Part 5: STRUCTURE DETERMINATION

a. Since the molecular formula of X is C₅H₁₀O, the molecular weight is (5 x 12.011) + (10 x 1.008) + (1 x 15.999) = 86.134 g/mol (1.5 mark)

b. For a hydrocarbon, the formula for the index of hydrogen deficiency, IHD = 0.5 ( 2c+2-h), therefore, the IHD = 0.5 (2 x 5 + 2 - 10) = 1 (1 mark)
FYI : The IHD = pi + r (pi bonds plus rings) there can be 1 units of unsaturation, which could mean a ring or a double bond.

c. Given the molecular formula, the IR data (3200-3600 cm^-1) indicates an -OH With only 4 H and 3 C types, X must be cyclopentanol, structure shown below. (2.5 marks for X)

d. Since X is an alcohol, we need a base that has an associated pKa of > 15. Good answers would be amide ions (R₂N-) or hydride (H-) or a carbanion system (R-)

e. Since Y is a constitutional isomer f X it has the same molecular formula. Y needs to have an sp3 C with 4 different groups attached and be an ether (ROR). (3D structure = 1 mark, name = 1.5 marks)

f. Z has the same molecular formula as X. Since Z has only 2 H and 3 C types, it must be symmetrical, the pKa = 20 suggests a ketone... has to be pentan-3-one to fit the types requirements (2.5 marks)

Common errors:

Not reading the questions.... so drawing structures that did not fit the molecular formula, not writing a balanced equation (attempting to drawing a mechanism instead),

Did not know what an empirical formula was the simplest ratio of the elements. (High school / 1st yr knowledge)
Incorrectly counting or not checking the number of H and or C types in Y and / or Z.
Drawing structures for X that did not fit the molecular formula of C₈H₁₀ and the IHD.
Note that IHD defines the number of rings + pi bonds present for a given molecular formulae. The answer to part b should have helped you avoid this!

Could not name Y correctly, ignoring the chirality that was specifically referred to. Hence but be named as R or S!
Incorrect numbering, incorrect alphabetisation, wrong root name (e.g. wrong root, mixing up phenyl with benzyl).

Drawing structures for Z that did not fit all the data provided

Suggesting the conjugate acid rather than the base or bases that are not strong enough to deprotonate terminal alkynes (such as HO- or RO-). Think about what the pKa refers to. Amines (e.g. NH₃ or RNH₂) have pKa about 10.