( Finally ( , ( ( the ( least one ) ) ( ( attacks ( ( another ( chlorine molecule ) ) ( to ( produce ( ( ( a ( sulfonyl chloride ) ) and ) ( ( a ( new ( chlorine atom ) ) ) ( which ( continues ( the ( reaction chain ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Finally)) (, ,) (NP (DT the) (JJS least) (NN one)) (VP (VBZ attacks) (S (NP (DT another) (NN chlorine) (NN molecule)) (VP (TO to) (VP (VB produce) (NP (NP (DT a) (JJ sulfonyl) (NN chloride)) (CC and) (NP (NP (DT a) (JJ new) (NN chlorine) (NN atom)) (SBAR (WHNP (WDT which)) (S (VP (VBZ continues) (NP (DT the) (NN reaction) (NN chain))))))))))) (. .)))
Finally, the least one attacks another chlorine molecule to produce a sulfonyl chloride and a new chlorine atom which continues the reaction chain.
(ROOT (S (ADVP (RB Replace)) (NP (NP (DT another)) (PP (IN in) (NP (DT a) (NN molecule)))) (VP (VBZ happens) (PP (TO to) (NP (NNS atoms))) (PP (IN during) (NP (DT a) (NN substitution) (NN reaction)))) (. .)))
Replace another in a molecule happens to atoms during a substitution reaction.