Two things to keep in mind with avocados: they produce very variable seedlings from pits and they need well draining soil.
From a recently purchased package of 6 avocados I put all 6 pits on the surface of soil in pots and one simply rotted and the other 5 split and produced roots and shoots. The heights of the stalks before producing leaves varied from 3 inches to about 16 inches, so having a long stalk and no leaves is not that unusual. Later on when well established you can prune them back.
The dead leaves are probably a result of having soil that is too heavy and lacking air pores. Avocados need to be moderately watered in a somewhat humid environment, but with that they need to have very free draining soil. Consequently you are looking for something like a cactus mix with a high proportion of sand and small rocky pieces. Regular peaty mixes will hold on to too much water and exclude air from the roots. Unlike many tropical plants that have thick leaves with a shiny waxy surface avocados have thin leaves that transpire easily and can lose a lot of moisture very quickly, calling on the roots to be healthy and active; transpiration rate will go up considerably if the air is dry.