QUESTIONS
SECTION A
1. The structure of glycerol (a by-product of soap manufacture) is shown below:
H H H
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H — C — C — C — H
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OH OH OH
What is the:
(a) molecular formula
(b) Empirical formula
(c) IUPAC nomenclature
2. Identify the reaction process induced by each of the following reagents:
(a) Br₂/H₂O
(b) hot concentrated NaOH(aq)
(c) H₂/Ni catalyst
(d) CH₃COCl/AlCl₃
(e) Cu, 300°C
3a) Define allotropy.
3b) Give the formula of three (3) known allotropes of carbon.
4. The structures of the amino acids serine and cysteine are shown below:
CH₂OH CH₂SH
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H₂N — C — COOH H₂N — C — COOH
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H H
serine cysteine
a) i) Serine exists as a pair of stereoisomers. Explain the meaning of the term optical isomerism.
ii) State the condition necessary for a carbon atom to be described as a chiral centre.
b) Give the formulae of the peptides which are formed when serine and cysteine react together.
5. In the bromination of hexane (C₆H₁₄):
(a) what is the likely mechanism for the process
(b) what type of bond fission is involved
(c) how many different structural isomers can be produced for C₆H₁₃Br
(d) draw the molecular formula of one of the isomers and name it
6. How many pi (π) electrons are present in each of the following molecules?
(a) But-1-yne
(b) Penta-1,3-diene
(c) Cyclopentene
(d) Octane
(e) Butanone
7. List five (5) uses of petrochemical products in everyday life.
8. Classify the following substituent groups as "activating" or "deactivating" in aromatic substitution reactions.
(a) −CH₃
(b) −Cl
(c) −CHO
(d) −OCH₃
(e) −CN
9. Give the molecular formula and name of the second, third, fourth, fifth and sixth member of the alkanol (alcohol) homologous series.
10. Give the structure of the second member of the following homologous series:
(a) Alkene
(b) Alkanal
(c) Alkanoate ester
(d) Alkanenitrile
(e) Alkylbenzene
SECTION B: Attempt Any TWO (2) Questions in this Section
11a) Explain, with illustrative examples, what is meant by the terms:
(i) hydrolysis (ii) hydrogenation (iii) sulphonation (iv) polymerisation
11b) List five (5) methods of preparing alkenes and give one example equation for each.
12a) An organic compound (0.45g) was analysed and found to contain 0.180g of carbon, 0.030g of hydrogen and the rest oxygen. What is the empirical formula, molecular formula and molecular structure of the compound? (given relative molar mass = 90)
12b) Give the structure of the organic product obtainable in the reaction of propanoic acid with:
(i) Phosphorus(V) chloride
(ii) Ethanol/concentrated H₂SO₄
(iii) Sodium metal
(iv) Lithium tetrahydridoaluminate
(v) Ammonia (heated)
ANSWERS
Question 1
Counting atoms in the structure: 3 carbons in a chain, each bearing one OH group, plus H atoms: C1 has 2H (terminal CH₂OH), C2 has 1H, C3 has 2H (terminal CH₂OH); total H = 2+1+2+3(OH) = 8
(a) Molecular formula: C₃H₈O₃
(b) Empirical formula: Ratio C:H:O = 3:8:3 (already in lowest terms since GCD=1) → C₃H₈O₃
(c) IUPAC nomenclature: Propane-1,2,3-triol
Question 2
(a) Br₂/H₂O: Electrophilic addition (adds across a C=C double bond to give a bromohydrin) or, with an aromatic compound, electrophilic substitution (bromination, no catalyst needed for activated rings e.g. phenol)
(b) Hot concentrated NaOH(aq): Nucleophilic substitution/hydrolysis (converts haloalkanes to alcohols) — with a haloalkane substrate, SN1/SN2 hydrolysis to form an alcohol
(c) H₂/Ni catalyst: Hydrogenation/reduction (adds hydrogen across C=C or C≡C multiple bonds, or reduces carbonyl groups, in the presence of a nickel catalyst)
(d) CH₃COCl/AlCl₃: Friedel-Crafts acylation (electrophilic substitution introducing an ethanoyl/acetyl group onto an aromatic ring)
(e) Cu, 300°C: Catalytic dehydrogenation (oxidises a primary alcohol to an aldehyde, or a secondary alcohol to a ketone, by removal of hydrogen)
Question 3
a) Allotropy: Allotropy is the phenomenon whereby an element exists in two or more distinct physical/structural forms (allotropes) in the same physical state, arising from different arrangements of atoms, while remaining the same element.
b) Three known allotropes of carbon: Diamond, Graphite, Fullerene (C₆₀/Buckminsterfullerene)
Question 4
a) i) Optical isomerism: Optical isomerism is a form of stereoisomerism in which two compounds have the same molecular formula and connectivity but differ in their spatial arrangement such that one is a non-superimposable mirror image of the other; the two forms (enantiomers) rotate plane-polarised light in opposite directions.
ii) A carbon atom is described as a chiral centre (asymmetric carbon) if it is bonded to four different atoms or groups, giving no plane of symmetry in the molecule about that carbon.
b) When serine and cysteine react together (condensation, losing H₂O to form a peptide bond):
Ser-Cys: HOCH₂CH(NH₂)CO—NH—CH(CH₂SH)COOH
Cys-Ser: HSCH₂CH(NH₂)CO—NH—CH(CH₂OH)COOH
Question 5
a) The likely mechanism is free radical substitution (free radical bromination), proceeding via initiation (homolytic fission of Br₂ by UV light/heat), propagation (chain reactions involving hexane and Br· radicals), and termination steps.
b) Homolytic (homolysis) bond fission is involved — each atom of the breaking Br–Br bond retains one electron, generating two reactive bromine free radicals (Br·).
c) Hexane (CH₃CH₂CH₂CH₂CH₂CH₃) has 3 chemically distinct types of carbon-hydrogen environments by symmetry (C1≡C6, C2≡C5, C3≡C4), so substitution of one H by Br can occur at 3 distinct positions.
Number of structural isomers of C₆H₁₃Br from monobromination of hexane: 3 (1-bromohexane, 2-bromohexane, and 3-bromohexane)
d) One isomer — 2-bromohexane:
CH₃—CHBr—CH₂—CH₂—CH₂—CH₃
Question 6
(a) But-1-yne (CH₃CH₂C≡CH): triple bond = 1σ+2π → 2 π electrons
(b) Penta-1,3-diene (CH₂=CH—CH=CH—CH₃): 2 double bonds → 4 π electrons
(c) Cyclopentene (one C=C in the ring): 2 π electrons
(d) Octane (fully saturated, no multiple bonds): 0 π electrons
(e) Butanone (CH₃COCH₂CH₃, one C=O): 2 π electrons
Question 7
Five everyday uses of petrochemical products:
- Plastics (packaging, containers, household items) — from polymerised alkenes.
- Synthetic fibres (nylon, polyester) for clothing and textiles.
- Synthetic rubber for tyres and industrial products.
- Detergents and soaps manufactured from petrochemical feedstocks.
- Fuels (petrol, diesel, kerosene) for transportation and heating.
Question 8
(a) −CH₃ : Activating (weakly activating, ortho/para-directing)
(b) −Cl : Deactivating (weakly deactivating but ortho/para-directing — an exception to the usual pattern)
(c) −CHO : Deactivating (deactivating, meta-directing)
(d) −OCH₃ : Activating (strongly activating, ortho/para-directing)
(e) −CN : Deactivating (deactivating, meta-directing)
Question 9
| Member | Formula | Name |
|---|---|---|
| 2nd | C₂H₅OH | Ethanol |
| 3rd | C₃H₇OH | Propan-1-ol |
| 4th | C₄H₉OH | Butan-1-ol |
| 5th | C₅H₁₁OH | Pentan-1-ol |
| 6th | C₆H₁₃OH | Hexan-1-ol |
Question 10
(a) Alkene (propene, 2nd member): CH₃—CH=CH₂
(b) Alkanal (ethanal, 2nd member): CH₃—CHO
(c) Alkanoate ester (methyl methanoate, simplest as 2nd conceptually — using methyl ethanoate as 2nd member of the series by carbon count): CH₃—COO—CH₃
(d) Alkanenitrile (ethanenitrile, 2nd member): CH₃—C≡N
(e) Alkylbenzene (ethylbenzene, 2nd member): C₆H₅—CH₂CH₃
Question 11
a) i) Hydrolysis: The breakdown of a compound by reaction with water (often acid or base catalysed), splitting a larger molecule into smaller ones.
Example: CH₃COOC₂H₅ + H₂O ⇌(H⁺) CH₃COOH + C₂H₅OH
ii) Hydrogenation: The addition of hydrogen gas to a compound, typically across a multiple bond, usually in the presence of a metal catalyst (Ni, Pt, or Pd).
Example: CH₂=CH₂ + H₂ →(Ni) CH₃CH₃
iii) Sulphonation: The introduction of a sulphonic acid group (−SO₃H) into an organic molecule, typically an aromatic ring, using fuming/concentrated sulphuric acid.
Example: C₆H₆ + H₂SO₄ →(heat) C₆H₅SO₃H + H₂O
iv) Polymerisation: The process by which many small monomer molecules combine to form a large macromolecule (polymer), either by addition (no by-product) or condensation (with loss of a small molecule, e.g., water).
Example: nCH₂=CH₂ → (CH₂—CH₂)ₙ (polyethene, addition polymerisation)
b) Five methods of preparing alkenes:
-
Dehydration of alcohols (using conc. H₂SO₄ or Al₂O₃ catalyst, heated):
CH₃CH₂OH →(conc.H₂SO₄, 170°C) CH₂=CH₂ + H₂O -
Dehydrohalogenation of haloalkanes (using alcoholic KOH, heated):
CH₃CH₂Br + KOH(alc) →(heat) CH₂=CH₂ + KBr + H₂O -
Cracking of larger alkanes (thermal or catalytic cracking):
C₈H₁₈ →(heat/catalyst) C₄H₈ + C₄H₁₀ -
Partial hydrogenation of alkynes (using Lindlar's catalyst):
CH≡CH + H₂ →(Lindlar's Pd) CH₂=CH₂ -
Dehalogenation of vicinal dihaloalkanes (using zinc dust):
CH₂BrCH₂Br + Zn →(heat) CH₂=CH₂ + ZnBr₂
Question 12
a) Mass of O = 0.45 − 0.180 − 0.030 = 0.240g
Moles: C = 0.180/12 = 0.015; H = 0.030/1 = 0.030; O = 0.240/16 = 0.015
Ratio C:H:O = 0.015 : 0.030 : 0.015 = 1 : 2 : 1
Empirical formula: CH₂O (empirical mass = 12+2+16 = 30)
Molecular mass/empirical mass = 90/30 = 3
Molecular formula: C₃H₆O₃
Molecular structure: This corresponds to lactic acid (2-hydroxypropanoic acid), with structure:
CH₃—CH(OH)—COOH
b) Reaction of propanoic acid (CH₃CH₂COOH) with:
i) Phosphorus(V) chloride (PCl₅): Converts the carboxylic acid to an acyl chloride:
CH₃CH₂COOH + PCl₅ → CH₃CH₂COCl + POCl₃ + HCl (product: propanoyl chloride, CH₃CH₂COCl)
ii) Ethanol/concentrated H₂SO₄: Esterification (acid catalysed), producing an ester:
CH₃CH₂COOH + C₂H₅OH ⇌(conc.H₂SO₄) CH₃CH₂COOC₂H₅ + H₂O (product: ethyl propanoate, CH₃CH₂COOC₂H₅)
iii) Sodium metal: Reacts with the acidic O−H, liberating hydrogen gas and forming a salt:
2CH₃CH₂COOH + 2Na → 2CH₃CH₂COONa + H₂↑ (product: sodium propanoate, CH₃CH₂COONa)
iv) Lithium tetrahydridoaluminate (LiAlH₄): Reduces the carboxylic acid to a primary alcohol:
CH₃CH₂COOH + 4[H] →(LiAlH₄) CH₃CH₂CH₂OH + H₂O (product: propan-1-ol, CH₃CH₂CH₂OH)
v) Ammonia (heated): Forms the ammonium salt first, which on heating dehydrates to form the amide:
CH₃CH₂COOH + NH₃ → CH₃CH₂COONH₄ →(heat) CH₃CH₂CONH₂ + H₂O (product: propanamide, CH₃CH₂CONH₂)