Markovnikov and Anti-Markovnikov

[shushumiga]

Can someone explain me the Markovnikov and Anti-Markovnikov rules?? I am not trolling. I read both explanation in the TBR Orgo book but could not understand them. I would appreciate that.

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[milski]

It might be helpful to discuss what exactly confuses you about them. On the surface, they're rather simple. You are doing an addition of HX across a double bond. Reactions which follow Markovnikov's rule add the H to the less substituted (more H) carbon atom, the X to the more substituted C. Anti-Markovnikov reactions will do the opposite.

For example let's take CH3CH=CH2. Markovnikov addition of HX will get you CH3-CXH-CH3. Anti-Markovnikov will get you CH3-CH2-CH2X.

Markovnikov reactions usually go through a carbocation which is explains how the more substituted carbon end up receiving the X.

 

[MedPR]

It might be helpful to discuss what exactly confuses you about them. On the surface, they're rather simple. You are doing an addition of HX across a double bond. Reactions which follow Markovnikov's rule add the H to the less substituted (more H) carbon atom, the X to the more substituted C. Anti-Markovnikov reactions will do the opposite.

For example let's take CH3CH=CH2. Markovnikov addition of HX will get you CH3-CXH-CH3. Anti-Markovnikov will get you CH3-CH2-CH2X.

Markovnikov reactions usually go through a carbocation which is explains how the more substituted carbon end up receiving the X.

Yea, It really depends on the reaction mechanism. Radical additions (like HX and Hydroboration) are typically anti-markovnikov.

You kind of have to know the reaction mechanism (or at least what adds first/leaves first) in order to know if it will be markovnikov or anti-markovnikov.

However, if they give you a product, you can usually tell that it's markovnikov if the addition occured at the most stable carbocation. For instance, a tertiary alkyl halide is probably markovnikov, while a secondary halide probably isn't.

 

[milski]

Keep in mind that after you're past intro ochem, it's very unlikely that someone would as you if something is Markovnikov or Anti-Markovnikov. You'll have the reagents, you'll have to figure out the products (or do the synthesis problem) and for that you need to have a good idea of what the mechanism of the reaction is. So don't worry about it so much as a single topic, worry about knowing the reactions that you are expected to know.

 

[MT Headed]

Mnemonic for markovnikov: The rich get richer, the poor get hydrogens.

Mnemonic for anti-markovnikov (borrowed from EK Audio Osmosis): Captialist peroxide PIGS!
(The scenario here is some guy with a russian accent and russian music in the background is always adding things markovnikov, except when peroxide is used as a reagent (HBr, ROOR or BH3:THF,H2O2,OH-) everything goes anti-markovnikov and the russian guy gets real mad)

 

[Golaso]

For the Markovnikov rule it's easy to remember that if you have an assymetric alkene reacting with HX then basically rich gets richer and poor get poorer (even in orgo this phrase applies lol) the way I like to think about it is as follow:
let say you have R2C=CH2 alkene, alkyl groups (R2) are always electron donating groups so basically the 2 R in my example are pushing electrons toward the right side of the molecule making it more electron dense and that way you will have the right side attacking the Hydrogen in HX. The left side of the molecule is now positive and will accept the X ion.

Anti-Markovnkov, still didn't get to it but I suppose its the opposite...

 

[chiddler]

uh this thread is about 3 months old.

 

[blackmarble]

i know this thread is now 2 years old, but i didn't want to make a new thread.

are markovnikov/anti-markovnikov topics still on the exam?

 

[Czarcasm]

i know this thread is now 2 years old, but i didn't want to make a new thread.

are markovnikov/anti-markovnikov topics still on the exam?

No, since alkene reactions where removed from the AAMC outline. Most prep books for whatever reason still talk about it, but it's not worth the time or effort and infact, would be counterproductive in preparing for the current MCAT.

 

[blackmarble]

No, since alkene reactions where removed from the AAMC outline. Most prep books for whatever reason still talk about it, but it's not worth the time or effort and infact, would be counterproductive in preparing for the current MCAT.

great, thanks. super relieved because it's actually super confusing.

 

[Czarcasm]

great, thanks. super relieved because it's actually super confusing.

Yeah, I bet. If you are short on time: definitely focus on lab techniques outlined in AAMC; NMR/IR/mass spec. For NMR and IR know the common values. For mass spec, really just know what is and how to identify the m+ ion peak. For reactions, focus on all the key ones: SN1/SN2, free radical halogenation; Carbonyl compounds and their derivatives (focus on understanding their reactivates and not so much on specific reactions); Also, some basic nomeclature, identifying chiral centers, and R/S configuration is worth noting. It can seem overwhelming, but for MCAT orgo, the questions are usually very straight forward so I wouldn't freak out about it too much.

 

[blackmarble]

Yeah, I bet. If you are short on time: definitely focus on lab techniques outlined in AAMC; NMR/IR/mass spec. For NMR and IR know the common values. For mass spec, really just know what is and how to identify the m+ ion peak. For reactions, focus on all the key ones: SN1/SN2, free radical halogenation; Carbonyl compounds and their derivatives (focus on understanding their reactivates and not so much on specific reactions); Also, some basic nomeclature, identifying chiral centers, and R/S configuration is worth noting. It can seem overwhelming, but for MCAT orgo, the questions are usually very straight forward so I wouldn't freak out about it too much.

thanks, that's really helpful. a bit short on time, but have most of what you listed down was mostly just worried about alkenes since they're kind of standalone in a way (oxymercuration/demercuration, markovnikov) and i didn't really want to deal with them lol. just need to get the carbohydrates down and ill be feeling alright!

 

[chemtopper]

Can someone explain me the Markovnikov and Anti-Markovnikov rules?? I am not trolling. I read both explanation in the TBR Orgo book but could not understand them. I would appreciate that.

Markovnikov and anti Markovnikov rules are based on the same concept and favors formation of more stable carbocation.Electrophile(E+ like H+ or electron deficient molecule like BH3) gets bonded with carbon of the double bond( in alkenes) so that more substituted carbon gets the positive charge or E+ goes to the carbon with more H on it.Now if this E+ is H+ then it becomes Markovnikov and if it is other than H+ then it becomes anti markovnikov rule.
Alkenes undergo electrophilic addition reaction and when H+ is the electrophile (reagents like HX or H2O/H+ ) follow Markovnikov rule and add H+ on least substituted Carbon.However if electrophile is other than H+ like in BH3(hdroboration) ,B is more electropositive than Hydrogen so here Hydrogen will go to more substituted carbon and B of the BH3 will get added on less substituted carbon and this called as anti Markovnikov rule.Free radicals formed with peroxides are also electrophile and take the position of H+ and will follow anti markovnikov rule.