Most vs. least substituted carbon

[Revilla]

I used to confuse these when I was in Organic Chem as well. Can someone clear up the language for me once and for all? When we say "most substituted carbon," are we talking about the carbon attached to the most hydrogens?

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[rocuronium]

The substitution refers to substituting FOR a hydrogen atom. Every time you replace a hydrogen atom with something else, you are increasing the substitution of that carbon.

 

[RySerr21]

I used to confuse these when I was in Organic Chem as well. Can someone clear up the language for me once and for all? When we say "most substituted carbon," are we talking about the carbon attached to the most hydrogens?

think the exact opposite. the carbon attached to the least hydrogens. if a carbon has 3 hydrogens on it, it only has room for 1 other group to bind, so it is monosubstituted.

CH3-CH2-CH3


the carbon on the very left is monosubstitutded (it is attached to one other element besides a hydrogen)

the carbon in the middle is disubstituted (it is attached to two other elemets besides hydrogen)

the carbon on the very right is the same exact as the one on the very left.

 

[J ROD]

One word: Tert-butyl.

 

[Revilla]

Thanks guys! I'm trying to reconcile what you're saying with my Organic review notes. I have here that for alcohol additions in acidic conditions, the alcohol attaches to the least substituted carbon. Is this backwards?

 

[J ROD]

Thanks guys! I'm trying to reconcile what you're saying with my Organic review notes. I have here that for alcohol additions in acidic conditions, the alcohol attaches to the least substituted carbon. Is this backwards?

I guess you are talking about adding an alcohol to an alkene:

The reaction with propene

This is typical of the reaction with unsymmetrical alkenes. An unsymmetrical alkene has different groups at either end of the carbon-carbon double bond.

If sulphuric acid adds to an unsymmetrical alkene like propene, there are two possible ways it could add. You could end up with one of two products depending on which carbon atom the hydrogen attaches itself to.

However, in practice, there is only one major product.


This is in line with Markovnikov's Rule which says:

When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.

In this case, the hydrogen becomes attached to the CH2 group, because the CH2 group has more hydrogens than the CH group.

Notice that only the hydrogens directly attached to the carbon atoms at either end of the double bond count. The ones in the CH3 group are totally irrelevant.

 

[Revilla]

I'm still a little confused, but I think I have to get through the whole section before I can explain my line of reasoning. Thanks for all your help, guys!

 

[BlackSails]

I'm still a little confused, but I think I have to get through the whole section before I can explain my line of reasoning. Thanks for all your help, guys!

The most substitued carbon is the carbon with the most things substituted for hydrogen.

So methane is degree 0.

n-Butane has 2 primary carbons and 2 secondary carbons. (The ones on the ends are the primary carbons)

Isobutane has 3 primary carbons and 1 tertiary carbon (the center one).

neo-Pentane has 4 primary carbons and one quartery carbon (the center one).

 

[TJames]

Thanks guys! I'm trying to reconcile what you're saying with my Organic review notes. I have here that for alcohol additions in acidic conditions, the alcohol attaches to the least substituted carbon. Is this backwards?

Yes, this is backwards.

The OH ends up on the more highly substituted carbon. When the double bond is protonated the H goes to the less substituted carbon and the + charge is on the most substituted, and then the OH goes to the carbon with the +.