cyclicstudies: Organic Chemistry: Electrophilic Additions to AlkenesHey all! I wanted to put togethe
cyclicstudies: Organic Chemistry: Electrophilic Additions to AlkenesHey all! I wanted to put together a few review guides for the reactions I learned in my organic chemistry 1 class. I’m starting off with the alkene-related reactions, specifically with electrophilic additions. Despite being super hard to fully conceptualize (I still have trouble with them!!), they’re very important to recognize, because the same patterns repeat with other reaction types, particularly with alkynes. I’ll go through each reaction type here with a quick mechanism and jot down important bits of info. But before I start, I used a lot of abbreviations, so here’s a key:R+ is carbocationR is any group with a carbonIM is intermediateX is halogenE+ is electrophileRDS is rate-determining stepHOMO is highest occupied molecular orbitalLUMO is lowest unoccupied molecular orbitalSquiggly lines indicate enantiomers formed due to alternating wedges and dashesAddition of HXIntroducing Markovnikov’s rule: the formation of a more stable R+ is favored b/c it’s a lower energy processAlkene additions proceed faster via a more stable R+ IM (kinetics)Tri-substituted carbons are favored over mono-substituted carbonsX is added to the more substituted carbonAlways watch for stereochemistry! Enantiomers can be formed as productsCarbocation Rearrangements Carbocation rearrangements will occur so the R+ is as stable as possibleThat’s why the Markovnikov product won’t always be the major productRearrangements usually occur through hydride shifts, but alkyl shifts can also happen if that’s the only possible routeRemember: always check to see where the positive charge is at− if you can shift a hydrogen or methyl group over to make that carbon extra substituted, then it’s probably rightWith the orbital alignment (aka alkyl shift), there’s a filled-empty orbital overlap, where the migrating bond is aligned with the empty 2p orbitalRing ExpansionsSome questions are gonna ask you to do ring expansions, so always think alkyl shifts (again, this is so we form the most stable R+)NUMBER. YOUR. CARBONS. This will get messyNote that even though the di-substituted carbon doesn’t change after the first arrow, the shape changes from cyclopentane to cyclohexane, so it’s thermodynamically more stableHydration This follows Markovnikov’s rule− the OH group is always on the more substituted carbon of the alkeneAlso this is pretty much a repeat of HX addition, but with some acid-base actionPro-tip: your catalyst, H3O+, is always gonna be the electrophile, and your nucleophile is always gonna be the double bond, so START THERE!There’s gotta be water after that first step (hydration, duh)Your water is now a nucleophile, and it’s gonna start attacking the R+Remember to follow through with any possible R+ rearrangements− we won’t need it here b/c the R+ is already tri-substitutedYour catalyst MUST be regenerated!! *insert acid-base reaction*Side note: alkene dehydration is the reverse of hydration and is an E1 process DON’T ADD H2O/H3O+ UNDER DEHYDRATION CONDITIONSInstead, use a strong acid, like concentrated H2SO4Oxymercuration-DemercurationThis is essentially an alkene “hydration” w/o the rearrangement partWe don’t have rearrangement b/c the mercurinium ion doesn’t have a carbonWhen the mercurinium ion bridge breaks, the nucleophile (in the second example, water), attacks the MORE substituted carbon, whereas the HgOAc attacks onto the LESS substituted carbonThe OH and HgOAc add ANTI to each other, but then the H from BH4- changes orientation on the LESS substituted carbon (forming diastereomers)Side note: as a variation, you can also use an alcohol (ROH) instead of H2O in the oxymercuration stepHydroboration/ OxidationThe double bond nucleophile attacks the empty orbital in BH3The BH3 attaches onto the LESS substituted carbon, so the positive charge builds on the MORE substituted carbonI like drawing the third H in BH3 as an extension of BH2, so I can visually see the hydride shiftThe OH from the peroxide basically replaces the BH2The specific steps with BH3 get messy, so I was taught a revised method− basically, a concerted addition of BH3 w/ an internal H- transferThere is regioselectivity in the sense that the partial positive charge follows Markovnikov’s ruleThere is stereospecificity due to the syn (same side) addition of H and BH2 in the hydroboration productAlkene HalogenationNote that I2 doesn’t work here as a possible halogenMarkovnikov regioselectivity is presentThere is ANTI stereospecificity, as in the two individual X’s from X2 attach themselves as opposite wedges and dashesAlkene Halohydrin FormationThis is very similar to the previous alkene halogenation, except the second X in X2 doesn’t attach onto the final productRather, the nucleophile replaces the second XMarkovnikov regioselectivity is also present (this is SN1-like)There is ANTI stereospecificity yet again (this is SN2-like)And that’s it! Well, this is a good chunk of alkene reactions (and a pretty brief version of it). Regardless, I hope this is helpful! I hope to follow up with extra review mechanisms for additional alkene and alkyne reactions. Thanks for reading!! -- source link