Dichloroacetic is a stronger acid than chloroacetic acid, and trichloroacetic acid is even stronger. The inductive effects of chlorine be clearly seen when looking at the electrostatic potential maps of acetic acid Left and trichloroacetic acid Right. The O-H bond in trichloroacetic acid is highly polarized, as shown by the dark blue color making it a much stronger acid than acetic acid.
Because inductive effects are not transmitted effectively through covalent bonds, the acid-strengthening effect falls off rapidly as the number of sigma bonds between the carboxylic acid and the electron-withdrawing group increases. A distance of three sigma bonds almost completely eliminates chlorine's inductive effect in 4-chlorobutanoic acid, giving it a similar pK a value to unsubstituted butanoic acid. Alkyl groups hydrocarbons are inductively electron-donating.
In this case, the inductive effects pushes electron density onto the carboxylate anion, producing a destabilizing effect, decreasing the acidity of the carboxylic acid. Lengthening the alkyl chain of a carboxylic acid can increase this inductive effect but it no longer decreases the acidity further after the chain is about three carbons long. Extensive research has been performed on the acidity of substituted benzoic acids. Benzoic acid itself is a somewhat stronger acid than acetic acid.
The carboxyl group of benzoic acid is attached to an sp 2 -hybridized carbon which is more electronegative and electron-withdrawing than the sp 3 -hybridized carbon attached to acetic acid. In Section 2. The same effect is seen in acrylic acid which is also more acidic than acetic acid. In Section Electron-donating substituents increase the electron density of the aromatic ring, activating it, and increasing the rate of electrophilic substitutions.
Electron-withdrawing groups decreased electron density, deactivating the aromatic ring, and reduced the rate of electrophilic substitution. In a similar fashion, substituents have an effect on the acidity of benzoic acids. Deactivating substituents, such a nitro group -NO 2 , in the ortho or meta position remove electron density from the aromatic ring, and also from the carboxylate anion. This stabilizes the negative charge of the conjugate base, increasing the acidity of the carboxylic acid.
The opposite effect occurs with electron donating substituents. The conjugate base of benzoic acid is destabilized by electron-donating substituents in the meta or ortho position making the carboxylic acid less acidic. A closer look at the effects of electron-withdrawing and electron-donating substituents in the meta or ortho position can be seen in the pK a values of benzoic acid as shown in the table below.
Notice the trend in the following table where electron donating substituents X at the meta and para positions lead to weaker acids while those having more electron withdrawing groups generate stronger acids. Almost all ortho-substituents increase the acid strength of a benzoic acid regardless of whether they are electron-donating or electron-withdrawing.
The conjugate base of benzoic acid is destabilized by electron-donating groups EDG. This makes the acid less acidic by pushing more electron density toward the negative charge in the carboxylate. Electron-donating groups activate the benzene ring to electrophilic attack and make benzoic acids less acidic.
Notice the trend in the following table where electron donating substituents X at the para position lead to weaker acids while those having more electron withdrawing groups, further down the table, generate stronger acids. Steven Farmer Sonoma State University. Objectives After completing this section, you should be able to list a given series of carboxylic acids in order of increasing or decreasing acidity. Electron-donating groups The conjugate base of benzoic acid is destabilized by electron-donating groups EDG.
Several examples of electron donating groups. If phenol would have two more of that same withdrawing group attached, would it be simply more acidic? Or, should another factors be considered to determine the acidity? Yes and no, with more electron withdrawing groups at ortho and para positions would certainly increase acidity but other factors do play a role.
For example, In trinitrobenzoic acid. This is the most important factor that plays role in reducing the acidity. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
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