(a) Consider the following reaction sequence. (i) What process leads/tathe formation of K? (ii) Write the formula of K. (iii) Write the structural formula o...

(a) Reaction sequence. The diagram shows: C₂H₅OH treated with conc. H₂SO₄ at 180°C gives K; K with KMnO₄/H₂O gives L; K with a catalyst and heat gives A_n; K with Cl₂ gives M.

(a)(i) Process forming K: dehydration of ethanol (removal of water). Ethanol loses a molecule of water to form ethene.

\[ \text{C}_2\text{H}_5\text{OH} \xrightarrow[180^{\circ}\text{C}]{\text{conc. H}_2\text{SO}_4} \text{C}_2\text{H}_4 + \text{H}_2\text{O} \]

(a)(ii) Formula of K: C₂H₄ (ethene).

(a)(iii) L. Ethene with cold dilute KMnO₄/H₂O is oxidised (hydroxylated) to a diol. Structural formula: CH₂(OH)–CH₂(OH). Name: ethane-1,2-diol (ethylene glycol).

(a)(iv) A_n. With a catalyst and heat, ethene molecules join by addition polymerization to form poly(ethene). A_n is polyethene (polythene), (C₂H₄)_n.

\[ n\,\text{CH}_2{=}\text{CH}_2 \rightarrow \ [\!-\text{CH}_2-\text{CH}_2-\!]_n \]

(a)(v) M. Ethene adds chlorine across its double bond. Structure: CH₂Cl–CH₂Cl. Name: 1,2-dichloroethane.

\[ \text{CH}_2{=}\text{CH}_2 + \text{Cl}_2 \rightarrow \text{CH}_2\text{Cl}{-}\text{CH}_2\text{Cl} \]

(b)(i) Carbohydrates are organic compounds made up of carbon, hydrogen and oxygen (the hydrogen and oxygen present in the ratio 2:1 as in water), having the general formula C_x(H₂O)_y; they are polyhydroxy aldehydes or ketones.

(b)(ii) Examples:

• I. Monosaccharide: glucose (or fructose).
• II. Disaccharide: sucrose (or maltose).
• III. Polysaccharide: starch (or cellulose).

(c) Simple sugar (open-chain glucose, containing a terminal C=O and several –C(H)(OH)– groups).

(c)(i) Functional group responsible for its reducing action: the aldehyde group, –CHO (the terminal C=O).

(c)(ii) Observations:

• I. With Fehling's solution, warmed: the blue solution gives a brick-red precipitate of copper(I) oxide, Cu₂O, because the aldehyde group reduces Cu²⁺ to Cu⁺.
• II. With a few drops of concentrated H₂SO₄: the sugar is dehydrated and turns black (charring), leaving a black spongy mass of carbon as water is removed.

(c)(iii) Equation for (c)(ii)II (dehydration of glucose by concentrated sulphuric acid):

\[ \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{conc. H}_2\text{SO}_4} 6\text{C} + 6\text{H}_2\text{O} \]

(d) Molecular formula of hydrocarbon Z (molar mass 78).

Amount of carbon (from CO₂):

\[ n(\text{CO}_2) = \frac{3.385}{44} = 0.0769\ \text{mol} \Rightarrow n(\text{C}) = 0.0769\ \text{mol} \]

Amount of hydrogen (from H₂O):

\[ n(\text{H}_2\text{O}) = \frac{0.692}{18} = 0.0384\ \text{mol} \Rightarrow n(\text{H}) = 2 \times 0.0384 = 0.0769\ \text{mol} \]

Mole ratio C : H

\[ \text{C} : \text{H} = 0.0769 : 0.0769 = 1 : 1 \]

Empirical formula = CH, of mass \(12 + 1 = 13\).

\[ n = \frac{78}{13} = 6 \]

Therefore the molecular formula of Z is C₆H₆ (benzene).