For Bootstrap 4 I find the following very handy because:
It is the combination of col and col-auto which does the magic. So you don't have to define a col width (like col-2,...)
<div class="row">
<div class="col">Left</div>
<div class="col-auto">Right</div>
</div>
Ideal for aligning words, icons, buttons,... to the right.
An example to have this responsive on small devices:
<div class="row">
<div class="col">Left</div>
<div class="col-12 col-sm-auto">Right (Left on small)</div>
</div>
Check this Fiddle https://jsfiddle.net/Julesezaar/tx08zveL/