What is macbeth's reaction when duncan reveals his plan to name his son malcolm as the heir to the throne

Macbeths reaction is to prevent Malcolm from becoming king - leading him to think about committing murder and treason.   "The prince of Cumberland: that is a step On which I must fall down, or else o'erleap For in my way it lies." this is showing how Malcolm is in his way of becoming king as the witches prophesied.  

When Duncan reveals his plan to name his son Mlacolm as the heir to the throne, Macbeth starts speaking aside and secretly reveals that Duncan's plans are in the way of his own desires.

Explanation: In Act 1, scene 4 from Shakespeare's Macbeth, Macbeth along with Banquo, Ross and Angus goes to see Duncan. In this meeting, the King reveals that he will name his son Malcolm Prince of Cumberland. When Macbeth finds out about this, he speaks aside and starts thinking that Malcolm's future title would give Duncan's son enough power to become King one day. As a result, Macbeth secretly reveals that Duncan's plan goes against his biggest ambition but, at the same time, he hopes no one can see his dark desires ("For in my way it lies. Stars, hide your fires;  Let not light see my black and deep desires").

a. the scene is structured as a dialogue between two characters in their own informal language.