Would it be possible to have an organism to be based on alcohol, rather than water? Complexity does not matter, but a complex organism would be favourable. their environment would be in oceans of isopropyl alcohol on a relatively warm planet with very thin CO2 atmosphere and less than half earth's gravity.
Not naturally, no.
Isopropyl alcohol is a pretty good solvent for both polar and non-polar molecules, but not salts. So, it is certainly plausible to imagine a complex engineered biosystem, maybe with ionic cell membranes, that depends on it. Maybe you've got a two-phase system where cells form from bubble of isopropyl alcohol suspended in saltwater with a monolayer membrane.
But naturally-occurring pools, let alone oceans, or mostly-isopropyl-alcohol in which abiogenesis could occur spontaneously? No. Any natural abiotic processes that could produce significant quantities of isopropyl alcohol would also produce even larger quantities of a slew of other simple carbohydrates. You would end up with an enormously complex mixture of an ocean--and sure, life might form in that ocean, using a complex solvent or multiple interacting solvent phases, and once primitive life is established it might well go on to specialize on a purer, self-produced or at least self-segregated internal solvent... but I see no strong reason for such life to prefer specialization on and biotic production of oceanic quantities of isopropyl alcohol, over much simpler substrates like methanol or formaldehyde.
I’m sorry but isopropyl is typically used against microbial life; any bacteria (and presumably less complex Protists and Archaea, etc) and fungi are likely to be annihilated, introducing H2O will likely only seal their fate. However comments on the web imply that a pure solution free of water (90%+) may be less destructive and allow certain spore forms to lie dormant but this is something that goes beyond my knowledge of organic chemistry and I can really only quote the Internet.
The situation that you describe - oceans of isopropyl alcohol - are unlikely to occur naturally and if they did would be unlikely to remain uncontaminated for long.
The simple answer is we don't know and can't really tell with any certainty if organisms could evolve because the theoretical possibilities within chemistry and biology are so vast. I suspect not.
That said if the ocean was contaminated as it surely would be with water and other chemicals from erosion and reaction with the environment it is possible that a life form based on something other than isopropyl alcohol might evolve and extract the compounds that it needed from the ocean in the same way that Earth based life extracts oxygen from ours.
Yes and no in the same time. Alcohol and water mix in all proportions, so the world you create with pure alcohol oceans would quickly change to a mixture of water and alcohol. And we know that there are organisms that thrive in this mixture. We use them to make fermented beverages.