According to the common definitions:
Rhetoric is persuasive speach.
Dialectic is sorting truth from opinion.
Truth is what persuades the justifiability of any and all opinion, and as such it is rhetorical in nature as the interplay of opinions sets the necessary definition to what something is and is not.
This tension of "being" and "nonbeing" resulting in definition is the persuasive power of any and all truth and as such necessitates dialog as an exhibition of force.
This force simply is form, this form being a continuum we call quality where any and all truth is irrational until it is divided into parts. This division, while a continuum in itself, into parts is the tautology that lends itself into an inherent circularity where one proposition is observed through many.
Philosophy is thus grounded into who can make the most impenetrable sphere of reasoning on any given point, yet fails to admit to this geometric origin as the sole underlying factor that determines its success or failure.
Dialect in its continual atomism not just negates itself under a perpetual manner of "assuming through division", but it effectively leads to the very multiplication of problems and languages its seeks to avoid. It is contradictory by nature with contradiction reflecting a basic seperation or tension of propositions.
Even the standard dialect, embodied through debate, never results in a form of agreement between parties but rather a creation of further tautologies that justify each parties stance. Under these terms it merely acts as a form of self-persuasion and the creation of a logical sphere allowing further dialect to bounce off any given stance when reinforced well enough by its recurssion of tautologies... or recursion if recursion.
In these respects the nature of Rhetoric, as persuasive, gains its groundings in a form of hypnotism where the same thing is repeated again and again from different angles. This hypnosis, through repitition, is the nature of truth itself where what is deemed true is observed as such if it is persuasive.
Dialectic is thus a self negating universal "or" function of the psyche, continually dividing assumptions until rhetoric is given form by uniting this chaotic mess much in the same manner a religious ritual gives rise to unity in ths psyche or between groups.
Rhetoric is thus a process of binding, with its persuasive quality being grounded in the simple presentation of a form. This form, persuasive by nature due to its self referential quality, acts as a filter through which one percieved the world and a such negates the dialectic as having any coherent value in and of itself other than a strict creation of perpetual fallacies that even it falls into.