This reaction is a reduction since hydrogen is added to the starting compound. T

Question

This reaction is a reduction since hydrogen is added to the starting compound. The reverse of this reaction would be oxidation and the I-naphthylmethanol can be converted into the aldehyde by reacting with an oxidizing agent. These types of reactions can form either the aldehyde or the carboxylic acid (structure below) depending on the oxidizing reagent used. Could IR be used to determine in the aldehyde to the acid was the product? How could you tell the difference between the two? Repeat this analysis using^1H NMR spectroscopy and indicate clearly what peaks in the NMR spectrum would be the most useful in differentiating between these products. If the reactions described in question 4 gave not one product but a mixture of both the aldehyde and the acid, would NMR or IR be able to tell you how much of each compound was present? If so specifically describe how the ratio of the two products could be determined.

Explanation / Answer

4. To differentiate aromatic aldehydes and acids by IR spectroscopy is rather difficult but not impossible.

Usually, the carbonyl of carboxyl group appears between 1700-1720 and the carbonyl of aldehyde appears bit downfield between 1725-1740. But normal ranges differ by a large extent depending on the neighboring groups.

There will be two bands between 2720-2750 and 2820-2850 in IR spectrum of aldehydes. Which is a characteristic feature due to (C-H str).

The O-H str of acid group is observed between 3300 - 3100. But this is not observed in aromatic acids as clearly as it does in aliphatic acids.

But both aldehyde and acid can be differentiated by analysis of 1H NMR spectra.

Although the aromatic protons appear in the similar regions (6.8-9.5 ppm), the functional group protons resonate differently.

i) Aldehyde proton resonates at 10-11 ppm.

ii) Carboxyl proton appears at 13-14 ppm.

Therefore, the by this striking feature, we can differentiate between the aldehyde and carboxylic acid derivatives of naphthalene.

5. When a mixture of both are obtained during a reaction, IR cannot be used as a distinguishing feature to estimate how much each compound is present. But whereas, NMR can be useful.

The ratio of both the products in the mixture can be estimated by taking the ratio of the integrals of both the peaks that appear between 10-11 and 13-14 ppm (i.e., the peaks of aldehyde and acid protons).

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