What is the salt used as support for IR? What is the support used for analyze li
ID: 1063415 • Letter: W
Question
What is the salt used as support for IR? What is the support used for analyze liquid samples using IR? Why should you not use water to clean the IR plates for liquid samples? How will you recognize aromatic compounds in IR? Can you differentiate steriosomers using IR? What is the volume of solvent recommended to use in NMR? What kind of solvents is used in NMR? How would you recognize aromatic compounds by NMR? When preparing a sample for NMR, would you check the solubility of your compound in deuterated or non-deutered solvents? What is the more polar, deuterated or non-deuterated water? Which material are the NMR tubes made of Is Friedel-Craft a nucleophilic aromatic substitution or an electrophillic aromatic substitution?Explanation / Answer
1. Usually, potassium halides like potassium chloride, KCl or potassium bromide, KBr can used as the support for IR spectroscopy. Infact, IR plates are made out of KCl or KBr because potassium halides provides transmission in the range of 0.3-20 µm.
2. The question looks a bit unclear. Are you asking for how to prepare liquid samples for analysis by IR? I think that’s the question.
In case of analysis of liquid samples, a pair of clean and highly polished salt plates (KCl or KBr) is used. A drop of the sample is placed on the lower plate and the upper plate is carefully placed on the lower placed so as not to introduce any air bubbles. Such placing of the plates one on top of the other allows for the formation of a thin layer between the plates. The excess sample must be wiped off with a clean kimwipe.
3. Water should never be used to clean salt plates. Alkali halides are highly soluble in water. Salt plates are usually made of alkali halides like KCl or KBr. Such salts are highly soluble in water and washing of the salt plates with water will lead to washing away of the support material and hence, decay of the salt plates.
4. The C=C stretches for aromatic compounds occur around 1600-1585 cm-1 and 1500-1400 cm-1. The C-H stretching vibrations due to in-plane C-H stretching occur around 1250-1000 cm-1. The characteristics vibrations are weak bands around 2000 cm-1 (known as overtones) and weak out-of-plane C-H stretching vibrations around 900-675 cm-1.
5. IR spectroscopy gives peaks corresponding to stretching or bending vibrations of functional groups in organic molecules like –OH, C=O, N-H, C-H, etc. Since a pair of stereoisomers will have the same functional groups with the same atom connectivity, hence IR spectroscopy cannot be used to distinguish between a pair of stereoisomers.
6. A small volume of the solvent is used for a typical 5mm NMR tube. The volume of solvent is usually 0.6-0.8 mL. Smaller solvent volumes can be used, but will require extensive shimming.
7. NMR solvents must not contain any hydrogen. In NMR solvents, the hydrogen atoms are replaced by deuterium. Hence, deuteriated solvents must be used in NMR. The cheapest and most easily available NMR solvent is Chloroform-d, CDCl3. Other NMR solvents like DMSO-d6, (CD3)2SO and deuteriated water, D2O are also used.
8. In aromatic compounds, the protons attached to the aromatic ring are shifter downfield and appear around 6.9-7.5. The aromatic protons appear downfield because the of the ring current of the aromatic ring and the protons attached to the aromatic ring are in the plane of the current and hence shifted downfield. Moreover, the aromatic protons undergo multiple splitting due to combined interactions with other ring protons.
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