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chemistry-9701 | as-a-level
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1.
Group 17
2.
Electrochemistry
3.
Chemical Energetics
4.
Group 2
5.
Atoms, Molecules and Stoichiometry
6.
Nitrogen Compounds
7.
Halogen Compounds
8.
Chemistry of Transition Elements
9.
Hydroxy Compounds
10.
Equilibria
11.
Nitrogen and Sulfur
12.
Carbonyl Compounds
13.
Chemical Bonding
14.
Electrochemistry
15.
Introduction to Organic Chemistry
16.
Genetic technology
17.
Atomic Structure
18.
Polymerisation (Condensation Polymers)
19.
Carboxylic Acids and Derivatives
20.
Introduction to A Level Organic Chemistry
21.
Reaction Kinetics
22.
Organic Synthesis
23.
Hydrocarbons (Arenes)
24.
Equilibria
25.
Analytical Techniques
26.
Halogen Compounds (Arenes)
27.
Hydroxy Compounds (Phenol)
28.
Polymerisation
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Hydrocarbons
30.
States of Matter
31.
The Periodic Table: Chemical Periodicity
32.
Organic Synthesis
33.
Reaction Kinetics
34.
Analytical Techniques
35.
Chemical Energetics
36.
Group 2
37.
Carboxylic Acids and Derivatives (Aromatic)
Reactions of Phenol with Bases, Sodium and Diazonium Salts
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Reactions of Phenol with Bases, Sodium and Diazonium Salts
Introduction
Phenol, an aromatic hydroxy compound, plays a pivotal role in organic chemistry due to its unique reactivity. Understanding its interactions with bases, sodium, and diazonium salts is essential for students pursuing AS & A Level Chemistry (9701). These reactions not only elucidate the fundamental properties of phenol but also pave the way for its diverse applications in synthesis and industry.
Key Concepts
Reaction of Phenol with Bases
Phenol (\chemfig{C6H5OH}) exhibits acidic properties, albeit weaker compared to mineral acids like HCl or H2SO4. This acidity arises from the ability of the hydroxyl group to donate a proton, forming the phenoxide ion. The reaction with bases is a classic example of acid-base chemistry in organic compounds.
When phenol reacts with a strong base, such as sodium hydroxide (NaOH), it undergoes deprotonation:
$$
\chemfig{C6H5OH + NaOH \rightarrow C6H5O^- Na^+ + H2O}
$$
The phenoxide ion (\chemfig{C6H5O^-}) is resonance-stabilized, which delocalizes the negative charge over the aromatic ring, enhancing the stability of the product. This reaction is fundamental in facilitating further chemical transformations, such as nucleophilic substitutions.
**Factors Influencing the Reaction:**
1. **Nature of the Base:** Stronger bases, like NaOH or KOH, more effectively deprotonate phenol compared to weaker bases.
2. **Solvent Effects:** Polar solvents stabilize the ionic phenoxide ion, driving the equilibrium towards product formation.
3. **Temperature:** Higher temperatures can favor the formation of the phenoxide ion by increasing the reaction rate.
**Applications:**
- **Synthesis of Salts:** The phenoxide ion serves as a precursor for various phenolate salts, which are useful intermediates in organic synthesis.
- **Industrial Processes:** Understanding this reaction is crucial in industries where phenol is used, such as in the production of plastics and pharmaceuticals.
Reaction of Phenol with Sodium
The reaction of phenol with sodium metal exemplifies a redox process where sodium serves as a reducing agent. This reaction is less common due to the vigorous nature of sodium's reactivity but is significant in demonstrating the fundamental interactions between metals and organic compounds.
When phenol reacts with sodium, hydrogen gas is typically evolved alongside the formation of sodium phenoxide:
$$
\chemfig{C6H5OH + Na \rightarrow C6H5O^- Na^+ + \frac{1}{2}H2}
$$
**Mechanism:**
1. **Electron Transfer:** Sodium donates an electron to phenol, leading to the formation of sodium phenoxide and hydrogen gas.
2. **Formation of Sodium Phenoxide:** Similar to the reaction with bases, the phenoxide ion is stabilized through resonance.
**Safety Considerations:**
- **Vigorous Reaction:** The reaction can be exothermic and may produce flammable hydrogen gas, necessitating appropriate safety measures.
- **Controlled Conditions:** Conducting the reaction under inert atmospheres or using excess solvent can mitigate risks.
**Applications:**
- **Organic Synthesis:** Sodium phenoxide acts as a strong nucleophile in various substitution reactions.
- **Material Science:** Sodium phenoxide can be used in the synthesis of conducting polymers and other advanced materials.
Reaction of Phenol with Diazonium Salts
Diazonium salts, such as benzenediazonium chloride (\chemfig{C6H5N2^+ Cl^-}), react with phenol through azo coupling, a process that forms azo compounds with significant industrial relevance, particularly as dyes.
The general reaction can be represented as:
$$
\chemfig{C6H5OH + C6H5N2^+ Cl^- \rightarrow C6H5N=N-C6H4OH + HCl}
$$
**Mechanism:**
1. **Activation of Phenol:** Phenol's hydroxyl group activates the aromatic ring, especially the ortho and para positions, towards electrophilic substitution.
2. **Formation of Azo Bond:** The diazonium salt acts as an electrophile, coupling with the activated phenol to form the azo linkage (\chemfig{N=N}).
**Factors Affecting the Reaction:**
1. **pH:** A slightly alkaline environment enhances the reaction by increasing the nucleophilicity of phenol.
2. **Temperature:** Lower temperatures stabilize the diazonium salt, preventing its decomposition.
3. **Substituents:** Electron-donating groups on the aromatic ring facilitate easier coupling.
**Applications:**
- **Dye Industry:** Azo dyes synthesized through this reaction are extensively used for coloring textiles, leather, and other materials.
- **Analytical Chemistry:** Diazotization of anilines followed by coupling with phenol derivatives aids in the synthesis of various analytical reagents.
Advanced Concepts
In-depth Theoretical Explanations
The reactivity of phenol with bases, sodium, and diazonium salts can be further understood through molecular orbital theory and the concept of resonance stabilization. The phenol molecule possesses a hydroxyl group attached to an aromatic ring, enabling electron delocalization.
**Resonance Stabilization of Phenoxide Ion:**
Upon deprotonation by a base, the resulting phenoxide ion exhibits resonance stabilization:
$$
\chemfig{C6H5O^-} \leftrightarrow \chemfig{C6H5O^-} \quad \chemfig{<->} \quad \chemfig{C6H5O^-}
$$
This delocalization lowers the overall energy of the ion, making it more stable and increasing phenol's apparent acidity.
**Molecular Orbital Considerations:**
1. **HOMO and LUMO:** The Highest Occupied Molecular Orbital (HOMO) of phenol interacts with the Lowest Unoccupied Molecular Orbital (LUMO) of reactants, facilitating bond formation or breaking.
2. **Electrophilic and Nucleophilic Sites:** The activated positions on the aromatic ring determine the pathway of electrophilic substitutions, especially in azo coupling reactions.
Complex Problem-Solving
**Problem:** Predict the major product when phenol reacts with benzenediazonium chloride in the presence of a mild base. Explain the regioselectivity observed in the reaction.
**Solution:**
1. **Activation of Phenol:** The hydroxyl group activates the ortho and para positions on the aromatic ring.
2. **Electrophilic Attack:** The diazonium ion (\chemfig{N2^+}) seeks the most electron-rich positions, favoring ortho and para substitution.
3. **Regioselectivity:** Due to steric hindrance and potential hydrogen bonding, para-substitution is often favored over ortho-substitution, leading to the predominant formation of para-azo-phenol.
**Answer:** The major product is para-azo-phenol (\chemfig{p-C6H4N=N-C6H5OH}).
Interdisciplinary Connections
The reactions of phenol with bases, sodium, and diazonium salts extend beyond pure chemistry, influencing various fields:
1. **Material Science:** Azo compounds synthesized from phenol are integral in producing azo dyes, which have applications in creating colored polymers and responsive materials.
2. **Environmental Science:** Understanding the degradation pathways of phenolic pollutants involves similar reaction mechanisms, aiding in developing effective remediation strategies.
3. **Pharmaceuticals:** Phenolic compounds are prevalent in drug design, where their reactivity is harnessed for synthesizing bioactive molecules.
Moreover, the principles of acid-base chemistry and electrophilic substitution are foundational in biochemistry, influencing enzyme activity and metabolic pathways.
Comparison Table
| Aspect | Reaction with Bases | Reaction with Sodium | Reaction with Diazonium Salts |
|---|---|---|---|
| Reactants | Phenol and strong bases (e.g., NaOH) | Phenol and sodium metal (Na) | Phenol and diazonium salts (e.g., C6H5N2^+Cl^-) |
| Products | Phenoxide ion and water | Sodium phenoxide and hydrogen gas | Azo-phenol and HCl |
| Reaction Type | Acid-base neutralization | Redox reaction | Electrophilic substitution (azo coupling) |
| Applications | Formation of phenolate salts for synthesis | Nucleophilic substitutions, material synthesis | Production of azo dyes, analytical reagents |
| Key Features | Resonance stabilization of phenoxide ion | Evolution of hydrogen gas, vigorous reaction | Formation of azo bond (\chemfig{N=N}), regioselectivity |
Summary and Key Takeaways
- Phenol's reactivity with bases, sodium, and diazonium salts illustrates fundamental acid-base and substitution mechanisms.
- Phenoxide ion formation is central to many subsequent chemical transformations.
- Azo coupling with diazonium salts is crucial for synthesizing commercially important dyes.
- Understanding these reactions enhances comprehension of phenol's role in various industrial and synthetic applications.
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Examiner Tip
Tips
To remember the reactivity of phenol, use the mnemonic "Phenol's Phenomenal Phenoxide" to associate phenol with its key reaction product. Additionally, always consider resonance structures when predicting the stability of intermediates, which is crucial for mastering exam questions on aromatic substitution.
Did You Know
Did You Know
Phenol was the first aromatic compound to be synthesized by the famous chemist Friedrich Benz in the 19th century. Additionally, phenol derivatives are not only used in dyes but also play a critical role in the production of antiseptics and disinfectants, impacting public health significantly.
Common Mistakes
Common Mistakes
Incorrect: Assuming phenol is as acidic as strong mineral acids.
Correct: Recognizing that phenol is weakly acidic and requires strong bases for effective deprotonation.
Incorrect: Overlooking the regioselectivity in azo coupling, leading to unexpected product formation.
Correct: Understanding that para-substitution is generally favored due to less steric hindrance.
FAQ
Why is phenol more acidic than benzene?
Phenol is more acidic than benzene because the resulting phenoxide ion is resonance-stabilized, delocalizing the negative charge over the aromatic ring, which stabilizes the deprotonated form.
What role does sodium play in the reaction with phenol?
Sodium acts as a reducing agent, donating electrons to phenol, which facilitates the formation of sodium phenoxide and hydrogen gas in a redox reaction.
How does pH affect azo coupling with diazonium salts?
A slightly alkaline pH enhances azo coupling by increasing the nucleophilicity of phenol, thereby promoting effective reaction with diazonium salts.
What are the safety precautions when reacting phenol with sodium?
Ensure the reaction is conducted in a controlled environment to manage the exothermic nature and flammable hydrogen gas. Use appropriate protective equipment and conduct the reaction under an inert atmosphere if possible.
Can phenol react with weak bases?
Phenol requires strong bases to effectively deprotonate and form the phenoxide ion. Weak bases are generally insufficient for this purpose.
What industrial applications rely on phenol’s reactions?
Industries producing plastics, pharmaceuticals, and dyes heavily rely on phenol's reactions with bases, sodium, and diazonium salts for synthesizing various products.
1.
Group 17
2.
Electrochemistry
3.
Chemical Energetics
4.
Group 2
5.
Atoms, Molecules and Stoichiometry
6.
Nitrogen Compounds
7.
Halogen Compounds
8.
Chemistry of Transition Elements
9.
Hydroxy Compounds
10.
Equilibria
11.
Nitrogen and Sulfur
12.
Carbonyl Compounds
13.
Chemical Bonding
14.
Electrochemistry
15.
Introduction to Organic Chemistry
16.
Genetic technology
17.
Atomic Structure
18.
Polymerisation (Condensation Polymers)
19.
Carboxylic Acids and Derivatives
20.
Introduction to A Level Organic Chemistry
21.
Reaction Kinetics
22.
Organic Synthesis
23.
Hydrocarbons (Arenes)
24.
Equilibria
25.
Analytical Techniques
26.
Halogen Compounds (Arenes)
27.
Hydroxy Compounds (Phenol)
28.
Polymerisation
29.
Hydrocarbons
30.
States of Matter
31.
The Periodic Table: Chemical Periodicity
32.
Organic Synthesis
33.
Reaction Kinetics
34.
Analytical Techniques
35.
Chemical Energetics
36.
Group 2
37.
Carboxylic Acids and Derivatives (Aromatic)
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