GRE Subject Test: Chemistry : Halide Chemistry

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

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Substitution And Elimination Mechanisms

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

Possible Answers:

A tertiary carbocation

A polar aprotic solvent

A primary halide

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 #1 : Halide Chemistry

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:

Addition reaction

Halogenation reaction

Elimination reaction

Substitution reaction

Oxymercuration/demercuration 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 #8 : 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, only

I and III

II and III

I and II

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 #2 : Halide Chemistry

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:

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

Doubling the concentration of the halide only 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 concentrations of both the hydroxide and the halide 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.

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: