Organic Chemistry : Organic Concepts

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #3 : Help With Addition Reactions

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Classify the type of reaction given.

Possible Answers:

Elimination

Rearrangement

Substitution

Addition

Correct answer:

Addition

Explanation:

An addition reaction is a reaction in which the reactants react to combine and form one product. It is the opposite of an elimination reaction. In the reaction given, the reactants hydrochloric acid and propyne combine to form the product 2-chloropropene.

Example Question #4 : Help With Addition Reactions

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Classify the type of reaction given above.

Possible Answers:

Addition

Rearrangement

Elimination

Substitution

Correct answer:

Addition

Explanation:

An addition reaction is a reaction in which the reactants react to combine and form one product. It is the opposite of an elimination reaction. In the reaction given, the reactants hydrobromic acid and propene combine to form the product 2-bromopropane.

Example Question #31 : Organic Concepts

What is the major product of the following reaction?

Image1

Possible Answers:

None of these

Correct answer:

Explanation:

The reaction shown is a Markovnikov addition of a hydracid (HX) across a double bond. According to Markovnikov's rule, the hydrogen gets added to the lesser-substituted carbon in the double bond, and the halide (in this case, ) gets added to the more-substituted carbon. Thus, the correct answer is .

Example Question #1 : Help With Substitution Reactions

Which of the following reaction conditions favors an SN1 mechanism?

Possible Answers:

Strong base

Protic solvent

Weak base

Weak electrophile

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 #2 : Help With Substitution Reactions

Which of the following reaction conditions favors an SN2 mechanism?

Possible Answers:

Strong base

Protic solvent

Weak base

Tertiary alkyl halide substrate

Strong nucleophile

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 #3 : Help With Substitution Reactions

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

Possible Answers:

Aprotic solvent

Strong nucleophile

Concerted mechanism

Stereochemical inversion of the carbon attacked (backside attack)

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 #4 : Help With Substitution Reactions

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

Possible Answers:

Formation of a racemic mixture of products

Use of a protic solvent

Use of a strong nucleophile

Formation of a carbocation intermediate

Unimolecular reaction rate

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 #5 : Help With Substitution Reactions

In a substitution reaction __________.

Possible Answers:

two sigma bonds are broken and one pi bond is formed

one sigma bond is broken and one sigma 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

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 #3 : Help With Substitution Reactions

Screen shot 2015 07 07 at 6.44.18 pm

Predict the major product of the given reaction.

 

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Possible Answers:

II

I

III

IV

None of these

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 #37 : Organic Concepts

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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

R configuration

SN2

R configuration

SN2

S configuration

SN1

Racemic mixture

SN1

S 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:

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