Organic Chemistry Practice Problems and Problem Sets
oxidation
Compound A (C5H12O) is oxidized using aqueous chromium (Jones reagent) to compound B (C5H10O2), which is then treated with methanol under acidic conditions to yield compound C (C6H12O2) and water.
The 1H NMR of compound C is shown below. Determine the structures of compounds A, B, and C.
Let's solve this NMR structure elucidation problem using steps similar to those used in problem 662.
1.Are there any hints?
Compound A has one oxygen and after treatment with aqueous chromium becomes compound B, which has two oxygens. This means A is probably an alcohol, B is probably a carboxylic acid.
Compound B is then treated with methanol under acidic conditions to form compound C. These are conditions for a Fischer esterification, so C is probably the methyl ester.
2.How many IHD are there?
Compound A: C5H12O = C5H12 should be C5H12 (CnH2n+2) so 0 IHD.
Compound B: C5H10O2 = C5H10 should be C5H12. Missing 2H, so 1 IHD.
Compound C: C6H12O2 = C6H12 should be C6H14. Missing 2H, so 1 IHD.
These IHD counts fit our assumptions from part 1).
3.Draw some structures and eliminate, learn, repeat.
Some clues from the NMR:
The isopropyl splitting pattern is present: d(6) (signal c at ~0.9 ppm) and multiplet(1) (signal b at ~2.4 ppm).
The s(3) at ~3.7 ppm is probably the methyl group from the methyl ester.
We know from before we have one IHD, and it's probably an ester.
So start drawing structures and eliminate those that don't fit the data!
MendelSet practice problem # 679 submitted by Matt on July 19, 2011.
Show how each compound can be prepared from an alkene containing 3 carbons (or less).
Each answer will involve the reaction of a Grignard with either a carbonyl or epoxide.
Note: epoxides are prepared from alkenes using a peroxy acid (epoxidation) such as mCPBA.
The trick to synthesis problems in second semester organic chemistry to recognize that alcohols ARE ketones ARE carboxylic acids. What do I mean? Alcohols, ketones/aldehydes, and carboxylic acids can all be easily converted using PCC or Jones Reagent (NaCr2O7/H2SO4).
For example, for a), the product is a ketone, but it may as well be an alcohol, because alcohols can be converted to ketones with PCC.
b) is similar, except the position of the alcohol (one away from the "bond cut", instead of directly connected to the cut as in a) ) indicates the starting material was an epoxide and not a carbonyl.
c) is just like b), except instead of PCC, use Jones Reagent to oxidize the alcohol all the way to a carboxylic acid.
MendelSet practice problem # 673 submitted by Matt on July 19, 2011.