Hydroxy Compounds – Complete Study Guide

Hydroxy Compounds – Complete Study Guide

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Key Notes
Tips & Tricks
Memorization Guide
Practice Questions
Chapter Exercises

Introduction to Hydroxy Compounds

Organic hydroxyl compounds are a class of organic compounds that contain one or more hydroxyl groups (-OH) attached to a carbon atom. The presence of a hydroxyl group significantly affects the properties and reactivity of these compounds.

Classification of Hydroxy Compounds

  • Alcohols: -OH group attached to a saturated carbon atom (sp³ hybridized)
    • Methanol (CH₃OH) – simplest alcohol, solvent, antifreeze
    • Ethanol (C₂H₅OH) – alcoholic beverages, solvent, fuel
    • Isopropanol (C₃H₇OH) – disinfectant, solvent
  • Phenols: -OH group attached directly to an aromatic benzene ring
    • Used in plastics, medicines, antiseptics, disinfectants
  • Naphthols: -OH group attached to a naphthalene ring (C₁₀H₇OH)
    • 1-Naphthol (α-naphthol): -OH on carbon 1
    • 2-Naphthol (β-naphthol): -OH on carbon 2
    • Used in dyes, pigments, pharmaceuticals

Preparation of Esters

Esters are organic compounds formed by the reaction between an acid and an alcohol. They are used in fragrances, flavorings, and pharmaceuticals.

From Acyl Chlorides

CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl
  • Reaction: Acyl Chloride + Alcohol → Ester + HCl gas
  • Mechanism: Nucleophilic attack by alcohol oxygen on electrophilic carbonyl carbon of acyl chloride
  • Key Features: Fast, exothermic, produces steamy HCl fumes
  • Conditions: Carried out at room temperature

Mechanism Details:

  1. Oxygen atom of alcohol (with lone pair) attacks electrophilic carbonyl carbon of acyl chloride
  2. Forms tetrahedral intermediate
  3. Chloride ion (good leaving group) departs
  4. Proton loss yields the final ester product

Preparation of Phenol

From Phenylamine (Aniline)

This is a two-step process:

  1. Diazotization (Below 10°C):
    C₆H₅NH₂ + NaNO₂/HNO₂ + HCl → C₆H₅N₂⁺Cl⁻

    Phenylamine reacts with nitrous acid (from NaNO₂ + HCl) below 10°C to form benzene diazonium chloride

  2. Hydrolysis (Warm up):
    C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl

    Benzene diazonium salt hydrolyzes when warmed to give phenol, nitrogen gas, and HCl

Critical Temperature Control:

The temperature must be maintained below 10°C during diazotization to prevent premature decomposition of the diazonium salt. If temperature exceeds 10°C, the diazonium salt decomposes to form phenol directly.

Acidity of Water, Phenol and Ethanol

Acidity Order: Phenol > Water > Ethanol

Compound Conjugate Base Stability Reason Kₐ Value Acidity
Phenol Phenoxide Ion Resonance Stabilization – negative charge delocalized into aromatic ring 1.3 × 10⁻¹⁰ Strongest Acid
Water Hydroxide Ion Charge localized but small size provides some stability 1.8 × 10⁻¹⁶ Medium
Ethanol Ethoxide Ion No Resonance. Negative charge localized, intensified by electron-donating ethyl group (+I effect) 1.26 × 10⁻¹⁶ Weakest Acid

Resonance in Phenoxide Ion

The phenoxide ion is stabilized by resonance, with the negative charge delocalized over the oxygen atom and the ortho and para positions of the benzene ring:

C₆H₅O⁻ ↔ Various resonance structures with negative charge on ring carbons

This resonance stabilization makes phenol a stronger acid than alcohols and water.

Reactions of Phenol

1. Reaction with Bases

  • With NaOH(aq):
    C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O

    Forms sodium phenoxide – proof of acidic nature

  • With Na metal:
    2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂

    Forms sodium phenoxide and hydrogen gas

2. Electrophilic Substitution Reactions

Phenol is more reactive than benzene due to the electron-donating -OH group which activates the ring.

(a) Nitration of Phenol

  • With dilute HNO₃ at room temperature:
    C₆H₅OH + HNO₃ → Mixture of o-Nitrophenol + p-Nitrophenol

    Forms a mixture of 2-nitrophenol and 4-nitrophenol

  • With concentrated HNO₃:
    C₆H₅OH + 3HNO₃ → 2,4,6-Trinitrophenol (Picric Acid) + 3H₂O

    Forms 2,4,6-trinitrophenol (picric acid)

(b) Bromination of Phenol

  • With Br₂ in non-polar solvent (CCl₄/CS₂):
    C₆H₅OH + Br₂ → Mixture of o-Bromophenol + p-Bromophenol

    Forms a mixture of 2-bromophenol and 4-bromophenol

  • With bromine water (Br₂(aq)):
    C₆H₅OH + 3Br₂ → 2,4,6-Tribromophenol + 3HBr

    Forms white precipitate of 2,4,6-tribromophenol – test for phenol

(c) Reaction with Diazonium Salt (Azo Coupling)

C₆H₅O⁻Na⁺ + C₆H₅N₂⁺Cl⁻ → C₆H₅-N=N-C₆H₄-OH (p-hydroxyazobenzene)

Forms azo compounds (yellow-orange dyes) under ice-cold conditions

Why different conditions than benzene?

The -OH group is a strong activating group that:

  • Increases electron density of the aromatic ring
  • Makes the ring more reactive toward electrophiles
  • Directs incoming groups to ortho and para positions (2,4,6)
  • Allows reactions under milder conditions without strong acids/catalysts

Reactions of Naphthols

Naphthols behave similarly to phenols in chemical reactions.

1. Reaction with Sodium Hydroxide

C₁₀H₇OH + NaOH → C₁₀H₇O⁻Na⁺ + H₂O

Forms naphthoxide salts

2. Reaction with Bromine Water

  • 1-Naphthol: Bromination at positions 2 and 4
  • 2-Naphthol: Bromination at position 1 (between OH group and other ring)

3. Reaction with Nitric Acid

Nitration occurs similarly to bromination

  • 1-Naphthol: Nitration at positions 2 and 4

4. Reaction with Diazonium Chloride

Forms azo compounds (intense orange-red precipitates)

  • 1-Naphthol: Reaction occurs at position 4
  • 2-Naphthol: Reaction occurs at position 1 (between OH group and other ring)

Essential Tips & Tricks

Classification Mnemonic

“A-P-N” Method:

  • Alcohols – Alkyl group
  • Phenols – Phenyl (Benzene) ring
  • Naphthols – Naphthalene ring

Ester Formation

“A-E” Rule: Acyl chloride + Ethanol (or any alcohol) gives an Ester

Remember: The reaction produces steamy HCl fumes – a key observation

Phenol Preparation

“Cold Salt, Warm Phenol”:

  • Cold (<10°C) to form the Diazonium Salt
  • Warm the salt to get Phenol

Temperature Tip: If you forget the temperature, you’ll get the wrong product!

Acidity Order

PWE Method: Remember Phenol > Water > Ethanol

“Resonance Rules Acidity”: The phenoxide ion’s resonance makes it stable, so phenol easily loses protons

Visual Tip: Draw the resonance structures of phenoxide to see why it’s stable

Electrophilic Substitution Positions

Smiley Face Method: Draw a benzene ring. The -OH group makes positions 2, 4, and 6 “smile” for electrophiles!

Positions: 2 and 6 (ortho), 4 (para)

Numbering Tip: Count positions clockwise or counterclockwise, but remember the -OH is at position 1

2-Naphthol Substitution

Reaction prefers the position opposite to the bulky other ring. For 2-naphthol, attack is at carbon 1.

Steric Hindrance: The other ring creates steric hindrance at position 4, so substitution occurs at position 1

Test for Phenol

Bromine Water Test:

  • Phenol + Bromine water → Decolorization + White precipitate
  • This is a positive test for phenol
  • Remember: White precipitate is 2,4,6-tribromophenol

Azo Dye Formation

“Ice-Cold Colors”: Azo coupling requires ice-cold conditions to form colorful dyes

Color Memory: Phenol gives yellow-orange, naphthols give intense orange-red colors

Memorization Techniques

Ultimate Mnemonic for Phenol Reactivity

“Phenol is A-OK!”

  • Acidic (reacts with NaOH/Na)
  • O– (Ortho) director
  • Kick-starts Electrophilic Substitution (very reactive)

Conceptual Summary

  1. Acidity is Key: Resonance in phenoxide explains why phenol’s chemistry differs from ethanol
  2. Phenol is Reactive: -OH group makes the ring a magnet for electrophiles (Br⁺, NO₂⁺)
  3. Position Matters: -OH always directs to Ortho (2,6) and Para (4) positions
  4. Tests for Phenol:
    • With FeCl₃: Violet coloration (not in your text but a classic test)
    • With Br₂ water: Decolorization + White PPT (2,4,6-tribromophenol)

Memory Palace Technique

Imagine a kitchen (your memory palace):

  • Alcohols are in the fridge (cold drinks)
  • Phenols are on the stove (used in manufacturing)
  • Naphthols are in the dye cabinet (used for colors)
  • Esters smell like fruits in the fruit bowl
  • Diazonium salts are in the freezer (need cold conditions)
  • White precipitate is like spilled salt on the counter

Acronym Method

Phenol Properties: “BRAVE”

  • Bromination – gives white precipitate
  • Resonance – stabilizes phenoxide
  • Acidic – stronger than water and ethanol
  • Very reactive – in electrophilic substitution
  • Electron-donating – OH group activates ring

Rhyme Method

“Phenol in water, makes it better,
Than ethanol, which is no go-getter.
With bromine water, white appears,
Confirming phenol, calming fears.
Ortho and para, it will direct,
Making benzene seem indirect.”

Multiple Choice Questions

1. Which of the following reagents is required to form an ester from an acyl chloride?

a) Water
b) Alcohol
c) Both alcohol and phenol
d) Phenol
Explanation: Esters are formed from acyl chlorides and alcohols. The reaction is: Acyl Chloride + Alcohol → Ester + HCl. Phenol can also form esters but the question specifically asks about the general reaction.

2. What is the product when phenylamine reacts with HNO₂ or NaNO₂ and dilute acid below 10°C?

a) Aniline
b) Phenol
c) Benzene diazonium chloride
d) Nitrobenzene
Explanation: Below 10°C, phenylamine forms benzene diazonium chloride. Phenol is formed only when this salt is warmed with water.

3. What happens when phenol reacts with sodium hydroxide (NaOH)?

a) Sodium phenoxide is formed
b) Phenol is reduced
c) Sodium phenolate is formed
d) No reaction occurs
Explanation: Phenol reacts with NaOH to form sodium phenoxide (C₆H₅O⁻Na⁺) and water. This reaction demonstrates the acidic nature of phenol.

4. Which of the following is produced when phenol reacts with sodium (Na)?

a) Phenyl sodium
b) Sodium phenoxide and hydrogen gas
c) Sodium phenoxide and oxygen gas
d) Phenolates and water
Explanation: Phenol reacts with sodium metal to form sodium phenoxide and hydrogen gas: 2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂

5. What is the product of nitration of phenol with dilute nitric acid (HNO₃) at room temperature?

a) Nitrobenzene
b) 2-Nitrophenol and 4-Nitrophenol
c) 3-Nitrophenol
d) Dinitrophenol
Explanation: Phenol with dilute HNO₃ at room temperature gives a mixture of 2-nitrophenol (ortho) and 4-nitrophenol (para). The -OH group directs to ortho and para positions.

6. Bromination of phenol with Br₂(aq) leads to the formation of:

a) Bromobenzene
b) 2-Bromophenol
c) 2,4,6-Tribromophenol
d) Benzene bromide
Explanation: Phenol with bromine water gives 2,4,6-tribromophenol as a white precipitate. This is a test for phenol.

7. Why is phenol more acidic than ethanol?

a) Phenol has a lower pH
b) Phenol’s phenoxide ion is stabilized by resonance
c) Ethanol is a weaker acid
d) Ethanol has a higher pKa
Explanation: The phenoxide ion undergoes resonance stabilization, delocalizing the negative charge into the aromatic ring, making it more stable than the ethoxide ion. This makes phenol a stronger acid.

8. Relative to water, phenol is:

a) Less acidic
b) More acidic
c) Equally acidic
d) Neither acidic nor basic
Explanation: Phenol is more acidic than water. The Kₐ of phenol is 1.3 × 10⁻¹⁰ while Kₐ of water is 1.8 × 10⁻¹⁶.

9. Which positions does the hydroxyl group of phenol direct electrophilic substitution to?

a) 2 and 3
b) 3 and 4
c) 2, 4, and 6
d) 2 and 4
Explanation: The -OH group is an ortho-para director. It directs incoming electrophiles to positions 2, 4, and 6 (where position 1 is the -OH group).

10. In the nitration of phenol, why are milder conditions required compared to benzene?

a) Phenol is less reactive than benzene
b) The hydroxyl group in phenol activates the ring
c) Benzene is more reactive
d) Phenol is inert
Explanation: The -OH group donates electrons to the ring, increasing electron density and making it more reactive toward electrophiles. This allows reactions under milder conditions.

Chapter Exercises – Complete Solutions

Concept Assessment Exercise 10.1

1. What is the acidity order of alcohol, phenol and water?

Answer: Phenol > Water > Ethanol

Explanation:

  • Phenol is the strongest acid because its conjugate base (phenoxide ion) is stabilized by resonance
  • Water is intermediate – its conjugate base (hydroxide ion) is stable due to small size
  • Ethanol is the weakest acid because its conjugate base (ethoxide ion) has no resonance stabilization and the electron-donating ethyl group destabilizes it

2. Alcohols and phenols both contain the -OH group. What is the difference between them?

Answer: The main difference is the carbon atom to which the -OH group is attached:

  • Alcohols: -OH group attached to a saturated carbon atom (sp³ hybridized) of an alkyl group
  • Phenols: -OH group attached directly to an aromatic benzene ring

Additional differences:

  • Phenols are more acidic than alcohols
  • Phenols undergo electrophilic substitution more readily than benzene
  • Phenols form colored complexes with FeCl₃ while alcohols do not

3. What happens when phenol is treated with bromine water?

Answer: When phenol is treated with bromine water:

  • The reddish-brown color of bromine water is decolorized
  • A white precipitate of 2,4,6-tribromophenol is formed
C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

Explanation: This reaction is used as a test for phenol. The -OH group activates the ring, making it highly reactive toward electrophilic bromination even without a catalyst.

Short Answer Questions

i. What is the diazonium salt formed when phenylamine reacts with nitrous acid below 10°C?

Answer: Benzene diazonium chloride (C₆H₅N₂⁺Cl⁻)

C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O

Explanation: This reaction is called diazotization and must be carried out below 10°C to prevent decomposition of the diazonium salt.

ii. Write the chemical equation for the reaction of phenol with sodium hydroxide.

Answer:

C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O

Product: Sodium phenoxide

Explanation: This reaction demonstrates the acidic nature of phenol. Phenol is a stronger acid than alcohols but weaker than carboxylic acids.

iii. What is the product when phenol reacts with bromine water?

Answer: 2,4,6-Tribromophenol (white precipitate)

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

Explanation: This is a test for phenol. The -OH group activates the ring toward electrophilic substitution, allowing triple bromination without a catalyst.

iv. Explain why phenol reacts differently with nitric acid compared to benzene.

Answer: Phenol reacts with nitric acid under milder conditions compared to benzene because:

  • The -OH group in phenol is a strong activating group that donates electrons to the ring
  • This increases electron density in the ring, making it more reactive toward electrophiles
  • Phenol undergoes nitration with dilute HNO₃ at room temperature, while benzene requires concentrated HNO₃ and H₂SO₄ at 50-60°C
  • The -OH group directs incoming groups to ortho and para positions

v. Why is phenol more acidic than ethanol?

Answer: Phenol is more acidic than ethanol because:

  • In phenol, the phenoxide ion formed after loss of proton is stabilized by resonance
  • The negative charge is delocalized over the oxygen atom and the ortho and para positions of the benzene ring
  • In ethanol, the ethoxide ion has no resonance stabilization and the electron-donating ethyl group further destabilizes it
  • The Kₐ of phenol (1.3 × 10⁻¹⁰) is higher than that of ethanol (1.26 × 10⁻¹⁶)

Long Answer Questions

1. Compare and contrast the reactivity of phenol, benzene, and ethanol in electrophilic aromatic substitution reactions. Explain the role of the hydroxyl group in phenol and its influence on these reactions.

Answer:

Compound Reactivity Conditions Required Positions of Substitution
Phenol Very high Mild conditions (dilute reagents, room temperature) Ortho and para (2,4,6)
Benzene Low Harsh conditions (conc. reagents, catalysts, heating) No specific directing effect
Ethanol No aromatic substitution Does not undergo aromatic substitution N/A

Role of -OH group in phenol:

  • The -OH group is a strong activating group that donates electrons to the aromatic ring through resonance
  • This increases electron density, especially at ortho and para positions
  • Makes the ring more nucleophilic and reactive toward electrophiles
  • Directs incoming electrophiles to ortho and para positions
  • Allows reactions to occur under milder conditions without strong acids or catalysts

2. Describe the acidity of phenol in terms of its ability to donate protons. Compare its acidity to that of water and ethanol and explain the molecular basis for the differences observed.

Answer:

Acidity Order: Phenol > Water > Ethanol

Molecular Basis:

  • Phenol:
    • After losing a proton, forms phenoxide ion (C₆H₅O⁻)
    • Phenoxide ion is stabilized by resonance – negative charge delocalized over oxygen and ring
    • This resonance stabilization makes phenol relatively acidic
    • Kₐ = 1.3 × 10⁻¹⁰
  • Water:
    • Forms hydroxide ion (OH⁻) after losing proton
    • Hydroxide ion is stable due to small size and high charge density
    • But has no resonance stabilization like phenoxide
    • Kₐ = 1.8 × 10⁻¹⁶
  • Ethanol:
    • Forms ethoxide ion (CH₃CH₂O⁻) after losing proton
    • No resonance stabilization
    • Electron-donating ethyl group (+I effect) destabilizes the ethoxide ion
    • Kₐ = 1.26 × 10⁻¹⁶ (weakest acid)

Conclusion: The resonance stabilization of phenoxide ion is the key factor that makes phenol more acidic than water and ethanol.

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