This set of Basic Organic Chemistry Questions and Answers focuses on “Nucleophilic Addition Reactions”.
1. What type of reaction takes place upon treatment of a ketone with HCN to form a cyanohydrin?
a) Nucleophilic addition
b) Nucleophilic substitution
c) Electrophilic addition
d) Electrophilic substitution
Explanation: The atoms of HCN add to the carbon-oxygen double bond of the ketone by nucleophilic attack of the cyanide anion on the electrophilic carbon of the carbonyl.
Explanation: Di-isobutyl aluminium hydride (DIBAH) is a selective reducing agent. It does not reduce esters to 1° alcohols (lithium aluminium hydride can be used to reduce esters to 1° alcohols).
Explanation: Ozonolysis of an alkene results in cleavage of the carbon-carbon double bond to form two carbonyl bonds. Ozonolysis of a cyclic alkene in which the double bond has a hydrogen atom on each of the carbon atoms of the carbon-carbon double bond will lead to the formation of a dialdehyde.
Explanation: Treatment of an alkyne with aqueous acid in the presence of Hg2+ results in a hydration reaction to form an enol, which tautomerizes to form a ketone. The regiochemistry of the addition is that predicted by Markovnikov’s rule, with the oxygen adding to the more substituted end of the carbon-carbon triple bond.
Explanation: Silver(I) oxide in aqueous ammonia (“Tollens reagent”) is a mild oxidizing agent which oxidizes aldehydes to carboxylic acids without reacting with carbon-carbon double bonds or many other functional groups.
Explanation: It is also important that products are identified accurately using IUPAC nomenclature: 4,7-Dimethyl-4-heptanol is not an IUPAC name.
Explanation: Lithium aluminium hydride (LiAlH4) is a reducing agent which reacts with ketones to give 2° alcohols. It does not react with carbon-carbon double bonds.
Explanation: 1° Amines react with ketones by nucleophilic addition to form a carbinolamine intermediate which undergoes dehydration to give an imine, not an 2° amine.
Explanation: Hydroxylamine reacts with ketones by nucleophilic addition followed by elimination to give an oxime.
Explanation: Reaction of a ketone with a phosphonium ylide results in a nucleophilic addition reaction to form a betaine which undergoes ring closure followed by elimination of a phosphine oxide to make a new carbon-carbon double bond of an alkene. This is a Wittig reaction. It is important to recognize that the new carbon-carbon double bond in the product is formed between the carbon atom of the carbonyl and the phosphorus-substituted carbon atom of the ylide. It is also useful to draw out the mechanism for this reaction to determine the structure of the product.
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