[rm.rahma4]

When learning about the carbocation of molecules and how a major product is formed with the ion which has greater stability I was told that the molecule attatching itself to the carbonium ion "knows" which carbon is more stable. I was wondering how this atom can "know" which product will be more stable?
Thank You

[kriggy]

Im not sure what you mean by your question. Usualy there is only single carbocation in the molecule. If the carbocation is primary, then it is unstable and it will/might rearange to more stable carbocation - tertiary. If you have the second reagent in the mixture, it will react with both carbocations but since the primary carbocation is not stable, there will be only little amount of the product formed from the primary carbocation.

[Babcock_Hall]

Assume that a particular electrophilic addition reaction can lead to two carbocations of unequal stability.  If so, the transition states often reflect the difference in stabilities.  The transition state of lower energy will form more quickly.

[Enthalpy]

Typical reaction steps used in chemical synthesis (that is, not detonations) are extremely improbable and rare.

That is, a molecule (or ion, radical...) can collide with a step candidate every nanosecond (gas) or picosecond (liquid), but the reaction takes an hour to complete. Consistently, the reaction step can need to cross an activation energy of 30kJ/mol for instance, but the temperature is only RT=3kJ, so the necessary heat is obtained in one attempt over exp(10) to make it simple. Even more difficult: the activation energy corresponds to the best possible combination or positions and orientations and conformations of both molecules.

So the species don't "know" nor "sense" what would be their best match. They just collide randomly; the encounters are rarely fertile when they happen to proceed at the most favourable carbon position, orientations and so on; and when the conditions are less favourable, say at a worse carbon, the encounters are even more sterile.