Ritalin (methylphenidate) is a substituted phenethylamine, and so are all amphetamines as well as MDMA. Cocaine, on the other hand, is a rather complicated ester, with no close relationship to any of the phenethylamines.

The similarities between Ritalin, amphetamine and MDMA should be clear when comparing their chemical diagrams:

(Methylphenidate / Ritalin)

(Methamphetamine)

(MDMA / Ecstasy)

As seen above, each of these phenethylamines is a variation on the same chemical skeleton:

(Chemical skeleton for substituted phenethylamines).

cocaine is vastly different, and does not include the same skeleton as MDMA, Ritalin, or amphetamines:

By the way, Ritalin was a blessing for me--for the first time in my life I was able to excel in school, and accomplish all the things I had wanted to do (such as homework and hobby projects) but was prevented from doing by my inability to focus. It's not for everyone, to be sure; and it is a very powerful medicine, which must be respected lest it wreak havoc rather than heal.