Chem 14D Hardinger S 2012 Fall Weeks 0 4 Midterm 1 Study Guide Lectures 1 7 Ionic Substitution Reactions SN2 Electron donor is the nucleophile Proton donor is the electrophile Reaction mechanisms a step by step accounting of bond changes in a reaction Substitution reaction a reaction in which any part of a molecule is replaced Leaving group an atom or group which breaks away from the rest of the molecule taking with it the electron pair which used to be the carbon leaving group bond Nucleophilic substitution bimolecular SN2 mechanism substitution at sp3 carbon when nucleophile carbon bond formation is simultaneous with leaving group carbon bond scission Bond making and breaking is at the same time Moment of collision o reactants must have sufficient energy and correct orientation o transitory period 5 atoms around carbon does not mean 10 electrons take into consideration partial pi bonds Reactions will always have back side attack no exceptions o Nucleophile finds where orbital is largest comes in from the back Inversion only occurs at the carbon bearing the leaving group Transition state highest energy points b n reactants and reaction mechanism step Increasing activation energy decreases reaction rate Increasing temperature increases reaction rate Structural factors that influence nucleophilicity o resonance atomic radius electronegativity inductive effects o better nucleophile will not be delocalized by resonance o resonance usually decreases nucleophilicity o only radius of atom providing electron pair to carbon matters o transfer of charge through a chain of atoms in a molecule by electrostatic induction What makes a good leaving group o Opposite role of the nucleophile These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Steric effect any effect on the molecule or reaction due to the size of atoms or groups Protic solvent solvent that is a hydrogen bond donor Aprotic solvent solvent that is not a hydrogen bond donor SN2 checklist o Nucleophile must be moderate better o Leaving group must be moderate better o Steric hindrance carbon bearing LG can t be tertiary o Solvent moderately polar aprotic Protic can be acceptable Nonpolar solvents rarely useful Carbocations A contraction of a carbon atom A structure w a positive formal charge on carbon due to an open octet Intermediate a species which is not starting material product or transition state Carbocations are common intermediates in organic mechanisms Structure of a carbocation o three attachments on carbon o trigonal planar o carbon is sp2 In general delocalizing the carboncation positive charge stabilizes the carbocation Secondary with resonance more stable than tertiary without resonance The Carbocation Fates o Capture a nucleophile Carbocation desires electron pair to fill octet Electron pair can be provided by a nucleophile o Be deprotonated form a pi bond Electron pair can be provided by an adjacent sigma bond o Carbocation rearrangement Electron pair can be provided by an adjacent sigma bond Carbocation desires electron pair to fill octet Carbocations are desperate and not fussy Ionic Substitution Reactions SN1 Every time you see a carbocation think of the three carbocation fates Compare our new mechanism with SN2 by starting with kinetics C LG bond ionization rate determining step Rate expression k R3C LG SN2 substitution at sp3 carbon in which C Nuc and C LG bonds change simultaneously no carbocation is formed SN1 substitution at sp3 carbon in which C LG bond is broken to give a carbocation intermediate before C Nuc bond is formed SN1 gives a mixture of inversion and retention SN2 always gives inversion A better nucleophile makes an SN1 reaction it has NO EFFECT A better leaving group makes an SN1 reaction faster More stable carbocations lower gibbs free energy faster SN1 reaction Solvolysis reaction in which solvent is a reaction and becomes part of a reactant product SN1 checklist o Leaving group moderate or better o Carbocation intermediate stability must be primary w resonance or better o Solvent must be polar protic usually preferred aprotic is ok o Nucleophile is irrelevant Elimination Reactions Elimination reaction a reaction in which a molecule loses atoms or groups from adjacent atoms resulting in a new pi bond Cis vs trans E vs Z Trans alkene isomer more stable than cis alkene isomer Internal alkenes are more stable than terminal alkenes Conjugation increases alkene stability Bonds to sp2 carbons are stronger Increasing stability due to degree of substitution Decreasing stability due to strain For alkene stability degree of substitution outweighs steric strain unless strain is severe Major elimination product more stable alkene Zaitsev s Rule E2 requires periplanar H C C LG Exceptions to Zaitsev s Rule o Increased steric hindrance at business end of base favors less substituted alkene o Hoffman elimination occurs when LG NR3 SR3 The E2 checklist o Base usually strong o Leaving group moderate or better o Molecular geometry H C and C LG periplanar Consider SN2 E2 no carbocation before SN1 E1 have carbocation intermediates