The correct order of increasing acid strength of the compounds is
(A) CH3CO2H
(B) MeOCH2CO2H
(C) CF3

Q: Why do we look at the conjugate base instead of the acid itself?

Because acidity is all about how easily a proton ($H^+$) can leave. If the resulting anion (conjugate base) is highly stable, the acid will happily give up its proton. It's like dropping a heavy backpack; you only do it if you feel comfortable without it!

Q: Why does the isopropyl group have a stronger +I effect than the methyl group?

The isopropyl group has two methyl groups attached to the central carbon. These methyl groups push electron density towards the central carbon, which in turn pushes a massive amount of electron density towards the carboxylate group. More alkyl groups mean a stronger +I effect.

Q: Isn't oxygen in the methoxy group supposed to donate electrons through resonance (+R effect)?

Great question! Oxygen *can* donate electrons via resonance, but ONLY if it is conjugated with a pi system (like a benzene ring or a double bond). Here, the methoxy group is attached to an $sp^3$ hybridized $CH_2$ group. There is no conjugation, so only its electronegativity (-I effect) operates.

Q: Why is fluorine's -I effect so dominant in compound C?

Fluorine is the most electronegative element in the entire periodic table. In compound C, you don't just have one, but *three* fluorine atoms pulling electron density away from the carboxylate group. This creates a massive stabilizing effect, making it a very strong acid.

Q: What if we had a chlorine atom instead of fluorine?

Chlorine is less electronegative than fluorine. So, a $-CCl_3$ group would still exert a strong -I effect and increase acidity, but it would be weaker than the $-CF_3$ group. The acid with $-CF_3$ would still win the acidity contest!

Question

The correct order of increasing acid strength of the compounds is

Options:

(A) B < D < A < C

(B) D < A < C < B

(C) D < A < B < C

(D) A < D < C < B

Sigma Percentile · Accepted Answer

### The Golden Rule of Acidity

When comparing the acidic strength of different organic compounds, the most crucial concept to remember is the **stability of the conjugate base**. An acid is only as strong as its conjugate base is stable.

Imagine an acid as a person carrying a heavy backpack (the proton, $H^+$). They will only drop the backpack if they feel comfortable and stable without it. When a carboxylic acid loses its proton, it forms a carboxylate anion ($R-COO^\ominus$). Our entire job boils down to analyzing how stable this resulting negative charge is.

### The Tug-of-War: Electronic Effects

To determine the stability of the carboxylate anion, we must look at the groups attached to it. These groups can either help stabilize the negative charge or make it worse.

**Electron-Withdrawing Groups (-I effect):** These groups act like helpful friends. They pull the negative electron density towards themselves, dispersing the charge over a larger area. This dispersion of charge highly **stabilizes** the anion, making the original compound a **stronger acid**.

**Electron-Donating Groups (+I effect):** These groups are like people adding more weight to an already heavy load. They push additional electron density onto the already negative oxygen atoms. This intensifies the charge, **destabilizing** the anion and making the original compound a **weaker acid**.

### Analyzing the Contenders

Let's break down the four compounds given in the problem:

**Compound (D): Isobutyric Acid**
Here, we have an isopropyl group attached to the carboxylate. The isopropyl group, $(CH_3)_2CH-$, has two methyl groups that push electron density towards the central carbon, which in turn pushes it towards the carboxylate group. This creates a **strong +I effect**, intensely destabilizing the conjugate base. Thus, compound (D) is the weakest acid.

**Compound (A): Acetic Acid**
Acetic acid has a simple methyl group ($CH_3-$). While a methyl group also exerts a +I effect, it is significantly weaker than the bulky isopropyl group. It still destabilizes the anion, but to a lesser extent than (D). Therefore, (A) is a stronger acid than (D).

**Compound (B): Methoxyacetic Acid**
In this compound, a methoxy group ($CH_3O-$) is attached to the $CH_2$ group. Oxygen is a highly electronegative atom, meaning it loves electrons. It exerts a **-I effect**, pulling electron density away from the carboxylate group. This disperses the negative charge and stabilizes the anion. Because of this stabilization, (B) is more acidic than (A).

**Compound (C): Trifluoroacetic Acid**
Finally, we have the trifluoromethyl group ($CF_3-$). Fluorine is the undisputed champion of electronegativity in the periodic table, and here we have three of them! They exert a **massive -I effect**, powerfully pulling electron density and highly stabilizing the carboxylate anion. This makes (C) the strongest acid of the group.

### The Final Verdict

By comparing the electronic effects, we can easily rank the acidic strengths. The strong +I effect in (D) makes it the weakest, followed by the moderate +I effect in (A). The -I effect in (B) provides stabilization, but the extreme -I effect of the three fluorines in (C) provides the maximum stabilization.

Therefore, the correct order of increasing acid strength is **(D) < (A) < (B) < (C)**.

Animated Solution for Chemistry - Organic Chemistry: The correct order of increasing acid strength of the compounds is

Select Answer:

Visualized Solution

$A c i d i c S t r e n g t h & C o nj ug a t e B a se$

$E l ec t r o ni c E f f ec t s$

$C o m p o u n d (D) : I so p r o p y l G r o u p$

$C o m p o u n d (A) : M e t h y l G r o u p$

$C o m p o u n d (B) : M e t h o x y G r o u p$

$C o m p o u n d (C) : T r i f l u or o m e t h y l G r o u p$

$F ina l O r d er o f A c i d i c S t r e n g t h$

The Sigma Insight: Bond Fission, Electronic Displacement and Hyperconjugation

The Golden Rule of Acidity

The Tug-of-War

Analyzing the Contenders

The Final Verdict

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