SN2 Substitution Practice Problems

Nucleophilic Substitution reactions are very important as they are the first type of reactions that teach us how to achieve functional group transformations in Organic Chemistry. Substitution reactions cover a broad spectrum of topics, starting from kinetics and thermodynamics that require your knowledge of rate laws, enthalpy of reaction, and energy diagrams in general. In order to fully understand the mechanisms and differences between SN2 and SN1 reactions, you need to strengthen your basis of General Chemistry and build up skills in Organic structures and curved-arrow mechanisms.

This quiz will help you assess and improve your comprehension of the SN2 mechanism. It starts with the kinetics of the SN2 reaction in question one and covers the energy diagrams, including questions about the reaction such as its activation energy, enthalpy, order, and the overall mechanism covered through questions 2-5. Question 2 is a comprehensive question that demonstrates how much information one energy diagram can convey.

The curved arrow pushing in SN2 mechanisms is covered in questions 6-7. Identify the nucleophile and the leaving group to draw the movement of electrons from the nucleophile to the electrophilic carbon, break off the bond with the leaving group, and last, pay attention to any stereochemistry involved in the reaction.

Print the energy diagrams and mechanisms and draw the answers to all the questions on paper before accessing the answers.

Practice

1.

Question 1 – The kinetics of SN2 reaction:

What happens to the rate of the SN2 reaction under each of the following changes of the concentration of the substrate (RX) and nucleophile (Nu)?

a) [RX] is doubled, and [Nu] stays the same.
b) Both, [RX] and [Nu] are doubled.
c) [RX] is doubled, and [Nu] is halved.
d) Both, [RX] and [Nu] are tripled.

Answer

The rate equation for this reaction is:

and the general rate equation for the SN2 mechanism is:

Rate = k[RX][Nu]

a) The rate doubles
b) The rate increases by a factor of 4 (2 x 2 = 4)
c) Rate stays the same (2 x ½ = 1)
d) The rate increases 9 times (3 x 3 = 9)

2.

Question 2 – The Energy Diagram of SN2 reaction:

 Consider the SN2 reaction between 1-Bromobutane and the cyanide ion and the following questions according to the hypothetical values given in the energy diagram:

a) Label the reactants and products
b) label the substrate and the nucleophile
c) Label the transition state
d) Label the Ea, and ΔH°
e) Is the reaction endothermic or exothermic?
f) How many kJ/mol is the Ea, and ΔH°?
g) Is the reaction mechanism SN1 or SN2?
h) What is the rate equation of the reaction?
i) What is the overall order of the reaction?

Answer

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Solution

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3.

Question 3 – The Energy Diagrams of SN2 reaction:

Which of the following energy diagrams represents an endothermic SN2 reaction?

Answer

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Solution

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4.

Question 4 – – The Energy Diagram of SN2 reaction:

Which of the following energy diagrams represents an exothermic SN2 reaction?

Answer

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Solution

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5.

Question 5 – The Energy Diagram of SN2 reaction:

Draw an energy diagram for the following SN2 reaction. Label the axes, the Ea, the ΔH° and the transition state of the reaction. Assume the reaction is exothermic and ΔH° = -75 kJ/mol and Ea = 50 kJ/mol. Draw the structure of reactants and products on the diagram. You can put the reactants at any energy level and then draw the rest as asked in the exercise. All the numbers are hypothetical for learning the concepts.

Answer

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Solution

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6.

Question 6 – The curved arrow mechanism of SN2 reaction:

Draw the mechanism and show the products for the following SN2reactions:

Answer

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Solution

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7.

Draw the mechanism and show the products with correct stereochemistry for the following SN2 reactions:

Answer

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8.

Each of the following compounds can be prepared with an alkyl halide and a suitable nucleophile by SN2 substitution. Identify the alkyl halide and the nucleophile that would lead to each product. When more than one set of reactants can be used, determine which one is a better choice.

a)
Answer

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b)
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c)
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d)
Answer

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e)
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f)
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g)
Answer

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h)
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i)
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j)
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k)
Answer

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9.

Draw a curved arrow mechanism for the following synthesis:

 

Answer

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10.

Only product A is formed when the following compound is treated with a base. Explain this selectivity by drawing a plausible curved arrow mechanism:

Answer

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11.

Draw a curved arrow mechanism for the following reaction:

Answer

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