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Organic Chemistry – the Functional Group Approach Organic Chemistry Organic Chemistry – The Functional Group Approach OH Br alkane alcohol halide alkene (no F.G.) non-polar (grease, fats) polar (water soluble) non-polar (water insoluble) non-polar (water insoluble) tetrahedral tetrahedral tetrahedral trigonal O NH alkyne aromatic aldehyde/ketone imine non-polar (water insoluble) non-polar (water insoluble) polar (water soluble) polar (water soluble) linear flat trigonal trigonal YSU Organic Chemistry – The Functional Group Approach NH O HO OH H3CO OCH3 2 OH hydrate acetal amine carboxylic acid polar (water soluble) non-polar (water insoluble) polar (water soluble) polar(watersoluble) tetrahedral tetrahedral tetrahedral trigonal O O O O O OCH3 NH2 Cl O carboxylic ester carboxylic amide acyl halide acid anhydride polar (water-solube) polar (water soluble) non-polar (reacts w/water) non-polar (reacts w/water) trigonal trigonal trigonal trigonal YSU 1 Organic Chemistry – The Functional Group Approach OH Br alkane alcohol halide alkene (no F.G.) non-polar (grease, fats) polar (water soluble) non-polar (water insoluble) non-polar (water insoluble) tetrahedral tetrahedral tetrahedral trigonal O NH alkyne aromatic aldehyde/ketone imine non-polar (water insoluble) non-polar (water insoluble) polar (water soluble) polar (water soluble) linear flat trigonal trigonal YSU Organic Chemistry – The Functional Group Approach OH Br alkane alcohol halide alkene (no F.G.) non-polar (grease, fats) polar (water soluble) non-polar (water insoluble) non-polar (water insoluble) tetrahedral tetrahedral tetrahedral trigonal O NH alkyne aromatic aldehyde/ketone imine non-polar (water insoluble) non-polar (water insoluble) polar (water soluble) polar (water soluble) linear flat trigonal trigonal YSU 2 Carey Chapter 4 – Alcohols and Alkyl Halides Figure 4.2 – Electron density maps of CH3OH and CH3Cl YSU Alcohols and Halogens in Medicine and Nature OH OH Cl HN O2N Cl O Acetaminophen Valium Chloramphenicol YSU 3 4.2 IUPAC Nomenclature of Alkyl Halides • Functional class nomenclature pentyl chloride cyclohexyl bromide 1‐methylethyl iodide • Substitutive nomenclature 2‐bromopentane 3‐iodopropane 2‐chloro‐5‐methylheptane YSU 4.3 IUPAC Nomenclature for Alcohols 1‐pentanol 2‐propanol cyclohexanol 2‐pentanol 1‐methyl cyclohexanol 5‐methyl‐2‐heptanol YSU 4 4.4 Classes of Alcohols and Alkyl Halides Br Primary (1o) Cl OH OH I Secondary (2o) Cl CH3 Br Cl Tertiary (3o) (CH3)3COH CH2CH3 YSU 4.5 Bonding in Alcohols and Alkyl Halides Figure 4.1 YSU 5 4.5 Bonding in Alcohols and Alkyl Halides Figure 4.2 – Electron density maps of CH3OH and CH3Cl YSU 4.6 Physical Properties – Intermolecular Forces CH3CH2CH3 CH3CH2FCH3CH2OH propane fluoroethane ethanol b.p. ‐42 oC ‐32 oC78 oC YSU 6 4.6 Physical Properties – Intermolecular Forces Figure 4.4 YSU 4.6 Physical Properties – Intermolecular Forces Figure 4.4 YSU 7 4.6 Physical Properties – Water Solubility of Alcohols Alkyl halides are generally insoluble in water (useful in lab) YSU 4.6 Physical Properties – Water Solubility of Alcohols Solubility is a balance between polar and non‐polar characteristics YSU 8 4.6 Physical Properties – Water Insolubility Cholesterol –non‐polar alcohol Limited solubility in water Precipitates when to concentrated Results in gallstones Biochemistry involves a delicate balance of “like dissolves like” YSU 4.7 Preparation of Alkyl Halides from Alcohols and H-X R OH + H X R X + H O H alcohol hydrogen halide alkyl halide water Lab Conditions YSU 9 4.8 Mechanism of Alkyl Halide Formation Mechanism –a description of how bonds are formed and/or broken when converting starting materials (left hand side) to products (right hand side) Usually involves solvents and reagents, sometimes catalysts Curved arrows are used to describe the chemical changes YSU 4.8 Reaction of a Tertiary Alcohol with H-Cl Look for chemical changes –which bonds are formed or broken? learn the outcome of reaction in order to get going quickly recognize the nature of the organic substrate (1o, 2o, 3o?) be aware of the reaction conditions (acidic, basic, neutral?) YSU 10 4.8 Reaction of a Tertiary Alcohol with H-Cl YSU 4.8 Energetic description of mechanism - Step 1 : protonation Figure 4.6 YSU 11 4.8 Energetic description of mechanism - Step 2 : carbocation Figure 4.7 YSU 4.8 Energetic description of mechanism - Step 3 : trap cation Figure 4.9 YSU 12 4.9 Full mechanism “pushing” curved arrows H Cl H3C H3C H3C C O H H3C C Cl (+ H2O) H3C H3C H Cl Cl H3C H (- H2O) CH3 H3C C O H C H C CH H3C 3 3 Cl YSU 4.9 Full SN1 mechanism showing energy changes Figure 4.11 YSU 13 4.10 Carbocation structure and stability Figure 4.8 YSU 4.10 Carbocation structure and stability Hyperconjugation –the donation of electron density Figure 4.15 from adjacent single bonds YSU 14 4.10 Relative carbocation stability Figure 4.12 YSU 4.11 Relative rates of reaction of R3COH with HX Related to the stability of the intermediate carbocation: YSU 15 4.11 Relative rates of reaction of R3COH with HX Figure 4.16 Rate‐determining step involves formation of carbocation YSU 4.12 Reaction of methyl- and 1o alcohols with HX – SN2 Same bonds are formed and broken as in 3o case, however; o CH3 and 1 carbon cannot generate a stabilized carbocation kinetic studies show the rate‐determining step is bimolecular sequence of bond‐forming/breaking events must be different YSU 16 4.12 Reaction of methyl- and 1o alcohols with HX – SN2 Alternative pathway for alcohols that cannot form a good carbocation YSU 4.12 Geometry changes during SN2 http://www.bluffton.edu/~bergerd/classes/cem221/sn‐e/SN2.gif YSU 17 4.12 Energy profile for SN2 reaction YSU 4.13 Other methods for converting ROH to RX Cl SOCl2 OH PBr3 Br Convenient way to halogenate a 1o or 2o alcohol Avoids use of strong acids such as HCl or HBr o Via SN2 mechanism at 1 and CH3 groups YSU 18 4.14 Free Radical Halogenation of Alkanes heterolytic Possible modes of bond cleavage homolytic YSU 4.15 Free Radical Chlorination of Methane CH4 + Cl2 CH3Cl +HCl (~400oC) CH3Cl + Cl2 CH2Cl2 +HCl (~400oC) CH2Cl2 + Cl2 CHCl3 +HCl (~400oC) CHCl3 + Cl2 CCl4 +HCl (~400oC) YSU 19 4.16 Structure and stability of Free Radicals Figure 4.17 –Bonding models for methyl radical YSU 4.16 Structure and stability of Free Radicals Free radical stability mirrors that of carbocations Hyperconjugation is the main factor in stability Experimental evidence that radicals are flat (sp2) YSU 20 4.16 Bond Dissociation Energies (BDE) YSU 4.16 Bond Dissociation Energies (BDE) 104 58 83.5 103 YSU 21 4.17 Mechanism for Free Radical Chlorination of Methane YSU 4.17 Mechanism for Free Radical Chlorination of Methane YSU 22 4.17 Mechanism for Free Radical Chlorination of Methane YSU 4.17 Mechanism for Free Radical Chlorination of Methane YSU 23 4.18 Free Radical Halogenation of Higher Alkanes YSU 4.18 Free Radical Halogenation of Higher Alkanes Radical abstraction of H is selective since the stability of the ensuing radical is reflected in the transition state achieved during abstraction. Cl H CH2CH2CH2CH3 Cl H CHCH2CH3 CH3 Lower energy radical, formed faster YSU 24 4.18 Free Radical Halogenation of Higher Alkanes Figure 4.16 YSU 4.18 Bromine radical is more selective than chlorine radical Consider propagation steps – endothermic with Br∙, exothermic with Cl∙ YSU 25 4.18 Bromine radical is more selective than chlorine radical Bromination – late TS looks a Chlorination – early TS looks lot like radical less like radical YSU 26.
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