In my textbook, resonance is presented rather extensively in Chapter 1 (“Atomic and Molecular Structure”), ultimately teaching students how to draw all resonance structures of a given species. I like to teach resonance to that depth early in the course because it reinforces topics that are vital to student success throughout the entire year of organic chemistry: the octet rule, formal charge, and curved arrow notation. Because each new resonance structure requires valence electrons to be rearranged, each time students draw a resonance structure, they must re-consider whether an atom’s octet has been exceeded, and must also re-valuate the formal charges on atoms. And to properly indicate how to move those valence electrons, students must develop a working knowledge of curved arrow notation—the same curved arrow notation that will facilitate their mastery of reaction mechanisms later on. When it comes to practicing these important aspects of organic chemistry, the earlier and more frequent the better.
I also like to teach resonance in Chapter 1 because the topic tends to be formidable for many students. The rest of the material in Chapter 1 is largely review from general chemistry and is relatively light, giving students plenty of time to focus on, and master, resonance when they first encounter it. If it is delayed significantly, then students would first encounter resonance in the midst of other challenging topics, and that would risk compromising students’ abilities to learn each topic well. This idea—avoiding bombarding students with multiple topics at the same time—is a theme that runs throughout my book. It is the reason why nomenclature and physical properties are taught in their own chapters, separate from reactions and mechanisms. It is also the reason that reactions that proceed by the same or similar mechanisms are taught together in the same chapter. Even though I like to teach resonance early, I know that some instructors prefer to delay teaching the topic to significant depth until after students get their bearings in the course. This is something that is certainly feasible with my book, at least until Chapter 6, where resonance is vital to understanding the acid strengths of key functional groups such as carboxylic acids, phenols, and ketones.
Some instructors, moreover, prefer to teach conjugation before introducing resonance, given that a more complete understanding of MO theory gives students better insights into resonance. This is also something that can be done with my book. It would just require Chapter 14 (conjugation and aromaticity), which is essentially self-contained, to be taught immediately before Chapter 6 (acid/base chemistry).
Should you choose to delay the introduction of resonance, I would simply recommend caution. Be aware that the longer the introduction of resonance is delayed, the fewer opportunities students have to practice the octet rule, formal charge, and curved arrow notation. And be aware that, if resonance is introduced at a time students are learning other challenging topics, students could become overwhelmed.