Part 5: STRUCTURE DETERMINATION

Note that there is no point showing all possible answers here, so the most likely and potentially simplest answers are shown.

a. C₈H₈O₂ has a molecular weight (8 x 12.01) + (8 x 1.008) + (2 x 16.00) = 136.14 g/mol (1 mark)

b. The formula for index of hydrogen deficiency, IHD = 0.5 ( 2c+2-h), therefore, the IHD = 0.5 (2 x 8 + 2 - 8) = 5 (1.5 marks)

(note that an IHD of 5 is large for a small MF, so it might suggest a benzene ring as part of the structure)

c. To be soluble in 5% NaHCO₃ solution (solubility expt) it suggests a carboxylic acid :



Note that a phenol is not typically soluble in 5% NaHCO₃ solution.

d. IR of 3300cm^-1 suggests an -OH and soluble in NaOH solution but not NaHCO₃ solution suggests a phenol but not a carboxylic acid (3 marks)

e. pKa > 20 suggests some sort of an ester with alpha-H or a terminal alkyne but can't save any -OH (too acidic) (2 marks)

f. No peak between 1685-1800 cm^-1 implies no C=O (2 marks)

g. Oxygen containing functional groups that could be present in X with up to 2 x O atom and a unit of unsaturation could include : carboxylic acid, ester, aldehyde, ketone, alcohol, ether, epoxide, enol (1.5 marks)

Common errors:

• incorrect IHD calculation (e.g. forget to divide by 2)
• not understanding that isomers are molecules with the same molecular formula
• failing to follow the fundamental rules of valence
• not knowing that NaHCO₃ (solubility expt) is a weak base that can deprotonate a carboxylic acid to give a water soluble ionic salt
• indentifying FG that did not match MF (e.g. anyhydride which requires 3 O atoms)
• not knowing pKa values for common functional groups
• incorrect numbers of types of atoms in part d
• incorrect structures (not an isomer of X as the question asked)