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Chapter 3: Problem 130

Why is ortho fluoro phenol a weaker acid than ortho chlorophenol?

Short Answer

Expert verified
Answer: Ortho-fluorophenol is a weaker acid than ortho-chlorophenol because, despite fluorine being more electronegative than chlorine, the larger size of the chlorine atom allows for more effective electron delocalization in the anion formed after the proton is donated. This results in a more stable anion and a lower pKa value, making ortho-chlorophenol a stronger acid.

Step by step solution

01

Understanding acidity of phenol compounds

First, let's recall what it means for a compound to be acidic. A compound is considered an acid if it can donate a proton (H+ ion) to another molecule or ion. The acidity of a compound can be measured using the pKa value, where a lower pKa indicates a stronger acid, and a higher pKa indicates a weaker acid. Both ortho-fluorophenol and ortho-chlorophenol are phenol compounds, which means they have a hydroxyl (OH) group attached to a benzene ring. The acidity of a phenol compound mainly depends on the ease with which the OH group can donate its proton (H+ ion), which in turn depends on the stability of the anion generated after the proton is donated.
02

Investigating the inductive effect

The inductive effect refers to the redistribution of electron density through the sigma bonds in a molecule, caused by the difference in electronegativity between atoms. In the case of ortho-fluorophenol and ortho-chlorophenol, the electronegative halogen atoms (fluorine in ortho-fluorophenol and chlorine in ortho-chlorophenol) can pull electron density towards themselves through the sigma bonds, weakening the O-H bond in the hydroxyl group and making it easier for the proton to be donated.
03

Comparing electronegativity of fluorine and chlorine

Electronegativity is the ability of an atom to attract electron density towards itself in a bond. In general, the higher the electronegativity of an atom, the stronger the inductive effect it will have on the molecule. Fluorine is more electronegative than chlorine, with an electronegativity of 3.98 compared to 3.16 for chlorine. Therefore, we might initially expect the inductive effect to be stronger in ortho-fluorophenol, resulting in a more acidic compound. However, there are other factors we need to consider that may affect acidity.
04

Considering the size of the halogen atom

While electronegativity plays a role in the inductive effect and acidity, we cannot ignore the size of the halogen atom. Larger atoms, like chlorine, are more polarizable than smaller atoms like fluorine, which results in more effective electron delocalization in the anion. In ortho-chlorophenol, the chlorine atom can effectively disperse the negative charge generated when the proton is donated, which stabilizes the anion. This stabilization makes it easier for ortho-chlorophenol to donate its proton and gives it a lower pKa value, making it a stronger acid.
05

Concluding the comparison

So, although fluorine is more electronegative than chlorine, the larger size of the chlorine atom means it can more effectively stabilize the anion formed after the proton is donated, resulting in a lower pKa for ortho-chlorophenol. Therefore, ortho-fluorophenol is a weaker acid than ortho-chlorophenol.

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Most popular questions from this chapter

When phenol is exposed to air for a long period or treated with chromic acid, it is oxidized to (a) p-benzoquinone (b) glyoxal (c) \(\mathrm{CO}_{2}\) (d) oxalic acid

$$ \mathrm{HCHO}+\mathrm{HCl} \stackrel{\mathrm{ZnCl}_{2}}{\longrightarrow} \mathrm{P} $$ The organic compound P formed in the above reaction is

Statement 1 Phenol gives p-bromophenol as the major product when treated \(\mathrm{Br}_{2}\) dissolved in carbon disulphide at low temperature. and Statement 2 p-Bromophenol has lower \(\mathrm{pK}_{\text {a }}\) value than phenol.

Identify the wrong statement among the following related to phenol. (a) Phenol is a weaker acid than carbonic acid. (b) When distilled with zinc, phenol gives benzene. (c) Among the three isomers of nitrophenol, the one that is least soluble in water is \(\mathrm{p}\) -nitrophenol. (d) Formation of phenol from chlorobenzene is an example of nucleophilic aromatic substitution.

The major product formed by the reaction of \(\mathrm{N}\) -phenylbenzamide with \(\mathrm{Br}_{2} / \mathrm{Fe}\) is
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