Download Chapter 4 - The Study of Chemical Reactions

Copyright 2003 Prentice-Hall, Inc. All rights reserved.
Chapter 4 - Reactions of Alkenes
1. Sect. 4.3 Transition state for electrophilic addition to an alkene A useful method to
approximate a transition state structure is to examine the reactant and product, note which bonds
must be broken and/or made, and then write a state with partial bond making and/or breaking,
sort of halfway in between the reactants and products. Use ChemDraw to carefully draw the
transition state for the addition of H+ to propene (p. 147). The H+ is bonding to a carbon, the π
bond is breaking, and a partial positive charge is building up. Hint: The lowercase Greek delta
"δ" results from changing the "d" to the Symbol font (Text>Font>Symbol). Your drawing
should rationalize the regioselectivity of the reaction.
2. Sect 4.7 Build a model of a halonium ion. In Chem3D, type CyclopropylC in the text box
from the Text Building tool and hit Enter. Minimize the energy. Replace one of the methylene
groups (CH2) of the cyclopropane ring with a bromine by typing Br in the text box from the Text
Building tool, selecting one of the CH2 groups, and hitting Enter. Chem3D will not minimize the
energy of this structure because it doesn't recognize the divalent bromine, but we can consider
the model to be similar to a bromonium ion. Examine the model from various perspectives.
Note that the backside of the bromonium ion is sterically more accessible to the attack by the
nucleophile. Go on to the next Exercise.
3. Problem 17a and b The halonium ion model in Exercise 2 is presumably an intermediate in
these reactions. Note that the two carbon atoms of the double bond are not identical. Since the
more highly substituted carbon bears more positive charge than the less substituted carbon,
attack by the nucleophile occurs at the carbon bearing the methyl group (p. 160). This is
especially pertinent when other nucleophiles are present such as water, alcohol, halide salts, etc.
4. Sect 4.10 Transition state for free radical addition to an alkene Use ChemDraw to
carefully draw the transition state for the addition of Br⋅ to propene, the first propagation step on
p. 169. Your drawing from Exercise 1 can be modified by changing the H to Br and replacing
the + charge with a dot representing an electron. Your drawing should rationalize the
regioselectivity of the reaction.
5. Problem 35 One approach to this problem would be to draw 2-methyl-2-butene in
ChemDraw. Duplicate the structure. Examine the two carbons of the double bond and notice
that one would become a tertiary carbocation and one would become a secondary one. Add the
reagents in each problem. Where applicable, decide what is the electrophile and what is the
nucleophile. If the reaction involves free radicals (peroxide present), decide what free radical
initiates the addition.
6. Problem 42 As in Problem 35, draw 1-methylcyclohexene in ChemDraw and duplicate the
structure. Add the reagents in each problem. Where applicable, decide what is the electrophile
and what is the nucleophile. If the reaction involves free radicals, decide what free radical
initiates the addition.
7. To add visual reinforcement to your study of some of the reactions in this chapter, consider a
model of 2-methyl-2-butene as you answer the following questions. Number each carbon atom
in the longest chain to label your answer.
a. In the electrophilic addition of HCl, which carbon is attacked by the electrophile, H+?
b. In the free radical addition of HBr (peroxides, ROOR) to 2-methyl-2-butene, which carbon is
attacked by the free radical, Br⋅?
c. In the reaction of bromine (Br2) in saturated aqueous sodium chloride, at which carbon will
the bromine appear?
d. In the reaction of bromine in saturated aqueous sodium chloride, at which carbon will the
chlorine appear?
8. The following pertain to reactions of 1-hexene.
a. To which carbon would the OH be attached after the oxymercuration-demercuration?
b. To which carbon would the OH be attached after the hydroboration-oxidation?
c. To which carbon would the boron become attached in the hydroboration step?
d. To which carbon would the mercury become attached after the oxymercuration?
9. The Summary of Reactions of Alkenes on p. 176 contains a wealth of useful information.
Consider reproducing the reactions for which you are responsible using ChemDraw. Use
propene as a substrate to summarize regiochemistry. A standard format might be to put an
organic reactant, reaction conditions above and below an arrow, and a product. Once they are
prepared, these equations can be used in many different ways. A sheet could give the complete
equation. Another could be prepared that has deleted the products and thus gives practice in
completing a reaction. Another could be prepared that has deleted the reaction conditions and
would give practice in synthesis.
10. Sect 4.11 a) Use Chem3D to verify the steric energy differences between cis- and trans-2butene.
• Select the Text Building tool, click in the model window, and type "cis-2-butene" and press
Enter or Return.
• From the MM2 menu, choose Minimize Energy and click Run.
• Note the steric energy for the frame.
• Repeat for trans-2-butene.
• Which is more stable, cis- or trans-2-butene?
b) Compare the steric energies of: Z-2-pentene, Z-4-methyl-2-pentene, and Z-4,4-dimethyl-2pentene. A fast way to build these models is to build from your cis-2-butene models. Replace an
H atom using the Text Building tool. Click the hydrogen atom, type "C" in the text box, and
press Enter or Return. Additional methyl groups can be added by double-clicking on the
appropriate hydrogen atom. Minimize the energy and examine the results.
c) Compare the steric interactions of a methyl group with: another methyl group, an ethyl
group, an isopropyl group, and a tert-butyl group (see a and b).
11. Examine the steric interactions in cis- and trans-2-butene (see problem 10) using spacefilling models.
• Build the model as in 10 (a).
• From the View menu, choose Settings and Model Display.
• Under Model Type select Space Filling.
• Close the Settings window.
• Examine the model from various perspectives.
• Chem3D will show you the steric hindrance with the Show Close Contacts command in the
Show Measurements submenu from the Analyze menu. The Measurements table appears
listing the atoms that are in close contact. The box to the left of each listing in the
Measurements table is called a Record Selector. When you select any of the close contact
records in the Measurements table the corresponding atoms in the model are selected.
12. Problem 38 If you made ChemDraw drawings of the C6H12 alkenes in Problem 30 in
Chapter 3, you can open the file to help you answer these questions. The stability of alkenes is
related to the number of non-hydrogen substituents attached to the double bond. Examine your
drawings to determine which alkene has the greatest number of alkyl substituents and which has
the fewest.