Question

Dimedone is a cyclic ketone. If Dimedone has M+ = 140 and contains 2 double bond(s) and 1 ring(s); what is its molecular formula?

Answer 1

Mo.Wt.= 140 by rule of 13 (divide 140 by 13,Take quotient as no.of C and quotient + remeinder as no.of H) hence its come out as C₁₀H₂₀ .

Given that its ketone so must contain Oxygen(s) and also known to have 1Ring and 2 double bond i.e total 3 Unsaturation sites.Taking this into account ,

First just 1-O atom added to formula. For that we need to substract 1C and 4H from C₁₀H₂₀ as it counts mass 16 as that of 1-O hence formula will be C₉H₁₆O.Still no.of unsaturation sites 2 only in C₉H₁₆O (you can count).Again need to add 1O-atom.Then formula will be C₈H₁₂O₂.

This formula fits all reqiurnments i.e in C₈H₁₂O₂. No.of unsaturation sites 3(since parent Hydrocarbon formula would be C₈H₁₈ so 6 H less i.e 3 sites of unsaturation. Formula as I remember is given in image file.

Molecular formula of dimeone C₈H₁₂O₂.