It was slow moving first period, with a half-chance for Manuel Lanzini before Mesut Ozil opened the scoring for the Gunners after 24 minutes. There was hope for West Ham early in the second period, with a good chance and save from Ashley Fletcher and Darren Randolph respectively. However a avalanche of goals from Sanchez, Andy Carroll, Alex Oxlade-Chamberlain and Sanchez again secured an excellent three points for Arsene Wenger's side.
Slow to get into gear
Both these sides were at opposite ends of the table ahead of this derby clash at the London Stadium, but both were equal on performance in the opening periods of the contest.
A half-chance emerged in the 13th minute after good work from Nacho Monreal, with the Spaniard putting a dangerous ball across the face of goal. It looked all set up for Sanchez to tap home, but Winston Reid was there to block.
The Hammers came the closest in the 19th minute, after Dimitri Payet was allowed to show his magic after good hold-up play by Fletcher. The Frenchman played Lanzini through on goal, although the Argentine was forced wide by Laurent Koscielny, he still produced a decent save from Petr Cech.
Drawing first blood
However it didn't take long for the visitors to get into first gear, and they proved critical as they took the lead 24 minutes in. It was poor from the Hammers as Angelo Ogbonna's clearance was deflected off Francis Couquelin and into the path of Sanchez. The Chilean turned off the gas, with his touch sending Reid flying, and he squared it to Ozil to tap it past Randolph.
Sanchez then looked to turn scorer in the 36th minute, when he took it beyond the on-rushing Randolph, but a heavy touch allowed Reid to get back and protect the ball.
From up to down
Slaven Bilic's side looked a lot more positive as they came out for the second period, and looked like that they might have opportunity to get themselves back in the game.
Fletcher was full of confidence after his first Hammers goal in the 4-1 defeat to Manchester United, and it showed has he fizzed one from distance in the 55th minute but was just wide of the mark.
It looked to be all but over the 69th minute, when Monreal managed to get the ball to substitute Aaron Ramsey. However he was thwarted by Randolph, who went one better to save Ozil's volleyed follow-up.
However it seemed their fate was sealed in the 72nd minute, when Arsenal added their second of the match. It was an excellent solo effort from Sanchez, as in a blink of an eye he had turned Arthur Masuaku and had given Randolph no chance even though was very much against him.
It looked like the three points were wrapped up from there, but contest was far from over as four goals came within a astonishing seven minute period.
The first seemed to be nail in the coffin, as it proved too easy for Sanchez as he let one rip from 18 yards and his low effort beat Randolph, and sent Hammers fans out in their droves.
Those who stayed were treated in the 83rd minute with a slight glimmer of hope. Payet was very unlucky not to score with his free-kick clattering the woodwork, but he returning Carroll was there to turn home the header from close-range.
However any hope was extinguished a minute and in some spectacular style. Oxlade-Chamberlain looked to go one better than his Chilean team-mate, as he fired a rocket from 25 yards which left Randolph scratching his head.
The romp was complete in the 87th minute, and it was in some style. There was a hint of offside as Sanchez was put through, but it didn't bother the forward as he planted Randolph on the floor, before chipping it over the Irishman and into a empty net.