Question

The correct order of increasing acid strength of the compound: (1) \(\mathrm{CH}_{3} \mathrm{COOH}\) (2) \(\mathrm{MeOCH}_{2} \mathrm{CO}_{2} \mathrm{H}\) (3) \(\mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}\) (4) \((\mathrm{Me})_{2} \mathrm{CH} \mathrm{CO}_{2} \mathrm{H}\) (a) \(2<4<1<3\) (b) \(4<1<3<2\) (c) \(4<1<2<3\) (d) \(1<4<3<2\)

Short answer

The correct order of increasing acid strength is option (c) \(4 < 1 < 2 < 3\).

Step-by-step solution

Identify the Functional Groups

Examine the given compounds and their functional groups. All compounds are carboxylic acids, indicated by the presence of the carboxyl group \((-COOH)\).

Analyze the Electron-Donating and Withdrawing Groups

Consider how the substituents attached to the carbon chain influence acidity. Electron-withdrawing groups, like -CF extsubscript{3} in compound (3), increase acidity by stabilizing the negative charge on the conjugate base. Electron-donating groups, like -CH extsubscript{3} or -OCH extsubscript{2}-, tend to decrease acidity.

Determine Acid Strength Due to Inductive Effect

Using the inductive effect, rank the compounds in order of electron-withdrawing strength of their substituents: 1. Compound (3), CF extsubscript{3}COOH, has the strongest electron-withdrawing group (-CF extsubscript{3}) and thus is the most acidic. 2. Compound (1), CH extsubscript{3}COOH, with a weakly electron-donating group (-CH extsubscript{3}), is moderately acidic. 3. Compound (4), (Me) extsubscript{2}CHCOOH, with two methyl groups, is less acidic because of increased electron donation. 4. Compound (2), MeOCH extsubscript{2}COOH, has an electron-donating group that can slightly decrease acidity.

Compare Acid Strength Order with Options

From the analysis, the acid strength order is \(4 < 1 < 2 < 3\). Thus, find a matching option from the given choices.

Select the Correct Option

The correct option that matches the order of increasing acid strength is (c) \(4 < 1 < 2 < 3\).

Key concepts

Carboxylic Acids

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group \((-COOH)\). This group is a combination of a carbonyl \(C=O\) and a hydroxyl group \((-OH)\). It is the source of the acidic properties in these molecules.

Carboxylic acids are notable for their ability to donate a proton \(H^+\), resulting in the formation of a carboxylate ion. This process is what defines their acidity. The more easily a proton is donated, the stronger the acid. Factors influencing this proton donation include additional groups attached to the molecular skeleton, which can have varying effects on stability and acidity.

Electron-Withdrawing Groups

Electron-withdrawing groups are chemical substituents that pull electron density away from other parts of the molecule. When attached to carboxylic acids, these groups can increase the acid's strength. They stabilize the negatively charged carboxylate ion, which forms after the acid loses a proton.

Consider \( ext{CF}_3\) in \(\text{CF}_3\text{COOH}\): this group is highly electronegative, meaning it draws electrons towards itself. This makes the carboxylate ion more stable after dissociation because the negative charge is better accommodated by the electron-withdrawing nature of \( ext{CF}_3\). In essence, the presence of such groups on a carboxylic acid enhances its acidity by making the action of losing a proton more favorable.

Inductive Effect

The inductive effect refers to the ability of an atom or group of atoms to affect the charge distribution in a molecule through sigma bonds. It is especially pronounced in organic chemistry when considering acidic strength.

When applied to carboxylic acids, the inductive effect can either strengthen or weaken the acid, depending on whether the substituents are electron-withdrawing or electron-donating. Electron-withdrawing groups create a negative inductive effect, stabilizing the ion by reducing the overall electron density. Meanwhile, electron-donating groups exert a positive inductive effect, destabilizing the ion and reducing acidity.

The order of acidity among the given compounds mainly stems from the differences in their substituents and the direction of the inductive effect exerted by these substituents.

Chemical Substituents Influence

Chemical substituents attached to the main molecular backbone play a crucial role in defining the properties of a compound, such as acidity in carboxylic acids.

Each substituent can have either an electron-withdrawing or electron-donating nature. This nature profoundly affects the acid's stability after proton loss. For instance:

• Methyl groups \((-CH_3)\) are electron-donating, which can reduce acidity as they push electron density back into the acid group.
• Alkoxy groups like \(\text{MeOCH}_2\)- also have electron-donating properties, albeit to different extents.
• Trifluoromethyl groups \((-CF_3)\) are strong electron-withdrawing groups, which enhance acidity by stabilizing the resulting anion.

Understanding the influence of these substituents helps us predict the behavior of carboxylic acids under various conditions and provides insights into designing more potent acids by modulating their chemical environment.