Organic Chemistry – The Functional Group Approach Br OH 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 aldehyde/ketone alkyne aromatic imine non-polar (water insoluble) non-polar (water insoluble) polar (water soluble) polar (water soluble) YSU YSU linear flat trigonal trigonal Organic Chemistry – The Functional Group Approach NH 2 O HO OH H 3 CO OCH 3 OH hydrate acetal amine carboxylic acid polar (water soluble) non-polar (water insoluble) polar (water soluble) polar (water soluble) tetrahedral tetrahedral tetrahedral trigonal O O O O O OCH 3 NH 2 Cl O carboxylic ester carboxylic amide acyl halide acid anhydride polar (water-solube) non-polar (reacts w/water) polar (water soluble) non-polar (reacts w/water) YSU trigonal YSU trigonal trigonal trigonal 1
Organic Chemistry – The Functional Group Approach Br OH 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 aldehyde/ketone alkyne aromatic imine non-polar (water insoluble) non-polar (water insoluble) polar (water soluble) polar (water soluble) YSU YSU linear flat trigonal trigonal Organic Chemistry – The Functional Group Approach Br OH 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 aldehyde/ketone alkyne aromatic imine non-polar (water insoluble) non-polar (water insoluble) polar (water soluble) polar (water soluble) YSU trigonal YSU linear flat trigonal 2
Carey Chapter 4 – Alcohols and Alkyl Halides Figure 4.2 – Electron density maps of CH 3 OH and CH 3 Cl YSU YSU Alcohols and Halogens in Medicine and Nature OH OH Cl HN O 2 N Cl O Acetaminophen Valium Chloramphenicol YSU 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 YSU 4.3 IUPAC Nomenclature for Alcohols 1 ‐ pentanol 2 ‐ propanol cyclohexanol 5 ‐ methyl ‐ 2 ‐ heptanol 2 ‐ pentanol 1 ‐ methyl cyclohexanol YSU YSU 4
4.4 Classes of Alcohols and Alkyl Halides Br Primary (1 o ) Cl OH OH I Cl Secondary (2 o ) CH 3 Br Cl (CH 3 ) 3 COH Tertiary (3 o ) CH 2 CH 3 YSU YSU 4.5 Bonding in Alcohols and Alkyl Halides Figure 4.1 YSU YSU 5
4.5 Bonding in Alcohols and Alkyl Halides Figure 4.2 – Electron density maps of CH 3 OH and CH 3 Cl YSU YSU 4.6 Physical Properties – Intermolecular Forces CH 3 CH 2 CH 3 CH 3 CH 2 F CH 3 CH 2 OH propane fluoroethane ethanol b.p. ‐ 42 o C ‐ 32 o C 78 o C YSU YSU 6
4.6 Physical Properties – Intermolecular Forces Figure 4.4 YSU YSU 4.6 Physical Properties – Intermolecular Forces Figure 4.4 YSU YSU 7
4.6 Physical Properties – Water Solubility of Alcohols Alkyl halides are generally insoluble in water (useful in lab) YSU YSU 4.6 Physical Properties – Water Solubility of Alcohols Solubility is a balance between polar and non ‐ polar characteristics YSU 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 YSU 4.7 Preparation of Alkyl Halides from Alcohols and H-X R OH + H X R X + H O H water alcohol hydrogen halide alkyl halide Lab Conditions YSU 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 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 (1 o , 2 o , 3 o ?) be aware of the reaction conditions (acidic, basic, neutral?) YSU YSU 10
4.8 Reaction of a Tertiary Alcohol with H-Cl YSU YSU 4.8 Energetic description of mechanism - Step 1 : protonation Figure 4.6 YSU YSU 11
4.8 Energetic description of mechanism - Step 2 : carbocation Figure 4.7 YSU YSU 4.8 Energetic description of mechanism - Step 3 : trap cation Figure 4.9 YSU YSU 12
4.9 Full mechanism “pushing” curved arrows H Cl H 3 C H 3 C O H H 3 C C H 3 C C Cl (+ H 2 O ) H 3 C H 3 C H Cl Cl H 3 C H (- H 2 O ) CH 3 H 3 C C O H C H 3 C CH 3 H 3 C Cl YSU YSU 4.9 Full S N 1 mechanism showing energy changes Figure 4.11 YSU YSU 13
4.10 Carbocation structure and stability Figure 4.8 YSU YSU 4.10 Carbocation structure and stability Hyperconjugation – the donation of electron density Figure 4.15 from adjacent single bonds YSU YSU 14
4.10 Relative carbocation stability Figure 4.12 YSU YSU 4.11 Relative rates of reaction of R 3 COH with HX Related to the stability of the intermediate carbocation: YSU YSU 15
4.11 Relative rates of reaction of R 3 COH with HX Figure 4.16 YSU Rate ‐ determining step involves formation of carbocation YSU 4.12 Reaction of methyl- and 1 o alcohols with HX – SN2 Same bonds are formed and broken as in 3 o case, however; CH 3 and 1 o 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 YSU 16
4.12 Reaction of methyl- and 1 o alcohols with HX – SN2 Alternative pathway for alcohols that cannot form a good carbocation YSU YSU 4.12 Geometry changes during SN2 http://www.bluffton.edu/~bergerd/classes/cem221/sn ‐ e/SN2.gif YSU YSU 17
4.12 Energy profile for SN2 reaction YSU YSU 4.13 Other methods for converting ROH to RX Cl SOCl 2 OH PBr 3 Br Convenient way to halogenate a 1 o or 2 o alcohol Avoids use of strong acids such as HCl or HBr Via S N 2 mechanism at 1 o and CH 3 groups YSU YSU 18
4.14 Free Radical Halogenation of Alkanes heterolytic Possible modes of bond cleavage homolytic YSU YSU 4.15 Free Radical Chlorination of Methane CH 4 + Cl 2 CH 3 Cl + HCl (~400 o C) CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl (~400 o C) CH 2 Cl 2 + Cl 2 CHCl 3 + HCl (~400 o C) CHCl 3 + Cl 2 CCl 4 + HCl (~400 o C) YSU YSU 19
4.16 Structure and stability of Free Radicals Figure 4.17 – Bonding models for methyl radical YSU 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 (sp 2 ) YSU YSU 20
4.16 Bond Dissociation Energies (BDE) YSU YSU 4.16 Bond Dissociation Energies (BDE) 104 58 83.5 103 YSU YSU 21
4.17 Mechanism for Free Radical Chlorination of Methane YSU YSU 4.17 Mechanism for Free Radical Chlorination of Methane YSU YSU 22
4.17 Mechanism for Free Radical Chlorination of Methane YSU YSU 4.17 Mechanism for Free Radical Chlorination of Methane YSU YSU 23
4.18 Free Radical Halogenation of Higher Alkanes YSU 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 CH 2 CH 2 CH 2 CH 3 Cl H CHCH 2 CH 3 CH 3 Lower energy radical, formed faster YSU YSU YSU YSU 24
4.18 Free Radical Halogenation of Higher Alkanes Figure 4.16 YSU YSU 4.18 Bromine radical is more selective than chlorine radical Consider propagation steps – endothermic with Br ∙ , exothermic with Cl ∙ YSU 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 YSU 26
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