ORGANIC - BROWN 8E CH.8 - HALOALKANES, HALOGENATION AND RADICALS

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2 CONCEPT: ALKYL HALIDES Alkyl halides are named by naming them as a substituent before the root chain and indicating their location. Prefixes: -F, -Cl -Br -I Alkyl halides have NO when it comes to numbering the direction of the chain. EXAMPLE: Name the following compounds: a. b. Page 2

3 CONCEPT: RADICAL INITIATORS Chemical bonds can be cleaved in two ways: Heterolytically (ionic cleavage) and homolytically (radical cleavage). We use full curved arrows to indicate the movement of electrons and a half arrow to indicate. Homolytic dissociation energy is much than a corresponding heterolytic dissociation energy. There are a few molecules that are known as radical initiators due to the fact that they contain relatively weak bonds that can be easily cleaved by homolysis. 1. Diatomic Halogen: 2. Peroxides: 3. N-bromosuccinimide (NBS): Page 3

4 CONCEPT: RADICAL STABILITY Radicals are electron deficient, therefore they also benefit from the stabilizing effects of. The relative stability of radicals is slightly different than that of carbocations: Unlike carbocations, allylic / benzylic radicals are ALWAYS most stable EXAMPLE: Determine which of the following radicals is most stable. Page 4

5 CONCEPT: RADICAL CHAIN REACTION Alkanes are the backbone of organic molecules, yet they are almost completely unreactive. The only reaction they undergo is radical halogenation, the gateway to the rest of organic synthesis. The Radical Chain Reaction Alkanes will react with diatomic halogens in the presence of heat, light or any other radical initiator. Initiation: Propagation: Termination: Page 5

6 EXAMPLE: Show the entire chain reaction mechanism for the following radical halogenation reactions, predicting which would be structure of the major alkyl halide produced. a. Page 6

7 CONCEPT: FREE ENERGY DIAGRAMS Atoms save energy by forming bonds. Free energy diagrams show overall changes in potential energy during reactions. Free energy diagrams give us information on the spontaneity and rate of reactions Thermodynamics describes favorability. Determined by Gibbs Free Energy Kinetics describes rate. Determined by the activation energy of the rate determining step. EXAMPLE: Describe the favorability and rate of the following free energy diagrams. Page 7

8 CONCEPT: GIBBS FREE ENERGY Predicts of reactions. Composed of three terms. Enthalpy is the sum of bond dissociation energies for the reaction. = bonds = Exothermic = bonds = Endothermic Entropy is a measure of disorder in the system. = More ordered = More disordered Temperature amplifies the effect of entropy on the overall favorability. Some reactions require more than one step to go to completion. The ΔG⁰ is the sum of all the steps. Transition states be isolated. They involve bonds being broken and made at the same time. Intermediates be isolated. They rest at a higher energy state than normal. Page 8

9 CONCEPT: ENTHALPY Bond dissociation energies describe the strength of chemical bonds. They can be determined experimentally. Enthalpy is the sum of bond dissociation energies for the reaction. = bonds = Exothermic = bonds = Endothermic EXAMPLE: Predict the sign and magnitude of ΔH⁰ for the following reaction. Give your answer in units of kilojoules per mole, and identify whether the reaction is expected to be endothermic or exothermic. PRACTICE: Predict the sign and magnitude of ΔH⁰ for the following reaction. Give your answer in units of kilojoules per mole, and identify whether the reaction is expected to be endothermic or exothermic. Page 9

10 CONCEPT: ENTROPY Even if a reaction is highly exothermic, the level of order it requires may make it statistically improbable. Entropy is the tendency of a system to take its most probable form. = More ordered = More disordered There are three phenomena that make reactions more probable. All are favored by 1. Increasing the Number of Molecules Reactions that create extra molecules are more probable since there are more ways to arrange them. 2. Phase Transition Transformation of solid to liquid or liquid to gas is more probable since the molecules will have a greater vibrational freedom. 3. Increasing Molecular Freedom of Motion Converting cyclic molecules to acyclic molecules are more probable since it increases freedom of rotation. Page 10

11 CONCEPT: RADICAL SELECTIVITY QUALITATIVE Selectivity is defined as the ability to only halogenate the carbons with most stable radical intermediates. Fluorination: Chlorination: Overall ΔH o = -432 Overall ΔH o = -101 No useful radical fluorination reactions. Call 911! The only useful radical chlorinations are reactions with a single type of - H Bromination: Iodination: Overall ΔH o = -26 Overall ΔH o = +53 The only useful method for selectively halogenating alkanes Not a spontaneous, don t even try it. Chiral Products are ALWAYS racemized Page 11

12 EXAMPLE: Draw the product of the following radical chlorinations. Would the following radical chlorinations be synthetically useful? (Yielding only one product). 1) 2) Page 12

13 EXAMPLE: Predict the following monobrominated products of the following radical brominations: a. b. Page 13

14 CONCEPT: RADICAL SELECTIVITY -- QUANTITATIVE Selectivity is defined as the ability to only halogenate the carbons with most stable radical intermediates. The Hammond Postulate explains why halogen radicals have differing selectivities. Radical Chlorination Radical Bromination Page 14

15 CONCEPT: THE HAMMOND-POSTULATE The Hammond-Postulate more accurately describes what transition states look like. Paraphrased version: Transition state that resembles reagents = Transition state that resembles products = EXAMPLE: Determine which species has higher energy in the following reactions. Identify the transition state as early or late, and correctly draw it. Page 15

16 CONCEPT: CALCULATING PERCENTAGE YIELDS We need equations to make quantitative predictions about the exact ratios of these products formed. Chlorination is non-selective, meaning that the difference between relative rates of halogenation is Bromination is highly selective, meaning that the difference between relative rates of halogenation is These ratios are only valid at room temperature. At higher temperature the ratios get EXAMPLE: Draw all of the monochlorination products of butane. Calculate the percentage yields of each product. Page 16

17 CONCEPT: CALCULATING PERCENTAGE YIELDS PRACTICE: Calculate the percentage yield of all monobromination products at room temperature. PRACTICE: Calculate the percentage yield of all monobromination products. Page 17

18 CONCEPT: ALLYLIC HALOGENATION Unlike carbocations, allylic / benzylic radicals are ALWAYS most stable Resonance plays a central role in the mechanisms of conjugated reactions (radical and carbocation intermediated). General Mechanism: Initiation: Propagation: Termination: Page 18

19 CONCEPT: ALLYLIC HALOGENATION Allylic Chlorination: Allylic Bromination: EXAMPLE: Predict the product(s) of the following reaction. Page 19

20 CONCEPT: RADICAL HYDROHALOGENATION Recall the hydrohalogenation mechanism: intermediate addition of bromine Compare to the radical mechanism that predominates in the presence of peroxide: intermediate addition of bromine Page 20

21 EXAMPLE: Provide the complete mechanism for the following radical hydrohalogenation. Page 21

22 EXAMPLE: Provide the MAJOR product for the following multi-step syntheses a. b. Page 22