Question #425ea

1 Answer
Jan 18, 2017

The primary kinetic isotope effect does not affect the reaction mechanism; rather, it gives information on the reaction mechanism.

Explanation:

Kinetic Isotope Effect

The kinetic isotope effect (KIE) refers to the change in rate caused by an isotopic substation in a molecule.

Frequently, the KIE refers to the effect of substituting #"D"# for #"H"# (the #k_"H"//k_"D"# ratio).

The #"C-H"# and #"C-D"# vibrations are quantized per the condition

#E = (n + 1/2)hf#, where #n = 0, 1, 2, …#

At ordinary temperatures, most of the molecules are in the #n = 0# state.

We see the maximum KIE when a #"C-H"# bond is being broken in the transition state, because the vibrational mode disappears.

Since #E_0("H") > E_0("D")#, #E_"a""(H") < E_"a""(D)"# and #k_"H" > k_"d"#.

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This is called a primary kinetic isotope effect.

The calculated value of #k_"H" // k_"d" ≈ 7#.

A secondary KIE occurs when the isotope is substituted at a position next to the bond being broken.

Usually, for a secondary KIE, #k_"H"//k_"D" < 1.5#.

Mechanistic information from KIEs

KIEs give useful information about the rate determining step in a mechanism.

Case 1

#"CH"_3"CH"_2"CH"_2"Br" stackrelcolor(blue)( "EtO"^"-", "EtOH"color(white)(m)) (→) "CH"_3"CH=CH"_2#

#"CH"_3"CD"_2"CH"_2"Br" stackrelcolor(blue)( "EtO"^"-", "EtOH"color(white)(m)) (→) "CH"_3"CD=CH"_2#

#k_"H"//k_"D" = 6.7#

This is consistent with an #"E2"# elimination in which the #"C-H/D"# bond is being broken in the rate determining step.

Case 2

#"CH"_3"CH"_2"C"("CH"_3)_2"Br" stackrelcolor(blue)( "H"_2"O", Δcolor(white)(m)) (→) "CH"_3"CH=C"("CH"_3)_2#

#"CH"_3"CD"_2"C"("CH"_3)_2"Br" stackrelcolor(blue)( "H"_2"O", Δcolor(white)(m)) (→) "CH"_3"CD= C"("CH"_3)_2#

#k_"H"//k_"D" = 1.4#

This is consistent with an #"E1"# elimination in which the #"C-H(D)"# bond is not being broken in the rate determining step.