( ( Reactions ( with oxygen ) ) ( ( ( are sometimes ) ( ( referred to ) ( as ( combustion reactions ) ) ) ) . ) )
(ROOT (S (NP (NP (NNS Reactions)) (PP (IN with) (NP (NN oxygen)))) (VP (VBP are) (ADVP (RB sometimes)) (VP (VBN referred) (S (VP (TO to))) (PP (IN as) (NP (NN combustion) (NNS reactions))))) (. .)))
Reactions with oxygen are sometimes referred to as combustion reactions.
(ROOT (S (NP (NNP Combustion) (NN reaction)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (NN substance)) (VP (VBZ reacts) (ADVP (RB quickly)) (PP (IN with) (NP (NN oxygen))))))) (. .)))