Question

69.77% Carbon, 11.63% Hydrogen and the rest Oxygen is present in an organic compound. The molecular mass of the compound is 86. The compound does not reduce Tollen’s reagent but gives an addition compound with sodium hydrogen sulphite and gives positive iodoform test. On strong oxidation it gives ethanoic acid and propanoic acid. Write the structure of possible compound.

Hint:

Positive iodoform test always indicates the presence of a \(-COCH_3\) group or a secondary alcohol oxidisable to it.

Answer 1

Step 1: Percentage composition.
Given: \[ %C = 69.77, \quad %H = 11.63, \quad %O = 100 - (69.77 + 11.63) = 18.60 \] Step 2: Calculate empirical formula.
\[ \text{For C: } \frac{69.77}{12} \approx 5.81 \] \[ \text{For H: } \frac{11.63}{1} \approx 11.63 \] \[ \text{For O: } \frac{18.60}{16} \approx 1.16 \] Dividing all by 1.16 (smallest): \[ C : H : O = 5.81/1.16 : 11.63/1.16 : 1.16/1.16 \approx 5 : 10 : 1 \] So, empirical formula = \(C_5H_{10}O\). Step 3: Molecular formula.
Empirical formula mass = \(5 \times 12 + 10 \times 1 + 16 = 86\).
Since molecular mass = 86, the molecular formula is the same: \[ C_5H_{10}O \] Step 4: Functional group tests.
\begin{itemize} \item Compound does not reduce Tollen’s reagent → Not an aldehyde. \item Forms addition compound with NaHSO$_3$ → Presence of carbonyl group. \item Positive iodoform test → Presence of methyl ketone group (\(-COCH_3\)). \end{itemize} Thus, compound must be a ketone with structure \(R-COCH_3\). Step 5: Oxidation products.
On strong oxidation, compound gives ethanoic acid and propanoic acid. This indicates the structure is pentan-2-one. \[ CH_3-CO-CH_2-CH_2-CH_3 \quad (Pentan\text{-}2\text{-}one) \] Step 6: Reaction equations.
(i) NaHSO$_3$ addition: \[ CH_3COCH_2CH_2CH_3 + NaHSO_3 \;\longrightarrow\; \text{Addition compound} \] (ii) Iodoform test: \[ CH_3COCH_2CH_2CH_3 + 3I_2 + 4NaOH \;\longrightarrow\; CHI_3 \downarrow + CH_3CH_2COONa + 3NaI + 3H_2O \] (iii) Oxidation: \[ CH_3COCH_2CH_2CH_3 \;\xrightarrow{[O]}\; CH_3COOH + CH_3CH_2COOH \] Conclusion:
The possible structure of the compound is: \[ \boxed{CH_3-CO-CH_2-CH_2-CH_3 \quad \text{(Pentan-2-one)}} \]