Percieved wisdom is to use a zoomier prime rather than a wide lens as a wide lens would distort your subject's pictures - i.e. it might make a nose look bigger etc. But rules are there to be broken - that might be perfect for your needs.
The idea is that a longer focal length is more flattering to your subject because it will tend to compress features. The opposite is true of wide angle lenses, which tend to accentuate features and have distortion.
you could use a telephoto to make your subject thinner....or a well placed wide angle to make their chest area humungous..
All humour aside...try out a few different lengths. I find that my 70-200 works quite well for portraits.
Remember that shooting with a lens on a less than full frame camera isn't changing the optics of the lens but rather crops the image... The same way as if you were to crop the image in PS latter. Cropping is not changing the lens you use, but rather changes the amount of the image you are viewing. I could shoot almost exclusively with a 70-200 f2.8 and a 50 f1.4. Portraits look great with both depending on the surroundings. but in short,,, yes 85mm is the ideal.