QUESTIONS
SECTION A
1. The structure of the main constituent in antifreeze is:
H OH H
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H — C — C — C — H
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OH H 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) concentrated HNO₃/H₂SO₄
(b) LiAlH₄
(c) acidified KMnO₄
(d) CH₃Cl/AlCl₃
(e) Na/liquid NH₃
3a) Define Isotopy.
3b) Give the formula of the three (3) known isotopes of carbon.
4. The structures of the amino acids alanine and glycine are shown below:
CH₃ H
| |
H₂N — C — COOH H₂N — C — COOH
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H H
alanine glycine
a) i) Alanine exists as a pair of stereoisomers. Explain the meaning of the term stereoisomers.
ii) State how you could distinguish between the stereoisomers.
b) Give the formulae of the peptides which are formed when alanine and glycine react together.
5. In the chlorination of decane (C₁₀H₂₄):
(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 C₁₀H₂₁Cl
(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) Propyne
(b) Buta-1,3-diene
(c) Cyclohexene
(d) Hexane
(e) Propanal
7. List five (5) primary petrochemical feed stocks in the petrochemical industry.
8. Classify the following substituent groups as "activating" or "deactivating" in aromatic substitution reactions.
(a) −NO₂
(b) −SO₃H
(c) −OH
(d) −COOH
(e) −NH₂
9. Give the molecular formula and name of the sixth, seventh, eighth, ninth and tenth member of the alkane homologous series.
10. Give the structure of the first member of the following homologous series:
(a) Alkyne
(b) Alkanone
(c) Alkanoic acid
(d) Alkanamide
(e) Alkanol
SECTION B: Attempt Any TWO (2) Questions in this Section
11a) Explain, with illustrative examples, what is meant by the terms:
(i) saponification (ii) esterification (iii) ethanoylation (iv) decarboxylation
11b) List five (5) main fractions of the fractional distillation of crude oil and their uses.
12a) An organic compound (0.30g) was analysed and found to contain 0.120g of carbon and 0.02g of hydrogen. What is the empirical formula, molecular formula and molecular structure of the compound? (given relative molar mass = 180)
12b) Give the structure of the organic product obtainable in the reaction of ethanamide with:
(i) Phosphorus(V) oxide
(ii) Dioxonitrite(III) acid
(iii) Bromine and potassium hydroxide solution
(iv) Lithium tetrahydridoaluminate
(v) Hydrochloric acid
ANSWERS
Question 1
The structure shown is butane-1,2,3,4-tetrol (erythritol-type), commonly identified as the tetraol constituent of antifreeze. Counting atoms: 4 carbons, 4 OH groups (4 O, 4 H from OH), plus H atoms on the carbons: C1 has 2H (terminal CH₂OH), C2 has 1H, C3 has 1H, C4 has 2H (terminal CH₂OH); total H = 2+1+1+2+4(OH) = 10
(a) Molecular formula: C₄H₁₀O₄
(b) Empirical formula: Ratio C:H:O = 4:10:4 = 2:5:2 → C₂H₅O₂
(c) IUPAC nomenclature: Butane-1,2,3,4-tetraol
Question 2
(a) Concentrated HNO₃/H₂SO₄: Nitration (electrophilic substitution introducing a −NO₂ group, e.g., into benzene to form nitrobenzene)
(b) LiAlH₄: Reduction (reduces carbonyl compounds — aldehydes, ketones, carboxylic acids, esters, amides, nitriles — to alcohols or amines)
(c) Acidified KMnO₄: Oxidation (oxidises primary alcohols to carboxylic acids, secondary alcohols to ketones, and alkenes to diols/cleavage products)
(d) CH₃Cl/AlCl₃: Friedel-Crafts alkylation (electrophilic substitution introducing a methyl group onto an aromatic ring)
(e) Na/liquid NH₃: Birch reduction (reduces aromatic rings to non-conjugated dienes, e.g., converts benzene to 1,4-cyclohexadiene)
Question 3
a) Isotopy: Isotopy is the phenomenon whereby atoms of the same element (having the same atomic/proton number) exist with different mass numbers, due to differing numbers of neutrons in the nucleus, while retaining identical chemical properties.
b) Three known isotopes of carbon: ¹²C, ¹³C, ¹⁴C
Question 4
a) i) Stereoisomers: Stereoisomers are compounds having the same molecular formula and the same sequence of bonded atoms (constitution), but differing in the three-dimensional spatial arrangement of their atoms/groups in space. In alanine's case, this arises because the central (alpha) carbon is bonded to four different groups (CH₃, H, NH₂, COOH), making it a chiral centre, giving rise to two non-superimposable mirror-image forms (enantiomers).
ii) The stereoisomers (enantiomers) can be distinguished by their effect on plane-polarised light using a polarimeter: one isomer rotates the plane of polarised light clockwise (dextrorotatory, D/+) while its mirror-image rotates it anticlockwise by an equal amount (levorotatory, L/−); all other physical/chemical properties are otherwise identical (except when interacting with other chiral molecules, e.g., in biological systems).
b) When alanine and glycine react together (condensation, losing H₂O to form a peptide bond), two different dipeptides can form depending on which amino/carboxyl groups combine:
Ala-Gly: CH₃CH(NH₂)CO—NH—CH₂COOH
Gly-Ala: H₂NCH₂CO—NH—CH(CH₃)COOH
(These are two distinct dipeptide products, differing in which amino acid contributes the free amino end vs the free carboxyl end.)
Question 5
a) The likely mechanism is free radical substitution (free radical chlorination), proceeding via initiation (homolytic fission of Cl₂ by UV light), propagation (chain reactions involving decane and Cl· radicals), and termination steps.
b) Homolytic (homolysis) bond fission is involved — each atom of the breaking Cl–Cl bond retains one electron, generating two reactive chlorine free radicals (Cl·).
c) Decane (C₁₀H₂₂ — note: the question states C₁₀H₂₄ but the correct saturated alkane formula for decane is C₁₀H₂₂, CₙH₂ₙ₊₂ for n=10) has 5 chemically distinct types of carbon-hydrogen environments (by symmetry: C1≡C10, C2≡C9, C3≡C8, C4≡C7, C5≡C6), so substitution of one H by Cl can occur at 5 distinct positions.
Number of structural isomers of C₁₀H₂₁Cl from monochlorination of decane: 5 (1-chlorodecane, 2-chlorodecane, 3-chlorodecane, 4-chlorodecane, and 5-chlorodecane)
d) One isomer — 2-chlorodecane:
CH₃—CHCl—CH₂—CH₂—CH₂—CH₂—CH₂—CH₂—CH₂—CH₃
Question 6
(a) Propyne (CH₃—C≡CH): triple bond = 1σ+2π → 2 π electrons
(b) Buta-1,3-diene (CH₂=CH—CH=CH₂): 2 double bonds → 4 π electrons
(c) Cyclohexene (one C=C in the ring): 2 π electrons
(d) Hexane (fully saturated, no multiple bonds): 0 π electrons
(e) Propanal (CH₃CH₂CHO, one C=O): 2 π electrons
Question 7
Five primary petrochemical feedstocks:
- Ethene (ethylene)
- Propene (propylene)
- Butadiene
- Benzene
- Methane (natural gas) (Naphtha may also be cited as a primary feedstock)
Question 8
(a) −NO₂ : Deactivating (strongly deactivating, meta-directing)
(b) −SO₃H : Deactivating (deactivating, meta-directing)
(c) −OH : Activating (strongly activating, ortho/para-directing)
(d) −COOH : Deactivating (deactivating, meta-directing)
(e) −NH₂ : Activating (strongly activating, ortho/para-directing)
Question 9
| Member | Formula | Name |
|---|---|---|
| 6th | C₆H₁₄ | Hexane |
| 7th | C₇H₁₆ | Heptane |
| 8th | C₈H₁₈ | Octane |
| 9th | C₉H₂₀ | Nonane |
| 10th | C₁₀H₂₂ | Decane |
Question 10
(a) Alkyne (ethyne): H—C≡C—H
(b) Alkanone (propanone): CH₃—CO—CH₃ (the first ketone possible, since a ketone requires at least 3 carbons)
(c) Alkanoic acid (methanoic acid): H—COOH
(d) Alkanamide (methanamide): H—CONH₂
(e) Alkanol (methanol): CH₃—OH
Question 11
a) i) Saponification: The alkaline hydrolysis of an ester (typically a fat or oil, a triglyceride) using aqueous NaOH or KOH, producing a salt of the fatty acid (soap) and glycerol.
Example: Fat/oil (glyceryl tristearate) + 3NaOH → 3C₁₇H₃₅COONa (soap) + glycerol
ii) Esterification: The reaction between a carboxylic acid and an alcohol (usually with an acid catalyst, e.g., concentrated H₂SO₄), producing an ester and water.
Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
iii) Ethanoylation (acetylation): The introduction of an ethanoyl (acetyl, CH₃CO−) group into a molecule, typically replacing an H atom on an −OH or −NH₂ group, usually using ethanoyl chloride or ethanoic anhydride.
Example: C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ + CH₃COOH
iv) Decarboxylation: The removal of a carboxyl group (−COOH) from a carboxylic acid (usually as CO₂), typically by heating the sodium salt of the acid with soda lime, producing an alkane with one fewer carbon.
Example: CH₃COONa + NaOH →(heat, soda lime) CH₄ + Na₂CO₃
b) Five main fractions of fractional distillation of crude oil and their uses:
| Fraction | Approx. Boiling Range | Use |
|---|---|---|
| Refinery gas (LPG) | Below 25°C | Domestic/industrial fuel (cooking gas) |
| Petrol (Gasoline) | 25–75°C | Fuel for cars/petrol engines |
| Naphtha | 75–150°C | Petrochemical feedstock for plastics/chemicals |
| Kerosene | 150–240°C | Fuel for jet engines/aircraft, domestic heating/lighting |
| Diesel oil (Gas oil) | 240–350°C | Fuel for diesel engines, heating |
| Fuel oil/Lubricating oil/Bitumen (residue) | Above 350°C | Ship/power station fuel, lubrication, road surfacing |
(Any five distinct fractions with correct uses.)
Question 12
a) Mass of O = 0.30 − 0.120 − 0.02 = 0.16g
Moles: C = 0.120/12 = 0.01; H = 0.02/1 = 0.02; O = 0.16/16 = 0.01
Ratio C:H:O = 0.01 : 0.02 : 0.01 = 1 : 2 : 1
Empirical formula: CH₂O (empirical mass = 12+2+16 = 30)
Molecular mass/empirical mass = 180/30 = 6
Molecular formula: C₆H₁₂O₆ (glucose)
Molecular structure: This corresponds to glucose, an aldohexose, with structure:
HOCH₂—(CHOH)₄—CHO (open-chain form), or more specifically the straight-chain structure:
CH₂OH—CHOH—CHOH—CHOH—CHOH—CHO
b) Reaction of ethanamide (CH₃CONH₂) with:
i) Phosphorus(V) oxide (P₂O₅): Acts as a dehydrating agent, removing water from the amide to form a nitrile:
CH₃CONH₂ →(P₂O₅) CH₃CN + H₂O (product: ethanenitrile/acetonitrile, CH₃C≡N)
ii) Dioxonitrite(III) acid (HNO₂): Reacts with the amide's NH₂ group, converting it to a carboxylic acid with loss of N₂ and H₂O:
CH₃CONH₂ + HNO₂ → CH₃COOH + N₂ + H₂O (product: ethanoic acid, CH₃COOH)
iii) Bromine and potassium hydroxide solution: Hofmann degradation/bromination reaction, converting the amide to a primary amine with one fewer carbon:
CH₃CONH₂ + Br₂ + 4KOH → CH₃NH₂ + 2KBr + K₂CO₃ + 2H₂O (product: methanamine, CH₃NH₂)
iv) Lithium tetrahydridoaluminate (LiAlH₄): Reduces the amide to a primary amine:
CH₃CONH₂ + 4[H] →(LiAlH₄) CH₃CH₂NH₂ + H₂O (product: ethanamine, CH₃CH₂NH₂)
v) Hydrochloric acid: Acid hydrolysis of the amide, producing a carboxylic acid and ammonium chloride:
CH₃CONH₂ + HCl + H₂O → CH₃COOH + NH₄Cl (product: ethanoic acid, CH₃COOH)