Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following orders of acid strength if correct? (a) \(\mathrm{RCOOH}>\mathrm{HC} \equiv \mathrm{CH}>\mathrm{HOH}>\mathrm{ROH}\) (b) \(\mathrm{RCOOH}>\mathrm{ROH}>\mathrm{HOH}>\mathrm{HC} \equiv \mathrm{CH}\) (c) \(\mathrm{RCOOH}>\mathrm{HOH}>\mathrm{ROH}>\mathrm{HC} \equiv \mathrm{CH}\) (d) \(\mathrm{RCOOH}>\mathrm{HOH}>\mathrm{HC} \equiv \mathrm{CH}>\mathrm{ROH}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{RCOOH} > \mathrm{HOH} > \mathrm{ROH} > \mathrm{HC} \equiv \mathrm{CH}\) is the correct order of acid strength.

Step 1: Analyze the functional groups

Consider the functional groups of each compound: - \(\mathrm{RCOOH}\) is a carboxylic acid- \(\mathrm{HC} \equiv \mathrm{CH}\) is an alkyne- \(\mathrm{HOH}\) is water- \(\mathrm{ROH}\) is an alcoholTo determine the acid strength, we need to consider the capacity of these compounds to donate a proton (\(H^+\)). Generally, strong acids readily donate protons.

Step 2: Identify the strongest acid

Carboxylic acids (\(\mathrm{RCOOH}\)) are known to be stronger acids compared to alcohols, water, and alkynes because the carboxylate ion (formed after donation of a proton) is resonance-stabilized. Therefore, \(\mathrm{RCOOH}\) is expected to be the strongest acid among the options.

Step 3: Compare water and alcohol

Water (5 5 ) is potentially a stronger acid than an alcohol (5 5 ) because when water donates a proton, it forms a more stable hydronium ion ( 5 5 ). Alcohols have alkyl groups that provide electron-donating effects (via hyperconjugation and inductive effects), making the conjugate base less stable.

Step 4: Evaluate alkyne acidity

Alkynes (5 5 5 -5 5 5 ) have terminal hydrogen atoms that can be acidic because of their sp hybridized carbon, which is more electronegative and holds the conjugate base more stably compared with sp2 or sp3 hybridized carbons. However, they are typically less acidic compared to water which contains more polar bonds.

Step 5: Establish correct order of acid strength

Taking into account the steps above, the correct order should be: carboxylic acid (5 5 ), water (5 5 ), alcohol (5 5 ), and then alkyne (5 5 5 -5 5 5 ). Therefore, based on the given options, (c) RCOOH > HOH > ROH > HC 5 5 is the correct order of acid strength.

Key Concepts

Carboxylic Acids

Carboxylic acids are organic compounds containing a carboxyl group \((\text{–COOH})\). This group plays a significant role in the acid's properties due to its ability to donate protons easily. When carboxylic acids donate a proton (\(\text{H}^+\)), they form a carboxylate ion. A key aspect of their strength as an acid is attributed to this carboxylate ion being resonance-stabilized.
This means the negative charge after losing the proton is spread out over multiple atoms, making the ion more stable. Because of this stability, carboxylic acids have a greater tendency to donate protons compared to many other organic molecules. Thus, they are generally considered strong acids among various organic functional groups.

Alkynes

Alkynes are hydrocarbons that contain a carbon-carbon triple bond \((\text{C}\equiv\text{C})\). This triple bond consists of one sigma bond and two pi bonds. In terminal alkynes, where the hydrogen is attached to the carbon, this hydrogen is relatively acidic.
This stems from the sp hybridization of the carbon, which holds electrons more closely and makes the proton more positive. However, alkynes are not typically very strong acids when compared to carboxylic acids or water. The stability of their conjugate base is less than that of water, reducing their ability to donate protons.The acidic properties of alkynes make them useful in some chemical reactions, but they are not strong acids in general chemical comparisons.

Alcohols

Alcohols are organic compounds that are characterized by the presence of a hydroxyl group \((\text{–OH})\) bonded to a carbon atom. This hydroxyl group can donate a proton, but it does so less readily than water. The introduction of the alkyl group in alcohols provides electron-donating effects.
This occurs through both hyperconjugation and inductive effects, where electrons are pushed towards the oxygen. This electron-donating nature makes the conjugate base of an alcohol less stable. As a result, alcohols are weaker acids compared to water. This weaker acidity means they are often used as solvents and have various chemical applications due to their moderate reactivity.

Water as an acid

Water \((\text{H}_2\text{O})\) can act as both an acid and a base. As an acid, it can donate a proton to form a hydronium ion \((\text{H}_3\text{O}^+)\), which is a well-stabilized species.
This ability to form stable hydronium ions means water has a certain level of acidity. Despite being a weaker acid than carboxylic acids, water is still more acidic than alcohols due to better stabilization of its conjugate base. Water's dual nature as both an acid and base makes it unique and essential in a wide range of chemical processes.

Proton donation

Proton donation is a fundamental concept in understanding acidity. A molecule's ability to donate a proton \(\text{(H}^+\text{)}\) determines its acidic strength. Strong acids, such as carboxylic acids, readily release \(\text{H}^+\text{)}\) ions, making them effective proton donors.
The acidic strength lies in the stability of the conjugate base formed after the proton is released. More stable conjugate bases result in stronger acids. Factors influencing this include the presence of electronegative atoms, resonance stabilization, and the hybridization state of the atoms involved. Proton donation is crucial in titration, buffer solutions, and many biochemical pathways.

Conjugate base stability

Conjugate base stability directly influences acid strength. When an acid donates a proton, it leaves behind its conjugate base, typically bearing the negative charge. The more stable this conjugate base, the more willing the acid is to donate a proton.
Several factors can enhance this stability, such as resonance effects, electron-withdrawing groups, and hybridization. For instance, carboxylic acids form conjugate bases that are resonance-stabilized, making them strong acids. Conversely, alkyl groups in alcohols diminish the stability of the conjugate base through electron-donating effects, reducing their acid strength. Conjugate base stability is a key concept in predicting and understanding acidity in chemical systems.