Why will this TS not have negative enthalpy?

[Albert Chong_1L]

Hi I was wondering why transition states always have positive enthalpy changes despite TS that involve solely bond formation.

One example would be the second step of electrophilic addition of HBr to Propene in which the second step involves a Br anion attaching to a carbocation.

Since the formation of bonds releases energy, why would this TS still have a positive enthalpy change?

Thanks.

[Chem_Mod]

Hi Albert,

Even though forming bonds is energetically favorable, it still takes some energy to get the reaction to occur. It will most likely result in an extremely small hump (relative to the first) for the nucleophilic attack, however it still requires some energy to get it moving.