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 $C_2H_4$C₂H₄).

Answer: (b) Ethanol.

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

Answer: (a) concentrated sulphuric acid at about $170^\circ C$170° C. The acid removes water (dehydration): $C_2H_5OH\xrightarrow{conc.\ H_2SO_4,\ 170^\circ C}C_2H_4+H_2O$C₂H₅OH→ (conc. H₂SO₄, 170° C)C₂H₄+H₂O. Ethene can also be made by dehydrohalogenation: $C_2H_5Br+KOH\ (alc.)\rightarrow C_2H_4+KBr+H_2O$C₂H₅Br+KOH (alc.)→ C₂H₄+KBr+H₂O.

Answer: (b) 2.

$C_4H_{10}$C₄H₁₀ 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 $-CH_2-$-CH₂- 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) $CH_3-COOH$CH₃-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.

$CH_3COONa+NaOH\xrightarrow{CaO,\ \Delta}CH_4\uparrow+Na_2CO_3$CH₃COONa+NaOH→ (CaO, Δ)CH₄↑+Na₂CO₃

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 $C_2H_6$C₂H₆); undergoes substitution reactions.

Unsaturated hydrocarbon: contains at least one C=C or C≡C multiple bond (e.g. ethene $C_2H_4$C₂H₄); undergoes addition reactions.

Test - bromine water (or dilute alkaline $KMnO_4$KMnO₄):

• Ethene rapidly decolourises reddish-brown bromine water: $C_2H_4+Br_2\rightarrow C_2H_4Br_2$C₂H₄+Br₂→ C₂H₄Br₂.
• 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):
$CH_2=CH_2+H_2\xrightarrow{Ni}CH_3-CH_3$CH₂=CH₂+H₂→ (Ni)CH₃-CH₃ (ethane)

(ii) With bromine:
$CH_2=CH_2+Br_2\rightarrow CH_2Br-CH_2Br$CH₂=CH₂+Br₂→ CH₂Br-CH₂Br (1,2-dibromoethane)

(iii) With chlorine: $CH_2=CH_2+Cl_2\rightarrow CH_2Cl-CH_2Cl$CH₂=CH₂+Cl₂→ CH₂Cl-CH₂Cl (1,2-dichloroethane)

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.

$CaC_2+2H_2O\rightarrow Ca(OH)_2+C_2H_2\uparrow$CaC₂+2H₂O→ Ca(OH)₂+C₂H₂↑

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$2C₂H₂+5O₂→ 4CO₂+2H₂O
2. Addition of hydrogen (unsaturated, adds across the triple bond):
   $C_2H_2+2H_2\xrightarrow{Ni}C_2H_6$C₂H₂+2H₂→ (Ni)C₂H₆ (ethane)

Ethyne also decolourises bromine water, confirming unsaturation.

(a) Isomers of $C_4H_{10}$C₄H₁₀:

1. n-Butane (straight chain): $CH_3-CH_2-CH_2-CH_3$CH₃-CH₂-CH₂-CH₃
2. 2-Methylpropane (iso-butane, branched):
   $CH_3-CH(CH_3)-CH_3$CH₃-CH(CH₃)-CH₃, i.e. a propane chain with a $-CH_3$-CH₃ branch on the middle carbon.

Both have molecular formula $C_4H_{10}$C₄H₁₀ but different structures, so they are structural (chain) isomers.

(b)(i) Substitution of methane with chlorine (sunlight/diffused light, first step):
$CH_4+Cl_2\xrightarrow{sunlight}CH_3Cl+HCl$CH₄+Cl₂→ (sunlight)CH₃Cl+HCl

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

(b)(ii) Hydrolysis of bromoethane (boiled with aqueous alkali): $C_2H_5Br+NaOH\ (aq)\rightarrow C_2H_5OH+NaBr$C₂H₅Br+NaOH (aq)→ C₂H₅OH+NaBr

(i) X is ethanol, $C_2H_5OH$C₂H₅OH (or $C_2H_6O$C₂H₆O); on oxidation it gives Y = ethanoic acid (acetic acid), $CH_3COOH$CH₃COOH (or $C_2H_4O_2$C₂H₄O₂).

(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$2C₂H₅OH+2Na→ 2C₂H₅ONa+H₂↑

(iv) Esterification (conc. $H_2SO_4$H₂SO₄ catalyst):
$CH_3COOH+C_2H_5OH\xrightleftharpoons[H_2SO_4]{}CH_3COOC_2H_5+H_2O$CH₃COOH+C₂H₅OH[H₂SO₄]CH₃COOC₂H₅+H₂O

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