# Why is phenolphthalein at pH 10 non-planar?

Phenolphthalein is colorless when pH = 0-8.2 and pink when pH = 8.2-12.

Why does the ring bearing the carboxylate (COO-) group not lying on the same plane as the other 2 rings?

• In fact, the other two rings aren't exactly in the same plane either. – Ivan Neretin Sep 17 '17 at 12:14
• I suggest drawing it out on paper with constant bond lengths and perfect $120^\circ$ angles. ;) – Jan Sep 17 '17 at 12:36
• @Jan I suggest making this an answer. – Oscar Lanzi Sep 17 '17 at 18:14
• @OscarLanzi Yes, but I didn’t have access to ChemDraw when I commented (and won’t until tomorrow morning). In the meanwhile, DSVA has provided a very nice answer that I could hardly add anything to … – Jan Sep 18 '17 at 15:10