CHEM 2212 Day 07 Radical Reactions PDF

CHEM 2212 – Day 7: Radical Reactions Dr. Nicholas Llewellyn September 5, 2024 SquareCap Join Code: WNC6KQ Radical Chlorination of Alkanes Regiochemistry of Radical Chlorination Two possible regioisomers are formed by the radical chlorination of butane. The “expected” product...

CHEM 2212 – Day 7: Radical Reactions Dr. Nicholas Llewellyn September 5, 2024 SquareCap Join Code: WNC6KQ Radical Chlorination of Alkanes Regiochemistry of Radical Chlorination Two possible regioisomers are formed by the radical chlorination of butane. The “expected” product distribution is based simply on probability – there are six ways to form 1-chlorobutane (6 methyl hydrogens that could be replaced) and four ways to form 2-chlorobutane (four methylene hydrogens that could be replaced). Radical Stability and Rate of Formation Hyperconjugation in Radicals Radicals aren’t as strongly stabilized by hyperconjugation as carbocations Regiochemistry of Radical Chlorination Based on relative reactivity (3.8:1), would predict 79% 2-chlorobutane and 21% 1-chlorobutane. OBSERVED 71% 29% Reactivity AND Probability Matter Chlorination regiochemistry is a product of both reactivity of a particular hydrogen and the probability that hydrogen collides and reacts. Number of equivalent hydrogen atoms x Relative reactivity Relative amount of primary radical formed Relative amount of secondary radical formed 6 equivalent H atoms x 1.0 reactivity = 6.0 4 equivalent H atoms x 3.8 reactivity = 15 6.0/21 = 29% 15/21 = 71% SquareCap #1: (JOIN CODE: WNC6KQ) How many possible products (ignoring stereoisomers) could result from the monochlorination of methylcyclohexane? (All products will have formula C7H13Cl) A) 1 B) 2 C) 3 D) 5 E) 7 SquareCap #2 (JOIN CODE: WNC6KQ) Given the following relative reactivity values, predict the LEAST abundant product resulting from monochlorination of methylcyclohexane: 1°: 1 2°: 3.8 3°: 5 Cl Cl A B Cl Cl Cl C D E Radical Bromination Radical Bromination is Highly Selective! Bromine radicals are less reactive than chlorine radicals, because the H-Br bond formed (88 kcal/mol) is weaker than the H-Cl bond formed in chlorination (103 kcal/mol). This leads to greater radical selectivity. Reactivity-Selectivity Principle Radical formation is exothermic with chlorine and endothermic with bromine. More stable radical really important with bromine! Reactivity-Selectivity Principle Hammond’s Postulate: The transition state more strongly resembles the higher energy species (starting material or product) SquareCap #3 (JOIN CODE: WNC6KQ) Bromine isn’t perfect! Calculate the percent yield for the product shown below, given the following relative reactivity values: 1°: 1 2°: 82 3°: 1600 A) 5% Br B) 29% Br2 C) 41% hν D) 71% The Stereochemistry of a Radical Substitution Reaction A new asymmetric center is formed, so the product is a racemic mixture. Racemization A pair of enantiomers is formed if a hydrogen atom is removed from an asymmetric center. What About Fluorine and Iodine? Fluorine radicals react too violently to be useful. Iodine radicals are too unreactive to propagate. Utility of Peroxides: Anti-Markovnikov Addition Recall from CHEM 2211 that haloacids add to alkenes to give the more substituted halide. In the presence of a peroxide, however, the less substituted halide is preferred. Radical Addition to Alkenes The weak O-O bond is cleaved in the first initiation step. The resulting alkoxy radicals abstract hydrogen from H-Br to yield Br radicals, which then propagate in the chain reaction. The Stereochemistry of a Radical Addition Reaction A new asymmetric center is formed, so the product is a racemic mixture. Radicals Resist Rearrangement Open carbocations readily rearrange. Radicals, with their half-filled p- orbitals, rarely exhibit migration of hydrogen or ordinary alkyl groups. HBr Br Radical Addition of HBr is Unique HCl and HI exhibit Markovnikov addition with or without peroxide. Bromine Radicals Hit the Sweet Spot To Do Attend Recitation 3 (Exam 1 Review) this week Submit Recitation 3 to Gradescope by 5:00 PM on Sat. 9/7. Please review the submission instructions on eLC. Your submission must be formatted exactly the same as the original worksheet PDF (all pages included in same order, same page breaks, etc). If you worked on a print copy, use the Gradescope app to take and submit photos or create a PDF using CamScanner, Adobe Scan, etc. Read the rest of Chapter 12 (12.6 – end) Practice a little every day!