CHEM 2513 Lecture Notes - Lecture 12: Nucleophile, Hinder, Sigma Pi Sigma

Lab 12 Substitution and Elimination Reactions: Unimolecular Elimination

QUESTIONS:

1) At the end of the experiment, which of the following can go into the organic waste container?

I. Excess cyclohexane

II. Excess cyclohexene

III. Anhydrous magnesium sulfate

I, II and III

I and II only

I only

II only

2) The "1" in SN1 and E1 stands for?

The rate only depends on one species.

The mechanism has one step.

The rate depends on two species.

3) Why is it necessary to add anhydrous magnesium sulfate to your distillate?

To aid in recrystallization.

To remove any excess protons from solution.

It re-protonates the final product.

The distillate is an azeotrope with water

4) What will the major product of an E1 reaction with 2-methyl-3-pentanol be?

5-methyl-6-pentane

4-methyl-3-pentene

4-methyl-2-pentene

2-methyl-2-pentene

A) Which alkyl halide reacts the fastest by the S_N2 mechanism?

a) (CH₃)₂CHBr b) CH₃Br c) CH₃CH₂Br d) (CH₃)₃CBr

B) Which alkyl halide undergoes an S_N2 reaction with sodium cyanide at the fastest rate?

a) CH₃CH₂Cl b) CH₃CH₂I c) CH₃CH₂Br d) CH₃CH₂F

C) What does the "2" in S_N2 refer to?

a) The reaction produces two products. One product is formed with inversion of configuration and the other product is formed with retention of configuration.

b) The reaction takes place in two steps

c) The rate of the reaction depends upon the concentration of two reactants.

d) The reaction produces two products that are related as diastereomers.

D) What product(s) will be formed in the S_N2 reaction of (R)-2-bromopentane with sodium cyanide?

a) (S)-2-cyanopentane

b) (R)-2-cyanopentane

c) A racemic mixture of both enantiomers of 2-cyanopentane

E) Which two diastereomeric products will be formed in the solvolysis reaction shown below?

F) Which of the following carbocations will be the most stable and most likely carbocation formed during the solvolysis of 1-chloro-2-methylcyclohexane in CH₃OH?

G) What does the "1" in S_N1 refer to?

a) The rate of the reaction depends upon the concentration of only one reactant.

b) The reaction takes place in one step.

c) The reaction produces only one product.

d) The product that's formed has either retention or inversion of configuration.

H) What product(s) will be formed in the S_N2 reaction shown below?

I) Which product(s) will be formed in the S_N2 reaction shown below?

J) What is the order of decreasing stability of the carbocations shown below?

Nucleophilic Substitution – Synthesis of an Alkyl Halide

Introduction

Alkyl halides can be prepared from alcohols by reactions with hydrogen halides (HCl, HBr, or HI) via nucleophilic substitution. In this type of reaction, the nucleophile displaces a leaving group from a carbon atom of an organic substrate (here the alcohol once protonated). Both electrons of the new bond to the carbon are provided by the nucleophile while the leaving group departs with both electrons of its bond to the substrate. There are two limiting mechanisms that are most common in describing nucleophilic substitution reactions at sp3 carbon centres : (1) a direct displacement (SN2) process and (2) a process that generates an intermediate carbocation (SN1). Which mechanism is a better description depends on the structure of the molecule containing the leaving group, the nature of the leaving group, the size of the nucleophile, and the polarity of the solvent. This experiment is a near-perfect example of an SN1 substitution. H3C OH CH3 H3C + HCl H3C Cl CH3 H3C + H2O (Equation 1) In this reaction, you will prepare 2-chloro-2-methylpropane (t-butyl chloride) from 2- methyl-2-propanol (t-butyl alcohol) using HCl as the strong acid and chloride source. Note that HCl first activates the alcohol in an acid/base reaction before the SN1 reaction proceeds. The final product is characterized by its yield, boiling point, and density. The presence of a halide atom is confirmed with a silver nitrate test.

1) State which of the electrophiles given below will react preferentially by i) SN1, ii) by SN2, or iii) capable of reacting by either of the two mechanisms depending on the given conditions. How can you affect those conditions to favour SN1 or SN2? Reason your predictions based on the structures of the compounds. Br-CH3, Br-CH2CH3, Br-CH(CH3)2, Br-C(CH3)3, Br-CH2-C5H6; C5H6 = phenyl

2) Rank the nucleophiles given below based on high, mediocre, and low nucleophilicity and reason why based on their structures. HO-CH3, H2N-CH3, KO-CH3, KO-C(CH3)3

3) Name two solvents that are commonly used for SN2 reactions. because they considerably slow down SN1 reactions.

4) Can you ever have only SN2 or only SN1?

5) Iodine is a better leaving group than bromine. But iodine is a better nucleophile than bromine. Why is that?

6) Using orbitals, draw the transition state of the SN2 reaction between sodium cyanide and bromomethane. Which orbitals are interacting with which? Why is the bond breaking? Why is the orientation important? Is there another angle besides backside attack that could theoretically work based on orbital geometry? Why doesn’t this happen?

7) Name four reagents you can use to dry solvent. Why do we use magnesium sulfate in this lab instead of them?

8) Draw a mechanism with all the proper arrows for the reaction from this experiment.

9) What are we removing with the water wash? Why don’t we just add the sodium bicarbonate directly to the reaction mixture instead of doing a water wash?