Proton nmr I have a peak near 2.5 ppm with 6 protons and 2 neighbors and a peak near 1.2 ppm with 4 protons and 3 neighbors. The compound is also given C6H10O3 How should i proceed to get the structure with only this information ?

I have also been given 8, 28, and 170 ppm.

I have also been given 8, 28, and 170 ppm.

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Dec 25, 2017

Answer:

Warning! Long Answer. The compound is propionic anhydride.

Explanation:

Preliminary analysis

You know the formula is #"C"_6"H"_10"O"_3"#.

An alkane with six carbon atoms has the formula #"C"_6"H"_14"#.

The degree of unsaturation #U# is

#U = (14-10)/2 = 4/2 = 2#

Therefore, the compound contains two rings and/or double bonds.

#""^1"H NMR"#

The spectrum has 10 protons and only two peaks. The molecule must have a symmetrical structure.

A peak with 2 neighbours and aone with 3 neighbors corresponds to an ethyl group (#"C"_2"H"_5#, triplet-quartet pattern).

The #"6H:4H"# pattern tells us there are two ethyl groups.

However, I think you have the assignments reversed. The methyl group should have the smaller chemical shift.

andromeda.rutgers.edu

A methyl group is normally at 0.9 ppm. Something is pulling it downfield
to 1.2 ppm.

A methylene group is normally at 1.3 ppm. Something is pulling it downfield
to 2.5 ppm.

We see from the table that a #"CH"_3# next to a #"C=O"# is shifted downfield to 2.2 ppm (a shift of 1.3 ppm).

We could expect a similar 1.3 ppm shift for a #"CH"_2# group; from 1.2 ppm to 2.5 ppm.

This is just where the #"CH"_2# group appears.

We now know that the partial structure is

#"CH"_3"CH"_2"(C=O)"# + #"(O=C)CH"_2"CH"_3#

These fragments add up to #"C"_6"H"_10"O"_2"#.

There is only one #"O"# atom left to insert. It must go in the middle:

#"CH"_3"CH"_2"(C=O)-O-(O=C)CH"_2"CH"_3#

The compound is propionic anhydride.

Confirmation:

1. The compound has two double bonds.

2. Three #""^13"C"# NMR signals tell us there are three different carbon environments,

image.slidesharecdn.com

We should expect to see

  • #"CH"_3color(white)(mm)# at 10 ppm
  • #"CH"_2 color(white)(mm)# at 30 ppm
  • #"(C=O)O"# at 170 ppm

We see peaks at 8 ppm, 28 ppm, and 170 ppm. This is consistent with propionic anhydride.

3. The #""^13"C"# NMR spectrum of propionaldehyde is

www.chemicalbook.com

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Dec 24, 2017

Answer:

Your question might be "1.2 ppm with 6 protons and 2.5 ppm with 4 protons."

Explanation:

The spectra has only two peaks for #10# protons. It implies that the molecule has a symmetrical structure.

#1.2# ppm shift corresponds to a methyl group, and #2.5# ppm shift to a metylene group next to a carbonyl group. (See my previous post).

I surfed the Internet and found the "answer": Propionic anhydride.

However, the structure of propionic anhydride is inconsistent with "2.5 ppm with 6 protons and 2 neighbors and a peak near 1.2 ppm with 4 protons and 3 neighbors".

If the conditions are "#color(red)"1.2"# ppm with 6 protons and 2 neighbors" and "#color(blue)"2.5"# ppm with 4 protons and 3 neighbors", the answer would be perfect.

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