problem solving with spectroscopy

Spectroscopy Problem Solving

Dr. Chris, UP Feb 2016

Chemical Shift δ

equivalent protons

Protons that can be transferred to another one by mirror or rotation are equivalent => They have the same δ and do not couple

How many signals ?

Identify the protons of ethylacetate

Find the position of Br and I in bromo-iodo-benzene

13C (decoupled)

Solution

Less deshielded, doublet

Less deshielded, triplet

More deshielded, doublet

More deshielded, triplet

Find the structure by IR, 1H and 13C

Find functional groups

Compare to:

More de-shielding

CDCl3

Solution

CH3 singlet

1.9 ppm

CH2 triplet

3.9 ppm CH2 quintet

1.45 ppm

CH2 multiplet 1.25 ppm

CH3 triplet

0.8 ppm

δ goes down

13C NMR Because the 13C isotope is present at only 1.1% natural abundance, the probability of finding two adjacent 13C carbons in the same molecule of a compound is very low. As a result spin-spin splitting between adjacent non-equivalent carbons is not observed. However, splitting of the carbon signal by directly bonded protons is observed, and the coupling constants are large, ranging from 125 to 250 Hz. Methyl groups appear as quartets, methylenes as triplets, methines as doublets, and unprotonated carbons as singlets. Commonly, splitting of the signal by protons is eliminated by a decoupling technique which involves simultaneous irradiation of the proton resonances at 300 MHz while observing the carbon resonances at 75 MHz.

Identify the 13C peaks of Ethylacetate:

CDCl3

How many 19F signals ?