The prototypical surface reaction of 10,10'-dibromo-9,9'-bianthryl (DBBA) towards the 7-atom-wide armchair graphene nanoribbon (7-AGNR) on the Au(111) surface has been investigated by means of vibrational spectroscopy, thermal desorption spectroscopy and density functional theory. Specifically, a direct correlation between annealing temperature and the formation of various intermediate species is derived. By comparing IR spectra with results from DFT calculations an identification of reaction intermediates has been achieved, allowing for a precise mapping of individual reaction steps. Thereby, we identify a prior unknown partially dehalogenated and strongly tilted DBBA* mono-radical species (DBBA-1Br) after mild annealing (380-450 K). This inclined adsorption geometry stabilizes the second Br atom, preventing full dehalogenation. According to our calculations dimerization of DBBA-1Br reverses this inclination and provides an efficient way to enable abstraction of the second Br atom and initiate polyanthracene chain formation by means of DBBA* addition. As a competitive process to dimerization a minor amount of the DBBA* mono-radicals recombine with previously released bromine atoms leading to associative DBBA desorption. In the course of thermal processing hydrogen release signals planarization of the twisted anthracene units of DBBA by means of cyclodehydrogenation and formation of the final 7-AGNR. Based on the results of this work an in-depth understanding of the on-surface synthesis of 7-AGNR on Au(111), starting with a DBBA molecular precursor, has been attained.