# Replacing factors that are only dependent on a single specific variable

I have expressions with terms that are products of factors. Some of the factors are expressions that depend on a single specific variable. I would like to replace those factors with constants and create a separate list with the replaced constants. An example:

(L Myb)/EIy + (L Myb Rzc^2 (12 + la^2 - 6 la Coth[la/2]))/(12 GJ la^2)


This example only has two terms. The second term has the factor

(12 + la^2 - 6 la Coth[la/2])/la^2


that is a function of la. I would like to replace all factors that only depend on la with constants, so turning the expression into

(L Myb)/EIy + c1 (L Myb Rzc^2)/(12 GJ)


and into a list

c1 -> (12 + la^2 - 6 la Coth[la/2])/la^2


How can I do this? In particular, I am struggling to match "any complicated" function of la.