Organic Chemistry : Using Sodium Borohydride

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Organic Reducing Agents

As a reducing agent,  donates a(n) __________ to a ketone or aldehyde.

Possible Answers:

hydride ion

hydrogen molecule

proton

hydrogen atom

electron

Correct answer:

hydride ion

Explanation:

Sodium borohydride donates a hydride ion to a ketone or aldehyde. In order to form a ketone or aldehyde, a nucleophile must attack the carbonyl group. This is because the ketone or aldehyde has an electrophilic carbon—a nucleophile must attack it in order for any reaction to occur. A hydride ion is the only answer choice that plays the role of a nucleophile.

Example Question #2 : Organic Reducing Agents

Which of the following reaction conditions will selectively reduce the ketone in the following compound, retaining the alkene functionality?

                                          Q8

Possible Answers:

Pd and H2 in hexanes

Pd, BaSO4, and H2 in hexanes

LiAlH4 in THF

CeCl3 and NaBH4 in MeOH

NaBH4 in MeOH

Correct answer:

CeCl3 and NaBH4 in MeOH

Explanation:

The correct choice, CeCl3 and NaBH4 in MeOH, shows reagents know as "Luche conditions," which are able to modify the reactivity of sodium borohydride to reduce the carbonyl to an alcohol without affecting alkene groups. This occurs as the cerium ion coordinates strongly to the carbonyl oxygen, which subsequently greatly enhances the electrophilicity at the carbonyl carbon. Nucleophilic attack of the hydride readily occurs, simultaneously destroying the electropilicty of the beta carbon of the alkene, such that it will not be reduced by the hydride reagent.

 

The incorrect answer choices would give various undesired products as detailed below:

NaBH4 in MeOH

Use of unmodified sodium borohydride would result in a 1,4 conjugate addition reaction, saturating the alkene, with a subsequent reduction of the ketone to an alcohol.

LiAlH4 in THF

Use of lithium aluminum hydride would give the same product as use of unmodified sodium borohydride, following the same reduction mechanism.

Pd and H2 in hexanes

This reagent will give reduction of the alkene only.

Pd, BaSO4, and H2 in hexanes

This reagent combination, known as Lindlar's catalyst, will also reduce the alkene only. This reagent is typically used to selectively reduce an alkyne to an alkene.

 

 

Example Question #3 : Organic Reducing Agents

Which of these can be reduced by sodium borohydride?

Possible Answers:

2-butanol

3-pentanone

Propanoic acid

None of these

2-butene

Correct answer:

3-pentanone

Explanation:

Sodium borohydride is a reducing agent with formula . It is a reducing agent, but it is not extremely strong. It reduces ketones to alcohols, but it does not affect carboxylic acids. 3-pentanone is the only ketone of the given choices. It would be reduced to 3-pentanol.

Example Question #1 : Using Sodium Borohydride

Which of the following statements is false?

Possible Answers:

 can be used to reduce an acid halide into a primary alcohol

 can be used to reduce an aldehyde into a primary alcohol

 can be used to reduce a ketone into a secondary alcohol

None of these

Correct answer:

None of these

Explanation:

These are all true uses of .

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