Organic Chemistry : Help with Substitution Reactions

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #32 : Organic Concepts

Which of the following reaction conditions favors an SN1 mechanism?

Possible Answers:

Weak electrophile

Protic solvent

Strong base

Weak base

Strong nucleophile

Correct answer:

Protic solvent

Explanation:

SN1 reactions occur in two steps. First, the leaving group leaves, forming a carbocation. Next, the weak nucleophile attacks the carbocation (beware of rearrangements during this step). The protic solvent stabilizes the carbocation intermediate. 

Example Question #33 : Organic Concepts

Which of the following reaction conditions favors an SN2 mechanism?

Possible Answers:

Strong nucleophile

Weak base

Tertiary alkyl halide substrate

Protic solvent

Strong base

Correct answer:

Strong nucleophile

Explanation:

SN2 reactions undergo substitution via a concerted mechanism. Thus, no carbocation is formed, and an aprotic solvent is favored. Furthermore, tertiary substituted substrates have lowest reactivity for SN2 reaction mechanisms due to steric hindrance.

Example Question #1 : Help With Substitution Reactions

Which of the following characteristics does not reflect an SN2 reaction mechanism?

Possible Answers:

Aprotic solvent

Stereochemical inversion of the carbon attacked (backside attack)

Strong nucleophile

Concerted mechanism

Tertiary substrate

Correct answer:

Tertiary substrate

Explanation:

SN2 reaction mechanisms are favored by methyl/primary substrates because of reduced steric hindrance. No carbocation is formed via an SN2 mechanism since the mechanism is concerted; thus a strong nuclephile is used.

Example Question #2 : Help With Substitution Reactions

Which of the following characteristics does not reflect an SN1 reaction mechanism?

Possible Answers:

Use of a strong nucleophile

Use of a protic solvent

Unimolecular reaction rate

Formation of a carbocation intermediate

Formation of a racemic mixture of products

Correct answer:

Use of a strong nucleophile

Explanation:

All of the given answers reflect SN1 reactions, except the claim that SN1 reactions are favored by weak nucleophiles.

SN1 reactions occur in two steps and involve a carbocation intermediate. The product demonstrates inverted stereochemistry (no racemic mixture). Tertiary substrates are preferred in this mechanism because they provide stabilization of the carbocation.

Example Question #36 : Organic Concepts

In a substitution reaction __________.

Possible Answers:

two sigma bonds are broken and one pi bond is formed

one pi bond is broken and two sigma bonds are formed

one pi bond is broken and one pi bond is formed

one sigma and one pi bond are broken, and two sigma bonds are formed

one sigma bond is broken and one sigma bond is formed

Correct answer:

one sigma bond is broken and one sigma bond is formed

Explanation:

Substitution reactions—regardless of the mechanism—involve breaking one sigma bond, and forming another sigma bond (to another group).

Example Question #37 : Organic Concepts

Screen shot 2015 07 07 at 6.44.18 pm

Predict the major product of the given reaction.

 

Screen shot 2015 07 02 at 8.06.19 pm

Possible Answers:

I

IV

II

None of these

III

Correct answer:

IV

Explanation:

 is an extremely useful reagent for organic synthesis in instances where an alcohol needs to be converted to a good leaving group (bromine is an excellent leaving group).  reacts selectively with alcohols, without altering any other common functional groups. This makes it ideal for situations in which a molecule contains acid-sensitive components that prevent the use of a strong acid to protonate a target alcohol.

While the mechanisms differ,  reactions are similar to SN2 reactions in that they both invert the configuration at the site of attack. The configuration about the carbon adjacent to the alcohol in the given reactant is S. After substitution, the configuration of the major product is R, as is the case in molecule IV.

Example Question #38 : Organic Concepts

Screen shot 2015 07 07 at 7.53.45 pm

Predict the most likely mechanism for the given single-step reaction and assess the absolute configuration of the major product at the reaction site.

Possible Answers:

SN1

S configuration

SN1

R configuration

SN2

S configuration

SN1

Racemic mixture

SN2

R configuration

Correct answer:

SN2

R configuration

Explanation:

Based on the given reagents and the specification that the reaction takes place in a single step, it may be concluded that the reaction occurs by an SN2 or E2 mechanism. Since the compound lacks any moderately acidic hydrogen, an SN2 reaction is more likely. The absolute configuration at the reaction site in the initial compound is S, which is converted to R as a result of the "back-side attack" characteristic of all SN2 reactions. The major product is shown below:

Screen shot 2015 07 07 at 8.02.18 pm

Example Question #41 : Organic Concepts

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Which reagent(s) are required to carry out the given reaction?

Possible Answers:

 

Correct answer:

 

Explanation:

This problem involves the synthesis of a Grignard reagent. Grignard reagents are easily created in the presence of halo-alkanes by adding magnesium in an inert solvent (in this case ). Once we have created our Gringard, it can readily attack a carbonyl. In this case, our Grignard attacks carbon dioxide to create our desired product.

Example Question #42 : Organic Concepts

Which of the following statements is true regarding an  reaction?

Possible Answers:

The configuration at the site of the leaving group becomes inverted

All of these

An  reaction is best carried out in a protic solvent, such as water or ethanol

The nucleophile that is substituted forms a pi bond with the electrophile

Correct answer:

An  reaction is best carried out in a protic solvent, such as water or ethanol

Explanation:

An  reaction is most efficiently carried out in a protic solvent. An inverted configuration site is characteristic of an  reaction and the substituted nucleophile does not form a pi bond in an  reaction.

Example Question #3 : Help With Substitution Reactions

Sn1 problemSn1 product

By which of the following mechanisms does the given reaction take place?

Possible Answers:

Correct answer:

Explanation:

In this question, we're given the reactant and product as well as the reagent being used in the reaction, and we're being asked to identify which reaction mechanism will correctly lead us from reactant to product.

To begin, it's important to notice that the reactant contains a tertiary bromine and the product contains a methoxy group in place of where the bromine was. Thus, we can conclude that a substitution reaction has taken place. If an elimination reaction had taken place, then there would have been a double bond in the product.

Now we need to identify which kind of substitution has occurred. Since the leaving group is attached to a tertiary carbon, we know that a stable carbocation will be generated upon dissociation. Therefore, we would expect this to be an  reaction.

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