CHM 234: Radical Reaction and Alkynes (part 1). Week 1
CHM 234: Radical Reaction and Alkynes (part 1). Week 1 CHM 234
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This 7 page Class Notes was uploaded by Alvin Notetaker on Monday January 18, 2016. The Class Notes belongs to CHM 234 at Arizona State University taught by Pillai in Fall 2015. Since its upload, it has received 48 views. For similar materials see General Organic Chemistry II in Chemistry at Arizona State University.
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Date Created: 01/18/16
Radical Reaction: I.ntroduction to Radicals: There are two types of bond cleavage: Heterolytic bond cleavage andHomolytic bond cleavage. Heterolytic bond cleavage (forms ion) Homolytic bond cleavage (forms radicals) Radical stabilityincrease when less hydrogen is bonded to the radicalized carbon. (This means that the radical prefers tertiary the most and methyl the least.) II.Stereochemistry of Halogenation: On the radicalized carbon, the halogen abstraction will bond on either side of the the carbon with equal proportion. (Radical takes the place of the hydrogen on the most stabilize carbon) III.Allylic Bromination: C=C present will change the regioselectivity of the halogenation reaction. The radical will be 2 placed on the carbon next to the C(sp). This is called allylic positio (arrow). The radicalized molecule will be stabilized by resonance. NBS can be use to avoid halogenation addition reaction IV. Addition of HBr in the Presence of Peroxide: AntiMarkovnikov Addition Mechanism: Initial Steps: Propagation Steps: (The radical is on a secondary radical carbon (stable)) Termination: Thermodynamic Considerations HBr in presence of peroxide s spontaneous HCl and HI are not generally ot spontaneous so, Only HBr obeys antiMarkovnikov. Alkynes (part 1): I. Structure and Properties: Carbons are sp hybridized. Consist of1 sigma bond and 2 pi bonds. Sigma takes the orbitalspsp Pi takes the orbitapp Have a high character II. Nomenclature 1. Find the parent carbon chain 2. Start the number at the nearest triple bond 3. Parent chain ends with yne. Pent2yne , 2pentyne but1yne , 1butyne Functional groups have higher priority than alkynes excepether and halides. 5bromo2methylhept3yne , 5bromo2methyl3heptyne If there are two triple bonds, it calldiyne and for a three triple bontriyne. 6methylhept1,4diyne With double and triple bond presence, whichever comes first near the end will start the numbering. Both double bond and triple bond have equal priority, but double goes first if there is a tie. By the rule, ene goes first then yne by the alphabetization. The second “e” in ene drops out when writing the name. hept1en5yne hex1en5yne Common Names Terminal Alkyne Internal Alkyne III. Acidity Alkynes are more acidic because of the electron density is closer to the nucleus than the alkane and alkene which provides more stability Deprotonation of alkynes needs to be greater than pKa = 25. Amide Ion (NH 2) Terminal Alkynes can be deprotonated by a strong base like amide. (Note: equilibrium favors weak acid and weak base.) Sodium amide is used to get acetylide salt. Hydroxide ion (−OH) and Alkoxide ion (−OR) These two base are not strong enough to deprotonate the alkyne. IV. Preparation of Alkynes Prepared through dehydrohalogenation elimination reaction (E2) 1. Eliminate 2 HX molecules 2. Strong base is needed for the second step, vinyl halide, (sp hybridization present) because it is less reactive than thalkyl halide (sp hybridization present) These compounds is called g eminal dihalide or vicinal.
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