I wish to comment on two points. I hope that these comments are helpful to you.
1. Focusing your argument
Avoid cramming too much information into the opening paragraph. Perhaps save the discussion of the chemical structures of Agent Orange for later in your essay. At this later point in your essay, you may find that you can write at least one full paragraph on how the chemical structures are important to consider. Consider sharpening your thesis statement; perhaps change “consequences” to “negative consequences” in the second-to-last sentence. Perhaps also consider changing “destroying Vietnam ecosystem” to “damaging the Vietnam ecosystem” in the final sentence.
2. Using sources responsibly
If you use sources, as a rule, you must cite them (in other words, you must name your sources and give them due credit). Also, as a rule, you must put quotation marks around material that is taken word-for-word from another source. Your opening line – “code name for one of the herbicides and defoliants….” – is taken word-for-word from Wikipedia.com. Other material in this first paragraph is also taken (without credit) from this source. Please talk with your teacher before turning in anything. Make sure that you know what is expected of you in terms of using sources (e.g. are you expected to cite your sources?). The enotes link given below discusses how to avoid plagiarism.
Best of luck with your essay! You’ve chosen a very interesting topic that should work well for a chemistry course.