(a) Consider the following reaction sequence.
(ii) Write the formula of K.
(iii) Write the structural formula of L and name L.
(v) Write the structure of M and name M.
(ii) Give one example each of a I. monosaccharide; II. disaccharide; III. polysaccharide.
(ii) State what would C = O be observed when I. the compound is mixed with Fehling's solution and boiled;
I. few drops of concentrated H\(_2\)SO\(_4\) is added to the sample of the compound. H?C?OH
(iii) Write an equation for the reaction in (c)(ii)(II).
(d) A hydrocarbon Z with molecular mass 78 on combustion gave 3.385 g of CO\(_2\) and and 0.692 g of H\(_2\)O. Determine the molecular formula of Z. [ H = 1, C = 12, O = 16]
(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 An; 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) An. With a catalyst and heat, ethene molecules join by addition polymerization to form poly(ethene). An 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 Cx(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).
