The touchpad in my Alienware 13 R2 used to work normally in Ubuntu 14.04 and 15.04, but it gets totally unresponsive when upgraded to Ubuntu 16.04. It seems this is a frequently-met problem on laptops that using Synaptics touchpad. After searching for solutions for a while, I found a solution from a dicussion thread in the Ubuntu bug report center:

Hi guys I found an solution:

Type in the terminal

sudo su
echo 'blacklist i2c_hid' >> /etc/modprobe.d/blacklist.conf
depmod -a
update-initramfs -u

and reboot

Hope did work for you!!

And this truely works for my laptop.