What is the effect of changing all the hydrogen atoms in benzene to deuterium? For example, which of the following reactions occurs at a faster rate?
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$\begingroup$ If we were to still compare, the $Ar H$ molecule would take the $NO_2 +$ ion easiest right? $\endgroup$– kushalNov 7, 2020 at 14:26
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2 Answers
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Page 513 of Organic Reaction Mechanisms by Bansal references the work of Melander in Arkiv för Kemi, 2, 211 (1950). This states that the rates of nitration for benzene vs benzene-d6 are about the same, implying that the addition of the nitronium ion is rate controlling and not the weakening of the C-H / C-D bond. However in case of sulphonation H containing benzene reacts faster than D containing benzene.
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$\begingroup$ This would imply an early transition state for the nitronium-benzene complex. $\endgroup$ Mar 18, 2016 at 1:43
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This is an example of an inverse kinetic isotope effect.
When a transition state increases in coordination then differences gained from vibrational bending energy for the $\ce{C-D}$ vs $\ce{C-H}$ bond favor $\ce{C-D}$, lowering the transition state energy relative to starting materials, and increasing reaction rate.
In this case, the intermediate is the protonated benzene changing a reaction center from $\mathrm{sp^2}$ to $\mathrm{sp^3}$, increasing coordination.
The perdeuterated benzene will react faster. The effect will probably be on the order of half again as faster.
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$\begingroup$ "order of half again as faster.", can you explain it a bit $\endgroup$– user10153Mar 18, 2016 at 6:51
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$\begingroup$ I would guess the rate constant to be 50% higher. However, Beerhunter's data says my guess is incorrect. $\endgroup$ Mar 18, 2016 at 21:20
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$\begingroup$ I would refer to the book and see what happens $\endgroup$– user10153Mar 19, 2016 at 9:50
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