Why Acid/Base Equilibrium Disfavors Strong Mineral Acids Despite Carbocation Formation

Why Acid/Base Equilibrium Disfavors Strong Mineral Acids Even with Carbocation Formation

Acid/base equilibrium generally disfavors the presence of strong mineral acids like HCl or HNO3, even if it leads to the formation of a carbocation. This occurs because equilibrium favors the side with lower-energy, more stable molecules. Strong acids almost fully dissociate, producing very stable conjugate bases, which shifts the equilibrium away from reformation of the strong acid.

Equilibrium and Stability in Acid-Base Reactions

Acid-base reactions establish an equilibrium where the position depends on molecular stabilities. The following points explain why strong mineral acids are disfavored at equilibrium:

• Equilibrium favors weaker acids: The side with a weaker acid and base corresponds to lower-energy species. This principle holds since equilibrium always seeks the state with minimized free energy.
• Strong acids produce stable conjugate bases: HCl and HNO3 dissociate to chloride (Cl−) and nitrate (NO3−), both highly stabilized ions. These ions lower the system’s overall energy, pushing the equilibrium toward dissociation.
• Salt formation drives the reaction: The formation of stable salts releases lattice enthalpy, a significant thermodynamic driver. For instance, NaCl and water from HCl and NaOH reactions form stable, low-energy compounds that rarely revert.

Formation of Carbocations and Equilibrium Considerations

Carbocation formation following protonation may initially seem to contradict equilibrium principles but does not invalidate them:

• Upon protonation of an alkene, the carbocation forms.
• The carbocation is highly reactive and promptly attacked by a Lewis base, such as Cl−, producing a stable adduct.
• This final product’s formation removes intermediate carbocations, pulling the equilibrium forward by Le Chatelier’s principle.

This mechanism explains why even strong acids like HCl can initiate carbocation formation under kinetic control, though thermodynamically, the reaction still favors stable end-products over free strong acids.

Adduct Formation Between Carbocation and Mineral Conjugate Base

The carbocation and mineral conjugate base can indeed form an adduct. This process stabilizes the positively charged carbocation:

• The Lewis base (e.g., Cl−) attacks the carbocation, forming a covalent bond.
• This adduct is more stable than the free carbocation.
• Its formation shifts equilibrium by consuming reactive carbocations.

Adduct formation helps explain observed reaction kinetics and the ability of strong acids to promote reactions without remaining intact as free species.

Summary of Key Points

• Acid/base equilibrium favors lower-energy, more stable species, meaning weaker acids predominate at equilibrium.
• Strong mineral acids dissociate completely, yielding stable conjugate bases and driving equilibrium away from the acid.
• Carbocation formation is transient; rapid capture by the conjugate base forms stable adducts.
• These adducts pull the equilibrium ahead according to Le Chatelier’s principle.
• Thermodynamic drivers like lattice enthalpy and product stability dominate equilibrium positioning.