Question 26 of 32intermediate🔍 AnalyzeShort Answer2 marks

Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.

Correct Answer

The decreasing order of acidic behaviour is: Ethyne > Benzene > n-hexane.

Reason: The acidic behaviour of hydrocarbons is determined by the hybridisation of the carbon atom bonded to the hydrogen. Ethyne has sp hybridised carbons (50% s-character), benzene has sp2 hybridised carbons (33.3% s-character), and n-hexane has sp3 hybridised carbons (25% s-character). Since s-electrons are closer to the nucleus, a higher percentage of s-character increases the electronegativity of the carbon atom. This makes the C-H bond more polar and the conjugate base more stable, thereby increasing the acidic character. As alkanes, alkenes, and alkynes follow the trend CH≡CH>H2C=CH2>CH3−CH3\text{CH} \equiv \text{CH} > \text{H}_2\text{C} = \text{CH}_2 > \text{CH}_3 - \text{CH}_3, ethyne is the most acidic, followed by the aromatic benzene, and finally the alkane n-hexane.

Exercise: EXERCISES | Q: 9.18 | (Page Page 33)
For More Understanding

Explanation

The textbook context highlights that alkynes like ethyne show acidic behaviour by reacting with active metals like sodium to form ethynides, a reaction not shown by alkanes and alkenes. The trend CH≡CH>H2C=CH2>CH3−CH3\text{CH} \equiv \text{CH} > \text{H}_2\text{C} = \text{CH}_2 > \text{CH}_3 - \text{CH}_3 is given. To place benzene, we apply the concept of hybridisation: ethyne (sp) is more acidic than benzene (sp2), which is more acidic than n-hexane (sp3) due to the decreasing percentage of s-character.

Solution Steps

  1. Step 1: Identify the hybridisation of the carbon atoms in each compound. Ethyne is sp hybridised, benzene is sp2 hybridised, and n-hexane is sp3 hybridised.

  2. Step 2: Relate hybridisation to s-character. sp has 50% s-character, sp2 has 33.3%, and sp3 has 25%.

  3. Step 3: Conclude that higher s-character leads to greater electronegativity of the carbon atom, making the hydrogen more easily lost as a proton (more acidic). Thus, the order is Ethyne > Benzene > n-hexane.

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