TTh_12 - Organic Lecture Series CH 310/318 M LECTURE 12 Textbook Assignment Chapter 6 Contd Todays Topics Alkenes Reactions Alkenes Notice

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Organic Lecture Series 1 CH 310/318 M LECTURE 12 Textbook Assignment: Chapter 6- Cont’d Today’s Topics: Alkenes Alkenes - Reactions Reactions Notice & Announcements: Organic Lecture Series 2 Addition of HX Addition of HX • Carried out with pure reagents or in a polar solvent such as acetic acid . • Addition is regioselective – Regioselective Regioselective reaction: reaction: An addition or substitution reaction in which one product is formed in preference to all others that might be formed. Markovnikov Markovnikov ’s rule: rule: In the addition of HX or H 2 O to an alkene, H adds to the carbon of the double bond having the greater number of hydrogens. 1-Bromopropane (not observed) 2-Bromopropane Propene + + CH 3 CH=CH 2 HBr 3 CH-CH 2 H 3 2 Br Br H
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Organic Lecture Series 3 HBr HBr + 2 + 2 -Butene Butene • A two-step mechanism Step 1: Proton transfer from HBr to the alkene gives a carbocation intermediate. Step 2: Reaction of the sec- butyl cation (an electrophile) with bromide ion (a nucleophile) completes the reaction. CH 3 CH=CHCH 3 HB r 3 CH-CHCH 3 H Br + + δ− δ+ slow, rate determining sec-Butyl cation a 2° carbocation 3 CHCH 2 3 3 2 3 + fast Bromide ion (a nucleophile) sec-Butyl cation (an electrophile) 2-Bromobutane Br Organic Lecture Series 4 Carbocations Carbocations Carbocation 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 3° depending 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.
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Organic Lecture Series 5 Carbocations Carbocations Carbocations: 4. Have bond angles of approximately 120° about the positively charged carbon. 5. Use sp 2 hybrid orbitals to form sigma bonds from carbon to the three attached groups. 6. The unhybridized 2 p orbital lies perpendicular to the sigma bond framework and contains no electrons. Organic Lecture Series 6 • The structure of the tert-butyl cation. Carbocations Carbocations
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Organic Lecture Series 7 – relative stability – Methyl and primary carbocations are so unstable that they are never observed in solution. Methyl cation (methyl) Ethy l cation (1°) Isopropyl cation (2°) tert -Butyl cation (3°) Increasing carbocation stability + ++ + C H H CH 3 C 3 3 H C 3 3 3 C H H H Carbocation Carbocation Stability Stability Organic Lecture Series 8 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.
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This note was uploaded on 10/28/2010 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas at Austin.

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TTh_12 - Organic Lecture Series CH 310/318 M LECTURE 12 Textbook Assignment Chapter 6 Contd Todays Topics Alkenes Reactions Alkenes Notice

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