Triazolopyrimidine herbicides

The invention provides herbicidal triazolesulphonamides, processes for their preparation and compositions containing them, the compounds being of the formula: and the salts thereof, where Ra represents hydrogen and Rb represents a group R³CONHCR²=CR¹-, or Ra and Rb together represent a chain of formula<br /> -C(R³)=N-C(R²)=C(R¹)- or<br /> -C(=Y)-N(R⁶)-C(R²)=C(R¹)-, where R¹, R² and R³, which may be the same or different, each represent hydrogen, hydroxy, mercapto, halo, cyano, amino, alkylamino, dialkylamino, or substituted or unsubstituted alkyl, aryl, aralkyl, alkoxy or alkylthio, Y is oxygen or sulphur, and R⁶ is hydrogen or a substituted or unsubstituted alkyl or aryl group; or R¹ and R² together represent -CH=CH-CH=CH- or -(CH₂)n- where n is 3, 4 or 5; R⁴ represents substituted phenyl or a substituted or unsubstituted aralkyl or heteroaryl group; and R⁵ represents hydrogen, acyl, carboxy, lkoxycarbonyl, alkylsulphonyl, or a substituted or unsubstituted alkyl, alkenyl, alkynyl, aryl or aralkyl group, or a group of the formula: where Ra and Rb are as defined hereinbefore.