140A Chapter 6 Notes

140A Chapter 6 Notes - Chemistry 140A Winter 2011 (K....

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Chemistry 140A Winter 2011 (K. Albizati) 1 Chapter 6 Properties and Reactions of Haloalkanes The carbon – halogen bond is an important one in organic chemistry, especially in organic synthesis. However, it also has a dark side. Alkyl halides = haloalkanes (for now) The lengths and strengths of C-X bonds: C-F 110 kcal/mole 1.38Å C-Cl 85 kcal/mole 1.78Å C-Br 70 kcal/mole 1.93Å C-I 57 kcal/mole 2.14Å Further, halogen atoms increase in size as you go down the Periodic Table – iodine is huge but fluorine is about the same size as a little hydrogen. Of course, halogens are much more electronegative than carbon, resulting in highly polarized C-X bonds in all cases: This polarization is responsible for one of the main reactivity modes of haloalkanes. Where is the electrophilic center of haloalkanes?
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Chemistry 140A Winter 2011 (K. Albizati) 2 Nucleophilic Substitution Reactions The main reaction between haloalkanes and nucleophiles results in “displacement” of the halogen by a nucleophilic species (that is, a substitution) Nu : or Nu - = generalized nucleophiles Generalized Nucleophilic Substitution Reactions The halogen (X) is called a “leaving group” in reactions in which its bond to carbon is broken. Halogens (except F) are very good leaving groups. They are readily displaced with a variety of nucleophilic species.
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Chemistry 140A Winter 2011 (K. Albizati) 3 Best to just look at a bunch of examples: CH 3 Cl + OH CH 3 OH + Cl CH 3 CH 2 I + O CH 3 CH 2 O I CH 3 CH 3 + Br + S Br CH 3 + SCH 3 CH 3 CH 2 I + CH 3 CH 2 N I + NH 3 H H H I C N + C I + N Audience Participation Time
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Chemistry 140A Winter 2011 (K. Albizati) 4 Mechanisms in Organic Chemical Reactions We have already seen one reaction mechanism is detail – radical halogenations. Again, a mechanism is the step-by-step explanation in molecular terms of how reactants become products. There may be several steps in a mechanism, but, in fact, there are relatively few different kinds of fundamental steps in mechanisms. Some fundamental types of steps which are common to many mechanisms:
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Chemistry 140A Winter 2011 (K. Albizati) 5 However, there are more aspects to these steps which are important (like stereochemistry) and we will look at these over the next few quarters. One Mechanism of Nucleophilic Substitution
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140A Chapter 6 Notes - Chemistry 140A Winter 2011 (K....

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