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Chapter 14 Summary Spring 12

Chapter 14 Summary Spring 12 - 1 Chapter 14 Summary Spring...

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1 Chapter 14 Summary Spring 12 Chapter 14: Do end of chapter homework: 32, 33, 34, 36 (after doing 14.6 you should realize that the ratio of the two products you get in parts a-d reactions will depend on reaction temperature!), 37 (assume part a is SN1but be careful with part d!), 38a, 39 for SN1 reactivity only, 41, 44, 45 a-b, 47, 48, 49 a and d only, 50 a and d only, 51 a and d only, 53 a only, 54, 56, 58 a and c (for both a and b don’t worry about the exact stereochemistry shown in the target molecules. Rather just get all the atoms hooked up the way they are illustrated) , 60 a, c, d, and f, 61, 62 a and b, 76 (don’t worry about stereochemistry in cyclohexene product). 14.1 : Overlap of Three Adjacent p Orbitals (Allyl systems): Read Carefully. Allylic substituents are bonded to the carbon that is bonded to an alkene carbon. You will see that allylic substituents have some “unexpected” chemistry/properties. o The allylic carbon- hydrogen bond is “unexpectedly” weak for a C sp3 -H 1s bond. o Allylic halides can “unexpectedly” dissociate to form “ primary ” carbocations. Thus, ally lic halides are suitable SN1 substrates (under proper SN1 conditions…which by the way you should be familiar with!). o The allylic hydrogen is “unexpectedly” acidic for a hydrogen bonded to an sp 3 carbon. Homolytic bond cleavage of the allylic carbon-substituent bond produces a resonance stablilized allylic radical. (See the first reaction on page 611). Heterolytic bond cleavage of an allylic carbon-halide bond produces an allylic resonance stabilized allylic cation. (See the second reaction on page 611). Heterolytic bond cleavage of an allylic carbon-hydrogen bond (by a strong enough base) can produce an allylic anion. A free allylic anion would be resonance stabilized like the radical or carbocation counterparts. (See the third reaction on page 611). Take a look at the “partial electron density” diagram in the middle of page 612. This shows that the extra electron density (or lack of electron density) is shared by the terminal carbons of the allyl system. Note : terminal carbons of an allyl system are not necessarily the terminal carbons of a parent chain . 14.2 : Radical Allylic Halogenation: Read Carefully but ONLY worry about allylic halogenation with NBS (Do exercises 1 and 2. Note for 14.2 c see the solution WE do in LECTURE!). Since the allylic carbon- hydrogen bond is relatively weak it can participate in “radical allylic halogenation” reactions. You know from chapter 12 that X 2 reagents (Br 2 and Cl 2 ) will add to an alkene to give “anti - vicinal dihalides”. Remember this is not a radical reaction pathway. However, if the X 2 reagent is present in a very low concentration then another reaction pathway (an allylic radical halogenation pathway) is followed. It is beyond the scope of this class to discuss why having a low or high molecular halogen concentration instigates different reaction pathways.
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