Combustion of Organic Compounds
Complete vs Incomplete Combustion
Complete Combustion
Excess \(\text{O}_2 \rightarrow \mathbf{\text{CO}_2 + \text{H}_2\text{O}}\)
\(\text{C}_8\text{H}_{18} + 12.5\text{O}_2 \rightarrow 8\text{CO}_2 + 9\text{H}_2\text{O}\)
Incomplete Combustion
Limited \(\text{O}_2 \rightarrow \mathbf{\text{CO or C (soot)} + \text{H}_2\text{O}}\)
\(\text{C}_4\text{H}_{10} + 4.5\text{O}_2 \rightarrow 4\text{CO} + 5\text{H}_2\text{O}\)
Addition Reactions of Alkenes
The C=C double bond is a region of high electron density (π electrons) that is susceptible to attack by electrophiles.
| Reaction | Reagent | Conditions | Product |
|---|---|---|---|
| Hydrogenation | H₂ | Ni catalyst, 150°C | Alkane |
| Halogenation | Br₂ (or Cl₂) | Room temperature | Dihalogenoalkane |
| Hydration | H₂O (steam) | H₃PO₄ catalyst, 300°C | Alcohol |
| Hydrohalogenation | HBr (or HCl) | Room temperature | Halogenoalkane |
Bromine Water Test for Alkenes
Bromine water test for unsaturation (\(\text{C}=\text{C}\)):
- Alkenes: Rapidly decolourise orange/brown bromine water to colourless (electrophilic addition: \(\text{CH}_2=\text{CH}_2 + \text{Br}_2 \rightarrow \text{CH}_2\text{BrCH}_2\text{Br}\)).
- Alkanes: No reaction in the dark (remains orange/brown; reacts only under UV light via radical substitution).
Markovnikov's Rule & Carbocation Stability
When an asymmetric hydrogen halide (\(\text{HX}\)) adds to an asymmetric alkene, the hydrogen atom attaches to the double-bond carbon with the greater number of hydrogen atoms ("the rich get richer").
Mechanism basis: Proceeds via the more stable carbocation intermediate (\(3^\circ > 2^\circ > 1^\circ\)) due to the electron-donating inductive effect (\(+I\)) of surrounding alkyl groups.
Two-Step Electrophilic Addition Mechanism
- Step 1 (Slow): The high electron density of the \(\text{C}=\text{C}\;\pi\) bond induces a dipole in \(\text{Br}_2\) (\(\text{Br}^{\delta+} - \text{Br}^{\delta-}\)). The \(\pi\) electrons attack \(\text{Br}^{\delta+}\), causing heterolytic fission of the \(\text{Br}-\text{Br}\) bond → forms a carbocation intermediate and \(\text{Br}^-\).
- Step 2 (Fast): The lone pair on \(\text{Br}^-\) attacks the positively charged carbon → forms 1,2-dibromoethane.
Substitution Reactions
Substitution Reactions of Alkanes
In substitution reactions, an atom or functional group in a molecule is replaced by a different atom or group:
\[\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl} + \text{HCl}\]Three-Stage Free-Radical Substitution
- Initiation (UV light): Homolytic fission: \(\text{Cl}_2 \xrightarrow{h\nu} 2\text{Cl}^\bullet\)
- Propagation (chain reaction):
• \(\text{Cl}^\bullet + \text{CH}_4 \rightarrow {}^\bullet\text{CH}_3 + \text{HCl}\)
• \({}^\bullet\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet\) - Termination (removes radicals):
• \(\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2\)
• \({}^\bullet\text{CH}_3 + \text{Cl}^\bullet \rightarrow \text{CH}_3\text{Cl}\)
• \({}^\bullet\text{CH}_3 + {}^\bullet\text{CH}_3 \rightarrow \text{C}_2\text{H}_6\)
Produces a mixture of products (poly-substitution possible: CH₂Cl₂, CHCl₃, CCl₄)
SN1 vs SN2 Nucleophilic Substitution
| Feature | SN2 Mechanism | SN1 Mechanism |
|---|---|---|
| Substrate Preference | Primary (1°) halogenoalkanes | Tertiary (3°) halogenoalkanes |
| Number of Steps | 1 step (concerted via transition state) | 2 steps (via carbocation intermediate) |
| Rate Law | \(\text{Rate} = k[\text{RX}][\text{Nu}^-]\) | \(\text{Rate} = k[\text{RX}]\) |
| Stereochemistry | Walden Inversion (backside attack) | Racemic Mixture (planar \(\text{C}^+\)) |
Secondary halogenoalkanes can undergo both SN1 and SN2.
Oxidation of Alcohols
Oxidation Pathways of 1°, 2°, and 3° Alcohols
Reagent: Acidified potassium dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}^+\)) → Colour change: Orange (\(\text{Cr}_2\text{O}_7^{2-}\)) → Green (\(\text{Cr}^{3+}\)).
- Primary (1°) Alcohol: \(\text{R-CH}_2\text{OH} \xrightarrow{\text{distil}} \text{Aldehyde (R-CHO)} \xrightarrow{\text{reflux}} \text{Carboxylic Acid (R-COOH)}\)
- Secondary (2°) Alcohol: \(\text{R-CH(OH)-R'} \xrightarrow{\text{reflux}} \text{Ketone (R-CO-R')}\)
- Tertiary (3°) Alcohol: Resistant to oxidation (no \(\alpha-\text{H}\) on carbon → solution remains orange).
Condensation & Esterification
Esterification and Ester Hydrolysis
Esterification (Condensation): \(\text{Carboxylic acid} + \text{Alcohol} \xrightleftharpoons{\text{conc. }\text{H}_2\text{SO}_4\text{, heat}} \text{Ester} + \text{H}_2\text{O}\)
- E.g. \(\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}\) (ethyl ethanoate).
- Acid hydrolysis: Ester + \(\text{H}_2\text{O} \xrightleftharpoons{\text{H}^+} \text{Acid} + \text{Alcohol}\).
- Alkaline saponification: Ester + \(\text{NaOH} \rightarrow \text{Carboxylate salt} + \text{Alcohol}\).
Elimination Reactions of Halogenoalkanes
Halogenoalkanes undergo elimination of \(\text{HX}\) to form alkenes when heated with a strong base in ethanol (e.g. \(\text{hot ethanolic NaOH}\)):
\[\text{CH}_3\text{CH}_2\text{Br} + \text{OH}^-(\text{ethanol}) \xrightarrow{\Delta} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} + \text{Br}^-\]Reaction Condition Control: Aqueous \(\text{NaOH}\) → Substitution (\(\text{Alcohol}\)); Ethanolic \(\text{NaOH} + \text{Heat}\) → Elimination (\(\text{Alkene}\)).
Organic Reaction Conditions & Distinctions
- Reaction Conditions: Always specify catalyst and temperature (e.g. UV for radical substitution, \(\text{H}_3\text{PO}_4/300^\circ\text{C}\) for steam hydration, \(\text{Ni}/150^\circ\text{C}\) for hydrogenation).
- Distillation vs Reflux: To collect aldehydes, distil immediately; to obtain carboxylic acids, heat under reflux.
- Tertiary Alcohols: State they resist oxidation (not "cannot react under any conditions").
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