Elimination Reactions of Alkyl Halides PDF
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Uploaded by AffluentNovaculite2115
The University of Texas at Rio Grande Valley
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Summary
This document covers the topic of elimination reactions of alkyl halides, discussing factors that influence the reaction, such as the use of strong bases and bulky substrates. It explains the concept of regioselectivity and stereoselectivity in elimination reactions and the preferred products formed as a result of these factors.
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Elimination reactions of alkyl halides E2 reaction are concerted: the base removes a hydrogen from the neighboring carbon while the pair of electrons serves to form the C-C double bond, causing the leaving of the leaving group Elimination vs. Substitution Elimination is favored By using a s...
Elimination reactions of alkyl halides E2 reaction are concerted: the base removes a hydrogen from the neighboring carbon while the pair of electrons serves to form the C-C double bond, causing the leaving of the leaving group Elimination vs. Substitution Elimination is favored By using a strong base (pKa > 14, base must have a negative charge) OH- is a strong base while H2O is a weak base By using a bulky substrate (2o or 3o alkyl halide) By using a bulky base (branched) At high temperature (> 40 °C) If H20 is used, the main product is the substitution When OH- is used, the elimination product predominates Regioselective and stereoselective reactions Main Minor When more than one constitutional isomer can be produced and one predominates, the reaction is said to be regioselective In elimination reactions, the most substituted alkene is typically the major product because it is more stable than the least substituted alkene E2 eliminations are regioselective Determining degree of substitution in alkenes the most substituted alkene has the highest number of alkyl groups and the lowest number of hydrogens on the C-C double bond 20% Hoffmann 80 % Saitsev Main The Hoffmann product (least substituted alkene) can be favored by using a strong, bulky base Stereochemistry of E2 eliminations E2 eliminations happen when the E-hydrogen and the leaving group are anti-coplanar stereoselectivity When more than one stereoisomer can be produced and one predominates, the reaction is said to be stereoselective More than one stereoisomer can be produced when the E-carbon has 2 labile hydrogens E2 eliminations are stereoselective Only one stereosiomer can be formed because there is only 1 E-hydrogen Reaction is stereospecific E and Z nomenclature system assign priorities to the groups bound to each sp2 carbon Priorities are assigned based on the atomic weight of the groups directly connected to the sp2 carbons Z (zusammen) together E (entgegen) opposite
