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CHE 8A: Week 8 Notes

by: Mackenzie Hayes

CHE 8A: Week 8 Notes CHE 8A

Mackenzie Hayes

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These notes cover every bit of lecture with diagrams and snipits from the lecture notes
organic chemistry - brief
Sarah Lievens
Class Notes
organic, Chemistry
25 ?




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This 6 page Class Notes was uploaded by Mackenzie Hayes on Thursday May 19, 2016. The Class Notes belongs to CHE 8A at University of California - Davis taught by Sarah Lievens in Summer 2015. Since its upload, it has received 7 views. For similar materials see organic chemistry - brief in Biological Sciences at University of California - Davis.

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Date Created: 05/19/16
CHE 8A: Week 8 Notes ● Mechanisms​ → show flow of e-during a reaction ○ Hydrogen looks for another source of electrons b/c bromine is being a jerk and stealing them all, then bromine steals them again ■ End up with carbon plus (super unstable) ○ Carbocation has 6 e- (E+Lewis acid) +​ ■ Adding E​ to alkene to pi bond ○ Bromine donates electrons back to carbon - ​ ■ Adding Nu​ to carbocation ● ^entire process → E​lectrophilic Addition(to an alkene) ○ Works because sigma bond is stronger ○ Pi is weak and accessible ○ Forms 2 new sigma (stronger) bonds ● Can also look atreaction coordinates ○ Show changes in energy over reaction process ○ Transition states → maximum E points → bonds actually break / form ■ Last TINY amounts of time ■ Arrows between structures ○ Intermediate states → last for measurable amounts of time ■ → sometimes we can stop here and look at them ■ Actual drawing of molecules ○ Reactants: alkene and HBr ■ Will get a high energy at some point in graph ● Peak = transition state ● Point where some bonds are still dotted lines ■ Intermediate state: energy still high but still way lower than transition ● Happier because it knows where its bond is ■ Seconds peak is another transition state ● Bromine is deciding what to do ■ Drops VERY low for products: haloalkane ○ Overall exothermic reaction: ■ Can measure​ ΔH → enthalpy ● Δin internal E at a constant pressure or reaction ● ΔH =/= 0​ → reaction will tend to give products ○ Exothermic ● If ​H ~ 0​→ a reaction is ​quilibrium ■ Rate determining activation energy step ● Determines how fast the reaction will occur ● Point from point before to middle of peak ■ Exothermic ΔH ● Determines how much product → thermodynamics ● Not much we can do about ΔH / enthalpy → works spontaneously or it won’t ○ BUT we can affect the rate of the reaction ■ Can increase energy (add heat) ● More E for molecules (more likely to react when collide) ■ Add more stuff → more collisions between molecules ● More chances to react ● Can often speed up reactions by changing mechanisms ○ Add catalyst → affect a mechanism but it’s the same at start + end of rxn ■ Changes the steps → changes activation energy ■ Changes rate determining step ○ Add H​ but no reaction 2​ ■ Hard to break up H​2​ super high activation energy ○ Metal surfaces act as catalysts ■ Break the hydrogen bond so can react with alkenes ● Without H​ +or H​or H​(radical) ● Hydrogenation → ​ adding H​2​o pi bonds ○ Stereo-selective → only forms particular diastereomers ○ Achiral → H add on same side (syn addition) ○ Get a racemic buy ONLY CIS ● Can use this to talk about alkene stability ○ More exothermic ΔH → less stable starting molecule ○ Get exactly the same thing for all of them ■ The ΔH changes based on how exothermic it is ○ More substituents → more alkyl groups → more stable alkene ■ Neighbors help to be more stable so want more ○ Trans → more stable than cis ■ Cis has steric hindrance → trans doesn’t ● Which is easier to form? ○ One w/ more alkyl groups ○ Trans easier than cis ● Can add stuff that is not H​to alkenes → use electrophilic addition processes 2​ ○ Can add mineral acids HCl, HBr, HI (strong, nucleophilic acids) ■ NOT HF (weak) ○ What happens w/ larger molecules who get H? ■ Who gets Br/Cl/I? ■ Nu​ -goes to more substituted side of the alkene ● Markovnikov’s rule → a type of regioselectivity ○ (where do bonds form?) ■ 1​o​carbocation o​ ■ 3​ carbocation ● Alkyl groups don’t just stabilize alkenes - inductive donors → stabilize lots of things ○ Carbocations, e- donors by induction o​ o ​ o​ ○ 1​ < 2​< 3​→ more stable ○ More stable = easier to form = lower activation E (5/18) ● Electrophilic addition - form the more stable carbocation ○ Nucleophile goes to the more substituted side (Markovnikov’s rule) o ​ ○ No 1​ C ○ Stability ofC is because - ​ ■ Alkyl groups donate e​ (by induction) ● w/ C​ +or radicals special name → hyperconjugation ● How does this work? ○ Look at C ​ ○ Sp​2+ positive charge in P orbital → doesn’t have octet ■ Puppy left at home alone sad and will chew up furniture ■ Add alkyl group as a neighbor to line up some bonds o ​ ● Changes from methyl cation to 1​ cation ● Lines up for 3 sp​bonds (doesn’t have eclipsing strain) ● Electrons drift over and line up ■ Sp​3​eclipses with p orbital and electrons can drift over ● Temporary but lessens positive charge ■ Now puppy has a neighbor to check on it ■ More neighbors = more chances to line up = more stable +​ ● Typically prefer more stable C​→ lower E, less activation E ● Can do other variations on electrophilic addition -​ ○ Follow same regioselectivity + stereoselectivity +pushing ● Hydration → adding water (H​ O) -​ 2​ - ○ HSO​=4​trong acid but poor Nu​ ○ H​ O OK Nu​-but poor acid (cannot form C)​ 2​ ○ Oxygen over commits itself, so HSO​4 ​lps out ■ H​ SO​ formally a catalyst 2​ 4​ ● Can do a variation with alcohol instead of water ○ Forms ethers (ROH → organic water) ○ Exact same but with ROH as nucleophile not HOH ● Some carbocations are unstable → can rearrange ○ Grass greener on other side ○ If can shift a bond to be more stable - will do that o​ o​ o​ o​ o​ o ○ 2​ → 3​, 1→ 2​ but 3​-/-> 1​ ○ Get secondary carbocation as intermediate ■ Hydride shift to make tertiary carbocation ■ Can get hydride or alkyl shifts ● General rules of carbocation rearrangement ○ If it can rearrange it will ○ If we have a secondary next to a tertiary, it will go into the tertiary ○ Hydride shifts are more common ■ More likely because give more stable carbocations + ■ Give more alkyl groups at new C​ ○ Only alkyl shifts if no neighbor


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