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24 Cards in this Set

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What are some physical properties of haloalkanes
-bond strength of C-X decreases as size of X increases b/c size of halogen p orbital increases as go from F to I resulting in e- around halogen becoming more diffuse. Increase in the C-X bond length but decrease in C-X bond strength
-C-X bond is polarized. e- density aong C-X bond is displaced in the direction of X giving partial neg charge on X and partial pos charge on C
-haloalkanes have higher boiling pt than corresponding alkane. Due to dipole-dipole interaction from coulombic attraction bwt partial neg X and partial pos C, also greater London interactions as increase size of X, and increase in molecular weight causing outer e- not being held very tightly around the nucleus
Polarization of an atom
degree to which it electron cloud is deformed under the influence of an external electric field.
-more polarizable an atom , the more effectively it will enter into London attractions, and the higher the b.p.
Nucleophilic substitution
-haloalkanes have electrophilic carbon atom that will react w/ nucleophiles
-nucleophile attackes reagent and replaces the halide
-changes functional group in molecule
-polar reaction b/c includes charged species and polarized bonds
-opposite charges attract
-electron pairs flow in synchronous manner (as one pair arrives at the closed shell, the other departs preventing violation of octet rule)
substrate, leaving group
substrate: organic starting material that's target of attack by nucleophile
leaving group: group displaced
reactive nucleophiles
HO-, CH3O-, I-, N---C-, CH3S-, NH3, P(CH3)3
Bimolecular nucleophilic substitution (Sn2)
-two reactants interact in a single step
-bond-making takes place at same time as bond-breaking
-rate = k[Nu][electrophile]
(second-order process)
-stereospecific
frontside displacement
-nucleophile approaches substrate from the same sid as the leaving group, one group exchanging for the other
-product has same configuration as SM
backside displacement
-nucleophile approaches C from side opposite the leaving group
-product has opposite configuration as SM
-Sn2 reactions proceed by backside displacement to give inversion of configuration
transistion state
-in either front or backside displacement neg charge is distributed over both nucleophile and leaving group
-as nucleophile approaches back lobe of the sp3 hybrid orbital used by C to bind the halogen, the rest of molecule becomes planar at the TS by changing the hybridization at C to sp2
-as rxn proceeds, the inversion motion is completed, and the C returns to the tetrahedral sp3 config
stereospecific
process whose mechanisms requires that each stereoisomer of the SM transform into a specific stereoisomer of product
double inversion
two Sn2 processess that give desired result of a net retention of configuration
Leaving group ability
-relative rate at which it can be displaced
-correlated w/ its capacity to hold a neg charge (I->Br->Cl->F-)
-weak bases (conjugate base of strong acids) are good leaving groups b/c can accomadate neg charge well
Common Leaving Groups
-I-, Br-, Ms (mesylate ion-has several resonance structures), Tf (triflate ion-best), Ts (tosylate ion-like Ms), occasionally Cl-
Nucleophilicity
depends on:
charge
basicity
solvent
polarizability
nature of substituents
Nucleophilicity: charge
increasing neg charge increases nucleophilicity
-OH > H2O
-more neg attacking species, the faster the rxn should be
Nucleophilicity: basicity
-in same row of the periodic table, usually the more basic the more nucleophilic
-basicity is thermodynamics measured by equilibrum constant while nucleophilicity is kinetics quantified by comparing rates of rxns
-despite differences observe correlation bwt basicity and nucleophilicity in charged species vs neutral speces along a row of the periodic table
Nucleophilicity: periodic trends
-from left to right of the periodic table, nucleophilicity decreases (involves basicity)
-nucleophility increases down the periodic table a trend that directly opposes that expected from the basicity of the nucleophile (involves solvation)
Nucleophilicity: solvation
-generally solvation weakens the nucleophile by forming a shell of solvent molecules around the nucleophile and impeding its ability to attack an electrophile
-smaller anions are more tightly solvated than are larger ones b/c their charge is more concentrated
Polar Protic solvents
-methanol, ethanol, and water
-H is attached to electronegative atom Y resulting in highly polarized H-Y bonds
-H has proton like character and can interact particularly strongly w/ anionic nucleophiles
Polar Aprotic solvents
-Sn2 rxns
-lack positively polarized H
-since dont form H bonds, these solvent molecules solvate anionmic nucleophiles relatively weakly resulting in raised reactivity of nucleophile
Nucleophilicity: polarizability
-increasing polarizability improves the nucleophilic power
-nucleophilicity increases down periodic table even for uncharged species
-larger elements have larger, more diffuse, and more polarizable e- clouds that allow for more effective orbital overlap in the Sn2 TS
-lower TS energy and faster nucleophilic substitution
Nucleophilicity: Sterics
-sterically hindered nucleophiles are poorer reagents
-hinderance is built in to nucleophile in form of bulky substituents
Reversible Nucleophilic substitutions
-Sn2 rxn is reversible w/ halide ions which are both good leaving groups and nucleophiles
-result correlates w/ relative stabilities of the product and SM
-nucleophile that is strong base is incapable of acting as leaving group so displacement is essentially irreversible process (Keq is very large)
Structure and Sn2 Reactivity: The Substrate
-branching at the reacting C decreases the rate of the rxn b/c methyl group gives rise to steric repulsion w/ incoming nucleophile raising the TS energy
Me-X > 1-X > 2-X >> 3-X (no)
-lengthen chain by one or two C's reduces Sn2 reactivity further chain elongation has no effect b/c added C's dont increase steric hinderance around reacting C in TS
-branching next to reacting C slows substitution, branching at positions farther from site of rxn has smaller effect