Arrange the pKas of the hydroxy groups of methanol, trifluoroacetic acid, phenol and benzyl alcohol in ascending order:a)Trifluoroacetic acid < phenol < benzyl alcohol < methanolb)Trifluoroacetic acid < benzyl alcohol < phenol < methanolc)Phenol < trifluroacetic acid < benzyl alcohol < methanold)Trifluoroacetic acid < methanol < phenol < benzyl alcoholCorrect answer is option 'A'. Can you explain this answer?

Question

Arrange the pKas of the hydroxy groups of methanol, trifluoroacetic acid, phenol and benzyl alcohol in ascending order:a)Trifluoroacetic acid &#60; phenol &#60; benzyl alcohol &#60; methanolb)Trifluoroacetic acid &#60; benzyl alcohol &#60; phenol &#60; methanolc)Phenol &#60; trifluroacetic acid &#60; benzyl alcohol &#60; methanold)Trifluoroacetic acid &#60; methanol &#60; phenol &#60; benzyl alcoholCorrect answer is option 'A'. Can you explain this answer?

Answer

Ascending order of pKas of hydroxy groups

Trifluoroacetic acid: Trifluoroacetic acid is a strong acid and its pKa is approximately 0.23.

Phenol: Phenol is a weak acid and its pKa is approximately 9.95.

Benzyl alcohol: Benzyl alcohol is a weak acid and its pKa is approximately 15.4.

Methanol: Methanol is a weak acid and its pKa is approximately 15.5.

Therefore, the correct order of the pKas of the hydroxy groups in ascending order is:

Trifluoroacetic acid < phenol='' />< benzyl='' alcohol='' />< />

Explanation:

Trifluoroacetic acid is a strong acid because of the presence of three electron-withdrawing fluorine atoms which stabilize the negative charge on the carboxylate ion. Hence, the pKa of the hydroxy group in this compound is low.

Phenol is a weak acid because of the presence of the aromatic ring which stabilizes the negative charge on the phenoxide ion. However, the stabilization is not as effective as in benzoic acid because the lone pair on the oxygen atom is involved in resonance with the aromatic ring. Hence, the pKa of the hydroxy group in this compound is higher than in trifluoroacetic acid.

Benzyl alcohol is a weak acid because of the presence of the benzyl group which stabilizes the negative charge on the benzyloxide ion. However, the stabilization is not as effective as in phenol because the benzyl group is not as electron-withdrawing as the aromatic ring. Hence, the pKa of the hydroxy group in this compound is higher than in phenol.

Methanol is a weak acid because of the absence of any group which stabilizes the negative charge on the methoxide ion. Hence, the pKa of the hydroxy group in this compound is the highest among the given compounds.

Answer

A is correct because F in trifluoroacetic acid is an ewg( electron withdrawing group) CF3COOH and as we know carboxylic acid is stronger acid than alcohol and hence it's pka value is less .phenol(C6H5OH) is more acidic than benzyl alcohol(C6H5CH2OH) because CH2(erg group) attached to benzyl  alcohol decreases its acidity though they have the same nunber of  resonance structures.phenol and benzyl alcohol are more acidic than methanol because they are more resonance stabilised than methanol because of  the stable phenoxide ion (C6H5O-) From above discussion order of acidity isCF3COOH>C6H5OH>C6H5CH20H>CH30H As we know pka= -log ka and less the pka value more is the acidity Hence order is AHope this is helpful

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