Part 5: STRUCTURE DETERMINATION

a.   The elemental analysis data gives the empirical formula to be C5H10O.

b. Given the MW = 86 g/mol, then since C5H10O = (5 x 12)+(10 x 1)+(1 x 16) = 86, then the molecular formula = C5H10O.

c. Index of hydrogen deficiencies ( IHD) = 1 (this helps us know that the structures must contain 1 unit of unsaturation based on double bonds or rings).

d. The IR band at 1715 cm-1 implies a C=O, and in order to have 5 C types and 4 H types, the C=O can't have an H so it has to be a ketone...

pentan-2-one

e.
resonance in a ketone

f. The IR band at 1715 cm-1 implies a C=O, and in order to have 4 C types and 3 H types, the C=O can't have an H so it has to be a different ketone...
3-methylbutan-2-one

g. Stereocenters could mean chirality centers (C with 4 different groups attached) or alkenes that can be cis or trans, for example:

stereocenters

 

h. Since the IHD =1, if there are no sp2 atoms, then there will have to be a ring. The IR means no C=O an no OH, so to incorporate an O, it will have to be an ether.

ethers


Common errors:

a. Not being able to do elemental analysis calculations (high school material) so use the link above to review.

c. Calculation errors

d & f. Counting types of H or C and putting it into practice.

e. Poor understanding of resonance, breaking the rules for resonance structures by moving atoms to create enol type structures.

g. Not drawing 4 different groups attached to the proposed chirality center.

h. Ignoring the IHD and hence not drawing an isomer of C5H10O.