The Chemistry of Nitriles and Hydroxynitriles (Topic 19.2)
Hello future Chemists! This chapter introduces two very useful functional groups: Nitriles ($R-C\equiv N$) and Hydroxynitriles. Why are these molecules so important? Because they are key tools in Organic Synthesis, allowing us to extend the carbon chain of a molecule. Mastering their synthesis and reactions is vital for tackling complex A-Level questions!
1. Understanding Nitriles
1.1 Structure and Nomenclature
The functional group that defines a nitrile is the cyano group, which contains a carbon atom triple-bonded to a nitrogen atom ($\mathbf{-C\equiv N}$).
The carbon atom in the $C\equiv N$ group is part of the main carbon chain and is always assigned position 1 when naming.
- The naming convention uses the alkane name corresponding to the total number of carbon atoms (including the $C$ in $C\equiv N$) plus the suffix nitrile.
Example:
If the molecule is $\text{CH}_3\text{C}\equiv \text{N}$, it contains two carbon atoms (one methyl C and one nitrile C). It is named Ethanenitrile.
1.2 Preparation of Nitriles (Chain Lengthening)
Nitriles are typically prepared from halogenoalkanes in a crucial reaction that adds an extra carbon atom to the molecule (a process called chain lengthening).
Starting Material:
Halogenoalkane ($R-X$)
Reagent:
Potassium cyanide (KCN) (or sodium cyanide, NaCN)
Conditions:
Ethanol (solvent) and Heat (under reflux)
Reaction Type:
Nucleophilic Substitution
In this reaction, the cyanide ion ($\text{CN}^-$) acts as a powerful nucleophile. It attacks the $\delta+$ carbon atom in the halogenoalkane (which is bonded to the electronegative halogen), kicking out the halide ion.
Chemical Equation Example (1-chloropropane to Butanenitrile):
$$ \text{CH}_3\text{CH}_2\text{CH}_2\text{Cl} + \text{KCN} \xrightarrow{\text{Ethanol, Heat}} \text{CH}_3\text{CH}_2\text{CH}_2\text{CN} + \text{KCl} $$
Key Takeaway for Nitrile Preparation: This is your go-to method for adding one carbon atom to an organic chain. Remember the CN⁻ nucleophile and the ethanolic reflux conditions.
2. Hydroxynitriles: Synthesis and Mechanism
Hydroxynitriles are molecules containing both the hydroxyl group ($\text{-OH}$) and the cyano group ($\text{-CN}$). They are formed by reacting carbonyl compounds (aldehydes or ketones) with hydrogen cyanide.
2.1 Preparation of Hydroxynitriles
Starting Material:
Aldehyde or Ketone (Carbonyl Compounds)
Reagent:
Hydrogen cyanide (HCN)
Conditions:
KCN as a catalyst, room temperature
Reaction Type:
Nucleophilic Addition
Safety Note: Since HCN is a highly toxic gas, it is usually generated *in situ* (in the reaction mixture) using a solution of $\text{NaCN}$ or $\text{KCN}$ and a dilute acid. However, the syllabus often simplifies this and requires $\text{HCN}$ (with $\text{KCN}$ catalyst) in the equations. The actual reacting species is the cyanide ion, $\text{CN}^-$.
Example: Reaction of Ethanal to produce 2-hydroxypropanenitrile:
$$ \text{CH}_3\text{CHO} + \text{HCN} \xrightarrow{\text{KCN catalyst}} \text{CH}_3\text{CH(OH)CN} $$
2.2 The Mechanism: Nucleophilic Addition
The formation of a hydroxynitrile is a classic example of a nucleophilic addition reaction, characteristic of carbonyl compounds.
Step 1: Nucleophilic Attack
The $C=O$ double bond is highly polarised because oxygen is very electronegative ($\text{C}$ is $\delta+$, $\text{O}$ is $\delta-$). The highly reactive cyanide ion ($\text{CN}^-$) acts as the nucleophile, attacking the electron-deficient ($\delta+$) carbon atom.
This attack causes the $\pi$ bond of the $C=O$ group to break, and the electron pair moves to the oxygen atom, forming a negatively charged intermediate called the alkoxide ion.
(Mechanism using curly arrows is required for examination purposes.)
Step 2: Protonation
The unstable alkoxide ion ($\text{R}_2\text{C}(\text{O}^-)\text{CN}$) quickly reacts with a proton ($\text{H}^+$), usually provided by a molecule of $\text{HCN}$ or water, to form the final neutral hydroxynitrile product.
Quick Review: Stereochemistry
When an aldehyde (except methanal) or an unsymmetrical ketone reacts, the planar nature of the carbonyl group means the nucleophile ($\text{CN}^-$) can attack from either side with equal probability. This creates a new chiral centre (a carbon atom bonded to four different groups), resulting in a racemic mixture (a 50:50 mixture of two optical isomers).
Aldehydes/Ketones usually react via Addition mechanisms because they have a double bond ($\pi$ bond) that can easily break to accommodate new atoms. Remember: Cyanide addition to $\text{C=O}$ is always Nucleophilic Addition.
Key Takeaway for Hydroxynitrile Preparation: Carbonyl compound + HCN/KCN $\rightarrow$ Hydroxynitrile via Nucleophilic Addition. This also introduces a chiral centre!
3. Reaction of Nitriles: Hydrolysis
Nitriles are useful not just for chain lengthening, but because the $\text{-CN}$ group can be converted into other important functional groups, most commonly the Carboxylic Acid group. This process is called hydrolysis.
3.1 Hydrolysis of Nitriles to Carboxylic Acids
Hydrolysis means 'breaking with water'. The nitrile group ($\text{-C}\equiv \text{N}$) reacts with water when heated, usually under highly acidic or highly alkaline conditions.
This reaction is very important because it provides a way to synthesise carboxylic acids that are one carbon longer than the starting halogenoalkane.
Case A: Acidic Hydrolysis
- Reagents: Dilute acid (e.g., $\text{H}_2\text{SO}_4$ or $\text{HCl}$)
- Conditions: Heat (Reflux)
- Products: Carboxylic Acid and Ammonium Ion ($\text{NH}_4^+$)
$$ R-C\equiv N + 2H_2O + H^+ \xrightarrow{\text{Heat/Reflux}} R-COOH + NH_4^+ $$
Example: Butanenitrile hydrolysed to Butanoic acid.
Case B: Alkaline Hydrolysis
- Reagents: Dilute alkali (e.g., $\text{NaOH}$ or $\text{KOH}$)
- Conditions: Heat (Reflux)
- Intermediate Product: Carboxylate Salt ($R-\text{COO}^-$) and Ammonia ($\text{NH}_3$)
- Final Step: Acidification (adding dilute acid) is required to convert the carboxylate salt back into the neutral Carboxylic Acid ($R-\text{COOH}$).
$$ R-C\equiv N + H_2O + OH^- \xrightarrow{\text{Heat/Reflux}} R-COO^- + NH_3 $$
(Followed by acidification: $R-\text{COO}^- + \text{H}^+ \longrightarrow R-\text{COOH}$)
When asked for alkaline hydrolysis, you must state two steps:
1. Reflux with dilute alkali (produces the salt).
2. Followed by acidification (produces the acid).
Failure to include the second acidification step means you have produced a salt, not the carboxylic acid!
Key Takeaway for Nitrile Hydrolysis: Nitriles are synthetic intermediates. They can be converted to carboxylic acids using either hot aqueous acid or hot aqueous alkali followed by acidification.
4. Summary of Synthesis Routes Using Nitriles
Nitriles are fantastic synthetic intermediates because they link halogenoalkanes (AS material) and carbonyl compounds to carboxylic acids (A2 material), while achieving carbon chain extension.
Route 1: Halogenoalkane to Carboxylic Acid
This is a two-step synthesis that increases the chain length by one carbon atom.
Step A: Halogenoalkane $\rightarrow$ Nitrile (Nucleophilic Substitution using KCN/Ethanol/Heat)
Step B: Nitrile $\rightarrow$ Carboxylic Acid (Hydrolysis using hot aqueous Acid or Alkali/Acidification)
Route 2: Carbonyl Compound to Hydroxy Carboxylic Acid
This is an important route if you need a molecule containing two different functional groups (a hydroxy acid).
Step A: Aldehyde/Ketone $\rightarrow$ Hydroxynitrile (Nucleophilic Addition using HCN/KCN catalyst)
Step B: Hydroxynitrile $\rightarrow$ Hydroxy Carboxylic Acid (Hydrolysis of the $\text{-CN}$ group using hot aqueous Acid or Alkali/Acidification)
Did you know? The reaction using cyanide to convert an aldehyde or ketone into a hydroxynitrile, which then hydrolyses to an $\alpha$-hydroxy acid, is often used in industry to synthesize amino acids, which are the building blocks of proteins!
Nitriles and Hydroxynitriles: Quick Review
- Structure: Nitriles contain $\mathbf{-C\equiv N}$. Hydroxynitriles contain $\mathbf{-C(OH)CN}$.
- Nitrile Production: Halogenoalkane + KCN (Ethanol, Heat). This is Nucleophilic Substitution and lengthens the carbon chain.
- Hydroxynitrile Production: Carbonyl + HCN (KCN catalyst). This is Nucleophilic Addition.
- Nitrile/Hydroxynitrile Reaction: Hydrolysis (Acid or Alkali/Acidify) converts the $\mathbf{-CN}$ group into a $\mathbf{-COOH}$ (Carboxylic Acid) group.