Question

Question:When UMP is dissolved in T₂O, exchange of T for H occurs at the 5-position. Propose a mechanism for this exchange.

Short answer

When T₂O attacks ribose phosphate, tritium adds to C₅, lone pair of electrons on nitrogen atom moves to form a double bond between nitrogen and C₆. The double bond between C₅ and C₆ is also shifted.

Step-by-step solution

Attack of at ribose-5-phosphate

When T₂O attacks ribose phosphate, tritium adds to C₅, lone pair of electrons on nitrogen atom moves to form a double bond between nitrogen and C₆. The double bond between C₅ and C₆ is also shifted.

Step 2: Exchange of hydrogen with tritium

Now, the oxygen-tritium bond again attacks the hydrogen and it is lost. This bond is shifted and nitrogen regains its lone pair of electrons. Now at C₅, hydrogen is replaced with tritium. The whole mechanism is shown below: