📚 A-Level Chemistry (9701) Study Notes: Phenylamine and Azo Compounds
Hello future Chemists! This chapter takes us deep into the fascinating world of aromatic nitrogen compounds. We will explore Phenylamine (an aromatic amine) and its unique reactions, which are fundamental for creating brightly coloured substances called azo dyes—the backbone of the global textile industry. Let's dive into how the benzene ring changes the behavior of the amine group!
1. Phenylamine: Structure and Key Properties
1.1 What is Phenylamine?
Phenylamine (also known as aniline) is the simplest aromatic amine. It consists of an amino ($\text{–NH}_2$) functional group directly attached to a benzene ring ($\text{C}_6\text{H}_5$).
- Formula: $\text{C}_6\text{H}_5\text{NH}_2$
- Structure: The lone pair of electrons on the nitrogen atom of the $\text{NH}_2$ group interacts with the delocalised $\pi$-electron system of the benzene ring. This interaction is key to understanding its properties.
1.2 Step-by-Step Synthesis of Phenylamine (Two Stages)
We cannot attach an $\text{NH}_2$ group directly to benzene, so we use a two-step indirect method starting from benzene. This synthetic route requires AS Level knowledge (nitration) and a new A Level reduction reaction.
Step 1: Nitration of Benzene
The benzene ring is substituted with a nitro ($\text{–NO}_2$) group to form nitrobenzene.
- Reagents: Concentrated nitric acid ($\text{HNO}_3$) and concentrated sulfuric acid ($\text{H}_2\text{SO}_4$). (The sulfuric acid acts as a catalyst, forming the electrophile $\text{NO}_2^+$).
- Conditions: Temperature controlled at $50^\circ\text{C}$ to $60^\circ\text{C}$. (If the temperature is too high, further nitration occurs, giving unwanted side products.)
- Reaction Type: Electrophilic Substitution.
Step 2: Reduction of Nitrobenzene to Phenylamine
The nitrobenzene is reduced using a metal catalyst and acid.
Reagents and Conditions:
Reduction: Hot Tin ($\text{Sn}$) metal and concentrated Hydrochloric acid ($\text{HCl}$), followed by heating.
Analogy Alert: Think of the $\text{Sn}/\text{HCl}$ mixture as a powerful "de-oxidising" chemical hammer. It hits the $\text{NO}_2$ group and turns it into an $\text{NH}_3^+$ ion.
Neutralisation/Freeing the Amine: Aqueous Sodium Hydroxide ($\text{NaOH}(\text{aq})$) is added.
Why is the $\text{NaOH}$ needed? The initial product is the protonated form, phenylammonium chloride ($\text{C}_6\text{H}_5\text{NH}_3^+\text{Cl}^-$), because the reaction happens in strong acid ($\text{HCl}$). The strong alkali ($\text{NaOH}$) is needed to remove the proton and yield the neutral phenylamine ($\text{C}_6\text{H}_5\text{NH}_2$).
Key Takeaway for Synthesis: Phenylamine is made in two stages: Nitration (Electrophilic Substitution) followed by Reduction ($\text{Sn}/\text{HCl}$) and final neutralisation ($\text{NaOH}$).
2. Reactions of Phenylamine
2.1 Phenylamine as a Highly Activated Ring
The $\text{–NH}_2$ group is a powerful activating group. The lone pair on the nitrogen atom is partially donated into the benzene ring, significantly increasing the electron density, especially at the 2, 4, and 6 positions (ortho and para).
This increased electron density makes the ring highly susceptible to electrophilic substitution, even without a catalyst!
Reaction 1: Substitution with Aqueous Bromine ($\text{Br}_2(\text{aq})$)
- Reagents: Aqueous Bromine ($\text{Br}_2(\text{aq})$)
- Conditions: Room temperature.
- Observation: Phenylamine reacts immediately with bromine water to form a white precipitate.
Unlike benzene, which requires a halogen carrier like $\text{AlBr}_3$, phenylamine reacts instantly, substituting all three available ortho and para positions.
Product: 2,4,6-tribromophenylamine.
Did you know? This difference in reactivity is how you can chemically distinguish phenylamine (and phenol) from simple benzene or non-activated rings.
2.2 Diazotisation and the Formation of Phenol
This is a two-part reaction crucial for synthesizing dyes and phenol.
Part A: Diazotisation (Forming the Diazonium Salt)
Phenylamine reacts with nitrous acid ($\text{HNO}_2$) to form benzenediazonium chloride.
- Reagents: Nitrous acid ($\text{HNO}_2$) generated in situ (in the reaction flask) by mixing Sodium Nitrite ($\text{NaNO}_2$) and dilute acid (e.g., $\text{HCl}$).
- Crucial Condition: Temperature below $10^\circ\text{C}$ (usually $0^\circ\text{C}$ to $5^\circ\text{C}$).
Why the low temperature? Diazonium salts ($\text{ArN}_2^+\text{Cl}^-$) are highly unstable above $10^\circ\text{C}$ and decompose explosively, releasing nitrogen gas.
Equation Summary:
$$ \text{C}_6\text{H}_5\text{NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{< 10^\circ\text{C}} \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O} $$
Part B: Forming Phenol from the Diazonium Salt
If the benzenediazonium chloride solution is further warmed with water ($\text{H}_2\text{O}$), it decomposes, replacing the unstable diazonium group with a hydroxyl ($\text{–OH}$) group, producing phenol.
$$ \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{H}_2\text{O} \xrightarrow{\text{heat}} \text{C}_6\text{H}_5\text{OH} + \text{N}_2 + \text{HCl} $$
Memory Tip: Warm the diazonium salt, and Nitrogen Gas ($\text{N}_2$) flies off, leaving Phenol (like an unstable compound collapsing).
Key Takeaway for Reactions: Phenylamine is highly reactive towards substitution (like $\text{Br}_2$) and forms the crucial diazonium salt below $10^\circ\text{C}$.
3. Basicity of Phenylamine
Amines act as bases because the nitrogen atom has a lone pair of electrons capable of accepting a proton ($\text{H}^+$). The strength of the base depends on how readily this lone pair is available.
3.1 Comparing Basicity: Ethylamine, Ammonia, and Phenylamine
The syllabus requires you to compare the basicity of three species:
- Ethylamine ($\text{CH}_3\text{CH}_2\text{NH}_2$): The strongest base.
- Aqueous Ammonia ($\text{NH}_3$): The intermediate base.
- Phenylamine ($\text{C}_6\text{H}_5\text{NH}_2$): The weakest base.
3.2 Explaining the Relative Basicities
A. Ethylamine is Stronger than Ammonia
- Reason: Ethylamine has an alkyl group ($\text{CH}_3\text{CH}_2\text{–}$). Alkyl groups are electron-donating (positive inductive effect).
- Effect: The alkyl group pushes electron density onto the nitrogen atom, making the lone pair more negative and therefore more available to accept a proton ($\text{H}^+$).
B. Phenylamine is Weaker than Ammonia
- Reason: The lone pair on the nitrogen atom in phenylamine is delocalised (spread out) into the adjacent benzene ring.
- Effect: Because the lone pair is pulled into the aromatic system, it is less available to bond with an incoming proton ($\text{H}^+$). This makes phenylamine a much weaker base than both ethylamine and ammonia.
Analogy Alert (Basicity): Imagine the nitrogen lone pair is money.
- In Ethylamine, the ethyl group gives the nitrogen extra money (electron-donating), making it generous and ready to share ($\text{H}^+$).
- In Ammonia, the nitrogen keeps its own money.
- In Phenylamine, the benzene ring acts like a black hole, pulling the nitrogen's money away (delocalisation). The nitrogen is now poor and cannot afford to accept the proton.
Quick Review Box: Relative Basicity Trend
Ethylamine ($\text{strongest}$) > Ammonia ($\text{intermediate}$) > Phenylamine ($\text{weakest}$)
4. Azo Compounds and Coupling Reactions (Dyes)
Azo compounds are brightly coloured compounds commonly used as synthetic dyes and indicators. They are formed via a reaction between the unstable diazonium salt and an aromatic compound (like phenol).
4.1 The Azo Group ($\text{–N=N–}$)
The defining feature of an azo compound is the azo group: the $\text{–N=N–}$ linkage which connects two aromatic rings.
Identification: The azo group is responsible for the bright colour of the dye, as it acts as a chromophore (a part of a molecule that absorbs specific wavelengths of light).
4.2 The Coupling Reaction
Diazonium salts are unstable, but they can react quickly with certain activated aromatic rings (like phenol) to form stable azo compounds. This is called a coupling reaction.
Example: Coupling of Benzenediazonium Chloride with Phenol
- Reagents: Benzenediazonium chloride ($\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$) and Phenol in a solution of Sodium Hydroxide ($\text{NaOH}(\text{aq})$).
- Conditions: Maintained at a low temperature (below $10^\circ\text{C}$) to prevent the diazonium salt from decomposing. The reaction occurs in alkaline solution ($\text{NaOH}(\text{aq})$) because phenol reacts with the alkali to form the highly activated phenoxide ion ($\text{C}_6\text{H}_5\text{O}^-$).
- Product: An intensely coloured precipitate (often orange or yellow) is formed, known as an azo dye (specifically, $p$-hydroxyazobenzene).
Key Features of the Reaction:
- The reaction occurs at the para position of the phenol (the 4-position).
- The product is an aromatic compound linked by the $\text{–N=N–}$ group.
Significance: The syllabus states that azo compounds are often used as dyes, and other azo dyes can be formed via a similar route (e.g., using different primary aromatic amines or coupling agents). By altering the starting chemicals, chemists can produce a vast spectrum of colours.
Key Takeaway for Azo Dyes: Coupling reactions link a diazonium ion with an activated aromatic ring (like phenol/phenoxide) below $10^\circ\text{C}$ to form stable, intensely coloured azo compounds containing the $\text{–N=N–}$ group.
💡 Summary Review Checklist (Syllabus 34.2)
- Phenylamine Preparation: $\text{Benzene} \xrightarrow{\text{Nitration}} \text{Nitrobenzene} \xrightarrow{\text{Sn}/\text{HCl} \text{ and } \text{NaOH}(\text{aq})} \text{Phenylamine}$.
- $\text{Br}_2(\text{aq})$ Reaction: Phenylamine is highly activated; forms $2,4,6$-tribromophenylamine instantly (white ppt) at room temperature.
- Diazotisation: Phenylamine + $\text{NaNO}_2/\text{dilute acid}$ must be below $10^\circ\text{C}$ to form the diazonium salt ($\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$).
- Phenol Synthesis: Warming the diazonium salt solution yields phenol and $\text{N}_2$ gas.
- Basicity Order: Ethylamine > Ammonia > Phenylamine. Explained by the delocalisation of the nitrogen lone pair into the ring, making it less available for protonation.
- Azo Coupling: Diazonium salt + Phenol (in $\text{NaOH}(\text{aq})$) $\xrightarrow{< 10^\circ\text{C}}$ Azo dye ($\text{–N=N–}$ linkage). These are used as commercial dyes.