Depending on no. of -OH group present, Monohydric, dihydric and polyhydric.

Primary, secondary, tertiary depending on the type of C (sp³) which is attached to -OH

Allylic – OH group is attached to allylic carbon (sp³), e.g. CH₂=CH-CH-OH

Benzylic – OH group attached to C (sp³) attached to an aromatic ring

Vinylic – OH group attached to C (sp²) with C=C, e.g. CH₂=CH-OH

Depending on no. of -OH group present, Monohydric, dihydric and polyhydric

Symmetrical (Simple) – having the same groups (alkyl/aryl) attached to O

Unsymmetrical (Mixed) – different groups attached to O

Common Name – ‘alcohol’ attached to the name of the alkyl group, e.g. methyl alcohol, ethyl alcohol, etc.

IUPAC – The suffix – ‘ol’ attached to the name of alkane, e.g. methanol (CH₃OH), ethanol (C₂H₅OH), etc.

Simplest is phenol C₆H₅OH. It is the common name and also accepted in IUPAC nomenclature

Position of OH group is denoted by o (ortho), m (meta), p (para), or by numbering the cyclic carbons etc.

E.g. o-Cresol is 2-Methylphenol, Catechol is Benzene-1,2-diol, etc.

Common Name – word ‘ether’ follows the name of the alkyl groups in alphabetical order, e.g. Ethylmethyl ether, Diethyl ether, etc.

IUPAC Name – Named as alkoxy or aryloxy derivative of hydrocarbon. The bulkier group is taken as the parent hydrocarbon, e.g. Methoxymethane, Methoxybenzene, etc.

There is a bonding between sp³ hybridised orbitals of C and O

Bond Angle:

The C-O-H bond angle is 108.9°, i.e. less than tetrahedral (109°-28’)

It is due to repulsion between electron pairs of Oxygen

Bond Length:

The C-O bond length is 142 pm in Methanol

There is a bonding between sp² hybridised orbitals of C of aromatic ring and O

Bond Angle:

The C-O-H bond angle is 109° in Phenol

Bond Length:

The C-O bond length is 136 pm in Phenol.

It is less compared to Methanol due to sp² hybridization of Carbon and conjugation of pi electrons of the aromatic ring gives partial double bond character.

Bond Angle:

The C-O-C bond angle is 111.7 (Methoxymethane).

It is more than tetrahedral due to repulsion between the two R (bulky) groups

Bond Length:

The C-O bond length is 141 pm, almost the same as alcohols

1. From Alkenes:

a. Acid-catalyzed hydration- Addition of H₂O according to Markonikov’s rule

b. By hydroboration-oxidation- Reaction with diborane to give trialkyl boranes, which are oxidised by H₂O₂ and NaOH (aq)

The end product formed is opposite to Markonikov’s rule

1. From Haloarenes:

Chlorobenzene is treated with NaOH to form sodium phenoxide, which is then treated with acid to form phenol

C₆H₅Cl + NaOH → C₆H₅ONa + HCl → C₆H₅OH

1. By dehydration of alcohols:

When primary alcohols are treated with protic acids (H₂SO₄, H₃PO₄) ether is formed by the nucleophilic bimolecular reaction. This reaction depends on conditions, at 443 K alkene is a major product, whereas at 413 K ether is a major product

When alcohol is 2° or 3°, elimination reaction competes with SN₁ resulting in alkene as a major product

2. From Carbonyl Compounds:

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

Aldehydes → primary alcohols

Ketones → secondary alcohols

b. By reduction of carboxylic acids and esters

2. From Benzene sulphonic Acid:

The first step is the sulphonation of benzene with oleum. Benzene sulphonic acid thus formed is heated with molten NaOH to form sodium phenoxide and then it is acidified to from phenol

2. Williamson Synthesis:

Ether is formed in the reaction of sodium alkoxide with an alkyl halide

With 1° alkyl halide, SN₂ is preferred and ether is formed as a major product but with 2° or 3° elimination proceeds and alkene is the major product formed

Phenol can also be converted to ether by this method

3. From Grignard Reagents:

By the reaction aldehyde and ketones with Grignard reagent (R-Mg-X). There is a nucleophilic addition of Grignard reagent followed by hydrolysis

HCHO + RMgX → RCH₂OMgX + H₂O → RCH₂OH + Mg(OH)X

3. From Diazonium Salts:

Aniline (C₆H₅NH₂) when reacts with NaNO₂ + HCl forms Benzene diazonium chloride (C₆H₅N₂Cl), which on hydrolysis give phenol

4. From Cumene:

Cumene (isopropylbenzene) is oxidised to form cumene hydroperoxide and then treated with dil. acid to form phenol. Acetone is a by-product in the reaction

Boiling Point:

Increases with increase in the number of C atoms due to the increase in Van der Waals forces

Decreases with an increase in branching due to the decrease in surface area and Van der Waals forces

Boiling point is more compared to other compounds like alkanes, ethers, haloalkanes, etc. due to intermolecular hydrogen bonding

Boiling Point:

Boiling point is more compared to other compounds like arenes, ethers, haloarenes, etc. due to intermolecular hydrogen bonding

Boiling Point:

Less compared to alcohols due to the presence of intermolecular hydrogen bonding in alcohols

Solubility:

Solubility is due to the formation of hydrogen bonds with H₂O

Solubility decreases with an increase in the size of the alkyl group (hydrophobic)

Solubility:

Solubility is due to the formation of hydrogen bonds with H₂O

Solubility decreases with an increase in the size of the aryl group (hydrophobic)

Solubility:

Miscibility of ether is comparable to alcohol and more than alkane of same molecular mass due to formation of a hydrogen bond with water and O of ether

Strong Bronsted acids and can donate electrons to a strong base

Acidity is due to the polarity of the O-H bond. The electron releasing alkyl groups decrease the acidity so the order of acidity of alcohol is:

1° > 2° > 3°

Water is a better proton donor or stronger acid than alcohols

They also act as a proton acceptor or Bronsted bases due to unshared electron pairs on oxygen

Strong Bronsted acids and can donate electrons to a strong base

Benzene ring attached to the OH group acts as an electron withdrawing group. Electron pairs of oxygen are in conjugation with the double bond of benzene ring making the oxygen of OH positive

Phenols are stronger acids than water and alcohols

This can be explained by more stable phenoxide ion and more polar OH bond

Electron withdrawing group (-NO₂) in the substituted phenols make it more acidic and Electron donating groups (alkyl) decrease the acidity

Order of acidity:

Nitrophenol > Phenol > Cresol > Ethanol

Ethers are more acidic than hydrocarbons but less acidic than ketones and aldehydes

Ethers can act as Lewis or Bronsted bases. They form oxonium ion after protonation by strong acids

1. Reactions where alcohol reacts as a Nucleophile (O-H bond cleavage):

a. Reaction with metals to form the corresponding alkoxides and H₂

2R-O-H + 2Na → 2R-O-Na + H₂

b. Esterification: Alcohols react with carboxylic acids, acid anhydrides and acid chlorides to form esters

1. Reactions where phenol reacts as a Nucleophile (O-H bond cleavage):

a. Phenol reacts with metal or aq NaOH to form sodium phenoxide

b. Esterification: Phenols react with carboxylic acids, acid anhydrides and acid chlorides to form esters

Salicylic acid + Acetic anhydride → Aspirin (Acetylsalicylic acid)

1. There is no O-H bond in ethers

2. Reactions where alcohol reacts as an Electrophile (C-O bond cleavage):

a. Reaction with HX to form alkyl halides. This forms the basis of the Lucas test to differentiate between 1°, 2° and 3° alcohols.

3° alcohols form halides easily so turbidity is produced immediately

b. Reaction with PX₃ to form alkyl halides

c. Undergo dehydration to form alkene in the presence of protic acid, e.g. ethanol forms ethylene on reacting with H₂SO₄ at 443 K

2° and 3° alcohols undergo dehydration at comparatively milder conditions. The order of dehydration is:

3° > 2° > 1°

d. Oxidation or Dehydrogenation reaction to form aldehydes and ketones

Primary alcohols form aldehydes

Strong oxidising agents (KMnO₄), form carboxylic acid directly

CrO₃ and PCC (pyrimidine chlorochromate) are used to get aldehydes

Secondary alcohols form ketones on oxidation by CrO₃

Tertiary alcohols do not undergo oxidation. On treating with KMnO₄ at a higher temp, dissociation of C-C bond takes place resulting in a mixture of carboxylic acids with fewer carbon atoms

e. Alcohols also undergo dehydrogenation when heated with Cu at 573 K.

1° alcohol → Aldehyde

2° alcohol → Ketone

3° alcohol → Alkene (Dehydration)

2. Reactions where there is C-O bond cleavage:

Phenol reacts with Zn dust to form benzene

C₆H₅OH + Zn → C₆H₆ + ZnO

2. Reactions where there is C-O bond cleavage:

Ethers are comparatively less reactive. The C-O bond cleavage occurs under drastic conditions

Dialkyl ethers when react with HX give rise two alkyl halides

When alkyl aryl ethers react with HX, they form phenol and alkyl halide due to more stability of the aryl-oxygen bond

The reactivity order of HX is

HI > HBr > HCl

3. Electrophilic Substitution:

In Phenol, electrophilic aromatic substitution takes place at ortho (o) and para (p) position

a. Nitration with dil. HNO₃ at 298 K gives mixture of o- and p-Nitrophenols

p-Nitrophenol is less volatile due to intermolecular hydrogen bonding. In o-Nitrophenol, intramolecular hydrogen bonding is present

Nitration with conc. HNO₃ forms Picric acid (2,4,6-trinitrophenol)

b. Halogenation: monosubstituted phenols are formed in presence of CS₂ or CHCl₃

When treated with bromine water, white precipitate of 2,4,6-tribromophenol is formed

Kolbe’s Reaction

Phenol reacts with NaOH to form sodium phenoxide

Salicylic acid (2-Hydroxybenzoic acid) is formed when sodium phenoxide undergoes electrophilic substitution on reacting with CO₂

Reimer-Tiemann Reaction

C₆H₅OH + CHCl₃ + aq NaOH → C₆H₅CHO (Salicylaldehyde)

-CHO group is attached to o- position in phenol

Oxidation

Phenol gets oxidised in presence of Na₂Cr₂O₇ and H₂SO₄ to form Benzoquinone

3. Electrophilic Substitution:

In Aryl ethers also electrophilic aromatic substitution takes place at ortho (o) and para (p) position

Friedel Crafts Reaction:

Halogenation and Nitration takes place at o and p position

Anisole reacts with alkyl or acyl halide in presence of AlCl₃ (anhydrous) give o and p substituted product

Name

Common Name

Chemical Formula

Preparation

Properties and Usage

Methanol

Wood spirit

CH₃OH

Catalytic hydrogenation of CO (carbon monoxide)

Highly poisonous

Used in paints and varnishes

Ethanol

Ethyl alcohol

C₂H₅OH

Fermentation of glucose and sugar by enzymes like zymase and invertase

Used in industries to make wine and other alcoholic drinks

Ethanol is also used in sanitisers and paint industry

Phenol

C₆H₅OH

Prepared from cumene and oxidation of benzene and toluene

Precursor for various important compounds, plastic and other polymers

Used as an antiseptic

Methoxybenzene

Anisole

CH₃OC₆H₅

Prepared by Williamson synthesis from sodium phenoxide and methyl halide

It smells like anise seed

It is precursor to perfumes and pharmaceuticals