SCSS

HTML

<div class="content">
  <div class="a">
    title
  </div>
  <div class="b">
    here on mobile, sticky sidebar on desktop
  </div>
  <div class="a">
    title
  </div>
  <div class="a">
    title
  </div>
   <div class="a">
    title
  </div>
   <div class="a">
    title
  </div>
</div>

SCSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

.content {
  width: 100%;
  max-width: 800px;
  margin: 0 auto;
}

.a, .b {
  padding: 15px;
  width: 200px;
  height: 100px;
  margin: 0 auto;
  display: block;
}

.a {
  background: grey;
}

.b {
  background: red;
}

@media (min-width: 768px) {
  .a {
    position: sticky;
    top: 0;
    right: 0;
  }
}