In case of Ludwig's angina, if 2nd premolar and 1st molars are involved, then the first involved space is:
When an infection perforates bone to establish drainage, it seeks the path of least resistance. Since the outer cortical plate of the mandible is thick in the molar region, the lingual plate is the one most frequently perforated. According to the studies of Tschiassny, initial infection of the submandibular space, particularly in cases of the second and third molars, is due to the fact that the apices of these teeth are situated below the mylohyoid ridge in 65% of cases. He also noted that because the apices of the roots of the first molar are above this ridge in about 60% of the cases, infection of the sublingual space is most common in cases of infection of this tooth. 
Ref: Shafer's textbook of oral pathology 7th edition page 510
Submaxillary space
Sublingual space
Submental space
Parotid space