Extending Carbon Chains (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define nitrile
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Define nitrile
A nitrile is an organic compound containing the –CN functional group attached to a carbon chain. Nitriles are formed when a cyanide ion replaces a halogen in a halogenoalkane via nucleophilic substitution.
Why is the reaction of a halogenoalkane with ethanolic KCN useful in organic synthesis?
It extends the carbon chain by one carbon atom, allowing chemists to make a compound with more carbons than the original starting material. This makes it a valuable carbon–carbon bond forming reaction.
When bromoethane is heated under reflux with ethanolic KCN, the CN- ion acts as a .......... and the product formed is .......... .
When bromoethane is heated under reflux with ethanolic KCN, the CN- ion acts as a nucleophile and the product formed is propanenitrile.
What are the two steps in the nucleophilic addition of HCN to a carbonyl compound?
In step 1, the cyanide ion attacks the carbonyl carbon forming a negatively charged intermediate. In step 2, the oxygen in the intermediate reacts with aqueous H+ to form a 2-hydroxynitrile.
True or False?
Hydrolysis of a nitrile with dilute acid produces a primary amine.
False.
Hydrolysis of a nitrile with dilute acid produces a carboxylic acid and an ammonium salt, not a primary amine.
Why must alkali hydrolysis of a nitrile be followed by acidification?
Alkali hydrolysis produces a carboxylate salt, not a carboxylic acid. Acidification converts the carboxylate ion into the carboxylic acid product.
True or False?
Reducing a nitrile with LiAlH4 in dry ether produces a primary amine.
True.
LiAlH4 in dry ether reduces the –CN group to –NH2, converting the nitrile into a primary amine.
What reagents can be used to reduce a nitrile to a primary amine?
Either LiAlH4 in dry ether or H2 gas passed over a nickel catalyst can reduce the –CN group to –NH2, forming a primary amine.
What is a Friedel-Crafts reaction?
A Friedel-Crafts reaction is an electrophilic substitution reaction in which a hydrogen atom on a benzene ring is replaced by an alkyl or acyl group, using AlCl3 as a catalyst to generate the electrophile.
What are the three steps common to all Friedel-Crafts reactions?
The three steps common to all Friedel-Crafts reactions are:
Generating the electrophile
Electrophilic attack on the benzene ring
Regenerating aromaticity of the ring
In Friedel-Crafts alkylation, benzene reacts with a .......... in the presence of .......... to form an alkylbenzene.
In Friedel-Crafts alkylation, benzene reacts with a chloroalkane in the presence of AlCl3 to form an alkylbenzene.
Why does benzene require a catalyst such as AlCl3 for Friedel-Crafts reactions?
Benzene is aromatic and therefore very stable and unreactive toward electrophiles. The AlCl3 catalyst generates a sufficiently powerful electrophile to overcome this stability.
True or False?
Friedel-Crafts acylation of benzene produces a ketone.
True.
An acyl group (an alkyl group containing a C=O) is substituted into the benzene ring, and the product is a ketone.
How does Friedel-Crafts acylation differ from alkylation in terms of the organic reactant used?
Acylation uses an acyl chloride as the organic reactant, whereas alkylation uses a chloroalkane. Both reactions require an AlCl3 catalyst.
True or False?
Friedel-Crafts reactions are a type of nucleophilic substitution.
False.
Friedel-Crafts reactions proceed by electrophilic substitution, not nucleophilic substitution, because the benzene ring acts as the electron donor.
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