How can i make a dropdown menu with bootstrap frame? e.g I need to put class into one of the li element.

<nav id="nav" class="l-nav align-left dropdown">
                    <ul>
                        <li> <a href="/">Home</a> </li>
                        <li> <a id="dLabel" type="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true" href="/">Services <span class="caret"></span> </a>
                            <ul role="menu" aria-labelledby="dLabel" class="dropdown-menu">
                                <li> <a href="services/index" title="Domestic/residential">Domestic / residential</a> </li>
                                <li> <a href="services/soundproofing-walls" title="Soundproofing walls">Soundproofing walls</a> </li>
                                <li> <a href="services/soundproofing-floors" title="Soundproofing floors">Soundproofing floors</a> </li>
                                <li> <a href="services/soundproofing-ceilings" title="Soundproofing Ceilings">Soundproofing Ceilings</a> </li>
                                <li> <a href="services/clubs-pubs" title="Clubs &amp; Pubs">Clubs &amp; Pubs</a> </li>
                                <li> <a href="services/cinemas-recording-studios" title="Cinemas &amp; Recording Studios">Cinemas &amp; Recording Studios</a> </li>
                                <li> <a href="services/acousticfence–motorways-roads-sports-grounds-factories-industrial-estates" rel="Acoustic fence – motorways/roads/sports grounds/factories/industrial estates">Acoustic fence – motorways / roads / sports grounds / factories / industrial estates</a> </li>
                            </ul>
                        </li>
                        <li> <a href="gallery">Gallery</a> </li>
                        <li> <a href="testimonials">Testimonials</a> </li>
                        <li> <a href="contact.php">Contact Us</a> </li>
                    </ul>
                </nav>