Chem320 Review Sheet Exam 1

Chem320 Review Sheet Exam 1 - Exam 1 Review Sheet Chem 320...

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Exam 1 Review Sheet Chem 320 Spring 2011 Chapter 14: Delocalized Pi Systems Pi Systems - the whole theme of this chapter revolves around pi systems (compounds that contain at least 1 C=C); the first part of the chapter talks about just one bond, the second part talks about systems having 2 double bonds, esp those that are connected (conjugated) - having a C=C next to a carbon is stabilizing: - more stabilizing than a 3 ° radical - about as stable as a 2 ° carbocation (as evidenced by the speed of a SN1 reaction) - more stable than a “normal” anion (as evidenced by pK a values) - what makes this so stabilizing? Resonance delocalization of the radical/anion/cation on a C beside (adjacent to) the C=C; you can move the charge around so it’s shared by 3 carbons, spreading the charge/radical out and minimizing it - an allylic group is C=C-C-; a vinylic group is C=C- - if you react an allylic system (something that contains a C=C) with a radical source (NBS is a good and common one - it generates Br . , NCS, while less common, also exists and produces Cl . ), the H on the C next to the C=C (C=C-C- H ) will react with the Br . first; the radical can be delocalized, and either radical can be trapped by another Br . - you can carry out SN1 reactions with allylic halides; the carbocation formed can be delocalized by resonance; it can be trapped: - if you add H 2 O, you make an alcohol from the carbocation - if you started with the alcohol and added HX, you make the alkyl halide, RX - if you add an alcohol, you make an ether from the carbocation - you can also make organometallic reagents (lithium reagents, Grignard reagents, etc) with allylic compounds; the allylic H (C=C-C- H ) is more acidic than the vinylic Hs (C=C- H ), so it is removed with a base preferentially (pK a of allylic is 40, pK a of vinylic is 44) Dienes - dienes contain 2 C=Cs in a compound - if the 2 C=Cs are adjacent (C=C-C=C), it is called conjugated and the 2 pi systems are connected and the electron density overlaps and is shared between the two systems; this is more stable that just having 2 C=Cs in the molecule - if the 2 C=Cs share a common C (C=C=C), it is a cumulated diene; the two pi systems are not in conjugation/the electron density is not overlapping and is not shared between the two systems because the p orbitals that contain that density are perpendicular to each other and cannot overlap with each other to share the electrons that they hold - if the 2 C=Cs are not near each other (not adjacent to each other), they do not overlap either and there is no additional stabilization that comes from having 2 C=Cs
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- be able to name dienes according to the systems discussed in class - know the difference between s-cis or s-trans -- it refers to the relative orientations of the two double bonds as being both on the same side of the single bond that joins them (s- cis) or on the opposite sides of the single bond (s-trans) - if you react a conjugated diene with HBr (for example), you can get 1,2 or 1,4 addition
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Chem320 Review Sheet Exam 1 - Exam 1 Review Sheet Chem 320...

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