part two of Karty
part two of Karty CHEM 1200
Popular in Chemistry II
Popular in Chemistry
This 4 page Class Notes was uploaded by Alexi Martin on Friday May 6, 2016. The Class Notes belongs to CHEM 1200 at Rensselaer Polytechnic Institute taught by Dr. Alexander Ma in Spring 2016. Since its upload, it has received 18 views. For similar materials see Chemistry II in Chemistry at Rensselaer Polytechnic Institute.
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Date Created: 05/06/16
For exact examples, see powerpoint Multistep mechanisms More than 1 elementary step Form reaction intermediaries Need to find rate determining step Sn1 Reaction Unimolecular nucleophilic substitution Carbocation intermediate rate=k[alkyl halide] Racemization occurs 1. Formation of carbocation 2. Attack of the nucleophile Free energy diagram Two humps Carbocation is endothermic Rate SN reaction stability 3 C>2C>1 C>CH3 More stable requires less energy to form Better leaving group increases the rate of reaction Carbocations sp2 hybridized trigonal planar, lobes of empty p orbital both sides of the trigonal plane Nucleophile attacks either side Unimolecular elimination (E1) 2 groups lost halide and H Nucleophile acts as a base E1 and SN1 reactions same conditions mixture of products will be obtained E1 and SN1 in competition with each other E1 competes with SN1 Substitution results from nucleophilic attack on carbocation Ethanol serves as a base in elimination and as a nucleophile in substitution A carbocation React with own leaving group to return to a reactant React with nucleophile to form substitute product (SN1) Lose a proton to form an elimination product (E1) Rearrange to form a more stable cation SN1(Inversion only) and E2 are stereospecific racemic mixture of SN1 and E1 mixture stereoisomers carbocation rearrangements faster than other elementary steps with the right orientationthey compete Proton Transfer proton transfer fast acidic conditions no strong base, strong base conditions no strong acid Internal proton transfer not favorable intermediate where N+ and O internal proton transfer prohibited solvent mediated proton transfer 1 Molecularity Termolecular steps are unreasonable Bimolecular and unimolecular are reasonable (three reactants are not possible) Tautomerization reactions Proceeds slowly, solvent mediated Under basic conditions fast, base abstracts proton in first step Acidic conditions quickly acidic proton attacks carbonyl O 1st step with abstraction of proton by water Esterification 6 step mechanisms 1. Proton transfer 2. Nucleophilic addition 3. Proton transfer 4. Proton transfer 5 nucleophilic elimination 6 proton transfer Look at chart that reviews mechanisms Predict competing reactions SN2 SN1 E2 and E1 compete substrate with leaving group attacking species nucleophiles SN2 & SN1 bases E1 and E2 attacking species can have dual character Dual characteristics of attacking species Base has an atom with partial or full charge and lone pair electrons, forms a bond with H+ Nucleophile atom with partial or full negative charge and a lone pair electron, form a bond with carbocation or C atom Predominate mechanism all four reactions are viable, one predominates or favorable with adjustable conditions formation or major product depends on predominant mechanisms five factors:strength of attacking species, concentration, stability of leaving group, types of C atoms, solvent effects Rate determining step Elementary step that is slower than other steps SN1 and E1 1st step= departure of leaving group Slow and endothermic process 1st step is the rate determining step SN2 nucleophile forces off the leaving group by direct displacement (backside) SN1 nucleophile waits for leaving group to depart E2 base forces off leaving group by pulling off adjacent proton E1 base waits for leaving group to depart Sn2 bimolecular single step substitution Sn1 unimolecular multistep substitution E2 bimolecular single step (stereospecificanti coplanar) elimination E1 unimolecular multistep elimination Rate Sn2&E2 2 Sn2 rate=k[Nu:][RL] E2 rate=k[base:][RL] Requires meeting one collision of the nucleophile/base with substrate increases in [Nu:]/[base:] pr [RL] increases the rate of the reaction I.Strength of the attacking species (H adjacent C atoms) nucleophilicity vs basicity nucleophiles attack C bases beta protons nucleophilesstronger the nucleophile the faster of the Sn2 Reaction strength nucleophile little to no effect on rate of Sn1 , no nucleophile competing reactions,strong nucleophiles favor SN2 weak nucleophiles favor SN1 strong nucleophiles have full negative charge and weak nucleophiles are neutral SN2 nucleophilic strength stronger the nucleophile reacts faster strong base=strong nucleophile but not all strong nucleophiles are basic Basicity vs. nucleophilicity basicity defined by equilibrium constant for abstracting proton nucleophilicity is defined by rate of attack on the electrophilic C steric hinderance(bulkiness) hinders nucleophilicity more than it hinder basicity Trends in Nucleophilicity Charged nucleophile stronger than neutral counterpart: OH >H2O SH>H2S NH2>NH3 Nucleophilicity decreases across a period NH2>OH>F NH3>H2O CH3CH2CH3Ch2P>CH2Ch3S Increases down a column and polarizability increases I>Br>Cl>F SeH>SH>OH Ch3Ch2Ch3Ch2P<Ch3Ch2S Strength in attacking species (elimination reactions) Bases Stronger the base the faster the rate of E2 Strength of bases does not affect the rate of E1 Strong bases favor E2 and weak bases favor E1 Elimination strong bases strong as or stronger than HO II.Concentration Increase in concentration of nucleophile favor Sn2 reaction over Sn1 Decrease concentration nucleophile SN1>SN2 Increase concentration base E2>E1 Decrease concentration of base E1>E2 Weak nucleophile Sn1 regardless Weak base E1 regardless Stability of leaving group More stable faster reaction Good leaving group favor Sn1>Sn2 , E1>E1 Improving on leaving group can favor SN1 and E1 Leaving group stabilizes Best leaving groups 3 E withdrawing polarize C atom Stable C not strong base once they have left Polarizable to stabilize transition states (see powerpoint graphic) Type of C atom (substrate) C bonds to leaving group Methyl, primary,secondary,tertiary More alkyl groups attack to C that has the leaving group, slower Sn2 rate Methyl and primary C favor Sn2 3 disfavor 2 neutral Reactions Sn2 rate Ch3X>1>2>>3 3 halides do not react via Sn2 due to steric hinderance Steric effects of SN2 reactions nucleophile approaches from backside overlap back lobe of CX sp3 orbital Type of C atom (substrate) unlike Sn2, E2 take place with all 3 types of C rate of Sn1 and E1 increases as stability of carbocation increases 3 favors Sn1 and E1 2 neutral methyl and 1 disfavor Sn1 and E1 Benzylic and allylic effects substrate favor Sn1 and E1 Solvent effects protic solvents favor Sn1 and E1 cation H bond donor (water and OH) protic solvents like to attach themselves to nucleophiles and bases weaker attacking species substantially aprotic solvents favor Sn2 and E2 (DMSO acetone DMF) DO not weaken attacking species substantially Protic Polar have acidic H )OH or NH) solvated nucleophile decreases nucleophilicity Nucleophilicity in protic solvents increase as the size of the atom increases Example 1: Br increasing molarity SN2 not E2 Decreasing CH3OH SN1 and E1 Increasing OH molarity SN2 and E2 Aprotic solvents polar aprotic solvents do not have acidic protons and cannot form water some are acetonitrile DMF acetone and DMSO SN2 occurs faster 4
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