Hydroxymethylene cation

Novolacs are prepared with an excess of phenol over formaldehyde under acidic conditions (Fig. 7.6). A methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a hydroxymethylene cation (step 1 in Fig. 7.6). This ion hydroxyalkylates a phenol via electrophilic aromatic substitution. The rate-determining step of the sequence occurs in step 2 where a pair of electrons from the phenol ring attacks the electrophile forming a car-bocation intermediate. The methylol group of the hydroxymethylated phenol is unstable in the presence of acid and loses water readily to form a benzylic carbo-nium ion (step 3). This ion then reacts with another phenol to form a methylene bridge in another electrophilic aromatic substitution. This major process repeats until the formaldehyde is exhausted. 

The selective UV absorption of imidazole-4-carbaldehyde has been referred to resonance structures such as (92), which are improbable in the cation. In addition, a hydroxymethylene form can be excluded. 

The conjugated acid of formaldehyde, i.e., the hydroxymethylene cation, acts as an electrophile. 

Fructose can be readily transformed into hydroxymethylene-imidazole according to a reaction developed by Trotter and Darby [23]. Alkylation leads to imidazolium cations (Scheme 5.5-5) [24]. 

---