A naturally occurring compound A containing only C, H and O has the following elemental composition, — Physical Chemistry Chemistry Question
A Naturally Occurring Compound
A naturally occurring compound A containing only C, H and O has the following elemental composition, percentage mass,
C: 63.2 %, H: 5.3%, O: 31.5%.
4.1 Derive the empirical formula of compound A.
Model Answer
The empirical formula of A is C8H8O3.
Figure 1 The mass spectrum of compound A is shown in Figure 1.
4.2 What is the molecular formula for compound A?
Model Answer
The molecular formula of compound A: C8H8O3.
A solution of A in ether is shaken with an aqueous solution of NaOH. After this, no A remains in the ether phase. Another solution of A in ether is shaken with an aqueous solution of NaHCO3. A remains in the ether phase.
4.3 Which of the following classes of compounds does A belong to according to these experiments? Mark with an X.
alcohol [ ] phenol [ ] aldehyde [ ] ketone [ ]
acid [ ] ester [ ] ether [ ]
Model Answer
The compound A is a phenol.
Compound A gave rise to formation of a silver mirror with Tollens’ reagent (Ag(NH3)2 +).
4-1 Which of the following functional groups does this indicate the presence of in A?
Mark with an X.
hydroxy group of an alcohol [ ] hydroxy group of a phenol [ ]
carbonyl group of an aldehyde [ ] carbonyl group of a ketone [ ]
carboxylic group [ ] ester group [ ]
alkoxy group of an ether [ ]
Model Answer
Compound A forms a mirror with Tollen's reagent. This indicates the presence of carbonyl group of an aldehyde.
Figure 2a
The 1H NMR spectrum of compound A recorded at 300 MHz is shown in Figure 2a (solvent CDCl3 (7.27 ppm), reference tetramethylsilane). The signals at 3.9, 6.3 and 9.8...
Model Answer
It indicates exchange of oxygen-bonded hydrogen.
Model Answer
It indicates:
decrease in hydrogen bonding,
intermolecular hydrogen bonding.
Model Answer
Four possible structural formulas for compound A:
Model Answer
Formulas for the fragments lost corresponding to the peaks at 137 and 123 mass units in the mass spectrum: CH3, HC=O.
Model Answer
Two isomers having a lower pKa value than the others: