The reaction of nitroalkanes with nitrous acid ($HNO_2$) depends on the presence of $\alpha$-hydrogen atoms. Primary nitro compounds (having two $\alpha$-hydrogens) react to form nitrolic acids, which dissolve in alkali to give a red solution. Secondary nitro compounds (having one $\alpha$-hydrogen) react to form pseudonitrols, which give a blue colour. Tertiary nitro compounds ($R_3C-NO_2$) do not have any $\alpha$-hydrogen atoms attached to the carbon bearing the nitro group, and therefore, they do not react with nitrous acid.