Note: many eminent scholars have over many years denied that the book of Ecclesiastes has a well ordered structure. We can probably attribute that view to their lack of awareness of the significance of inverted parallelism in Scripture. The following structure is adapted from Dorsey.
Title: author mentioned in third person 1:1
A. the brevity and insignificance of life 1:2-2:26
B. wisdom’s failure to discover life’s meaning 1:12-2:26
C. regarding time 3:1-15
D. fear God 3:16-6:12
C’. time revisited 7:1-14
B’. wisdom’s failure revisited 7:15-10:19
A’. life’s brevity revisited 10:20-12:8
Conclusion: author mentioned in third person 12:9-14
Also, Labuschagne points out that Ecclesiastes has a very precise arithmetic structure and says “With regard to the numerical structure of Ecclesiastes Duane Christensen has remarked: “Qohelet appears to be the most finely crafted numerical composition in the Bible.”
6:12 is the mathematical centre of the book.