Ch_322a_6.011

Ch_322a_6.011 - A C B TS 1 TS 2 The overall rate of A C is...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Multi-Step Reactions and the Rate-Determining Step Many reactions in organic chemistry involve a sequence of steps , each with an energy barrier linking starting and product states. Each step proceeds with a unique rate constant. A B C k 1 k 2 a reaction intermediate Two general cases for the above two-step reaction sequence will be examined. The relative magnitudes of rate constants k 1 and k 2 are determined by the relative magnitudes of the free energies of activation (the "energy barriers") for each step. Note: B is an intermediate . It is not a transition state. This important difference is shown in the free energy diagrams below.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Two General Cases A B C k 1 k 2 slower faster Case A: k 1 < k 2 Step 1 (A B) is slower than step 2 (B C). The free energy of activation for step 1 is larger leading to a smaller rate constant. Free Energy Progress of Reaction
Background image of page 2
Background image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: A C B TS 1 TS 2 The overall rate of A C is controlled by step 1 which is the rate-determing step . slow step Note: A chemical intermediate , such as B, is an energy minimum in the free energy diagram. It persists for a finite time. A transition state is an energy maximum that is transitory, existing for perhaps the time of a bond vibration, ~10-12 s. Case B: k 1 &gt; k 2 A B C k 1 k 2 slower faster In this scheme, the first step is fast and reversible: A B (fast). When B is formed, it returns to A faster than it proceeds to C . The rate determining (slow) step is B C. Free Energy Progress of Reaction A C B TS 1 TS 2 faster slower B. The rate of formation of product C is Rate = k 2 [B] where the concentration of B is determined by the equilibrium A...
View Full Document

Page1 / 3

Ch_322a_6.011 - A C B TS 1 TS 2 The overall rate of A C is...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online