Stability and Reactivity of alkyl halides

[lamborghiniMD]

Which of the following would react the fastest with Chlorine in a radical reaction, assuming they all have comparable molecular weight?
a) primary carbon
b) secondary carbon
c) tertiary carbon

So I know the stability belongs to the carbon that has the most substitutents, but what about the reactivity?

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[shaboobly]

primary radical would prob be the most reactive because it's the least stable and less hindered by substituents.

 

[sciencebooks]

Which of the following would react the fastest with Chlorine in a radical reaction, assuming they all have comparable molecular weight?
a) primary carbon
b) secondary carbon
c) tertiary carbon

So I know the stability belongs to the carbon that has the most substitutents, but what about the reactivity?

In general,

Stability = thermodynamic product = tertiary because it's more substituted(like you said)
Fastest = kinetic product = primary because it's less hindered and easy to get in

So I agree with above, probably the primary carbon (though really radicals like to be on primary carbons much, much less).

 

[shaboobly]

In general,

Stability = thermodynamic product = tertiary because it's more substituted(like you said)
Fastest = kinetic product = primary because it's less hindered and easy to get in

So I agree with above, probably the primary carbon (though really radicals like to be on primary carbons much, much less).

it's like saying there's such thing a primary carbocation (yea theoretically, but it exists for such a small amount of time because it will just result in an intramolecular hydride shift it's so reactive).

 

[sciencebooks]

it's like saying there's such thing a primary carbocation (yea theoretically, but it exists for such a small amount of time because it will just result in an intramolecular hydride shift it's so reactive).

Speaking of hydride shifts, do they ever happen with radicals? I don't think I've heard of it happening but just curious.

 

[shaboobly]

Speaking of hydride shifts, do they ever happen with radicals? I don't think I've heard of it happening but just curious.

not a hydride shift as that would carry one too many electrons for the carbons, but something along the lines of one electron shifting from a tertiary -> secondary -> primary is definitely not out of the picture as it would increase the stability of the radical. you would just need to provide enough energy (increase temperature, and thus be considered a thermodyamic product probably) for the electron to move.

 

[sciencebooks]

not a hydride shift as that would carry one too many electrons for the carbons, but something along the lines of one electron shifting from a tertiary -> secondary -> primary is definitely not out of the picture as it would increase the stability of the radical. you would just need to provide enough energy (increase temperature, and thus be considered a thermodyamic product probably) for the electron to move.

That makes absolute sense. Thank you!

 

[V5RED]

Reaction rate is determined by the activation energy for the reaction.

The thermodynamic product is the one which is the most stable once formed.

The kinetic product is the one that is easiest to form. This is the product with the lowest activation energy barrier to formation. k=Ae^(-E/RT) where k is the rate constant, A is the Arrhenius factor, R is the gas constant, T is the temperature in Kelvin, and E is the activation energy.

Sometimes the thermodynamic product is also the product that is formed the fastest, sometimes it is not.

If the thermodynamic product is also the product with the lowest activation energy, then it will also be the kinetic product.

For free radical halogenation, the activation energy is lowest for tertiary carbons, so tertiary halogenation will occur the fastest. This is probably due to the stability of the radical intermediate formed in tertiary halogenation. That said, this question was unanswerable without looking up rate data or having memorized that the activation energy for primary alkanes is higher than tertiary alkanes.