Question

Consider the Arrhenius equation.

K= Ae^-Ea/RT

Discuss the effect of changing the basicity of the nucleophile would have on the activation energy, and therefore on the rate constant. Describe the molecularity and rate law of the reaction. How could this be determined experimentally?

Answer 1

Effect of changing the basicity

Arrhenius' equation gives the dependence of the rate constant k of a chemical reaction on the absolute temperature T (in kelvin), where R is the pre-exponential factor (or simply the prefactor), Eais the activation energy, and R is the universal gas constant.

It can be seen that either increasing the temperature or decreasing the activation energy (for example through the use of catalysts) will result in an increase in rate of reaction.

The change in any of the reaction conditions for example changing the basicity of nucleophile can greately affect the activation energy and thus rate constant of the reaction.Small changes in Ea can lead to large changes in rate and thus rate constants.

Role of nucleophile in partial bonds: The more complete the bond is, the more stable the transition state [TS], and as fast as reaction can possibly go, i.e higher rate and rate constant but a lower Ea- energy of activation (height of hill). The single most important factor which controls the nucleophilicity or basicity of any molecule or ion is the ability, desire, or driving force to share an e- pair. - Nucleophilicity: ability to share e- pairs with electrophile - Basicity: ability to share e- pair with H - Stronger bases are better nucleophiles * When comparing molecules with attacking atoms that vary greatly in size, the polarizability (ability to skew the electron cloud) of the atom and the reaction conditions determine whether the greater polarizability of the larger atoms makes up for their decreased basicity. * The relationship between basicity and nucleophilicity becomes inverted when the reaction is carried out in a protic solvent (the solvent molecules have a hydrogen bonded to a nitrogen or oxygen).

The nucleophilicity itself is affected by various factors such as resonance, atomic size, electronegativity, inductive effects, steric effects.

Resonance (can increase or decrease e- density at the atom that shares e- with the electrophile, but generally decreases nucleophilicity)

Atomic size: Smaller atoms are better nucleophiles because they have a more concentrated e- density, and thus a stronger driving force to share e-.

Electronegativity