Question

What happens when (i) n-butyl chloride is treated with alcoholic KOH. (ii) bromobenzene is treated with Mg in the presence of dry ether, (iii) chlorobenzene is subjected to hydrolysis, (iv) ethyl chloride is treated with aqueous \(\mathrm{KOH}\), (v) methyl bromide is treated with sodium in the presence of dry ether, (vi) methyl chloride is treated with KCN?

Short answer

(i) But-1-ene forms. (ii) Phenylmagnesium bromide forms. (iii) No reaction under normal conditions. (iv) Ethanol forms. (v) Ethane forms. (vi) Methyl cyanide forms.

Step-by-step solution

Alcoholic KOH with n-butyl chloride

When n-butyl chloride is treated with alcoholic KOH, an elimination reaction takes place. Alcoholic KOH serves as a strong base and facilitates the dehydrohalogenation process. This leads to the formation of but-1-ene as the major product. The reaction proceeds as: \[\text{C}_4\text{H}_9\text{Cl} + \text{KOH (alcohol)} \rightarrow \text{C}_4\text{H}_8 + \text{KCl} + \text{H}_2\text{O}\].

Formation of Grignard Reagent with Bromobenzene

When bromobenzene is treated with magnesium in the presence of dry ether, a Grignard reagent is formed. The reaction is a metal-halogen exchange process, which results in the formation of phenylmagnesium bromide: \[\text{C}_6\text{H}_5\text{Br} + \text{Mg} \xrightarrow{\text{Dry Ether}} \text{C}_6\text{H}_5\text{MgBr}\].

Hydrolysis of Chlorobenzene

Subjecting chlorobenzene to hydrolysis does not occur under normal conditions. Chlorobenzene is quite stable due to resonance and does not easily hydrolyze to form phenol. Special conditions are required for this transformation, such as high temperature and pressure with a strong base (e.g., molten NaOH).

Aqueous KOH with Ethyl Chloride

When ethyl chloride is treated with aqueous \( \text{KOH} \), a nucleophilic substitution reaction occurs. Water provides a medium for \( \text{KOH} \) to release \( \text{OH}^- \), facilitating the substitution of the chloride ion with a hydroxyl group, forming ethanol: \[\text{C}_2\text{H}_5\text{Cl} + \text{KOH (aq.)} \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{KCl}\].

Wurtz Reaction with Methyl Bromide

Methyl bromide reacts with sodium in the presence of dry ether, undergoing the Wurtz reaction. This coupling reaction leads to the formation of ethane: \[2 \text{CH}_3\text{Br} + 2 \text{Na} \rightarrow \text{C}_2\text{H}_6 + 2 \text{NaBr}\].

Reaction of Methyl Chloride with KCN

Methyl chloride reacts with \( \text{KCN} \) in a nucleophilic substitution reaction to form methyl cyanide (acetonitrile). The \( \text{CN}^- \) ion from \( \text{KCN} \) replaces the chloride ion in methyl chloride: \[\text{CH}_3\text{Cl} + \text{KCN} \rightarrow \text{CH}_3\text{CN} + \text{KCl}\].

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental type of reaction in organic chemistry. This process involves the exchange of an atom or group of atoms in a molecule with a nucleophile. A nucleophile is a chemical species that donates an electron pair to form a chemical bond with an electrophile, which is a species that accepts an electron pair.
In a typical nucleophilic substitution reaction, a molecule would have a leaving group, which is replaced by the incoming nucleophile. For example, when ethyl chloride reacts with aqueous KOH, the hydroxide ion (\( ext{OH}^-\)) acts as the nucleophile. It substitutes the chloride ion in ethyl chloride to form ethanol.
Some key points to remember about nucleophilic substitution include:

• The strength and concentration of the nucleophile can affect the rate of the reaction.
• The type of halide involved (like bromide, chloride, etc.) influences how easily it will leave the group.
• Solvent choice can play a crucial role in the reaction's outcome.

Elimination Reaction

Elimination reactions are another crucial aspect of organic chemistry. In an elimination reaction, two atoms or groups are removed from a molecule, resulting in the formation of a double or triple bond. This reaction is often catalyzed by a strong base.
When n-butyl chloride is treated with alcoholic KOH, an elimination reaction occurs. Alcoholic KOH acts as a strong base, facilitating a dehydrohalogenation process. This results in the removal of a hydrogen and a halide (such as chlorine), forming a double bond and producing but-1-ene.
Important points about elimination reactions include:

• They often require heat or a strong base to proceed.
• The stereochemistry of the reaction can affect which isomer is favored as the product.
• Competing with substitution reactions, the conditions must be controlled to favor elimination.

Grignard Reagent

The Grignard reagent is a pivotal tool in synthetic organic chemistry. This reagent is typically formed by reacting an organohalide with magnesium metal in the presence of dry ether. The resulting compound, usually in the form of RMgX, where R is an organic group and X is a halogen, is highly reactive and can be used to form carbon-carbon bonds.
For example, when bromobenzene is treated with Mg in dry ether, it forms phenylmagnesium bromide, a type of Grignard reagent. This process involves a metal-halogen exchange where magnesium inserts itself into the bond between carbon and bromine.
Key considerations when working with Grignard reagents:

• They are sensitive to moisture and require an anhydrous (water-free) environment.
• The resulting Grignard reagent can be used to add to carbonyl compounds, forming alcohols.
• These reactions often require careful temperature control to prevent unwanted side reactions.

Hydrolysis

Hydrolysis is the chemical breakdown of a compound due to reaction with water. It is a common method for converting halides into alcohols or acids. In some cases, this reaction is simple and occurs under mild conditions, but other compounds, like chlorobenzene, require more extreme conditions.
Chlorobenzene, for instance, does not easily undergo hydrolysis under standard conditions due to its resonance-stabilized structure, which makes the carbon-chlorine bond particularly strong. Special conditions, such as high temperatures and pressures along with a strong base like molten NaOH, are required to effectively perform hydrolysis on chlorobenzene.
Points to consider about hydrolysis:

• It's commonly used for functional group transformation, especially in laboratory settings.
• The type of bond involved in the molecule greatly affects the ease of hydrolysis.
• Hydrolysis can sometimes lead to the breakdown of other sensitive functional groups.

Wurtz Reaction

The Wurtz reaction is a coupling reaction where two alkyl halides react with sodium metal to form a new carbon-carbon bond, resulting in the formation of a larger alkane. This reaction is particularly useful for synthesizing higher alkanes from smaller ones.
For example, methyl bromide reacts with sodium in dry ether to produce ethane in the Wurtz reaction. This involves radical intermediates and the coupling of two alkyl radicals to form the new carbon-carbon bond.
Considerations for the Wurtz reaction include:

• The reaction often involves side reactions that may lead to mixtures of products.
• Dry ether is necessary to avoid unwanted side reactions and to stabilize intermediates.
• The reaction is most effective with primary halides, as secondary and tertiary halides can yield unwanted products.