Science — CBSE Class 10 board question

(i) Structural formula of benzene (C₆H₆):

Benzene has a ring of 6 carbon atoms with alternating double bonds:

$$\text{Hexagonal ring with alternating single and double bonds between carbon atoms, each C bonded to one H}$$

(A regular hexagon with alternating double bonds, each carbon bonded to one H atom.)

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(ii) Carbon has 4 electrons in its outermost shell. To form C⁴⁺, it would need to lose 4 electrons, requiring a very large amount of energy. To form C⁴⁻, it would gain 4 electrons, which would be difficult to hold in the nucleus with only 6 protons. Both processes are energetically unfavourable, so carbon forms covalent bonds instead.

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(iii) The bond formed between carbon and hydrogen is a covalent bond (non-polar covalent bond), formed by equal sharing of electrons.

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(iv) Ethanol is converted to ethanoic acid by oxidation, using an oxidising agent such as alkaline KMnO₄ or acidified K₂Cr₂O₇.

$$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{[\text{O}]} \text{CH}_3\text{COOH}$$

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(v)

$$\text{CH}_3\text{COOC}_2\text{H}_5 + \text{NaOH} \longrightarrow \text{CH}_3\text{COONa} + \text{C}_2\text{H}_5\text{OH}$$

(Sodium ethanoate + Ethanol)

Source: Carbon and its Compounds, Chapter 4, Sections 4.2.2, 4.4

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• Benzene structure: Draw the hexagonal ring clearly with alternating double bonds; examiners want to see the alternating bond pattern.
• C⁴⁺/C⁴⁻: Focus on two reasons — large energy needed to lose 4 electrons, and difficulty of nucleus holding 4 extra electrons. Mention that covalent bonding is preferred.
• C–H bond: Simply state "covalent bond"; adding "non-polar" earns extra credit.
• Ethanol → Ethanoic acid: Must state it is an oxidation reaction and name the oxidising agent. The arrow notation [O] is standard.
• Saponification equation: This is ester hydrolysis (saponification). Products are sodium ethanoate (sodium salt of acid) and ethanol. Names of products often fetch ½ mark each.