Before you can assemble the large macro-molecules necessary for life you must have a ready supply of basic organic molecules. Imagine a primitive ocean. You need tons of sugars, amino acids, purines, pyrimidines, etc. There are a number of severe obstacles that must be overcome in getting a suitable ocean:
Quantities - The first problem is overcoming the diluting effect of a vast primordial ocean. For example, a study which assumed use of the entire atmospheric supply of nitrogen for molecular formation indicates insufficient concentrations would result (Gish 1972, 10-11).
Synthesis vs destruction - For chemical bonds to form there needs to be an external source of energy. Unfortunately, the same energy that creates the bonds is much more likely to destroy them. In the famous Miller experiment (1953) that synthesized amino acids, a cold trap is used to selectively isolate the reaction products. Without this, the would be no products. This poses a challenge to simplistic early earth schemes where lightning simply strikes a primitive ocean. Where is the "trap" in such an ocean? Also, the creation of amino acids by a chemist in a laboratory is still much different from forming self-replicating life.
This point has not escaped the attention of evolutionists. "The physical chemist, guided by the proved principles of chemical thermodynamics and kinetics, cannot offer any encouragement to the biochemist, who needs an ocean full of organic compounds to form even lifeless coacervates [blobs]" (D. E. Hull, Nature, 186, 693 1960)(Gish 1972, 13)
Incompatibility - Another problem is that different molecules will react with one another. For example,
amino acids and sugars combine and destroy each other. In lab experiments the component chemicals are neatly separated from one another. How is this possible in a primitive ocean?
Even the simpler molecules are produced only in small amounts in realistic experiments simulating possible primitive earth conditions. What is worse, these molecules are generally minor constituents of tars: It remains problematical how they could have been separated and purified through geochemical processes whose normal effects are to make organic mixtures more and more of a jumble. With somewhat more complex molecules these difficulties rapidly increase. In particular a purely geochemical origin of nucleotides (the subunits of DNA and RNA) presents great difficulties. In any case, nucleotides have not yet been produced in realistic experiments of the kind Miller did. (Cairns-Smith, 1985, p. 90).