Organic Chemistry

Example

Name: 2-methylbut-2-ene.

• Parent chain: 4 carbons (butene).
• Double bond starts at carbon 2.
• Methyl substituent at carbon 2.

Structural Isomers

Chain isomers: different arrangements of the carbon skeleton.

Position isomers: same skeleton, different position of the functional group.

Functional group isomers: same molecular formula, different functional groups.

Example

C$_4$H$_8$O has multiple isomers: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, butanal, butanone, Methyl propanoate, ethyl ethanoate, etc.

Alkanes

Structure

• General formula: C$_n$H$_{2n+2}$
• Single covalent bonds only (C—C and C—H).
• sp$^3$ hybridisation, tetrahedral geometry (bond angles $\approx 109.5\degree$).
• Saturated hydrocarbons (maximum number of hydrogens).

Physical Properties

Reactions

Combustion

Complete combustion (excess oxygen):

Incomplete combustion (limited oxygen):

Or: produces carbon (soot) and water.

Halogenation (Free Radical Substitution)

Mechanism (free radical substitution):

1. Initiation: Cl$_2 \xrightarrow{\mathrm{UV}} 2\mathrm{Cl}^\bullet$ (homolytic fission)

2. Propagation:

• $\mathrm{Cl}^\bullet + \mathrm{CH}_4 \to \mathrm{HCl} + \mathrm{CH}_3^\bullet$
• $\mathrm{CH}_3^\bullet + \mathrm{Cl}_2 \to \mathrm{CH}_3\mathrm{Cl} + \mathrm{Cl}^\bullet$

3. Termination: radicals combine in various ways:

• $\mathrm{Cl}^\bullet + \mathrm{Cl}^\bullet \to \mathrm{Cl}_2$
• $\mathrm{CH}_3^\bullet + \mathrm{CH}_3^\bullet \to \mathrm{C}_2\mathrm{H}_6$
• $\mathrm{CH}_3^\bullet + \mathrm{Cl}^\bullet \to \mathrm{CH}_3\mathrm{Cl}$

Cracking

Breaking large hydrocarbons into smaller, more useful molecules.

Thermal cracking: high temperature, produces alkenes.

Catalytic cracking: uses a zeolite catalyst, lower temperature, produces branched alkanes and Cycloalkanes.

Alkenes

Structure

• General formula: C$_n$H$_{2n}$
• Contain at least one C=C double bond.
• sp$^2$ hybridisation, trigonal planar geometry around the double bond (bond angle $\approx 120\degree$).
• Unsaturated hydrocarbons.
• The double bond consists of one $\sigma$ bond and one $\pi$ bond.
• Restricted rotation about the C=C bond leads to cis-trans (E/Z) isomerism.

Physical Properties

Similar to alkanes of comparable molecular mass but with slightly higher boiling points due to the $\pi$ electron cloud.

Reactions

Addition Reactions

Hydrogenation:

Halogenation:

The bromine water is decolourised — this is the test for unsaturation.

Hydration (with acid catalyst):

Hydrohalogenation:

Markovnikov”s Rule

When HX adds to an unsymmetrical alkene, the hydrogen adds to the carbon with the greater number of Hydrogen atoms (the more substituted carbon gets the halogen).

Polymerisation

Alkenes undergo addition polymerisation:

Poly(chloroethene) — PVC.

Alkynes

Structure

• General formula: C$_n$H$_{2n-2}$
• Contain a C$\equiv$C triple bond.
• sp hybridisation, linear geometry around the triple bond.
• Terminal alkynes have an acidic hydrogen.

Reactions

Similar to alkenes but can undergo two successive addition reactions due to two $\pi$ bonds.

Benzene and Aromatic Chemistry

Structure of Benzene

• Formula: C$_6$H$_6$.
• Delocalised $\pi$ electron system (6 $\pi$ electrons above and below the ring).
• Planar hexagonal structure.
• All C—C bonds are equal length (intermediate between single and double).
• Represented by a hexagon with a circle inside (or alternating double bonds).

Stability

Benzene is more stable than predicted by Kekule’s structure because of delocalisation energy (resonance energy).

Electrophilic Substitution

Benzene undergoes electrophilic substitution (not addition) to preserve the aromatic system.

Nitration

Electrophile: $\mathrm{NO}_2^+$ (nitronium ion).

Halogenation

Electrophile: $\mathrm{Br}^+$ (generated by FeBr$_3$).

Friedel-Crafts Alkylation

Electrophile: $\mathrm{R}^+$ (carbocation).

Functional Groups

Alcohols (R—OH)

Classification:

• Primary ($1\degree$): the —OH carbon is attached to one other carbon.
• Secondary ($2\degree$): the —OH carbon is attached to two other carbons.
• Tertiary ($3\degree$): the —OH carbon is attached to three other carbons.

Reactions:

Oxidation of Alcohols:

Primary alcohol $\to$ Aldehyde $\to$ Carboxylic acid

Secondary alcohol $\to$ Ketone (stops here)

Tertiary alcohol $\to$ Not oxidised

Aldehydes and Ketones (C=O)

Aldehydes (terminal C=O): suffix -al. Can be oxidised to carboxylic acids (reducing agents).

Ketones (internal C=O): suffix -one. Cannot be further oxidised.

Tests:

• Tollens’ reagent: aldehydes give a silver mirror; ketones do not.
• Fehling’s solution: aldehydes give a brick-red precipitate; ketones do not.
• 2,4-DNPH: both aldehydes and ketones give an orange precipitate (test for carbonyl group).

Carboxylic Acids (R—COOH)

Properties:

• Weak acids (partially dissociate in water).
• Hydrogen bonding gives relatively high boiling points.
• Form dimers in non-polar solvents.

Reactions:

Esters (R—COO—R’)

Formed by condensation (esterification) of a carboxylic acid and an alcohol:

Catalysed by concentrated H$_2$SO$_4$.

Uses: flavourings, fragrances, plasticisers, solvents.

Amines (R—NH$_2$)

• Weak bases (the lone pair on nitrogen accepts a proton).
• Form salts with acids.
• Primary amines can be formed by nucleophilic substitution of halogenoalkanes with ammonia.

Reaction Mechanisms

Nucleophilic Substitution ($S_N1$ and $S_N2$)

$S_N2$ (Bimolecular)

• One-step mechanism.
• Concerted: bond breaking and forming happen simultaneously.
• Inversion of configuration (Walden inversion).
• Rate depends on both substrate and nucleophile concentration.

• Favoured by: primary substrates, strong nucleophiles, polar aprotic solvents.
• Sterically hindered substrates react slowly.

Mechanism:

1. Nucleophile attacks from the back of the C—X bond.
2. Transition state with partial bonds.
3. X$^-$ leaves.
4. Product has inverted configuration.

$S_N1$ (Unimolecular)

• Two-step mechanism.
• Rate depends only on substrate concentration.

• Step 1 (slow, rate-determining): R—X $\to$ R$^+$ + X$^-$ (carbocation formation).
• Step 2 (fast): R$^+$ + Nu$^-$ $\to$ R—Nu.
• Racemisation occurs (equal mixture of inverted and retained configuration).
• Favoured by: tertiary substrates, weak nucleophiles, polar protic solvents.

Elimination ($E1$ and $E2$)

Elimination reactions remove HX to form an alkene.

$E2$ (Bimolecular)

• Concerted one-step mechanism.
• Strong base removes a proton while X$^-$ leaves.
• Follows Zaitsev’s rule: the more substituted alkene is the major product.

$E1$ (Unimolecular)

• Two-step mechanism.
• Step 1: carbocation formation (rate-determining).
• Step 2: base removes a proton.

Factors Affecting $S_N$ vs $E$

Polymer Chemistry

Addition Polymers

Formed by addition polymerisation of alkenes (monomers with C=C bonds). No by-product.

Condensation Polymers

Formed when monomers join with the elimination of a small molecule ( water).

Polyesters

Monomer: dicarboxylic acid + diol.

Example: PET (polyethylene terephthalate) — used in fibres and bottles.

Polyamides (Nylons)

Monomer: dicarboxylic acid + diamine.

Proteins

Proteins are natural polyamides formed from amino acid monomers via peptide bonds.

Biodegradability

Identifying Monomers

To find the monomer from an addition polymer, break single C—C bonds alternately to recover the C=C Double bond.

For condensation polymers, identify the repeating unit and add back the eliminated molecule (H$_2$O).

IB Exam-Style Questions

Question 1 (Paper 1 style)

Name the following compound: CH$_3$CH(Cl)CH(CH$_3$)CH$_2$CH$_3$.

1. Longest chain: 5 carbons (pentane).
2. Number from the end nearest the substituent with the lowest number: Cl at C2, CH$_3$ at C3.
3. Name: 2-chloro-3-methylpentane.

Question 2 (Paper 2 style)

Compare the mechanisms of $S_N1$ and $S_N2$ reactions.

$S_N2$: One-step bimolecular mechanism. The nucleophile attacks the carbon bearing the leaving group From the opposite side, leading to inversion of configuration. The rate depends on both [substrate] And [nucleophile]. Favoured for primary substrates.

$S_N1$: Two-step unimolecular mechanism. The leaving group departs first to form a carbocation Intermediate, which is then attacked by the nucleophile. This leads to racemisation. The rate Depends only on [substrate]. Favoured for tertiary substrates.

Question 3 (Paper 2 style)

Ethanol can be oxidised to ethanal and then to ethanoic acid.

(a) Describe the conditions for each oxidation.

Ethanol $\to$ Ethanal: Use PCC (pyridinium chlorochromate) in CH$_2$Cl$_2$ at room temperature (mild Oxidation).

Ethanol $\to$ Ethanoic acid: Use acidified K$_2$Cr$_2$O$_7$ (potassium dichromate) under reflux (strong oxidation).

(b) How would you distinguish between ethanol, ethanal, and ethanoic acid?

• Tollens’ reagent: silver mirror with ethanal, no reaction with ethanol or ethanoic acid.
• Acidified K$_2$Cr$_2$O$_7$: orange to green with ethanol and ethanal, no change with ethanoic acid.
• NaHCO$_3$: bubbles of CO$_2$ with ethanoic acid, no reaction with ethanol or ethanal.

Question 4 (Paper 1 style)

Which compound is the major product when 2-methylpropene reacts with HBr?

By Markovnikov’s rule, H adds to the less substituted carbon (C1) and Br adds to the more Substituted carbon (C2):

Product: 2-bromo-2-methylpropane.

Question 5 (Paper 2 style)

Describe the electrophilic substitution mechanism for the nitration of benzene.

Generation of electrophile:

Mechanism:

1. The electron-rich benzene ring attacks the nitronium ion ($\mathrm{NO}_2^+$), forming a delocalised carbocation intermediate.

2. The intermediate loses a proton to HSO$_4^-$Regenerating the aromatic system and forming nitrobenzene.

Summary