Physical properties of nitro compounds

The structure of a nitro group can be represented as a resonance hybrid of two equivalent zwitter ionic polar structures.

The hybrid structure has a positively charged nitrogen and two equivalent negatively charged oxygens. Thus the two N-O bonds have equal bond lengths (121 pm in nitro methane) which is intermediate between N-O single bond (114 pm) and N = O double bond (146 pm).

i) Nitro compounds have large dipole moments compared to other simple organic compounds. Due to the polarity, their boiling points are unusually high in comparison with other compounds of some molecular mass.

ii) Lower nitro alkanes are colorless liquids with a pleasant smell. Higher members are solids. Aromatic nitro compounds like nitro benzene has a smell of bitter almonds.

iii) Nitro alkanes are sparingly soluble in water (lower members are relatively more soluble than the higher members). Aromatic nitro compounds are not soluble in water. They are soluble in organic solvents.

iv) The formal positive charge on nitrogen of nitro group makes it a strong electron withdrawing group and exerts a strong pull on neighboring electrons. The presence of nitro group in phenols enhances their acidity. In aromatic compounds, the nitro group deactivates then towards electrophilic substitution.