[Twickel]

Hi
I understand that tertiary substituted alkyl halides are favored in Sn1 reactions, since they form a more stable carbocation intermediate. Now what I don't understand is the energy profile for this reaction.

If the alkyl halide is more stable then the energy of the molecule is increased meaning delta G is larger and hence the reaction should form more slowly. So how is it that it is favored?

[fledarmus]

If the alkyl halide is more stable than what?

If I understand your question, you are saying that the carbocation intermediate would still be higher in energy than the alkyl halide, so none of the carbocation intermediate would form? Or that the equilibrium of the reaction forming the carbocation should lie on the side of the alkyl halide?

Where is the carbocation on the reaction pathway - what happens to it once it has formed? And how does the energy of that product compare to the starting material? Could Le Chatelier's principle be involved in any way?

[Dan]

In addition to what Fledarmus has said, I think you may be confusing :delta: G with activation energy.

Perhaps mark these on your energy profile, and maybe post what you have done for further clarification.

[Twickel]

Sorry for the vague post.

In the Sn2 reaction, the larger the substrate ( alkyl halide) the slower the reaction is since delta G is increased. So how can the tertiary alkyl halide provide a lower delta G for the Sn1 reaction. All I have to by is this line from my lecture note. factors that stabilise the carbocation intermediate also stabilise the transition state, this will lower its energy and hence Ea.

Does it effect the starting energy or the transition energy?

By delta G i meant the delta G++. So where the peaks of the transition states are.

[james_a]

The energy you're really concerned with minimizing is the transition energy (the height of the hill). The difference in energy from the starting alkyl halide to the transition state.

For different alkyl halides, you can assume that the differences in energies between the starting energies is small relative to the differences in the stabilities of their carbocations.

[fledarmus]

You are looking at two different reactions mechanisms (S_N1 and S_N2) and two different transition states now. That's fine, but be sure you are aware that this is what you are doing.

Imagine a primary alkyl halide going through an S_N2 reaction. Your transition state, which will have a higher energy than your starting materials, has your reactive center associated with both the halide and your nucleophile.

Now imagine the same primary alkyl halide going through an S_N1 reaction. Your transition state, still with a higher energy than your starting materials, has a reactive center which is not associated with either the halide or the nucleophile, and is positively charged.

Got those points drawn out in your head (or on paper)?

Good. Now imagine bulking up your halide - going up to secondary or tertiary alkyl halides, or even just putting a lot of substitutions in the alpha positions. You won't be changing the energy of your starting material much, but for the S_N2 reaction, it will take a lot more energy to get the intermediate associated with both the halide and the nucleophile due to the size of the groups you have to push aside. The energy of your transition state will rise considerably. On the other hand, for the S_N1 reaction, all those extra alkyl substituents on the reaction center will stabilize the carbocation, lowering the energy of that intermediate.

At some point the S_N1 intermediate has lower energy than the S_N2 intermediate, and substitutions take place by the S_N1 mechanism instead of the S_N2 mechanism.