Hydroxy Compounds: Alcohols and Phenols Notes

Hydroxy Compounds: Alcohols and Phenols

Objectives

• Describe structural and optical isomerism in hydroxy compounds.
• State the physical properties of hydroxy compounds.
• Classify alcohols into primary, secondary, and tertiary alcohols.
• Describe the preparation of alcohols (e.g., ethanol) from fermentation and ethene hydration.
• Understand alcohol reactions: oxidation, dehydration, reaction with Na, haloalkane formation, esterification, and acylation.
• Explain tests to determine alcohol class and type (Lucas test).
• State the uses of alcohols as antiseptic, solvent, and fuel.
• Explain the relative acidity of water, phenol, and alcohol, referencing inductive and resonance effects.
• Describe phenol preparation via the Cumene process.
• Understand phenol reactions with Na, NaOH, acyl chloride, and electrophilic substitution.
• Describe bromine water and aqueous iron(III) chloride tests for phenol.
• Explain phenol's use in cyclohexanol and nylon-6,6 manufacture.

Phenols

• Overview of Phenols:
  • Preparation
  • Chemical Properties
  • General Reactions
  • Electrophilic Substitution Reactions
  • Identification Tests

Industrial Preparation of Phenol (Cumene Process)

1. Alkylation of Benzene: Benzene reacts with propene in the presence of $AlCl_3$ to form isopropylbenzene (cumene).

   • $C<em>6H</em>6 + CH<em>2=CHCH</em>3 \rightarrow C<em>6H</em>5CH(CH<em>3)</em>2$

2. Oxidation of Cumene: Cumene is oxidized with $O_2$ to form cumene hydroperoxide.

   • $C<em>6H</em>5CH(CH<em>3)</em>2 + O<em>2 \rightarrow C</em>6H<em>5C(CH</em>3)_2OOH$

3. Decomposition and Rearrangement: Cumene hydroperoxide decomposes in the presence of dilute $H<em>2SO</em>4$ to form phenol and acetone.

   • $C<em>6H</em>5C(CH<em>3)</em>2OOH \xrightarrow{H<em>2SO</em>4} C<em>6H</em>5OH + CH<em>3COCH</em>3$

Laboratory Preparation of Phenol

1. From Aniline:

   • Aniline reacts with $NaNO_2$ and $HCl$ at 0-5°C to form arenediazonium salt.

   • $C<em>6H</em>5NH<em>2 + NaNO</em>2 + HCl \rightarrow C<em>6H</em>5N<em>2^+Cl^- + H</em>2O$

2. Hydrolysis of Arenediazonium Salt:

   • Arenediazonium salt reacts with $Cu<em>2O$, $Cu^+$, and $H</em>2O$ under heat to yield phenol.
   • $C<em>6H</em>5N<em>2^+Cl^- + H</em>2O \xrightarrow{Cu<em>2O, Cu^+, \Delta} C</em>6H<em>5OH + N</em>2 + HCl$

Properties of Phenols

• Attachment to Benzene Ring: The –OH group is directly attached to the benzene ring.
• Reactivity Difference: Phenol reactivity differs from ROH due to the oxygen atom's electron pair overlapping with the delocalized π electrons in the benzene ring.
• C-O Bond Character: The C-O bond in phenol has partial double bond character, making it stronger than the C-O bond in ROH.
• Charge Distribution: The partially negative charge on the O atom is delocalized over the carbon atoms in the benzene ring.
• Resonance: Delocalization of pi electrons results in a resonance hybrid structure of phenol.

Reactions of Phenols

• Nucleophilic Substitution: Loss of –OH is less likely in phenols compared to ROH.
  • Reaction with HX: Phenol (no reaction) < 1° < 2° < 3° < benzyl alcohol.
• Elimination: Not possible because it would destroy the stable aromatic system.
• Oxidation: Phenols are not easily oxidized due to the absence of an H atom at the C atom bearing the –OH group.
• Activating and Directing Group: The hydroxyl (–OH) group is a strong activating and ortho-para directing substituent in electrophilic substitution.
• Reactivity: The –OH group makes the benzene ring very reactive towards electrophiles ($E^+$), compared to benzene.

Electrophilic Substitution (ES) of Phenols

• Reactions occur readily without a Lewis acid catalyst.

1. Halogenation:

   • In non-polar solvents and at low temperatures, phenol reacts with halogens (chlorine and bromine) to form a mixture of ortho-substituted and para-substituted halophenols.

2. Halogenation in Aqueous Solution:
   *In aqueous solutions and at high temperatures, 2,4,6-trihalophenol is produced.

   *This reaction serves as a test for phenol, with 2,4,6-tribromophenol appearing as a white precipitate.

3. Nitration:

   • Nitration of phenol occurs readily without a catalyst at room temperature with dilute nitric acid, yielding o-nitrophenol and p-nitrophenol.

   • With concentrated nitric acid, 2,4,6-trinitrophenol (picric acid) is formed.

4. Sulfonation:

   • Products from the reaction of phenol with concentrated sulfuric acid depend on temperature, forming 2-hydroxybenzenesulfonic acid and 4-hydroxybenzenesulfonic acid.

Esterification

• Phenoxide Ion Formation: Phenoxide ion is obtained by dissolving phenol in NaOH (aqueous).
• Reaction with Acyl Chlorides/Anhydrides:
  • $PhOH + RCOCl \rightarrow RCOOR'$; $PhOH + RCOOCOR' \rightarrow RCOOR'$ (in the presence of NaOH(aq))
  • Phenol is a weaker nucleophile compared to alcohol due to the delocalization of lone pairs of O atom into the benzene ring, making the reaction with $RCOOH$ slow.
    *Phenoxide ion is a better nucleophile, reacting with acyl chloride or anhydride to form $RCOOR'$.

Identification Test for Phenol

• Iron(III) Chloride Test: Phenols react with iron(III) chloride to form a light purple complex.
  • $PhOH + FeCl_3(aq) \rightarrow \text{purple complex}$

Summary of Reactions of Phenol

1. Reaction of the O-H Bond

• Phenol reacts with acid anhydrides or acyl chlorides in the presence of pyridine to form esters.

2. Reaction of the Benzene Ring

• (i) Halogenation: Phenol reacts with $Br_2$ in water to give 2,4,6-tribromophenol.

  • $C<em>6H</em>5OH + 3Br<em>2 \rightarrow C</em>6H<em>2Br</em>3OH + 3HBr$

• (ii) Nitration: Phenol reacts with dilute $HNO_3$ to yield o-nitrophenol and p-nitrophenol.

  • $C<em>6H</em>5OH + HNO_3(aq) \rightarrow o-Nitrophenol + p-Nitrophenol$

Differentiation Test Example

• Question: Suggest a simple chemical test to differentiate between phenol and benzyl alcohol.

Key Concepts

• Physical properties
• Synthesis / preparation
• Reactions and predict product
• Alcohols vs. Phenols
• Classify, name, draw

References

• Comprehensive College Chemistry
• Organic Chemistry, 8th Edition, L.G. Wade Jr., Pearson (2013)
• Lecture note of Mr. Mohd. Hilmi Jaafar (2018/2019)