Presentation on theme: "Fischer-Rosanoff Convention"— Presentation transcript:
1 Fischer-Rosanoff Convention Before 1951, only relative configurations could be known.Sugars and amino acids with same relative configuration as (+)-glyceraldehyde were assigned D and same as (-)-glyceraldehyde were assigned L.With X-ray crystallography, now know absolute configurations: D is (R) and L is (S).No relationship to dextro- or levorotatory =>
3 Properties of Diastereomers Diastereomers have different physical properties: m.p., b.p.They can be separated easily.Enantiomers differ only in reaction with other chiral molecules and the direction in which polarized light is rotated.Enantiomers are difficult to separate =>
4 Resolution of Enantiomers React a racemic mixture with a chiral compound to form diastereomers, which can be separated.=>
7 Chapter 6 Alkyl Halides: Nucleophilic Substitution and Elimination Organic Chemistry, 5th Edition L. G. Wade, Jr.Chapter 6 Alkyl Halides: Nucleophilic Substitution and Elimination
8 Classes of HalidesAlkyl: Halogen, X, is directly bonded to sp3 carbon.Vinyl: X is bonded to sp2 carbon of alkene.Aryl: X is bonded to sp2 carbon on benzene ring.Examples:=>
9 Polarity and Reactivity Halogens are more electronegative than C.Carbon-halogen bond is polar, so carbon has partial positive charge.Carbon can be attacked by a nucleophile.Halogen can leave with the electron pair =>
10 Classes of Alkyl Halides Methyl halides: only one C, CH3XPrimary: C to which X is bonded has only one C-C bond.Secondary: C to which X is bonded has two C-C bonds.Tertiary: C to which X is bonded has three C-C bonds =>
12 DihalidesGeminal dihalide: two halogen atoms are bonded to the same carbonVicinal dihalide: two halogen atoms are bondedto adjacent carbons.=>
13 IUPAC Nomenclature Name as haloalkane. Choose the longest carbon chain, even if the halogen is not bonded to any of those C’s.Use lowest possible numbers for position.=>
14 Systematic Common Names Name as alkyl halide.Useful only for small alkyl groups.Name these:=>
15 “Trivial” Names CHX3 is a haloform. CX4 is carbon tetrahalide. CH2X2 called methylene halide..CHX3 is a haloform.CX4 is carbon tetrahalide.Examples:CH2Cl2 is methylene chlorideCHCl3 is chloroform -CHI3 is iodoformCCl4 is carbon tetrachloride
16 Preparation of RX Free radical halogenation (Chapter 4) - REVIEW Free radical allylic halogenationproduces alkyl halide with double bond on the neighboring carbon. LATER =>
17 Substitution Reactions The halogen atom on the alkyl halide is replaced with another group.Since the halogen is more electronegative thancarbon, the C-X bond breaks heterolytically andX- leaves.The group replacing X- is a nucleophile. =>
18 Elimination Reactions The alkyl halide loses halogen as a halide ion, and also loses H+ on the adjacent carbon to a base.The alkyl halide loses halogen as a halide ion, andalso loses H+ on the adjacent carbon to a base.A pi bond is formed. Product is alkene.Also called dehydrohalogenation (-HX).
20 Father of Physical Organic Chemistry Sir ChristopherIngoldFather of Physical Organic ChemistryCoined such names and symbols as:SN1, SN2, E1, E2, nucleophile, electrophileresonance effect, inductive effect/In print, he often attacked enemies vigorously and sometimes in vitrolic manner.
21 SN2 Mechanism Both reactants are involved in RDS Rate is first order in each reactantBoth reactants are involved in RDSNote: one-step reaction with no intermediateBimolecular nuleophilic substitution.Concerted reaction: new bond formingand old bond breaking at same timeINVERSION OF CONFIGURATION
22 SN2 Energy Diagram One-step reaction. Transition state is highest in energy. =>
23 Uses for SN2 Reactions Synthesis of other classes of compounds. Halogen exchange reaction.=>
24 SN2: Nucleophilic Strength Stronger nucleophiles react faster.Strong bases are strong nucleophiles, but not all strong nucleophiles are basic.=>
25 Trends in Nuc. StrengthOf a conjugate acid-base pair, the base is stronger: OH- > H2O, NH2- > NH3Decreases left to right on Periodic Table. Moreelectronegative atoms less likely to form new bond:OH- > F-, NH3 > H2OIncreases down Periodic Table, as size and polarizabilityincrease: I- > Br- > Cl-
40 How would you prepare the following from an alkyl halide? EXAMPLE 4How would you prepare thefollowing from an alkyl halide?retrosyntheticanalysis
41 SN1 Reaction Two step reaction with carbocation intermediate. Unimolecular nucleophilic substitution.Two step reaction with carbocation intermediate.Rate is first order in the alkyl halide,zero order in the nucleophile.Racemization occurs.
42 Formation of carbocation (slow) SN1 Mechanism (1)Formation of carbocation (slow)=>
43 SN1 Mechanism (2)Nucleophilic attackLoss of H+ (if needed)=>
44 SN1 Energy Diagram Forming the carbocation is endothermic Carbocation intermediate is in an energy well =>
45 Rates of SN1 Reactions3° > 2° > 1° >> CH3XOrder follows stability of carbocations (opposite to SN2)More stable ion requires less energy to formBetter leaving group, faster reaction (like SN2)Polar protic solvent best: It solvates ions strongly with hydrogenbonding
46 Stereochemistry of SN1Racemization: inversion and retention=>
47 RearrangementsCarbocations can rearrange to form a more stable carbocation.Hydride shift: H- on adjacent carbon bonds with C+.Methyl shift: CH3- moves from adjacent carbonif no H’s are available.
51 SN2 or SN1? Primary or methyl Tertiary Weak nucleophile (may also be solvent)Strong nucleophilePolar protic solvent, Ag+Polar aprotic solventRate = k[halide]Rate = k[halide][Nuc]Inversion at chiral carbonRacemization of optically active compoundNo rearrangementsRearranged products
52 E1 Reaction Two groups lost (usually X- and H+) Unimolecular eliminationTwo groups lost (usually X- and H+)Nucleophile acts as base)Also have SN1 products (mixtureSN1 and E1 have common first step.