Screencast 19.1
https://www.youtube.com/watch?v=3QMz7_iP5KA
In this screencast, Andrew Parsons defines resonance and before using resonance to explain reactivity.

Screencast 19.2
https://www.youtube.com/watch?v=617wKp3LFpw
In this screencast, Andrew Parsons discusses the significance of orbital overlap in reaction mechanisms.

Screencast 19.3
https://www.youtube.com/watch?v=rl4BJWy0T8U
In this screencast, Andrew Parsons walks you through guidelines for drawing reaction mechanisms.

Screencast 19.4
https://www.youtube.com/watch?v=BCowhSn4ofc
In this screencast, Andrew Parsons introduces you to different types of selectivity in reactions and the way in which they determine how reactions proceed.