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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.
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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. =>
=> * * D and L Assignments *
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.=>
Resolution of Enantiomers React a racemic mixture with a chiral compound to form diastereomers, which can be separated. =>
Organic Chemistry, 5th EditionL. G. Wade, Jr. Chapter 6Alkyl Halides: Nucleophilic Substitution and Elimination
=> Classes of Halides • Alkyl: 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:
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. =>
Classes of Alkyl Halides • Methyl halides: only one C, CH3X • Primary: 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. =>
=> Classify These:
=> Dihalides • Geminal dihalide: two halogen atoms are bonded to the same carbon • Vicinal dihalide: two halogen atoms are bonded • to adjacent carbons.
=> 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.
=> Systematic Common Names • Name as alkyl halide. • Useful only for small alkyl groups. • Name these:
“Trivial” Names • CH2X2 called methylene halide. . • CHX3 is a haloform. • CX4 is carbon tetrahalide. • Examples: • CH2Cl2 is methylene chloride • CHCl3 is chloroform -CHI3 is iodoform • CCl4 is carbon tetrachloride
Preparation of RX • Free radical halogenation (Chapter 4) - REVIEW • Free radical allylic halogenation • produces alkyl halide with double bond on the neighboring carbon. LATER =>
Substitution Reactions • The halogen atom on the alkyl halide is replaced with another group. • Since the halogen is more electronegative than • carbon, the C-X bond breaks heterolytically and • X- leaves. The group replacing X- is a nucleophile. =>
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, and • also loses H+ on the adjacent carbon to a base. • A pi bond is formed. Product is alkene. • Also called dehydrohalogenation (-HX).
Ingold Sir Christopher Father of Physical Organic Chemistry Coined such names and symbols as: SN1, SN2, E1, E2, nucleophile, electrophile resonance effect, inductive effect/ In print, he often attacked enemies vigorously and sometimes in vitrolic manner.
SN2 Mechanism • Rate is first order in each reactant • Both reactants are involved in RDS Note: one-step reaction with no intermediate • Bimolecular nuleophilic substitution. • Concerted reaction: new bond forming • and old bond breaking at same time INVERSION OF CONFIGURATION
SN2 Energy Diagram • One-step reaction. • Transition state is highest in energy. =>
Uses for SN2 Reactions • Synthesis of other classes of compounds. • Halogen exchange reaction. =>
=> SN2: Nucleophilic Strength • Stronger nucleophiles react faster. • Strong bases are strong nucleophiles, but not all strong nucleophiles are basic.
Trends in Nuc. Strength • Of a conjugate acid-base pair, the base is stronger: OH- > H2O, NH2- > NH3 • Decreases left to right on Periodic Table. More • electronegative atoms less likely to form new bond: • OH- > F-, NH3 > H2O • Increases down Periodic Table, as size and polarizability • increase: I- > Br- > Cl-
=> Bulky Nucleophiles Sterically hindered for attack on carbon, so weaker nucleophiles.
=> Solvent Effects (1) Polar protic solvents (O-H or N-H) reduce the strength of the nucleophile. Hydrogen bonds must be broken before nucleophile can attack the carbon.
Solvent Effects (2) • Polar aprotic solvents (no O-H or N-H) do not form hydrogen bonds with nucleophile • Examples:
Crown Ethers • Solvate the cation, so nucleophilic strength of the anion increases. Fluoride anion becomes a good nucleophile
=> SN2: Reactivity of Substrate • Carbon must be partially positive. • Must have a good leaving group • Carbon must not be sterically hindered.
=> Leaving Group Ability • Electron-withdrawing • Stable once it has left (not a strong base) • Polarizable to stabilize the transition state.
Structure of Substrate • Relative rates for SN2: CH3X > 1° > 2° >> 3° • Tertiary halides do not react via the SN2 mechanism, due to steric hindrance. =>
Effect of Beta Branching Isobutyl bromide Neopentyl bromide Propyl bromide NOTE: ALL ABOVE ARE PRIMARY
Miscellaneous Substrate All have sterically hindered backsides - No SN2 reactivity
Stereochemistry of SN2 Walden inversion =>
EXAMPLES S R
EXAMPLES 2 MESO
EXAMPLE 4 How would you prepare the following from an alkyl halide? retrosynthetic analysis
SN1 Reaction • Unimolecular nucleophilic substitution. • Two step reaction with carbocation intermediate. • Rate is first order in the alkyl halide, • zero order in the nucleophile. • Racemization occurs.
=> SN1 Mechanism (1) Formation of carbocation (slow)
=> SN1 Mechanism (2) • Nucleophilic attack • Loss of H+ (if needed)
SN1 Energy Diagram • Forming the carbocation is endothermic • Carbocation intermediate is in an energy well. =>
Rates of SN1 Reactions • 3° > 2° > 1° >> CH3X • Order follows stability of carbocations (opposite to SN2) • More stable ion requires less energy to form • Better leaving group, faster reaction (like SN2) Polar protic solvent best: It solvates ions strongly with hydrogen bonding
Stereochemistry of SN1 Racemization: inversion and retention =>
Rearrangements • Carbocations can rearrange to form a more stable carbocation. Hydride shift: H- on adjacent carbon bonds with C+. • Methyl shift: CH3- moves from adjacent carbon • if no H’s are available.
=> Hydride Shift
=> Methyl Shift
Interesting Rearrangement = PUSH PULL -AgBr push pull