[Big-Daddy]

With regards to electrophilic addition onto the C=C double bond, my understanding of Markovnikov's Rule is that it suggests that the electrophile (let's call it E) bonds to whichever C from the C=C is less substituted and the nucleophile (Nu) to whichever C is more substituted.

My question is, what does "more substituted" and "less substituted" mean? What are the exact rules for defining which C is which?

[discodermolide]

Count the number of substituents on the carbons:
R₃CH is more substituted than R₂CH₂

[Big-Daddy]

Count the number of substituents on the carbons:
R₃CH is more substituted than R₂CH₂

And these substituents can be anything, except H?

So then what if they have the same number of substituents? How does the size of the substituents come into effect?

[AWK]

http://en.wikipedia.org/wiki/Markovnikov_rule

[Big-Daddy]

http://en.wikipedia.org/wiki/Markovnikov_rule

discodermolide already mentioned the point made by the page. Can you answer my question regarding to clarification? (the one in the post above yours)

[discodermolide]

In this context H is not normally considered a substituent.
If the C has the same number of "R's" then I guess steric factors will start to influence the "Rule".

What do you think is a reasonable explanation?

[Big-Daddy]

In this context H is not normally considered a substituent.
If the C has the same number of "R's" then I guess steric factors will start to influence the "Rule".

What do you think is a reasonable explanation?

I'm just trying to get a systematic way to think about it, much like the Cahn-Ingold-Prelog Priority Rules are laid out.

So the first port of call is how many non-H substituents there are versus H substituents; whichever atom has more non-H substituents "wins" (is considered more substituted).

Among non-H substituents, how do we decide? Overall molecular weight of all the non-H substituents on one of the atoms combined, versus the same for the other?

I'm wondering if it really just follows the Cahn-Ingold-Prelog system and when it says "more substituents" it really means bigger substituents (as H is the smallest substituent possible), but I'm not too sure.

[curiouscat]

In this context H is not normally considered a substituent.
If the C has the same number of "R's" then I guess steric factors will start to influence the "Rule".

What do you think is a reasonable explanation?

I'm just trying to get a systematic way to think about it, much like the Cahn-Ingold-Prelog Priority Rules are laid out.

So the first port of call is how many non-H substituents there are versus H substituents; whichever atom has more non-H substituents "wins" (is considered more substituted).

Among non-H substituents, how do we decide? Overall molecular weight of all the non-H substituents on one of the atoms combined, versus the same for the other?

I'm wondering if it really just follows the Cahn-Ingold-Prelog system and when it says "more substituents" it really means bigger substituents (as H is the smallest substituent possible), but I'm not too sure.

Think about the underlying carbocation I guess. Ultimately the stability of that is what leads to the Rule.

Start thinking where the inductive etc. effect is stronger.

[discodermolide]

It is a rule and as such there are always exceptions. I don't think the Cahn-Ingold-Prelog system has anything to do with it. So basically there is not a systematic way to go about it.
More does not necessarily mean bigger. More just means more!

[Big-Daddy]

In this context H is not normally considered a substituent.
If the C has the same number of "R's" then I guess steric factors will start to influence the "Rule".

What do you think is a reasonable explanation?

I'm just trying to get a systematic way to think about it, much like the Cahn-Ingold-Prelog Priority Rules are laid out.

So the first port of call is how many non-H substituents there are versus H substituents; whichever atom has more non-H substituents "wins" (is considered more substituted).

Among non-H substituents, how do we decide? Overall molecular weight of all the non-H substituents on one of the atoms combined, versus the same for the other?

I'm wondering if it really just follows the Cahn-Ingold-Prelog system and when it says "more substituents" it really means bigger substituents (as H is the smallest substituent possible), but I'm not too sure.

Think about the underlying carbocation I guess. Ultimately the stability of that is what leads to the Rule.

Start thinking where the inductive etc. effect is stronger.

This seems like the accurate approach, but I'm not sure how to predict the stability of the different possible carbocations. For example, would a longer group (purely alkyl) be of greater stability than a smaller, purely alkyl group? What about a halogen compared to an alkyl group? And among halogens, are the bigger (I) or smaller (F) ones the "bigger"/more stable substituents? (i.e. which counts as "more substituted", RCH2Cl or RCH2Br)

[discodermolide]

In my opinion RCH₂Cl or RCH₂Br are the same in terms of "substituents".
The order of carbocation stability is well described. See for example http://en.wikipedia.org/wiki/Carbocation.
Halogens would tend to destabilise the cation, as far as I remember. But this effect will decrease with increasing electronegativity of the halogen. That is iodine is better than fluorine.

[curiouscat]

In this context H is not normally considered a substituent.
If the C has the same number of "R's" then I guess steric factors will start to influence the "Rule".

What do you think is a reasonable explanation?

I'm just trying to get a systematic way to think about it, much like the Cahn-Ingold-Prelog Priority Rules are laid out.

So the first port of call is how many non-H substituents there are versus H substituents; whichever atom has more non-H substituents "wins" (is considered more substituted).

Among non-H substituents, how do we decide? Overall molecular weight of all the non-H substituents on one of the atoms combined, versus the same for the other?

I'm wondering if it really just follows the Cahn-Ingold-Prelog system and when it says "more substituents" it really means bigger substituents (as H is the smallest substituent possible), but I'm not too sure.

Think about the underlying carbocation I guess. Ultimately the stability of that is what leads to the Rule.

Start thinking where the inductive etc. effect is stronger.

This seems like the accurate approach, but I'm not sure how to predict the stability of the different possible carbocations. For example, would a longer group (purely alkyl) be of greater stability than a smaller, purely alkyl group? What about a halogen compared to an alkyl group? And among halogens, are the bigger (I) or smaller (F) ones the "bigger"/more stable substituents? (i.e. which counts as "more substituted", RCH2Cl or RCH2Br)

Worst case, do a calculation. Find their total energies. Best way to settle the tie. Computational chemistry programs are dime a dozen these days.

[AWK]

http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/addene1.htm

[Big-Daddy]

In my opinion RCH₂Cl or RCH₂Br are the same in terms of "substituents".
The order of carbocation stability is well described. See for example http://en.wikipedia.org/wiki/Carbocation.
Halogens would tend to destabilise the cation, as far as I remember. But this effect will decrease with increasing electronegativity of the halogen. That is iodine is better than fluorine.

So the presence of halogens is even worse than the presence of H atoms, i.e. if you have C(Cl₂)=CH₂ and you're adding HI you would get CH(Cl₂)-CH₂Cl.

But F is the worst possible and I is only slightly worse a substituent than H. When I say "worse", I mean leading to the least stable carbocation and thus contributing the most to this C atom being the one from the C=C that receives the electrophile in the substitution rather than the nucleophile.

Correct?

[sankalpmittal]

In my opinion RCH₂Cl or RCH₂Br are the same in terms of "substituents".
The order of carbocation stability is well described. See for example http://en.wikipedia.org/wiki/Carbocation.
Halogens would tend to destabilise the cation, as far as I remember. But this effect will decrease with increasing electronegativity of the halogen. That is iodine is better than fluorine.

So the presence of halogens is even worse than the presence of H atoms, i.e. if you have C(Cl₂)=CH₂ and you're adding HI you would get CH(Cl₂)-CH₂Cl.

But F is the worst possible and I is only slightly worse a substituent than H. When I say "worse", I mean leading to the least stable carbocation and thus contributing the most to this C atom being the one from the C=C that receives the electrophile in the substitution rather than the nucleophile.

Correct?

I don't think you are correct. Perhaps , in absence of organic peroxides, and having an unsymmetrical alkene : C(Cl₂)=CH₂ , and unsymmetrical compound (not organic) :    HI , then as per markinikov rule , attach H on carbon having more H , and I on carbon having less H. So you will get :

(CCl₂I)-(CH₃)

Now analyze its stability....