# 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.

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#### Explanation

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Dec 25, 2017

Warning! Long Answer. The compound is propionic anhydride.

#### Explanation:

Preliminary analysis

You know the formula is $\text{C"_6"H"_10"O"_3}$.

An alkane with six carbon atoms has the formula $\text{C"_6"H"_14}$.

The degree of unsaturation $U$ is

$U = \frac{14 - 10}{2} = \frac{4}{2} = 2$

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

$\text{^1"H NMR}$

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 (${\text{C"_2"H}}_{5}$, triplet-quartet pattern).

The $\text{6H:4H}$ 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.

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 ${\text{CH}}_{3}$ next to a $\text{C=O}$ is shifted downfield to 2.2 ppm (a shift of 1.3 ppm).

We could expect a similar 1.3 ppm shift for a ${\text{CH}}_{2}$ group; from 1.2 ppm to 2.5 ppm.

This is just where the ${\text{CH}}_{2}$ group appears.

We now know that the partial structure is

$\text{CH"_3"CH"_2"(C=O)}$ + ${\text{(O=C)CH"_2"CH}}_{3}$

These fragments add up to $\text{C"_6"H"_10"O"_2}$.

There is only one $\text{O}$ atom left to insert. It must go in the middle:

${\text{CH"_3"CH"_2"(C=O)-O-(O=C)CH"_2"CH}}_{3}$

The compound is propionic anhydride.

Confirmation:

1. The compound has two double bonds.

2. Three $\text{^13"C}$ NMR signals tell us there are three different carbon environments,

We should expect to see

• ${\text{CH}}_{3} \textcolor{w h i t e}{m m}$ at 10 ppm
• ${\text{CH}}_{2} \textcolor{w h i t e}{m m}$ at 30 ppm
• $\text{(C=O)O}$ at 170 ppm

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

3. The $\text{^13"C}$ NMR spectrum of propionaldehyde is

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#### Explanation

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#### Explanation:

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Dec 24, 2017

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 "$\textcolor{red}{\text{1.2}}$ ppm with 6 protons and 2 neighbors" and "$\textcolor{b l u e}{\text{2.5}}$ ppm with 4 protons and 3 neighbors", the answer would be perfect.

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