GRE Subject Test: Chemistry : Other Groups

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #41 : Organic Chemistry, Biochemistry, And Metabolism

Which of these methods can be used to synthesize a secondary alcohol?

Possible Answers:

Reacting a ketone with lithium aluminum hydride, then adding water

All of these answer choices are correct

Reacting propene with water and acid

Adding a Grignard reagent to an aldehyde, then reacting with acid

Correct answer:

All of these answer choices are correct

Explanation:

All of these methods can successfully synthesize secondary alcohols.

Lithium aluminum hydride and sodium borohydride are strong and weaker reducing agents, respectively. Both are able to reduce ketones to secondary alcohols.

Adding water and acid to an alkene (such as propene) results in Markovnikov addition of a hydroxyl group, also creating a secondary alcohol.

Grignard reagents (organometallic halides) add to the carbonyl carbon of an aldehyde, adding an alkane group and forming an alcohol product.

Example Question #71 : Molecular Properties

Ephedrine (shown below) contains what type of amine?

Mcat_problem_set_1

Possible Answers:

Quaternary

Primary

Secondary

Neutral

Tertiary

Correct answer:

Secondary

Explanation:

A secondary amine is an amine (nitrogen atom) that is attached to two carbon-containing groups (alkyl groups or aryl groups). The nitrogen in ephedrine is attached to two alkyl groups, making it a secondary amine.

Primary amines are generally written as . Secondary amines are generally written as . A tertiary amine will be bound to three different R-groups. Quaternary amines require a positive charge on the nitrogen atom to accommodate a fourth R-group.

Example Question #51 : Organic Chemistry, Biochemistry, And Metabolism

Which of the following factors do NOT favor an SN2 reaction of an alkyl halide?

Possible Answers:

A primary halide

A polar aprotic solvent

A tertiary carbocation

A good nucleophile 

Correct answer:

A tertiary carbocation

Explanation:

The way the question is phrased, three answer choices must favor an SN2 reaction, while the "correct" answer is a factor that does not favor, or disfavors an SN2 reaction.

SN2 reactions are bimolecular, and thus their rate of reaction depends on both the substrate and the nucleophile, forming a high energy transition state in which the nucleophile will displace the substate's leaving group at an angle of 180o. The more sterically hindered the compound is, the higher in energy the transition state will be, and the slower the rate of reaction will be. Consequently, SN2 reactions are favored when the leaving group (a halogen in this case) is on a primary carbon center. Additionally, because the reaction is bimolecular, step two of the reaction will NOT occur without a good nucleophile to displace the leaving group. Finally, all SN2 reactions are favored by polar aprotic solvents.  

Because SN2 reactions proceed via a transition state, no carbocation intermediate is formed (that happens in SN1 reactions) and therefore the formation of any carbocation favors an SN1 reaction, not an SN2 reaction.

Example Question #2 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

Using the product of reaction 2, a scientist adds bromine gas to the reaction chamber. After the bromine and the alkene react, he finds that his product consists entirely of single bonds, with two bromine atoms on the carbon chain. What kind of reaction most likely took place?

Possible Answers:

Substitution reaction

Oxymercuration/demercuration reaction

Addition reaction

Elimination reaction

Halogenation reaction

Correct answer:

Addition reaction

Explanation:

The addition of bromine gas () to the reaction vessel would likely result in the addition of one half of the diatomic bromine to each carbon, eliminating the double bond and resulting in an alkyl halide chain.

Halogenation reactions refer to reactions between a halogen and an alkane, while addition reactions occur between a halogen and an alkene (such as the product in reaction 2).

Example Question #3 : Substitution And Elimination Mechanisms

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

Investigating reaction 2, you find that the reaction is initiated when a carbocation forms. Which of the following is likely true?

I. Concentration of the halide is the main determinant of reaction rate

II. The carbocation forms when the hydroxide removes the chlorine atom

III. The carbocation is planar

 

Possible Answers:

I and II

II and III

I, only

I and III

III, only

Correct answer:

I and III

Explanation:

The carbocation forms spontaneously with the loss of the chlorine atom. This is the rate determining step, thus, the concentration of the halide is the most important determinant of reaction rate. Carbocations form spontaneously in these reactions, and do not use the strong base to remove the halogen.

Example Question #62 : Organic Chemistry, Biochemistry, And Metabolism

Organic reactions can often be classified into two broad categories: substitution and elimination. Substitution reactions substitute one substituent for another. Elimination reactions typically form after the wholesale removal of a substituent, with no replacement. Below are examples of two types of reactions.

Reaction 1:

1

Reaction 2:

2

A scientist is studying the rate of reaction 1. He wants to double the rate of the reaction, but is unsure how to increase concentrations of the reactants. Which of the following is true?

Possible Answers:

Doubling the concentrations of both the hydroxide and the halide will quadruple the reaction rate

Reaction rate in this reaction is not determined by concentration

Doubling the concentration of the hydroxide only will quadruple the reaction rate

Doubling the concentration of the halide only will quadruple the reaction rate

Neither doubling the concentration of halide, nor doubling the concentration of hydroxide, will quadruple the reaction rate

Correct answer:

Doubling the concentrations of both the hydroxide and the halide will quadruple the reaction rate

Explanation:

Reaction 1 represents an SN2 reaction. The rate limiting step involves both reactants coming together to form a transition state. The rate of this reaction depends on the concentration of both the organic molecule and the nucleophile.

In contrast, reaction 2 is an E1 reaction, in which the rate limiting step is the removal of the leaving group to form a carbocation. In E1 and SN1 reactions, adjusting the concentration of the halide only is enough to affect the rate.

Example Question #271 : Organic Chemistry

Which of the following amino acids can participate in the formation of a disulfide bridge?

Possible Answers:

Alanine

Tyrosine

Aspartic acid

Cysteine

Phenylalanine

Correct answer:

Cysteine

Explanation:

Cysteine's R-group contains a sulfhydryl group (-SH), which can participate in the formation of a disulfide bridge in a protein's tertiary and/or quaternary structure. Cysteine is the only amino acid to contain a sulfur atom.

Example Question #1 : Hydrocarbon Reactions

Which substrate, when subjected to ozonolysis followed by treatment with dimethyl sulfide, would give only one hydrocarbon product?

Possible Answers:

1,7-octadiene

1,3-octadiene

4-octene

2-octene

3-octene

Correct answer:

4-octene

Explanation:

Ozonolysis is essentially used to cleave a compound at the location of a double bond. For the result to be a single product, the cleavage must occur at a point of symmetry.

4-octene, a symmetrical alkene, would give two equivalents of butanal upon ozonolysis. All of the other compounds are unsymmetrical and would give at least two different non-identical products.

Example Question #1 : Aromatic Chemistry

Untitled

What is the value of  from Huckel's rule for the given aromatic compound?

Possible Answers:

Correct answer:

Explanation:

Huckel's rule states that an aromatic compound must have  delocalized electrons. The electrons in each double bond are delocalized for this molecule. There are nine double bonds, and thus eighteen delocalized electrons.

If 4n+2=18, then n=4.

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