Organic Chem 1 Exam 2 Study Guide
Organic Chem 1 Exam 2 Study Guide CHEM 2323
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This 3 page Study Guide was uploaded by Malvi Mehta on Friday February 12, 2016. The Study Guide belongs to CHEM 2323 at University of Texas at Dallas taught by in Spring 2016. Since its upload, it has received 37 views. For similar materials see Introductory Organic Chemistry I in Chemistry at University of Texas at Dallas.
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Date Created: 02/12/16
Introductory Organic Chemistry I – Exam 2 Study Guide (Chapters 4 and 6) Chapter 4 Material Free radicals: reactive species with ODD numbers of electrons (unpaired electrons) Mechanism for a free radical chain reaction: o Initiation: generates a radical intermediate o Propagation: the intermediate reacts with a stable molecule to produce another radical as well as a product o Termination: side reactions that destroy the reactive intermediate and result in no free radicals; only product remains Bond Dissociation Enthalpy/Energy o Bond dissociation requires energy (+) o Bond formation releases energy () o In an overall reaction, total bond dissociation energy=(sum of bonds broken) – (sum of bonds formed) Energy Level Diagrams o Highest points on diagrams: transition states o Dips between transition states: intermediates o Ea or “activation energy” is the rate determining step! Selectivity in halogenation reactions o Primary hydrogens: C is bonded to 3 Hydrogens and 1 R group o Secondary hydrogens: C is bonded to 2 Hydrogens and 2 R groups o Tertiary hydrogens: C is bonded to 1 Hydrogen and 3 R groups o Chlorination – not very selective Cl will replace any type of hydrogen (primary, secondary, or tertiary) Exothermic process Lower difference in activation energy o Bromination – very selective! *NBS=bromination* Br will replace only the highest level hydrogen present in the mol ecule (tertiary>secondary>primary) Endothermic process Higher difference in activation energy Carbocations: a positively charged carbon o More highly substituated carbocations are MORE stable! o Also resonance makes carbocations more stable Carboanions: a negatively charged carbon o More highly substituated carboanions are LESS stable! o Also resonance makes carboanions more stable Allylic Hydrogens: hydrogens that are attached to a carbon which is attached to a double bond. *these are in the MOST stable positions and can often form resonance. Chapter 6 Material Review of Substitution and Elimination Reactions: SN2: bimolecular, nucleophilic substitution reactions o One step reaction (concerted reaction) o Stronger nucleophiles react faster o SN2 reactions proceed faster in polar APROTIC solvents Ex. Acetone, DMF o Reacts fastest with methyl halides (CH3X), then primary halides, then secondary halides Tertiary halides will never react SN2 o Result in a “walden inversion” of stereochem! (wedge turns into dash, and vice versa for leaving group position) SN1: unimolecular, nucleophilic substitution reactions o Two step reaction – first step is formation of a carbocation intermediate o Does not necessarily need a strong nucleophile o SN1 reactions proceed faster in polar PROTIC solvents Ex. Alcohols o Reacts fastest with tertiary halides, then secondary, primary, methyl o Results in rearrangements such as hydride or methyl SHIFTS in order to form a more stable carbocation with a greater # of substituents E2: bimolecular elimination reaction o Produces double bonds o Stronger bases react faster o Reacts fastest with tertiary halides o One step reaction (concerted) o Specific stereochemistry: halide and abstracted proton must be anti coplanar! E1: unimolecular elimination reaction o Produces double bonds o Does not necessarily need a strong base o Reacts fastest with tertiary halides o Rearranged products to form most stable form o Two step reaction – first step is formation of a carbocation intermediate *for elimination reactions, the more substituated double bonds are more stable! *make sure to review strong and weak nucleophiles and bases!
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