How can 1,2-, 1,3-, and 1,4-dinitrobenzene be distinguished by b. 13 C NMR spectroscopy?

Hello, everyone. Let's do this problem together. It says determine how we can distinguish 1213 and 14 di bromo benzene in carbon 13. And a Mar spectroscopy, we are given four different answer choices which vary in the number of signals that we would see in the spectra for these three compounds. So the answer choices gives us a hint that we should distinguish these compounds based on the number of signals. And we do that by determining the number of unique non equivalent carbons. And an important part of determining equivalent or non equivalent carbons would be identifying if we have a plane of symmetry, right? Because then carbons that mirror each other on either side of the plane of symmetry will be equivalent. So let's draw our three compounds. 1213 and 14 di bromo benzene. So we have all benzene rings that are dye substituted with two bromine atoms. So 12 is Ortho substituted, 13 is meta substituted and is per substituted, right. So let's look at 12 dia broma benzene. I see a plane of symmetry going between the bromine atoms across the benzene ring. So it is crossing between carbons, one and two and then crossing between carbons three and four. So we have three carbons on either side of the plane of symmetry, carbons one and two are mirroring each other. So they will represent signal a carbons three and five, oops three and six are mirroring each other. So that would be carbon B in carbons three and five, wait four and five, sorry would be signal C. All right. So that is three signals that we would see for this compound. OK. Moving on to 13 di bromo benzene, I see another plane of symmetry except this one goes through two carbons. So it goes through carbons two and five. So we will have one signal for carbon two, one signal for carbons, one in three, they both have that bromine atom, one signal, four carbons, four and six signal C and one signal for carbon five signal D. So that is four signals, 413, Di Broma benzene and now 14 diro of benzene has actually two planes of symmetry, right? One horizontal between, let me number my carbons in the benzene ring. So I don't mess it up again. One plane of symmetry going between carbons, two and three and then going through between carbons five and six horizontally and then one plane of symmetry vertical going through carbons, one and four where, where those bromine atoms are. So we will have one signal for the carbons that are on that vertical plaintiff symmetry where the brom means are, we will call that signal A and then the carbons that are in the corners of the intersection of these planes of symmetry. So carbons 235 and six will get their own signal because they are all equivalent to one another. So that will be signal B so we only have two signals, 414 Diro Bez. So that is three signals, four signals and two signals respectively. So that makes answer choice. C the correct answer because it says for 1213 and 14 dibromm benzene, it would have 34 and two signals respectively. That is it for this problem? I hope this video is helpful and thank you for watching.

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15. Analytical Techniques:IR, NMR, Mass Spect

Carbon NMR

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

Carbon NMR

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Problem 43b

Textbook Question

How can 1,2-, 1,3-, and 1,4-dinitrobenzene be distinguished by
b. ¹³C NMR spectroscopy?

Verified step by step guidance

Understand the concept: In 13C NMR spectroscopy, the chemical environment of each carbon atom in a molecule determines its chemical shift. The symmetry and electronic effects of substituents on the benzene ring will influence the number of unique carbon signals observed in the spectrum.

Analyze 1,2-dinitrobenzene (ortho isomer): This molecule has two nitro groups (-NO₂) attached to adjacent carbons on the benzene ring. Due to the lack of symmetry, all six carbons in the ring are in different chemical environments, resulting in six distinct signals in the 13C NMR spectrum.

Analyze 1,3-dinitrobenzene (meta isomer): In this isomer, the nitro groups are attached to carbons separated by one carbon. The molecule has a plane of symmetry passing through the carbon atoms between the nitro groups. This symmetry reduces the number of unique carbon environments to four, so the 13C NMR spectrum will show four distinct signals.

Analyze 1,4-dinitrobenzene (para isomer): Here, the nitro groups are attached to carbons opposite each other on the benzene ring. The molecule has a high degree of symmetry, with a plane of symmetry and rotational symmetry. This results in only three unique carbon environments, so the 13C NMR spectrum will show three distinct signals.

Compare the spectra: By counting the number of distinct signals in the 13C NMR spectra, you can distinguish between the three isomers. 1,2-dinitrobenzene will have six signals, 1,3-dinitrobenzene will have four signals, and 1,4-dinitrobenzene will have three signals.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It exploits the magnetic properties of certain nuclei, such as carbon-13 (13C), to provide information about the environment surrounding these nuclei. In 13C NMR, the chemical shifts observed are influenced by the electronic environment, allowing for differentiation between various carbon atoms in a molecule.

Chemical Shifts

Chemical shifts in NMR spectroscopy refer to the variation in resonance frequency of a nucleus due to its electronic environment. In 13C NMR, different substituents on the benzene ring, such as nitro groups, can cause shifts in the carbon signals. The position and number of these substituents in 1,2-, 1,3-, and 1,4-dinitrobenzene will lead to distinct chemical shifts, enabling their differentiation based on the resulting spectra.

Symmetry and Substitution Patterns

The symmetry and substitution patterns of a molecule significantly influence its NMR spectrum. In the case of dinitrobenzenes, the positions of the nitro groups (ortho, meta, and para) affect the symmetry of the molecule, which in turn impacts the number of unique carbon environments. This results in different peak patterns and intensities in the 13C NMR spectrum, allowing for the identification of each isomer based on their distinct substitution patterns.

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