Scott McDonald netted a double as 10-man Dundee United swept to their biggest Ladbrokes Championship win of the season with a 4-1 triumph over Brechin.United needed an 80th-minute strike from McDonald to edge past the bottom club on their previous trip to Tannadice and drew at Glebe Park in November, but the Australian drilled home the opener inside 60 seconds.Sam Stanton doubled United's lead after a seventh-minute corner and United were also quick off the mark in the second half - McDonald netting from close range inside four minutes before James Keatings converted a penalty three minutes later.Paul McMullan's second yellow card, for simulation, changed the game in the 57th minute and Jordan Sinclair struck a consolation from the spot seven minutes later.