QCM : Nucleophilic Substitution Mechanisms — 9 questions

Questions et réponses du QCM

1. What is nucleophilic substitution?

A reaction where an electrophile adds to an alkene to form an addition product
A process where an alkene reacts with a halogen to form a dihalogenoalkane
A reaction where a nucleophile replaces a leaving group in a molecule
A reaction where a molecule loses a proton to form a new compound

A reaction where a nucleophile replaces a leaving group in a molecule

Explication

Nucleophilic substitution is a reaction mechanism where a nucleophile donates a pair of electrons to replace a leaving group in a molecule, which is explicitly defined in the source content.

2. Which type of halogenoalkanes predominantly undergo the SN2 mechanism?

Secondary halogenoalkanes
Primary halogenoalkanes
All halogenoalkanes equally
Tertiary halogenoalkanes

Primary halogenoalkanes

Explication

SN2 reactions predominantly occur in primary halogenoalkanes because they have less steric hindrance, allowing the nucleophile to attack from the opposite side of the leaving group. Tertiary halogenoalkanes are hindered sterically and favor SN1 instead. Secondary halogenoalkanes can undergo both SN1 and SN2, but SN2 is most characteristic in primary halogenoalkanes.

3. What is the primary function of the SN1 mechanism in organic substitution reactions?

To enable elimination reactions to produce alkenes from halogenoalkanes
To facilitate substitution by forming a carbocation intermediate in tertiary halogenoalkanes
To promote a concerted attack by nucleophiles on primary halogenoalkanes
To prevent carbocation formation and favor direct nucleophilic attack

To facilitate substitution by forming a carbocation intermediate in tertiary halogenoalkanes

Explication

The SN1 mechanism's primary function is to facilitate substitution reactions, especially in tertiary halogenoalkanes, by forming a carbocation intermediate that allows the nucleophile to attack. This mechanism is characterized by carbocation stability and a two-step process, making it particularly important for tertiary substrates.

4. When was the SN2 mechanism for halogenoalkane reactions established relative to the SN1 mechanism?

SN2 was established in the 21st century
SN2 was established before SN1
SN2 was established after SN1
SN2 and SN1 were established simultaneously

SN2 was established before SN1

Explication

The SN2 mechanism was characterized and understood before the SN1 mechanism, primarily because its concerted, one-step process was easier to observe and explain. The development of the SN2 concept predates the more complex two-step SN1 mechanism.

5. How does the substitution of NH2 in halogenoalkanes differ from substitution with hydroxide ions (OH-)?

Substitution with NH2 occurs via an SN1 mechanism, whereas with OH- it always occurs via SN2.
Substitution with NH2 produces primary amines, while with OH- it produces alcohols.
Substitution with NH2 requires ethanolic NH3 in a sealed tube, while substitution with OH- occurs in aqueous solution.
Substitution with NH2 is a nucleophilic addition, while with OH- it is a nucleophilic substitution.

Substitution with NH2 requires ethanolic NH3 in a sealed tube, while substitution with OH- occurs in aqueous solution.

Explication

Substitution with NH2 typically requires ethanolic NH3 in a sealed tube to prevent competing reactions and to favor nucleophilic substitution leading to amines. In contrast, substitution with hydroxide ions (OH-) generally occurs in aqueous solution and leads to alcohol formation. The conditions and mechanisms differ significantly, making option 0 the correct comparison.

6. Who is credited with proposing or discovering cyanide substitution in organic chemistry?

Louis Pasteur
Dmitri Mendeleev
Robert Robinson
Marie Curie

Robert Robinson

Explication

Robert Robinson is recognized for his pioneering work involving cyanide compounds and their reactions, including cyanide substitution in organic synthesis. The other options are notable scientists but are not associated with cyanide substitution reactions.

7. What is the effect of using reflux conditions during the oxidation of primary alcohols with acidified potassium dichromate?

It prevents oxidation, leaving the alcohol unchanged.
It converts primary alcohols into ketones.
It results in the formation of carboxylic acids from primary alcohols.
It produces aldehydes that are distilled off immediately.

It results in the formation of carboxylic acids from primary alcohols.

Explication

Reflux conditions during oxidation allow the oxidation of primary alcohols to proceed fully to form carboxylic acids, as the reaction mixture is heated continuously and the products are not distilled off. Distillation would be used to isolate aldehydes at an earlier stage, but reflux promotes complete oxidation to acids.

8. What is the correct practical procedure to dehydrate an alcohol to produce an alkene in the laboratory?

Heat the alcohol with a catalyst such as Al2O3 or concentrated sulfuric acid, using reflux conditions.
Add excess oxygen to the alcohol and heat gently.
Mix the alcohol with dilute hydrochloric acid and stir at low temperature.
Heat the alcohol with water and a base like NaOH at room temperature.

Heat the alcohol with a catalyst such as Al2O3 or concentrated sulfuric acid, using reflux conditions.

Explication

The proper method to dehydrate an alcohol involves heating it with a catalyst such as Al2O3 or concentrated sulfuric acid, which acts as a dehydrating agent, under reflux conditions. This removes water from the alcohol, forming an alkene. The other options are incorrect because they do not involve dehydration conditions or catalysts, or they involve conditions that do not produce alkenes.

9. What is the characteristic functional group of esters formed during esterification?

(R)-COOH
(R)-OH
(R)-NH2
(R)-C(=O)-O-(R')

(R)-C(=O)-O-(R')

Explication

Esters are characterized by the functional group (R)-C(=O)-O-(R'), which consists of a carbonyl group (C=O) attached to an oxygen atom that is also bonded to an alkyl group. This group is the defining feature of esters formed during esterification.

Révisez avec les flashcards

Mémorisez les réponses avec 18 flashcards sur Nucleophilic Substitution Mechanisms.

Nucleophile — definition?

Electron pair donor in substitution.

Nucleophilic substitution — process?

Nucleophile replaces a leaving group.

SN2 mechanism — key feature?

One-step backside attack with inversion.

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