Organic Chemistry: Electrophilic Additions to Alkenes
Hey 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 carbocation
R is any group with a carbon
IM is intermediate
X is halogen
E+ is electrophile
RDS is rate-determining step
HOMO is highest occupied molecular orbital
LUMO is lowest unoccupied molecular orbital
Squiggly lines indicate enantiomers formed due to alternating wedges and dashes
Addition of HX
Introducing Markovnikov’s rule: the formation of a more stable R+ is favored b/c it’s a lower energy process
Alkene additions proceed faster via a more stable R+ IM (kinetics)
Tri-substituted carbons are favored over mono-substituted carbons
X is added to the more substituted carbon
Always watch for stereochemistry! Enantiomers can be formed as products
Carbocation Rearrangements
Carbocation rearrangements will occur so the R+ is as stable as possible
That’s why the Markovnikov product won’t always be the major product
Rearrangements usually occur through hydride shifts, but alkyl shifts can also happen if that’s the only possible route
Remember: 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 right
With the orbital alignment (aka alkyl shift), there’s a filled-empty orbital overlap, where the migrating bond is aligned with the empty 2p orbital
Ring Expansions
Some 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 messy
Note 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 stable
Hydration
This follows Markovnikov’s rule− the OH group is always on the more substituted carbon of the alkene
Also this is pretty much a repeat of HX addition, but with some acid-base action
Pro-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-substituted
Your 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 CONDITIONS
Instead, use a strong acid, like concentrated H2SO4
Oxymercuration-Demercuration
This is essentially an alkene “hydration” w/o the rearrangement part
We don’t have rearrangement b/c the mercurinium ion doesn’t have a carbon
When 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 carbon
The 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 step
Hydroboration/ Oxidation
The double bond nucleophile attacks the empty orbital in BH3
The BH3 attaches onto the LESS substituted carbon, so the positive charge builds on the MORE substituted carbon
I like drawing the third H in BH3 as an extension of BH2, so I can visually see the hydride shift
The OH from the peroxide basically replaces the BH2
The specific steps with BH3 get messy, so I was taught a revised method− basically, a concerted addition of BH3 w/ an internal H- transfer
There is regioselectivity in the sense that the partial positive charge follows Markovnikov’s rule
There is stereospecificity due to the syn (same side) addition of H and BH2 in the hydroboration product
Alkene Halogenation
Note that I2 doesn’t work here as a possible halogen
Markovnikov regioselectivity is present
There is ANTI stereospecificity, as in the two individual X’s from X2 attach themselves as opposite wedges and dashes
Alkene Halohydrin Formation
This is very similar to the previous alkene halogenation, except the second X in X2 doesn’t attach onto the final product
Rather, the nucleophile replaces the second X
Markovnikov 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!!
