The correct sequence of acidic strength of the following aliphatic acids in their decreasing order is: $CH_3CH_2COOH$, $CH_3COOH$, $CH_3CH_2CH_2COOH$, $HCOOH$
To determine the correct sequence of acidic strength for the given aliphatic carboxylic acids, let's analyze each compound:
\(HCOOH\) (Formic Acid): Formic acid is the simplest carboxylic acid, with a molecular formula of \(HCOOH\). Due to the absence of an electron-donating alkyl group, formic acid is the most acidic among aliphatic carboxylic acids.
\(CH_3COOH\) (Acetic Acid): It has a methyl group (\(CH_3\)), which exerts a slight electron-donating effect, reducing its acidic strength compared to formic acid.
\(CH_3CH_2COOH\) (Propionic Acid) and \(CH_3CH_2CH_2COOH\) (Butyric Acid): Both have larger alkyl groups than acetic acid, further decreasing the acidic strength due to the electron-donating effect increasing with the length of the alkyl chain.
The electron-donating effect of alkyl groups decreases the acidity because they destabilize the carboxylate ion formed after deprotonation. Thus, the more extended the alkyl chain, the weaker the acid.
Based on the analysis above, the sequence of acidic strength in decreasing order is:
\(HCOOH\) (Formic Acid) > most acidic
\(CH_3COOH\) (Acetic Acid)
\(CH_3CH_2COOH\) (Propionic Acid)
\(CH_3CH_2CH_2COOH\) (Butyric Acid) > least acidic
Therefore, the correct answer is: \(HCOOH > CH_3COOH > CH_3CH_2COOH > CH_3CH_2CH_2COOH\).
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The acidic strength of carboxylic acids is influenced by the electron-withdrawing or electron-donating effects of alkyl groups: HCOOH (formic acid) is the most acidic as it has no electron-donating alkyl group, which would reduce acidity. CH$_3$COOH (acetic acid) is less acidic because the methyl group (CH$_3$) is weakly electron-donating. CH$_3$CH$_2$COOH (propionic acid) is even less acidic due to the larger electron-donating ethyl group. CH$_3$CH$_2$CH$_2$COOH (butyric acid) is the least acidic because the longer alkyl chain has a stronger electron-donating effect. The correct order of acidic strength is: \[ \textbf{HCOOH > CH$_3$COOH > CH$_3$CH$_2$COOH > CH$_3$CH$_2$CH$_2$COOH}. \]
