Not much of a punch really. Ill take a "punch" like that any time over Ivanovic's vicious elbows. to me it looked more like Sanchez was trying to grab Ivanovic's shirt than punch him but I might not have seen the angle you did...
Accidentally? nonsense. Ivanovic doesn't have to see Benteke to know where he is as their bodies were touching and Ivanovic completely leads with his elbow as he knows Benteke is there before they jumped for the ball.On the other hand,when I closely looked foul on Benteke last year, you can see that Ivanovic eyes were set on ball and that he hadn't intention to harm opponent player. It simply happened accidentally and that's why he got yellow card for being careless.
To me Ivanovic's elbow looks a lot more vicious than what Sanchez did.
And Ivanovic is usually in the top 10 in yellows and reds so clearly he fouls more than the average defender and is definitely not as "clean as a right back can be" as you claimed.