On the surface alienation and detachment might seem to be describing the same thing. In both cases one is putting some sort of distance between "self" and "object." But the difference is that "detachment" is a positive process of recognizing the distinctions between "self" and not-self. But "alienation" represents opposition. The alienated individual or group no longer appreciates, values, or wants to be associated with some other group that is in actual fact materially connected to him/her or them. Thus, alienation while it might function similar to detachment is in reality it's opposite. The alienated individual is tied all the more thoroughly to the something that he is "alienated" from, and those ties are precisely the kind of negative ones that "detachment" is aiming to sever.
Let's bring this from the abstract to the concrete by example....
For example, when two people, say high school sweethearts (I have a lot of examples from Saturday night) are "alienated" from one another. That usually precedes the divorce and is a long miserable process. They fight over resources, they struggle, they argue, and in the end they seperate. But is the seperation ever complete? No. They fight over all the things that once bound them together; children, property, etceteras. And when they are done they are full of resentments of one another.
On the other hand detachment represents something different, at least in the Buddhist teachings. It is accompanied by "mindfulness" and "loving kindness" and represents the voluntary letting go not of the objective ties, but of the illusory, delusory, and subjective ones. You cannot claim detachment if you are full of anger and resentment. Whereas "alienation" represents nothing but.
Buddhism offers the world certain "techniques" of thought and self-control that are wonderful if properly understood.