Question

An organic compound contains 69.77% carbon, 11.63% hydrogen and rest oxygen. The molecular mass of the compound is 86. It does not reduce Tollens’ reagent but forms an addition compound with sodium hydrogensulphite and give positive iodoform test. On vigorous oxidation it gives ethanoic and propanoic acid. Write the possible structure of the compound.

Answer 1

(a) Finding the molecular formula of a compound:

Percentage of carbon = 69.77%

Percentage of hydrogen = 11.63%

∴ Percentage of oxygen = 100 – (69.77 + 11.63)

= 18.6%

C : H : O = `69.77/12 : 11.63/1 : 18.6/16`

= 5.81 : 11.63 : 1.16

∴ Simple ratio = 5 : 10 : 1

Empirical formula of the given compound = C₅H₁₀O

Empirical formula mass = 5 × 12 + 10 × 1 + 1 × 16 = 86

Molecular mass = 86  ...(Given)

Molecular formula = C₅H₁₀O × `86/86` = C₅H₁₀O

Thus, the molecular formula of the given compound is C₅H₁₀O.

(b) Determining the structure of a compound:

1. Since the given compound forms a divalent compound with sodium hydrogensulphite, it must be an aldehyde or a ketone.
2. Since the compound does not reduce Tollen's reagent, it cannot be an aldehyde. Hence, it must be a ketone.
3. Since the compound gives an iodoform test, the given compound is methyl ketone.
4. Since the given compound gives a mixture of ethanoic acid and propanoic acid on vigorous oxidation, methyl ketone is pentane-2-one. Its structure is as follows:

\[\begin{array}{cc}
\ce{O}\phantom{........}\\
||\phantom{........}\\
\ce{\underset{Pentan-2-one}{CH3 - C - CH2CH2CH3}}
\end{array}\]

(c) Details of the reactions involved:

\[\begin{array}{cc}
\phantom{.................}\ce{O}\phantom{...........................}\ce{OH}\phantom{.....}\ce{S\overset{-}{O}_3N\overset{+}{a}}\\
\phantom{..........}||\phantom{...............................}\backslash\phantom{...}/\\
\ce{CH3 - \underset{Pentan-2-one}{C - CH2}CH2CH3 + NaHSO3 -> C}\\
\phantom{..........................................}/\phantom{...}\backslash\\
\phantom{..................................}\ce{\underset{Sodium Hydrogen Sulphite Additive Products}{CH3CH2H2C \phantom{.....}CH3}}
\end{array}\]

\[\begin{array}{cc}
\ce{O}\phantom{....................................................................................}\\
||\phantom{....................................................................................}\\
\ce{CH3 - \underset{Pentan-2-one}{C - CH2}CH2CH3 + 3I2 + 4NaOH ->[Iodoform][reaction]\underset{(Yellow ppt)}{\underset{Iodoform}{CH3\downarrow}} + CH3CH2CH2COONa + 3NaI + 3H2O}
\end{array}\]

\[\begin{array}{cc}
\ce{O}\phantom{...............................................}\\
||\phantom{...............................................}\\
\ce{CH3 - \underset{Pentan-2-one}{C - CH2}CH2CH3->[K2Cr2O7][H2SO4] \underset{Ethanoic acid}{CH3COOH} + \underset{Propanoic acid}{CH3CH2COOH}}
\end{array}\]