According to Apple Support website these Mac models are compatible with OS X El Capitan:
MacBook (Early 2015)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
iMac (Mid 2007 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)
Any previous model is not compatible and you won't be able to install El Capitan on it. If you have any questions regarding Operating System compatibility you can always call us at 1-800-Geek-Squad (1-800-433-5778) 24 hours a day 7 days a week.
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Geek Squad Agent Marcos P. | Badge 87761
Consultation Agent | Best Buy Store 1090
Geek Squad Social Media Team | www.geeksquad.com/support
9.2.16