Organic unknowns: reactions, IR and molecular mass

What you’ll learn

• Combine functional-group observations with infrared and mass data.
• Explain which structural claims the evidence does and does not support.

Understand the idea

An organic identification needs several constraints. A molecular-ion peak can provide molecular mass when the peak has been correctly assigned; it does not alone give connectivity. Infrared data reveal bonds and functional groups. Use the supplied wavenumber ranges: a broad alcohol O–H band supports an alcohol, whereas a strong carbonyl band supports a C=O-containing compound. A carbonyl signal by itself does not distinguish an aldehyde, a ketone or a carboxylic acid.

Unit 2 reaction evidence narrows those possibilities. An aldehyde can give a red precipitate with Fehling's solution, while the simple ketones considered here do not. A carboxylic acid releases carbon dioxide with carbonate. Primary alcohol oxidation can produce an aldehyde and then an acid, whereas a secondary alcohol produces a ketone. Interpret the given supervised observations alongside molecular formula and spectra rather than proposing a hazardous sequence of tests.

Negative evidence matters only when the test is appropriate and has worked. For example, absence of an O–H band supports excluding an alcohol only if the spectrum covers the relevant region. If the identity of a molecular-ion peak is uncertain, fragmentation data or another mass determination can help. State a structure only when it satisfies all the supplied evidence; otherwise give the remaining possibilities.

Key formulae

$$\mathrm{CH_3CHO}+[\mathrm O]\rightarrow\mathrm{CH_3COOH}$$

$$\mathrm{CH_3CH(OH)CH_3}+[\mathrm O]\rightarrow\mathrm{CH_3COCH_3}+\mathrm{H_2O}$$

$$M=\frac{m}{n}=\frac{mV_m}{V}$$

Worked example

An unknown has molecular formula $\mathrm{C_3H_8O}$ and a broad IR band in the supplied alcohol O–H range, $3200$–$3600\ \mathrm{cm^{-1}}$. Its oxidation product has a carbonyl band but no alcohol or carboxylic-acid O–H band, and gives no red precipitate with Fehling's solution in a valid test. The candidate structures are propan-1-ol, propan-2-ol and methoxyethane. Identify the unknown.

1. The alcohol O–H band in the original unknown excludes methoxyethane, which is an ether.
2. The product's carbonyl band, absence of an acid O–H band and negative Fehling's test support a ketone rather than an aldehyde or carboxylic acid.
3. A secondary alcohol forms that ketone, so propan-2-ol fits; its product is propanone.

Answer: Propan-2-ol.