Editing Formaldehyde (section)

===Hydroxymethylation and chloromethylation===
Formaldehyde reacts with many compounds, resulting in [[hydroxymethylation]]:
:X-H + CH<sub>2</sub>O → X-CH<sub>2</sub>OH{{nbsp|5}}(X = R<sub>2</sub>N, RC(O)NR', SH).

The resulting hydroxymethyl derivatives typically react further. Thus, amines give [[hexahydro-1,3,5-triazine]]s:
:3{{nbsp}}RNH<sub>2</sub> + 3{{nbsp}}CH<sub>2</sub>O → (RNCH<sub>2</sub>)<sub>3</sub> + 3{{nbsp}}H<sub>2</sub>O

Similarly, when combined with [[hydrogen sulfide]], it forms [[trithiane]]:<ref>{{OrgSynth|last1=Bost|first1=R. W.|last2=Constable|first2=E. W.|title=''sym''-Trithiane|prep=cv2p0610|volume=16|pages=81|year=1936|collvol=2|collvolpages=610}}</ref>
:3{{nbsp}}CH<sub>2</sub>O + 3{{nbsp}}H<sub>2</sub>S → (CH<sub>2</sub>S)<sub>3</sub> + 3{{nbsp}}H<sub>2</sub>O

In the presence of acids, it participates in [[electrophilic aromatic substitution]] reactions with [[aromatic compound]]s resulting in hydroxymethylated derivatives:
:ArH + CH<sub>2</sub>O → ArCH<sub>2</sub>OH
When conducted in the presence of hydrogen chloride, the product is the chloromethyl compound, as described in the [[Blanc chloromethylation]]. If the arene is electron-rich, as in phenols, elaborate condensations ensue. With 4-substituted phenols one obtains [[calixarenes]].<ref>{{OrgSynth|authorlink1=C. David Gutsche|last1=Gutsche|first1=C. D.|last2=Iqbal|first2=M.|title=''p-tert''-Butylcalix&#91;4&#93;arene|prep=cv8p0075|year=1993|collvol=8|collvolpages=75}}</ref> Phenol results in polymers.