( ( ( A class ) ( of ( four ( tri-halogenated compounds ) ) ) ) ( often ( ( formed ( as ( by-products ( of chlorination ) ) ) ) . ) ) )
(ROOT (S (NP (NP (DT A) (NN class)) (PP (IN of) (NP (CD four) (JJ tri-halogenated) (NNS compounds)))) (ADVP (RB often)) (VP (VBD formed) (PP (IN as) (NP (NP (NNS by-products)) (PP (IN of) (NP (NN chlorination)))))) (. .)))
A class of four tri-halogenated compounds often formed as by-products of chlorination.
(ROOT (S (NP (EX There)) (VP (VBP are) (NP (NP (CD four) (NNS classes)) (PP (IN of) (NP (JJ biochemical) (NNS compounds))))) (. .)))