Chapter17Spectroscopy - . Typically very far downfield...

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KNOW THESE, PLUS BENZOIC ACID
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Spectroscopy IR : Let’s compare the IR spectra of butanoic acid to 1-butanol: CH 3 CH 2 CH 2 C OH O CH 3 CH 2 CH 2 CH 2 OH Strong C=O stretching absorption near 1710 cm –1 No C=O stretching absorption Very broad O–H stretching absorption between 2400–3600 cm –1 (obliterates C– H stretching region) O–H stretching absorption is narrower and centered near 3300 cm –1 Difference in appearance in the O–H stretching absorption is due to the stronger hydrogen bonding found in carboxylic acids vs. alcohols
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1 H NMR : Let’s look at the 1 H NMR spectrum of butanoic acid as a representative example: CH 3 CH 2 CH 2 C OH O a b c d c b a d a protons typically produce a signal between d 2.0–2.5 ppm (just like for aldehydes and ketones) Signals move progressively further upfield as protons become further removed from the carboxyl group Just like in alcohols, chemical shift of signal due to OH proton is highly variable (depends on concentration and acidity)
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Unformatted text preview: . Typically very far downfield (between d 913 ppm) , and often broad. How to distinguish from aldehydic proton, which is also very far downfield? D 2 O shake OH proton is exchangeable, while aldehydic proton is not 13 C NMR : Again, lets consider butanoic acid: CH 3 CH 2 CH 2 C OH O a b c d c b a d Very similar to what we observed for aldehydes/ketones. a carbon typically appears between d 2040 ppm. Signals move progressively farther upfield as the carbons become further removed from the carboxyl group. Signal for carbonyl carbon appears at relatively higher field than in aldehyde/ketone (~ d 180 ppm vs d 200220 ppm) Why further upfield , when the carbonyl carbon atom of a carboxylic acid is bonded to 2 electronegative oxygen atoms? Lone pairs on carboxylate oxygen lead to increased shielding....
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Chapter17Spectroscopy - . Typically very far downfield...

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