Isaiah 6 is strategically located between 1-5 and 7-12

Isaiah 1-5 raises a serious problem. Sinful, arrogant Israel is going to be the holy people of God to whom the nations will come to learn of God (Isa 2:1-4; 43:8-14; 49:5-6; Ezek 36:22-38). But how can this be? Isaiah 6 provides the solution. Sinful Israel can become servant Israel when the experience of Isaiah becomes the experience of the nation. When the nation has seen itself against the backdrop of God's holiness and glory, when the nation has received God's gracious provision for sin, then she can speak for God to a hungry world. (Ch. 60-66 follow immediately upon the promise of 59:21.) If ever Isaiah's experience should be duplicated on a national scale, then the promises of Isa 1:16-19; 2:1-4; 4:2-6 could be experienced.

Isaiah 6 is not merely the conclusion of ch.1-5. One of the marks of the book's style seems to be its smooth transitions, so smooth that it is frequently difficult to decide whether an element is the final one in the preceding segment or the initial one in the following. For just as ch.6 is the conclusion to ch.1-5, it is also the introduction to ch.7-12. In a real sense the, ch.7-12 are a fulfillment and an explication of the word given to Isaiah in his call--the hardening impact of Isaiah's gracious invitation to trust. In a real sense the destruction coming to Judah from Assyria's hand toward the end of the 8th century B.C. is caused by Ahaz's refusal of Isaiah's invitation (8:6-8; 6:11-12).

So it is impossible to link Isaiah 6 solely to ch.1-5 or solely to ch.7-12. It functions with both sections, both showing the way of hope for the future (ch1-5) and explaining the present situation (ch.7-12). It is a genuinely strategic chapter, shaping and defining the book as a whole.

The vision of 6:1-8 is clearly fundamental to the entire course of Isaiah's ministry and to the shape of his book. When Isaiah saw the Sovereign, he saw him in such a way as to change the shape of the rest of his life. The glory, majesty, holiness and the righteousness of God became the ruling concepts of his ministry. It also explains Isaiah's contempt for, and horror of, any kind of national or individual life which did not pay adequate attention to the one God.
The Book of Isaiah, Chapters 1-39 (p. 174-176) by John N. Oswalt.