Q.Complete the following reaction: pentan-2-one (CH3-CH2-CH2-CO-CH3), treated with propane-1,3-diol (HO-CH2-CH2-CH2-OH) in the presence of dry HCl, gives ?

Step 1. Pentan-2-one, CH3-CH2-CH2-CO-CH3, is a ketone; propane-1,3-diol, HO-CH2-CH2-CH2-OH, supplies the SAME TWO nucleophilic alcohol oxygens that two SEPARATE methanol molecules supply in the book's own acetal-formation example (Section 12.5.A.3), but here tethered together within one molecule.

Step 2. Under dry HCl catalysis, the ketone's carbonyl oxygen is protonated, activating the carbonyl carbon toward nucleophilic attack.

Step 3. One -OH of the diol attacks intermolecularly first, giving a hemiketal; the resulting hydroxyl is then protonated and leaves as water, generating a resonance-stabilised oxocarbenium ion.

Step 4. Because the diol's SECOND -OH is tethered just three atoms away, it attacks that same cationic carbon INTRAmolecularly (rather than requiring a second separate alcohol molecule, as in the simple two-methanol case), closing a six-membered ring. …

Recognise this as the same acetal/ketal-forming addition of two alcohol oxygens across a carbonyl (Section 12.5.A.3), except that here both oxygens come from the SAME diol molecule, so the second addi …

• Treating the diol's two -OH groups as though they must react with TWO separate ketone molecules (an intermolecular polymer) rather than recognising that the diol's own three-carbon tether lets its second -OH close back ont …