Unformatted text preview: Organic Lecture Series CH 310/318 M
Textbook Assignment: Chapter 6 Today's Topics: Alkenes-addition Rxns LECTURE 17 Notice & Announcements:
HW 4 Due on Monday 2/27 1 Organic Lecture Series Reactions of Alkenes Chapter 6 2 Organic Lecture Series Addition of HX Carried out with pure reagents or in a polar solvent such as acetic acid. Addition is regioselective Regioselective reaction: An addition or substitution reaction in which one product is formed in preference to all others that might be formed. Markovnikov's rule: In the addition of HX or H2O to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens.
3 Organic Lecture Series Carbocations
Carbocation: A species in which a carbon atom has only six electrons in its valence shell and bears a positive charge. Carbocations: 1. Are classified as 1 2 or 3depending on , , the number of carbons bonded to the carbon bearing the + charge. 2. Are electrophiles; that is, they are electron-loving. 3. Are Lewis acids.
4 Organic Lecture Series Carbocations
Carbocations: 4. Have bond angles of approximately 120 about the positively charged carbon. 5. Use sp2 hybrid orbitals to form sigma bonds from carbon to the three attached groups. 6. The unhybridized 2p orbital lies perpendicular to the sigma bond framework and contains no electrons. 5 Organic Lecture Series Carbocations The structure of the tert-butyl cation. 6 Organic Lecture Series Carbocation Stability relative stability Methyl and primary carbocations are so unstable that they are never observed in solution. 7 Organic Lecture Series Carbocation Stability
To account for the relative stability of carbocations- assume that alkyl groups bonded to a positively charged carbon are electron releasing and thereby delocalize the positive charge of the cation. This electron-releasing ability of alkyl groups arises from (1) the inductive effect, and (2) hyperconjugation.
8 Organic Lecture Series The Inductive Effect The positively charged carbon polarizes electrons of adjacent sigma bonds toward it. The positive charge on the cation is thus delocalized over nearby atoms. The larger the volume over which the positive charge is delocalized, the greater the stability of the cation. 9 Organic Lecture Series Hyperconjugation Involves partial overlap of the -bonding orbital of an adjacent C-H or C-C bond with the vacant 2p orbital of the cationic carbon. The result is delocalization of the positive charge. 10 Organic Lecture Series Carbocation Rearrangements In electrophilic addition to alkenes, there is the possibility for rearrangement if a carbocation is involved. Rearrangement: A change in connectivity of the atoms in a product compared with the connectivity of the same atoms in the starting material. 11 Organic Lecture Series Carbocation Rearrangements In addition of HCl to an alkene. In acid-catalyzed hydration of an alkene. 12 Organic Lecture Series Carbocation Rearrangements The driving force is rearrangement of a less stable carbocation to a more stable one. The less stable 2carbocation rearranges to a more stable 3carbocation by 1,2-shift of a hydride ion. 13 Organic Lecture Series Carbocation Rearrangements Reaction of the more stable carbocation (an electrophile) with chloride ion (a nucleophile) completes the reaction. 14 ...
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