By Tosin Omojola
The Chilean forward put the Gunners ahead before half-time as he controlled Mesut Ozil’s fine pass and calmly shot low from inside the area.
Gareth McCleary then leveled the scoreline for the Championship side as Wojciech Szczesny failed to keep out his low volley.
Reading’s dominance was only temporary; however, as Adam Federici pulled off a magnificent save to divert Gabriel’s header onto the crossbar.
Aaron Ramsey missed a golden opportunity to win the game in normal time as he struck the post from inside the six-yard box.
But despite his earlier heroics, Federici’s fine display ended in disappointment in extra-time as he let Sanchez’s low drive slip under his body and trickle across the line.
Arsenal had the chance to make sure of progression with eight minutes left to play but substitute Olivier Giroud could only hit the foot of the post with his low shot.