Level 2 — RecallGeneral Organic Chemistry (GOC)

General Organic Chemistry (GOC)

⏱ 30 minutes30 marksprintable — key stays hidden on paper

Difficulty Level: 2 (Recall — definitions, standard textbook problems, short derivations) Time Limit: 30 minutes Total Marks: 30

Instructions: Answer all questions. Draw structures where required. Use $...$ notation for any formulae.

Q1. Define the terms catenation and homologous series. Give the general molecular formula of the homologous series of alkanes and alkenes. (3 marks)

Q2. State the hybridisation of each carbon and the C–C–C bond angle in the following molecules: (3 marks) (a) $CH_4$ (b) $CH_2=CH_2$ (ethene) (c) $CH\equiv CH$ (ethyne)

Q3. Write the IUPAC names of the following compounds: (4 marks) (a) $CH_3-CH_2-CH(CH_3)-CH_2-OH$ (b) $CH_3-CH_2-CO-CH_3$ (c) $CH_2=CH-CH_2-CH_3$ (d) $CH_3-CH_2-COOH$

Q4. Identify the type of structural isomerism shown in each pair: (3 marks) (a) $CH_3CH_2CH_2CH_3$ and $(CH_3)_3CH$ (b) $CH_3-O-CH_3$ and $CH_3CH_2OH$ (c) 1-propanol and 2-propanol

Q5. Arrange the following carbocations in increasing order of stability and justify in one line: (3 marks) $CH_3^+,\quad CH_3CH_2^+,\quad (CH_3)_2CH^+,\quad (CH_3)_3C^+$

Q6. Define an electrophile and a nucleophile. Classify each of the following as electrophile (E) or nucleophile (Nu): $H_2O$ , $NO_2^+$ , $CN^-$ , $BF_3$ . (4 marks)

Q7. Explain the +I (inductive) and +M (mesomeric) effects with one example each. (3 marks)

Q8. (a) Distinguish between homolysis and heterolysis of a covalent bond, naming the intermediate formed in each. (2 marks) (b) Using curved-arrow notation, show the homolytic cleavage of $Cl-Cl$ . (1 mark)

Q9. State whether each molecule is chiral or achiral. For chiral ones mark the chiral centre with an asterisk. (2 marks) (a) $CH_3-CHBr-COOH$ (2-bromopropanoic acid) (b) $CH_3-CHCl-CH_3$ (2-chloropropane)

Q10. Define specific rotation and write its expression. What is a racemic mixture and what is its observed optical rotation? (2 marks)

End of Paper

Answer keyMark scheme & solutions

Q1. (3 marks)

Catenation: the self-linking ability of carbon atoms to form long chains, branched chains and rings through strong C–C covalent bonds. (1)
Homologous series: a series of compounds with the same general formula and same functional group, where successive members differ by a $-CH_2-$ unit and show a gradual gradation in physical properties. (1)
Alkanes: $C_nH_{2n+2}$ ; Alkenes: $C_nH_{2n}$ . (1)

Q2. (3 marks, 1 each)

(a) $CH_4$ : $sp^3$ , bond angle $109.5^\circ$ .
(b) Ethene: $sp^2$ , bond angle $120^\circ$ .
(c) Ethyne: $sp$ , bond angle $180^\circ$ .

Q3. (4 marks, 1 each)

(a) 2-methylbutan-1-ol (longest chain 4 C, OH at C1, methyl at C2).
(b) butan-2-one (4-C ketone, C=O at C2).
(c) but-1-ene (4-C chain, double bond at C1).
(d) propanoic acid (3-C carboxylic acid).

Q4. (3 marks, 1 each)

(a) Chain isomerism (same formula $C_4H_{10}$ , different carbon skeleton).
(b) Functional isomerism (ether vs alcohol, both $C_2H_6O$ ).
(c) Position isomerism (–OH at different positions on same chain).

Q5. (3 marks) Order of increasing stability: (2 for correct order) $CH_3^+ < CH_3CH_2^+ < (CH_3)_2CH^+ < (CH_3)_3C^+$ Justification: stability increases from methyl → 1° → 2° → 3° due to increasing +I effect and hyperconjugation of alkyl groups stabilising the positive charge. (1)

Q6. (4 marks)

Electrophile: electron-deficient species (positive/neutral) that accepts an electron pair. (1)
Nucleophile: electron-rich species (negative/neutral with lone pair) that donates an electron pair. (1)
Classification (2 marks, ½ each): $H_2O$ → Nu; $NO_2^+$ → E; $CN^-$ → Nu; $BF_3$ → E.

Q7. (3 marks)

+I effect: electron-releasing inductive effect through σ-bonds by groups like alkyl ( $-CH_3$ ); e.g. in propane the methyl pushes electron density. (1.5)
+M effect: electron-donating resonance effect by groups with lone pairs like $-OH, -NH_2, -\ddot{O}CH_3$ ; e.g. in aniline the $-NH_2$ donates its lone pair into the ring. (1.5)

Q8. (3 marks)

(a) Homolysis: bond breaks symmetrically, each atom keeps one electron → free radicals. Heterolysis: bond breaks unsymmetrically, one atom takes both electrons → ions (carbocation + carbanion / cation + anion). (2)
(b) $Cl\!:\!Cl \xrightarrow{h\nu} 2\,Cl^\bullet$ shown with two single-barbed (fishhook) arrows each moving one electron to a chlorine. (1)

Q9. (2 marks, 1 each)

(a) Chiral — C2 bears four different groups ( $H, Br, CH_3, COOH$ ): $CH_3-C^*HBr-COOH$ .
(b) Achiral — C2 has two identical $CH_3$ groups, so no chiral centre.

Q10. (2 marks)

Specific rotation $[\alpha] = \dfrac{\alpha}{l \times c}$ , where $\alpha$ = observed rotation, $l$ = path length (dm), $c$ = concentration (g/mL). (1)
Racemic mixture: an equimolar (1:1) mixture of two enantiomers; net observed optical rotation = 0 (optically inactive due to external compensation). (1)

[
  {"claim":"Alkane general formula gives C4H10 for n=4 (chain isomerism pair)","code":"n=4; H=2*n+2; result=(H==10)"},
  {"claim":"Alkene general formula gives C4H8 for n=4 (but-1-ene)","code":"n=4; H=2*n; result=(H==8)"},
  {"claim":"Ethene and ethyne bond angle sum equals 300 degrees","code":"result=(120+180==300)"},
  {"claim":"Racemic mixture net rotation is zero for equal +a and -a","code":"a=Symbol('a'); result=(simplify(a + (-a))==0)"}
]