Question

How many signals are there in nmr of propionaldehyde?

Answer 1

Answer:

Mark 〽 me Brainleist

Explanation:

There are three signals because there are three sets of equivalent protons. The positions of the signals in an NMR spectrum are based on how far they are from the signal of the reference compound. This information tells us the kind of proton or protons that are responsible for the signal.

Answer 2

Answer:

Explanation:

INTERPRETING C-13 NMR SPECTRA?

This page takes an introductory look at how you can get useful information from a C-13 NMR spectrum.

Important:  If you have come straight to this page via a search engine, you should be aware that this is the second of two pages about C-13 NMR. Unless you are familiar with C-13 NMR, you should read the introduction to C-13 NMR first by following this link.

Taking a close look at three C-13 NMR spectra

The C-13 NMR spectrum for ethanol

Note:  The nmr spectra on this page have been produced from graphs taken from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan.

It is possible that small errors may have been introduced during the process of converting them for use on this site, but these won't affect the argument in any way.

Remember that each peak identifies a carbon atom in a different environment within the molecule. In this case there are two peaks because there are two different environments for the carbons.

The carbon in the CH3 group is attached to 3 hydrogens and a carbon. The carbon in the CH2 group is attached to 2 hydrogens, a carbon and an oxygen.

So which peak is which?

You might remember from the introductory page that the external magnetic field experienced by the carbon nuclei is affected by the electronegativity of the atoms attached to them. The effect of this is that the chemical shift of the carbon increases if you attach an atom like oxygen to it. That means that the peak at about 60 (the larger chemical shift) is due to the CH2 group because it has a more electronegative atom attached.

Note:  In principle, you should be able to work out the fact that the carbon attached to the oxygen will have the larger chemical shift. In practice, given the level I am aiming at (16 - 18 year old chemistry students), you always work from tables of chemical shift values for different groups (see below).

What if you needed to work it out? The electronegative oxygen pulls electrons away from the carbon nucleus leaving it more exposed to any external magnetic field. That means that you will need a smaller external magnetic field to bring the nucleus into the resonance condition than if it was attached to less electronegative things. The smaller the magnetic field needed, the higher the chemical shift.

All this is covered in more detail on the introduction to C-13 NMR page mentioned above.

A table of typical chemical shifts in C-13 NMR spectra

carbon environment chemical shift (ppm)

C=O (in ketones) 205 - 220

C=O (in aldehydes) 190 - 200

C=O (in acids and esters) 160 - 185

C in aromatic rings 125 - 150

C=C (in alkenes) 115 - 140

RCH2O- 50 - 90

RCH2Cl 30 - 60

RCH2NH2 30 - 65

R3CH 25 - 35

CH3CO- 20 - 50

R2CH2 16 - 25

RCH3 10 - 15

Note:  I have no confidence in the exact values above. The table is a composite of three separate tables, with values which I have selected in order to make sense of the spectra I am talking about.

The values vary depending on the exact environment of the carbon, and these values should just be taken as an approximation. In an exam, your examiner should give you values which are consistent with the spectra they are asking you about.

In the table, the "R" groups won't necessarily be simple alkyl groups. In each case there will be a carbon atom attached to the one shown in red, but there may well be other things substituted into the "R" group.

If a substituent is very close to the carbon in question, and very electronegative, that might affect the values given in the table slightly.

For example, ethanol has a peak at about 60 because of the CH2OH group. No problem!

It also has a peak due to the RCH3 group. The "R" group this time is CH2OH. The electron pulling effect of the oxygen atom increases the chemical shift slightly from the one shown in the table to a value of about 18.