ch3 - Chemistry 140A Fall 2009 (K. Albizati) 1 CHAPTER 3...

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Unformatted text preview: Chemistry 140A Fall 2009 (K. Albizati) 1 CHAPTER 3 REACTIONS OF ALKANES ---- MECHANISMS OF CHEMICAL REACTIONS This chapter starts to introduce organic chemical reactivity --- the basis for understanding a lot of biology, biochemistry and medicine. Arguably, the most important chemical reaction in history: Gasoline (a mixture of hydrocarbons) + O 2 Spark CO 2 + CO + H 2 O + a lot of energy A lot of C-C and C-H bonds are broken, but a lot of very stable C-O bonds are formed. This reaction does not involve acid-base (electrophile-nucleophile) chemistry as we saw earlier. Instead, this reactions proceeds via radicals species which possess an unpaired electron and are quite reactive. Reaction Mechanism the step-by-step explanation in molecular terms of how reactants are transformed into products; involves the sequence of the breaking and forming of bonds and the identities of any reactive intermediates. This is a very important concept in organic and biochemistry. Radicals We saw in Chapter 1: H . . H + H H H o = -104 kcal/mol Heat is given off as two hydrogen atoms are combined to form the very stable hydrogen molecule. The opposite is true if you want to reverse the process and cleave a hydrogen molecule into 2 hydrogen atoms with unpaired electrons: Chemistry 140A Fall 2009 (K. Albizati) 2 H . . H + H H H o = 104 kcal/mol In this case heat is required to break the bond. This is an example of homolytic cleavage, which can be generalized as: A . . B + A B Radicals - species with unpaired electrons Some examples: Cl . . Cl + Cl Cl These are chlorine radicals. . H + C H H H H C H H H . This is a methyl radical. Whats this one called? C C . H H H H H Chemistry 140A Fall 2009 (K. Albizati) 3 Bonds can also be broken such that electron sharing is unequal: + A B Ions - charged species A + B- This mode of bond breaking is quite prevalent in solution phase chemical reactions. Back to homolytic cleavage: There is a quantity associated with any homolytic cleavage, the DH o or bond dissociation energy. Some examples: + C H H H H C H H H . H O H H OH . . + DH o = 119 kcal/mol H . DH o = 105 kcal/mol + C H H CH 3 H C H CH 3 H . H . DH o = 101 kcal/mol + C H H CH 3 CH 2 H C H CH 3 CH 2 H . H . DH o = 101 kcal/mol Chemistry 140A Fall 2009 (K. Albizati) 4 A primary radical DH is only weakly affected by what is a few atoms away. What happens to DH when you start to substitute closer to the radical? + C H CH 3 CH 3 H C CH 3 CH 3 H . H . DH o = 98.5 kcal/mol + C H CH 3 CH 3 CH C CH 3 CH 3 CH . H . DH o = 96.5 kcal/mol Although these appear to be small changes, they are actually quite significant and have a profound effect on chemical reactivity. Look at the sequence again: DH o Methyl radical formation 105 kcal/mol Primary radical formation 101 Secondary radical 98 Tertiary radical 96 A decreasing DH o indicates an easier C-H bond to break, indicating greater stability of the resulting radicals. Hence: Order of radical stability is (in general) Tertiary > Secondary > Primary >> Methyl 3 o > 2 o > 1 o > CH Why is this? Chemistry 140A...
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ch3 - Chemistry 140A Fall 2009 (K. Albizati) 1 CHAPTER 3...

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