acidity & H-bonds

👨🏻‍🏫 學術台化學

2011-01-02 3:46 pm

1. 3,3-dinitrophenol (M)< 4-nitrophenol (K)< 2-nitrophenol (L)
(in ascending order of pKa values)
ans: L is the weakest acid as intramolecular hydrogen bonds are formed between the -OH group and -NO2 group

another question:
4-hydroxybenzoic acid < benzoic acid < 2-hydroxybenzioc acid
(in ascending order of acidity)
ans: 2-hydroxybenzoic acid is the strongest acid because intramolecular hydrogen bonds are formed. This helps stabilize the carboxylate anion formed.

咁我就唔明點解both 2-nitrophenol and 2-hydroxybenzoic acid can form intramolecular hydrogen bonds
但係一個係strongest acid while another is a weakest acid
究竟如果個compound form到H-bond點考慮佢係help stabilize the anion formed定係相反?
同埋2-hydroxybenzoic acid form左H-bond後點解會stabilize 左個anion formed既?

唔該晒 !!!!!

更新1:

it should be 3,4-dinitrophenol(M) instead of 3,3-dinitrophenol(M) sorry for the typing error

回答 (2)

yns

2011-01-05 7:20 am

✔ 最佳答案

For acidity of molecules with Intramolecular hydrogen bonding, both the stability of the species concerned before and after deprotonation should be considered and compared. Before deprotonation, 2-Nitrophenol: √ Intramolecular H-bond between H of –OH and O of –NO22-Hydroxybenzoic acid: √ Intramolecular H-bond between O of –OH and H of –COOH √ Intramolecular H-bond between H of –OH and O of –COOHAfter deprotonation, 2-Nitrophenoxide: No Intramolecular H-bond between H of –OH and O of –NO22-Hydroxybenzoate: No Intramolecular H-bond between O of –OH and H of –COOH√ Intramolecular H-bond between H of –OH and O of –COOH(Ion-dipole attraction between H of –OH and –ve charge of COO-, much stronger!!!) Comparing the above extra stability originated from Intramolecular H-bond, a conclusion can be drawn: Relative Stability: 2-Nitrophenol > 2-NitrophenoxideRelative Stability: 2-Hydroxybenzoic acid < 2-Hydroxybenzoate=> Intramolecular H-bond inhibits the acidity of 2-Nitrophenol but strengthens the acidity of 2-Hydroxybenzoate. BTW, benzoic acids are always more acidic than phenol.

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己式庚辛

2011-01-03 10:02 am

3,3-dinitrophenol ?!?!
how come there's 3,3- position?
please clarify.

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