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. C₆H₁₀O₂ has a molecular weight (6 x 12.01) + (10 x 1.008) + (2 x 16.00) = 114.14 g/mol (1.5 marks)

b. The formula for index of hydrogen deficiency, IHD = 0.5 ( 2c+2-h), therefore, the IHD = 0.5 (2 x 6 + 2 - 10) = 2 (1.5 marks)

c. A pKa of 25 would suggest either a terminal alkyne or an ester is present :

(2 marks)

d. IR of 3300cm^-1 suggests an -OH and soluble in NaOH solution suggests a carboxylic acid (this relates to the solubility experiment). 5 types of C requires some symmetry. (2 marks)

e. No IR of 3300-3150 cm^-1 means no -OH and not a terminal alkyne, and a peak between 2300-2100 cm^-1 means it is an alkyne so it must be internal: (2 marks)

f. Need an ester, alkene and chiral center (4 marks)

Common errors:

• incorrect IHD calculation (e.g. forget to divide by 2 or didn't read the MF correctly)
• not understanding that isomers are molecules with the same molecular formula
• not knowing pKa values for common functional groups
• drawing structures that violated the rules of valence
• in part c (i) not having a pKa of 25 (e.g. ketone = 20, or a 1,3-diketone = 10) (ii) incorrect alkyne geometry and (iii) having an internal alkyne
• in part d (i) not having a low enough pKa to be totally deprotonated by NaOH (e.g. alcohol = 15) (ii) incorrect number of types of C
• in part e (i) a terminal alkyne (ii) having an -OH despite the IR data
• in part f (i) didn't understand a chirality center (e.g. assigning an sp2 C in an alkene or carbonyl as being chiral or not having 4 different groups attached to an sp3 C. (ii) couldn't apply the Cahn-Ingold-Prelog rules to assign configuration correctly (nomenclature !)