(a) (i) Give reason why carbon can neither form $\mathrm{C}^{4 + }$ cations nor $\mathrm{C}^{4 - }$ anions but form covalent compounds.

(ii) What is homologous series of carbon compound ? Write the molecular formula of any two consecutive members of homologous series of aldehydes.

(iii) Draw the structure of the molecule of cyclohexane $(\mathrm{C}_6\mathrm{H}_{12})$.

OR

(b) (i) Name a commercially important carbon compound having functional group -OH and write its molecular formula.

(ii) Write chemical equation to show its reaction with

(1) Sodium metal

(2) Excess conc. sulphuric acid

(3) Ethanoic acid in the presence of an acid catalyst

(4) Acidified potassium dichromate

Also write the name of the product formed in each case.

Part (a)

#### (i) Why Carbon forms Covalent Compounds

Carbon has an atomic number of $6$ with an electronic configuration of $2, 4$. It needs to gain or lose $4$ electrons to attain a stable noble gas configuration.

1. It cannot gain four electrons to form a $\mathrm{C}^{4-}$ anion because it would be extremely difficult for a nucleus with only $6$ protons to hold on to $10$ electrons ($4$ extra electrons).
2. It cannot lose four electrons to form a $\mathrm{C}^{4+}$ cation because it requires a very large amount of energy to remove four electrons, leaving behind a highly unstable cation with $6$ protons holding on to just $2$ electrons.

To overcome this, carbon shares its valence electrons with other atoms, forming stable covalent bonds.

#### (ii) Homologous Series and Aldehydes

• Homologous Series: A series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, resulting in similar chemical properties and a regular gradation in physical properties. Successive members differ by a $-\mathrm{CH}_2$ unit.
• Consecutive members of the aldehyde series:

1. Methanal: $\mathrm{HCHO}$
2. Ethanal: $\mathrm{CH}_3\mathrm{CHO}$

#### (iii) Structure of Cyclohexane $(\mathrm{C}_6\mathrm{H}_{12})$

Cyclohexane is a cyclic saturated hydrocarbon. Each carbon atom is single-bonded to two neighboring carbon atoms and two hydrogen atoms.

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OR

Part (b)

#### (i) Commercially Important Compound

• Name: Ethanol
• Molecular Formula: $\mathrm{C}_2\mathrm{H}_5\mathrm{OH}$ (or $\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}$)

#### (ii) Chemical Equations and Products

1. Reaction with Sodium metal:

$$2\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH} + 2\mathrm{Na} \rightarrow 2\mathrm{CH}_3\mathrm{CH}_2\mathrm{ONa} + \mathrm{H}_2 \uparrow$$

• Product Name: Sodium ethoxide (and Hydrogen gas)

1. Reaction with excess concentrated sulphuric acid (at $443\text{ K}$):

$$\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH} \xrightarrow{\text{Hot conc. } \mathrm{H}_2\mathrm{SO}_4, 443\text{ K}} \mathrm{CH}_2=\mathrm{CH}_2 + \mathrm{H}_2\mathrm{O}$$

• Product Name: Ethene

1. Reaction with Ethanoic acid in the presence of an acid catalyst (Esterification):

$$\mathrm{CH}_3\mathrm{COOH} + \mathrm{CH}_3\mathrm{CH}_2\mathrm{OH} \xrightarrow{\text{Acid catalyst}} \mathrm{CH}_3\mathrm{COOCH}_2\mathrm{CH}_3 + \mathrm{H}_2\mathrm{O}$$

• Product Name: Ethyl ethanoate (Ester)

1. Reaction with Acidified Potassium Dichromate (Oxidation):

$$\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH} \xrightarrow{\text{Acidified } \mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7, \text{Heat}} \mathrm{CH}_3\mathrm{COOH}$$

• Product Name: Ethanoic acid