Organic Chemistry — ICSE Class 10 Chemistry Important Questions

Answer: (b) $C_nH_{2n}$C_nH_2n.

Alkenes contain one C=C double bond and have the general formula $C_nH_{2n}$C_nH_2n (e.g. ethene $C_2H_4$C_2H_4).

Answer: (b) Ethanol.

Two carbons (eth-) with the $-OH$-OH alcohol group (suffix -ol) give ethanol.

Answer: (b) Bromine water.

Unsaturated ethene adds bromine ($CH_2=CH_2+Br_2\rightarrow CH_2Br-CH_2Br$CH_2=CH_2+Br_2→ CH_2Br-CH_2Br), decolourising bromine water; saturated ethane does not (in the dark).

Answer: (b) 2.

$C_4H_{10}$C_4H_10 exists as n-butane (straight chain) and iso-butane / 2-methylpropane (branched) - two structural isomers.

Answer: (c) A is true - alkanes are called paraffins (meaning 'little affinity'). R is false: alkanes are saturated and rather unreactive, undergoing substitution, not addition.

A homologous series is a family of organic compounds having the same general formula and the same functional group, in which consecutive members differ by a $-CH_2-$-CH_2- group (relative mass $14$14).

Characteristics (any two):

1. All members have the same general formula and can be prepared by similar methods.
2. They show a gradual change in physical properties with increasing molecular mass but have similar chemical properties.

(i) $CH_3-COOH$CH_3-COOH has two carbons and the $-COOH$-COOH group, so its IUPAC name is ethanoic acid (acetic acid).

(ii) $CH\equiv CH$CH CH has two carbons and a triple bond, so its IUPAC name is ethyne (acetylene).

Preparation: Anhydrous sodium acetate (sodium ethanoate) is heated with soda lime (a mixture of $NaOH$NaOH and $CaO$CaO) in a hard glass test tube. Methane gas is collected over water.

$CH_3COONa+NaOH\xrightarrow{CaO,\ \Delta}CH_4\uparrow+Na_2CO_3$CH_3COONa+NaOH→ (CaO, Δ)CH_4+Na_2CO_3

Nature of the reaction: it is a decarboxylation reaction, in which the carboxyl group is removed as carbonate and methane is liberated. (Soda lime provides the alkali; $CaO$CaO keeps $NaOH$NaOH dry and prevents it from attacking the glass.)

Saturated hydrocarbon: contains only single C-C bonds (e.g. ethane $C_2H_6$C_2H_6); undergoes substitution reactions.

Unsaturated hydrocarbon: contains at least one C=C or C≡C multiple bond (e.g. ethene $C_2H_4$C_2H_4); undergoes addition reactions.

Test - bromine water (or dilute alkaline $KMnO_4$KMnO_4):

• Ethene rapidly decolourises reddish-brown bromine water: $C_2H_4+Br_2\rightarrow C_2H_4Br_2$C_2H_4+Br_2→ C_2H_4Br_2.
• Ethane does not decolourise it (no reaction in the dark).

Decolourisation therefore indicates the unsaturated compound, ethene.

(i) With hydrogen (catalytic hydrogenation, $Ni$Ni, heat):
$CH_2=CH_2+H_2\xrightarrow{Ni}CH_3-CH_3$CH_2=CH_2+H_2→ (Ni)CH_3-CH_3 (ethane)

(ii) With bromine:
$CH_2=CH_2+Br_2\rightarrow CH_2Br-CH_2Br$CH_2=CH_2+Br_2→ CH_2Br-CH_2Br (1,2-dibromoethane)

(iii) With steam (catalyst, high pressure):
$CH_2=CH_2+H_2O\xrightarrow{H_3PO_4}CH_3-CH_2-OH$CH_2=CH_2+H_2O→ (H_3PO_4)CH_3-CH_2-OH (ethanol)

In each case the double bond opens and atoms add across it, showing the addition nature of unsaturated compounds.

Preparation: Water is added drop by drop from a dropping funnel onto pieces of calcium carbide in a flask; ethyne is evolved and collected over water.

$CaC_2+2H_2O\rightarrow Ca(OH)_2+C_2H_2\uparrow$CaC_2+2H_2O→ Ca(OH)_2+C_2H_2

Chemical properties:

1. Combustion (burns with a bright sooty flame; used in oxy-acetylene welding):
   $2C_2H_2+5O_2\rightarrow 4CO_2+2H_2O$2C_2H_2+5O_2→ 4CO_2+2H_2O
2. Addition of hydrogen (unsaturated, adds across the triple bond):
   $C_2H_2+2H_2\xrightarrow{Ni}C_2H_6$C_2H_2+2H_2→ (Ni)C_2H_6 (ethane)

Ethyne also decolourises bromine water, confirming unsaturation.

(a) Isomers of $C_4H_{10}$C_4H_10:

1. n-Butane (straight chain): $CH_3-CH_2-CH_2-CH_3$CH_3-CH_2-CH_2-CH_3
2. 2-Methylpropane (iso-butane, branched):
   $CH_3-CH(CH_3)-CH_3$CH_3-CH(CH_3)-CH_3, i.e. a propane chain with a $-CH_3$-CH_3 branch on the middle carbon.

Both have molecular formula $C_4H_{10}$C_4H_10 but different structures, so they are structural (chain) isomers.

(b)(i) Substitution of methane with chlorine (sunlight/diffused light, first step):
$CH_4+Cl_2\xrightarrow{sunlight}CH_3Cl+HCl$CH_4+Cl_2→ (sunlight)CH_3Cl+HCl

A hydrogen atom is replaced by a chlorine atom, giving chloromethane.

(b)(ii) Fermentation of glucose (enzyme zymase, $\approx35^\circ C$35^ C):
$C_6H_{12}O_6\xrightarrow{zymase}2C_2H_5OH+2CO_2\uparrow$C_6H_12O_6→ (zymase)2C_2H_5OH+2CO_2

(i) X is ethanol, $C_2H_5OH$C_2H_5OH (or $C_2H_6O$C_2H_6O); on oxidation it gives Y = ethanoic acid (acetic acid), $CH_3COOH$CH_3COOH (or $C_2H_4O_2$C_2H_4O_2).

(ii) X contains the hydroxyl group ($-OH$-OH, alcohol); Y contains the carboxyl group ($-COOH$-COOH, carboxylic acid).

(iii) Reaction of ethanol with sodium (liberates hydrogen):
$2C_2H_5OH+2Na\rightarrow 2C_2H_5ONa+H_2\uparrow$2C_2H_5OH+2Na→ 2C_2H_5ONa+H_2

(iv) Esterification (conc. $H_2SO_4$H_2SO_4 catalyst):
$CH_3COOH+C_2H_5OH\xrightleftharpoons[H_2SO_4]{}CH_3COOC_2H_5+H_2O$CH_3COOH+C_2H_5OH[H_2SO_4]CH_3COOC_2H_5+H_2O

The product is ethyl ethanoate (ethyl acetate), a sweet-smelling ester.