In this chapter, the radioactive tracers of the individual elements and their produc­tion are sketched briefly. In the case of tritium and C-14, the nuclear reactions, as well as the chemical syntheses of labeled organic compounds, are outlined.

Ion source

Figure 8.8 The scheme of a linear accelerator. Protons fly through the tubes operated with alternating voltage supply. The length of the tube is adjusted in relation to the frequency of alteration that the protons meet an accelerating voltage whenever they pass from one tube to the next.

8.6.1 Tritium

Tritium (3H) is the isotope of hydrogen and emits negative beta particles with 18.6 keV maximal energy. Its half-life is 12.3 years. Tritium is produced by the 6Li(n, a)3H nuclear reaction. A foil from an MgLi alloy is used as target; tritium is separated by heating. Tritium is applied as T2 gas or tritiated water (T2O) in the syntheses of different organic compounds.

• Alkanes can be produced by isotope exchange reactions of hydrogen in organic compounds:

Pd, heating

R — H 1 T2O ——— > R — T 1 THO (8.18)

• Alkane (methane) can be prepared in the reaction of tritiated water with aluminum carbide:

R — CHOH — COOH — NaO! CHI3 1 R21 (COOH)2 (8.19)

• Alkanes are formed in the Grignard reaction:

RMgX 1 T2O! RT 1 MgOTX (8.20)

• Double bonds can be saturated by tritium gas.

• Alcohols can be obtained by the reduction of carbonyl compounds with NaBT4 or LiAlT4:

4R2CO 1 NaBT4 1 2H2O! 4R2CTOH 1 NaBO2 (8.21)

• By the exchange of the labile hydrogen of malonic acid and decarboxylation, acetic acid is produced. In addition, the labeled malonic acid can be used in many organic syntheses.

• By the reaction of calcium carbide and tritiated water, tritium-labeled acetylene can be prepared, which can be a starting material in many organic syntheses.