The more stable the ion is, the more likely it is to form. One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring. This overlap leads to a delocalization which extends from the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localized on the oxygen, but is spread out around the whole ion.
Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen. However, oxygen is the most electronegative element in the ion and the delocalized electrons will be drawn towards it. That means that there will still be a lot of charge around the oxygen which will tend to attract the hydrogen ion back again.
That is why phenol is only a very weak acid. Why is phenol a much stronger acid than cyclohexanol? To answer this question we must evaluate the manner in which an oxygen substituent interacts with the benzene ring.
As noted in our earlier treatment of electrophilic aromatic substitution reactions, an oxygen substituent enhances the reactivity of the ring and favors electrophile attack at ortho and para sites. It was proposed that resonance delocalization of an oxygen non-bonded electron pair into the pi-electron system of the aromatic ring was responsible for this substituent effect. A similar set of resonance structures for the phenolate anion conjugate base appears below the phenol structures.
The resonance stabilization in these two cases is very different. An important principle of resonance is that charge separation diminishes the importance of canonical contributors to the resonance hybrid and reduces the overall stabilization. The contributing structures to the phenol hybrid all suffer charge separation, resulting in very modest stabilization of this compound. On the other hand, the phenolate anion is already charged, and the canonical contributors act to disperse the charge, resulting in a substantial stabilization of this species.
The conjugate bases of simple alcohols are not stabilized by charge delocalization, so the acidity of these compounds is similar to that of water. An energy diagram showing the effect of resonance on cyclohexanol and phenol acidities is shown on the right. Since the resonance stabilization of the phenolate conjugate base is much greater than the stabilization of phenol itself, the acidity of phenol relative to cyclohexanol is increased. Supporting evidence that the phenolate negative charge is delocalized on the ortho and para carbons of the benzene ring comes from the influence of electron-withdrawing substituents at those sites.
Comparing the strengths of weak acids The strengths of weak acids are measured on the pK a scale. Carboxylic acid are more acidic than phenols because the negative charge in carboxylate anion is more spread out as compared to the phenoxide ion as there are two electronegative O-atoms in carboxylate anion in comparison to one in phenoxide ion.
In the resosnce structures of carboxylate anion, the negative charge is present on the O-atoms while in resonance of phenoxide ion negative charge is also present on electropositive carbon atom, which leads to less stability of phenoxide ion than carboxylate anion.
This discussion on Why carboxylic acids are more acidic than phenols? The Questions and Answers of Why carboxylic acids are more acidic than phenols? If the answer is not available please wait for a while and a community member will probably answer this soon.
You can study other questions, MCQs, videos and tests for Class 12 on EduRev and even discuss your questions like Why carboxylic acids are more acidic than phenols? Apart from being the largest Class 12 community, EduRev has the largest solved Question bank for Class Upgrade to Infinity.
By continuing, I agree that I am at least 13 years old and have read and agree to the terms of service and privacy policy.
For Your Perfect Score in Class The Best you need at One Place. Start Your Infinity Experience. Forgot Password. Enter OTP. Required desktop or laptop with internet connection. Carboxylic acid is stronger acid than …. Carboxylic acid is stronger acid than phenol. A carboxylic acid can be represented by resonance structure and is stabilized as shown below Fig. Resonance structure of carboxylate ion. Related Questions:. What are antichlors. How can understand stoichiometric defects?
Explain defects? And type of defects. Which one is explosive in following Pcl3 Mgcl2, Sbcl3, Ncl3. High electronic configuration is. How to prepare for organic chemistry I don't know the basics of class 11th and there are so much reactions that I'm finding it really hard to just remember it!
0コメント