Calculate the chemical shifts for the aromatic protons using Table A6.3 in Appendix 6. DIAGRAM C

Calculate the chemical shifts for the aromatic protons using...

Key Concepts

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR is an analytical technique used for determining the structure of molecules by examining the magnetic properties of atomic nuclei. It provides information about the local chemical environment using the concept of resonance absorption of radiofrequency energy in a magnetic field.

Chemical Shift

Chemical shift is the measurement of the resonance frequency of a nucleus relative to a standard reference, usually reported in parts per million (ppm). It reflects how the electron cloud surrounding a nucleus shields it from the external magnetic field, and variations in chemical shift indicate differences in chemical environments.

Aromatic Proton Chemical Shifts

Aromatic protons typically exhibit characteristic chemical shifts due to the influence of delocalized pi electrons in the aromatic ring. These electrons create a ring current when placed in a magnetic field, which alters the local magnetic field experienced by the protons, usually causing them to appear further downfield compared to aliphatic protons.

Substituent Effects on Chemical Shifts

Substituents attached to an aromatic ring can affect the chemical shifts of nearby protons by either donating or withdrawing electron density. The electronic properties of these substituents, along with their positions relative to the protons in question, determine the extent of shielding or deshielding experienced, thereby influencing the observed chemical shifts.

Empirical Data and Predictive Tables

Reference tables, like the one mentioned (Table A6.3), compile empirical data on chemical shifts for various chemical environments. These tables are used as guidelines to predict the chemical shifts in NMR spectra by correlating the type and position of substituents and ring current effects with expected resonance frequencies.

Transcript

00:04 In this question, we are asked to explain the relative chemical shifts of the protons in the benzene ring of nitrobenzene.

00:14 So, nitrobenzine will look like this.

00:16 It has a benzene ring and a nitro group attached to it.

00:21 Nitro group is a electron withdrawing group.

00:25 Let's just keep that in mind.

00:27 And there will be hydrogens in these corners.

00:36 So let's try to identify which of these.

00:39 Protons are equivalent.

00:42 So these two protons that are in the orthoposition compared to the nitro group are equivalent to each other so we can give them a single label.

00:56 These two hydrogens are in the metaposition compared to the nitro group and they are equivalent to each other and this will be the third type of protons present coming from the paraposition relative to the nitro group.

01:18 So let's just use these terms to get to the answer.

01:24 So the orthometta and para protons have different chemical shifts and the question is asking us to explain why this is the case.

01:35 So this is the case because nitrobenzene has resonance structures due to the inductive effect that molecule can display.

01:48 So this is what a nitrobenzine molecule will look like in its normal state.

01:57 Let me draw that.

02:08 This will be the nitrogen.

02:10 There will be one oxygen with a single bond and an extra electron pair on it.

02:15 And then another oxygen with a double bond.

02:20 And then there will be a plus charge on the nitrogen.

02:25 So this bond over here can break towards this plus charge on the nitrogen because this double bond can break towards the oxygen.

02:42 This will form a resonant structure of nitrobenzine and it will look like this.

02:51 So nitrogen will have double bonds with the carbon in the ring.

03:01 And then there will be two oxygens with single bonds...

Please give Ace some feedback