Question

How could you convert butanoic acid into the following compounds? Write each step showing the reagents needed. (a) 1 -Butanol (b) 1-Bromobutane (c) Pentanoic acid (d) 1-Butene (e) Octane

Short answer

Use LiAlH₄ to reduce to 1-butanol, HBr or PBr₃ for 1-bromobutane, Grignard reaction for pentanoic acid, acid dehydration for 1-butene, and a coupling reaction for octane.

Step-by-step solution

Reduction to 1-Butanol

To convert butanoic acid (CH₃(CH₂)₂COOH) to 1-butanol (CH₃(CH₂)₃OH), use lithium aluminum hydride (LiAlH₄) as the reducing agent. This reagent will reduce the carboxylic acid group to a primary alcohol. The reaction should be performed in dry ether, and care should be taken to control the reaction conditions.

Conversion to 1-Bromobutane

To convert 1-butanol (CH₃(CH₂)₃OH) into 1-bromobutane (CH₃(CH₂)₃Br), carry out a substitution reaction using hydrobromic acid (HBr) or phosphorus tribromide (PBr₃) as the reagent. Either of these will replace the hydroxyl group (-OH) with a bromide ion (-Br).

Alkylation to Pentanoic Acid

To elongate the carbon chain and convert butanoic acid into pentanoic acid, perform a Grignard reaction. First, convert 1-bromobutane into butyl magnesium bromide by reacting it with magnesium in dry ether to form the Grignard reagent. Then react this Grignard reagent with carbon dioxide (CO₂) and subsequently protonate the carboxylate with dilute acid (such as aqueous HCl) to obtain pentanoic acid (CH₃(CH₂)₃COOH).

Dehydration to 1-Butene

To convert 1-butanol into 1-butene, perform a dehydration reaction using concentrated sulfuric acid (H₂SO₄) as the dehydrating agent under heat. This will remove the water molecule, converting the alcohol into an alkene.

Coupling to Octane

To synthesize octane from butanoic acid, first convert butanoic acid into a butyl Grignard reagent as done in Step 3. Then use a coupling reaction with another butyl bromide in the presence of a transition metal catalyst like nickel (Ni) or palladium (Pd). This method, known as the Kumada coupling or similar, allows for the synthesis of octane (CH₃(CH₂)₆CH₃).

Key concepts

Reduction Reaction

In the process of converting butanoic acid to 1-butanol, a reduction reaction takes place. Reduction reactions involve the gain of electrons or the loss of oxygen. Here, lithium aluminum hydride (LiAlH₄) acts as a powerful reducing agent. It selectively reduces carboxylic acids to primary alcohols.

• Lithium aluminum hydride provides hydride ions (H⁻), which attack the electrophilic carbon atom of the carboxylic group.
• This step replaces the double-bonded oxygen in the carboxylic group with a hydroxyl group (-OH), forming an alcohol.
• The reaction is conducted in dry ether to prevent any interaction with water that might deactivate the reducing agent.

It is crucial to manipulate the reaction conditions, such as temperature and solvent, carefully to drive the reduction effectively without unwanted reactions.

Substitution Reaction

To transform 1-butanol into 1-bromobutane, a substitution reaction is employed. In substitution reactions, one functional group in a molecule is replaced by another group.

• In this case, the hydroxyl group (-OH) in 1-butanol is replaced by a bromide ion (-Br).
• Hydrobromic acid (HBr) or phosphorus tribromide (PBr₃) are the typical reagents used for this conversion.
• These reagents work by first protonating the alcohol to make a better leaving group, allowing for the substitution to occur swiftly.

The substitution reaction enhances the compound's reactivity, making 1-bromobutane a useful intermediate for further chemical reactions.

Grignard Reaction

When extending butanoic acid's carbon chain to form pentanoic acid, a Grignard reaction is needed. Grignard reactions are used to form carbon-carbon bonds, which is essential in forming longer carbon chains.

• To begin, 1-bromobutane is transformed into a Grignard reagent by reacting it with magnesium in dry ether.
• The resulting organomagnesium compound reacts with carbon dioxide (CO₂), which introduces an additional carbon atom to the chain.
• Finally, the reaction mixture is treated with a weak acid to protonate the carboxylate ion, generating pentanoic acid.

This reaction showcases the utility of Grignard reagents in synthetic organic chemistry, particularly in constructing more complex molecules.

Dehydration Reaction

For converting 1-butanol into 1-butene, a dehydration reaction is utilized. Dehydration reactions involve the removal of water (H₂O) from a molecule, transforming alcohols into alkenes.

• In this case, concentrated sulfuric acid (H₂SO₄) serves as the dehydrating agent.
• The acid catalyzes the reaction by protonating the alcohol's hydroxyl group, allowing it to leave as a water molecule.
• Under heat, this elimination reaction results in the formation of 1-butene, an alkene with a double bond.

This type of reaction is fundamental in organic chemistry as a method of converting saturated hydrocarbons into more reactive, unsaturated ones with double bonds.

Coupling Reaction

To achieve the synthesis of octane from butanoic acid, a coupling reaction is performed. Couplings are key in forming carbon-carbon bonds between organic molecules.

• Initially, butanoic acid is converted into a butyl Grignard reagent, similar to the process in forming pentanoic acid.
• This Grignard reagent is then reacted with butyl bromide in the presence of a transition metal catalyst, such as nickel (Ni) or palladium (Pd).
• This coupling of the two butyl groups effectively forms octane, an eight-carbon alkane.

The Kumada coupling reaction exemplifies how complex hydrocarbons can be synthesized through strategic assembly of simpler molecular parts.