[joemok]

I was taught before that the stability affects the rate of the reaction of alkanes towards chlorination.
That is, as tertiary carbocations are more stable than secondary carbocations and primary carbocations, the rate towards chlorination is: Tertiary > Secondary >Primary.

But recently i rationalize that there is actually no relationship between kinetics and thermodynamics (that is stability), so i wonder if i can still use this kind of explanation in AL chemistry exams?

I am also curious that why the C--H bond weakens with more alkyl groups attaching to the carbon? Is it just experimental fact that cannot be deduced?

[movies]

There is definitely a relationship between kinetics and thermodynamics!

[joemok]

There is definitely a relationship between kinetics and thermodynamics!

I am confused. Can you clearly point out the relationship?

[movies]

You can't have one without the other.  A process may be thermodynamically favorable (for example, the conversion of diamond into graphite is thermodynamically downhll), but kinetics may prevent it from happening (the rate is immeasurably slow).

Think about an S_N2 displacement of a tertiary halide.  If you were to form a new C-C bond, then you would go way down in energy, but the sterics are so bad in that case that the reaction just does not occur.

[joemok]

You can't have one without the other.  A process may be thermodynamically favorable (for example, the conversion of diamond into graphite is thermodynamically downhll), but kinetics may prevent it from happening (the rate is immeasurably slow).

Thank you.
I also think like this, but i am confused that the textbooks use stability to account for faster reaction rates. This is ambiguous.

Recently, i found a textbook that it uses activation energy to explain the reaction rates of the chlorination of alkane. However, it only states experimental data of the bond strength(c--H) without explain in terms of structure. I am curious indeed can we get teh rough estimation of the relative strength of the c--h bonds from the structure?

[movies]

Activation energy will account for kinetic factors like sterics because it is defined by the energy difference between the starting material and the transition state structure.

I think the trick to understanding this is to recognize that just knowing where you start (energy of the starting materials) and where you finish (relative energy of the products) only tells you half of the story.  Using the argument of starting material relative energy to account for rate differences only applies when the TS are quite similar in energy.  If the two TS are very different, all bets are off.

I don't think you can really estimate C-H bond strength based solely on the structure, but we know enough about C-H bonds to compare to known values and make an educated guess as to which is the weakest and therefore (usually) most reactive C-H bond.

[joemok]

I don't think you can really estimate C-H bond strength based solely on the structure, but we know enough about C-H bonds to compare to known values and make an educated guess as to which is the weakest and therefore (usually) most reactive C-H bond.

I am not familiar with the guessing on which is the strongest or weakest among C--H bonds that will form primary radical, secondary radical  and tertiary radical. Can you illistrate how to deduce?

[Donaldson Tan]

The link between thermodynamics and kinetics is that thermodynamics (stability) is used to estimate the activation energy which will determine the rate constant.

[joemok]

The link between thermodynamics and kinetics is that thermodynamics (stability) is used to estimate the activation energy which will determine the rate constant.

Is that this is only applicable in some cases?

I found a useful tool, Polanyi-Hammond postulate. Is it the relationship?
I have another question, is this still a postulate?

[english]

I was taught before that the stability affects the rate of the reaction of alkanes towards chlorination.
That is, as tertiary carbocations are more stable than secondary carbocations and primary carbocations, the rate towards chlorination is: Tertiary > Secondary >Primary.

You're not dealing with cations here!  Alkanes are very unreactive. 

Your general trend for cations still holds for radicals, but this does not mean that you will necessarily get more chlorinated product from a 3° radical than a 1° radical.

I've actually worked an example before where it's the exact opposite.


Look up the reactivity-selectivity principle.

Chlorination is only good for molecules that have only one or two different kinds of carbon, i.e. cyclohexane.  Each carbon is the same.

Depending on the rate of alkyl radical formation, from the reaction of a chlorine radical with a 3°, 2°, or 1° position, as well as the probability of hydrogen removal (i.e. how many hydrogens are at that position), you will get varying yields of monochlorinated product.

Bromination, however, is selective, so the formation of a 3° is much lower energy than for 1°.

The relative rates of alkyl radical formation are so different at a given temperature for bromination and chlorination, and that's the main reason why bromination gives you preferentially more product from the most stable radical—thus bromination is said to be selective—whereas chlorination can give you varying yields from 1°, 2°, or 3° radicals—thus chlorination is said to be reactive.

[movies]

I am not familiar with the guessing on which is the strongest or weakest among C--H bonds that will form primary radical, secondary radical  and tertiary radical. Can you illistrate how to deduce?

Look for stabilizing groups like heteroatoms, olefins, etc.  An allylic radical is usually a good bet.

The link between thermodynamics and kinetics is that thermodynamics (stability) is used to estimate the activation energy which will determine the rate constant.

This is only half of the story!  You can't get at activation energy only using thermodynamic relative energies, you have to account for transition state energy which includes steric interactions.  It's not enough to know that your product is lower in energy than your starting material because you also need a mechanistic path to get from start to finish.

The Hammond postulate is a great way to think about transition states simply.  The idea is that if your transition state has a structure similar to what your reactants look like, then your TS is probably easy to get at from your starting materials.  The consequence is that you would likely have an exothermic reaction from a TS that is reactant like.  Alternatively, if the TS is more similar to what the products look like, you have to go a long way uphill to get to the TS.  This would usually be an endothermic process because you have to add a lot of energy to get to the TS.  There is also the possibility that the TS is not modelled effectively by either the starting material or product structures.  In this case, it is a lot more difficult to estimate.

The other case to consider is the Curtin-Hammett situation.  In such a case, if you have two rapidly interconverting intermediates prior to the TS, then often the products observed will result from reaction of the higher energy intermediate.  This is a consequence of what I mentioned before: you start higher on the hill of activation energy.  This can be confusing because we are often taught to just look at the lowest energy structures as intermediates.

[joemok]

You're not dealing with cations here!  Alkanes are very unreactive. 

Oh i say wrongly there. It should be radicals...
On the other hand, I know the final proportion also depends on the number of respective hydrogen atoms and the halogens.However, i am confused if it is appropriate to conclude faster rate from stabler radical formations for an individual hydrogen (not counting the whole molecule).
This explanation uses stability to account for faster rates, but we are taught also that we should not make direct linkage between thermodynamics and kinetics. Are they contradicting?

Also, I've seen some websites saying that Hammond postulate is not based on theories. Then, can we use it to explain? Is it appropriate?

[joemok]

Look for stabilizing groups like heteroatoms, olefins, etc.  An allylic radical is usually a good bet.

Is it alkane saturated? i don't understand why there is unsaturation. Can you illustrate with an example?

Also, I doubt if we can use the unproved Hammond postulate to explain for faster rates. This also contradicts with what we are taught in the lessons with kinetics: never directly link thermodynamics and kinetics....
It seems so inconsistent and contradicting.

[Ψ×Ψ]

There is definitely a relationship between kinetics and thermodynamics!

Equation, please?  That would clear up a bit of the fuzziness.   

[movies]

I don't understand what the problem is here.  What do you mean that the Hammond postulate is not based on theories?  That doesn't make sense to me.  The Hammond postulate is a statement related to transition state theory and the principle of least motion.  What are you looking for here?  Also, what do you mean that the Hammond postulate is unproven?  It has been applied to many, many reactions and works quite well when the transition states are similar to either the products or reactants.

Why are you hung up on insisting that kinetics and thermodynamics are independent?  Who told you that?  It's not true.  You have to be careful when you make arguments about either kinetics or thermodynamics because if you have some piece of kinetic evidence then you can't necessarily make a particular conclusion about the thermodynamics of the process, and vice versa.  You have to be very precise in what you are looking at when you are talking about kinetics and thermodynamics, but the two are definitely interrelated.  Think of it this way: thermodynamics gives you the starting point and the end point, but it can't get you from start to finish.  Kinetics gets you from start to finish.

I'm no physical chemist, so I don't have an equation handy...sorry 'bout that.

As for radical stability, I gave those two examples as types of radicals that are stabilized, you can make hyperconjugation arguments to account for the relative stability of primary, secondary, and tertiary radicals just as you would for the corresponding cations.