Portraits can be shot at any focal length, any distance. The 'right' portrait lens depends on 1) what you mean by 'portrait', 2) what effects you want to achieve, 3) comfortable working distance(s) for you and your subject(s), and 4) budget.
1) Are your portraits formal sittings or stealthy streetshots or informal snaps or what? General guidelines: On APS-C, shoot context portraits at 15-24mm; full-body at 24-40mm; 3/4-1/2 body at 40-60mm; head+shoulders or close headshots at 60-90mm; further headshots at 90-150mm.
2) Do you want thin-DOF soft romantic shots? Use a fast lens. Do you want sharp detail? Use as slower|macro lens. My personal preference for H&S and headshots is around 80/3.5 for best modeling of human features. My old M42 Sears-Tokina 55-135/3.5 (US$8) is my favorite people zoom. A Vivitar-LU 75/3.5 enlarger lens (US$7) on extension is my favorite H&S 'prime'. My Jupiter-9 85/2 ($more$) sees a fair amount of use too.
3) If you and/or subjects are uncomfortable with being too close, use a longer lens.
4) The modern equivalent of my afore-mentioned 55-135/3.5 is the DA*50-135/2.8 that only costs 100x as much. Ouch. We usually don't want zooms that are slow on the long end, not for portraiture -- not enough DOF control. But fast zooms aren't cheap. How much can you spend?
Portraits from full-body to far-headshot are best shot in the 35-135mm range. Preferred apertures are in the f/2-f/4 range. Fitting those parameters to a new AF zoom requires a large budget. Older MF zooms or a set of primes may be more cost-effective -- unless you NEED autofocus. Hay, it's only money!