Can I give the col-md-1.5 in bootstrap?

You cloud also simply override the width of the Column...

<div class="col-md-1" style="width: 12.499999995%"></div>

Since col-md-1 is of width 8.33333333%; simply multiply 8.33333333 * 1.5 and set it as your width.

in bootstrap 4, you will have to override flex and max-width property too:

<div class="col-md-1" style="width: 12.499999995%;
    flex: 0 0 12.499%;max-width: 12.499%;"></div>

As @bodi0 correctly said, it is not possible. You either have to extent Bootstrap's grid system (you can search and find various solutions, here is a 7-column example) or use nested rows e.g. http://bootply.com/dd50he9tGe.

In the case of nested rows you might not always get the exact result but a similar one

<div class="row">
    <div class="col-lg-5">
        <div class="row">
            <div class="col-lg-4">1.67 (close to 1.5)</div>
            <div class="col-lg-8">3.33 (close to 3.5)</div>
        </div>    
    </div>
    <div class="col-lg-7">
        <div class="row">
            <div class="col-lg-6">3.5</div>
            <div class="col-lg-6">3.5</div>
        </div>    
    </div>
</div>