GRE Subject Test: Chemistry : Halide Chemistry
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Example Questions
Example Question #1 : Reaction Mechanisms
Which of the following factors do NOT favor an SN2 reaction of an alkyl halide?
A good nucleophile
A tertiary carbocation
A polar aprotic solvent
A primary halide
A tertiary carbocation
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:
Reaction 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?
Halogenation reaction
Elimination reaction
Addition reaction
Substitution reaction
Oxymercuration/demercuration reaction
Addition reaction
The addition of bromine gas (
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 #31 : Reactions
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:
Reaction 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
III, only
II and III
I and II
I and III
I, only
I and III
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 #15 : Reaction 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:
Reaction 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?
Doubling the concentration of the halide only will quadruple the reaction rate
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
Reaction rate in this reaction is not determined by concentration
Neither doubling the concentration of halide, nor doubling the concentration of hydroxide, will quadruple the reaction rate
Doubling the concentrations of both the hydroxide and the halide will quadruple the reaction rate
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.
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