Screencast 20.1
https://www.youtube.com/watch?v=Thb1K_se084
In this screencast, Andrew Parsons walks you through the mechanisms behind the activation of alcohols.

Screencast 20.2
https://www.youtube.com/watch?v=gIVddEzZmdM
In this screencast, Andrew Parsons compares substitution and elimination reaction mechanisms.