Chapter 12: Introduction to Organic Chemistry – Hydrocarbons (Alkenes, Alkynes, Aromatic Compounds)

Structure and Physical Properties of Alkenes

Structural Features and Nomenclature of Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Their structure and properties are influenced by the presence of this double bond.

• Alkene Structure: Alkenes have the general formula CnH2n. The double bond is planar, and each carbon in the double bond is sp2 hybridized.

• Nomenclature: The name of an alkene is based on the longest chain containing the double bond. The suffix -ene is used (e.g., ethene, propene).

• Numbering: Number the chain so the double bond gets the lowest possible number.

• Geometry: The double bond restricts rotation, resulting in a planar geometry around the double bond.

Example: 2-butene: CH3-CH=CH-CH3

Addition Reactions of Alkenes

Alkenes undergo addition reactions, where reagents add across the double bond.

• Hydrogenation: Addition of H2 to form alkanes.

• Halogenation: Addition of Cl2 or Br2 to form dihalides.

• Hydrohalogenation: Addition of HX (HCl, HBr) to form alkyl halides.

• Hydration: Addition of H2O (with acid catalyst) to form alcohols.

Markovnikov’s Rule: In addition of HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens, and the halide attaches to the carbon with fewer hydrogens.

$\text{CH}_3\text{CH}=\text{CH}_2 + \text{HBr} \rightarrow \text{CH}_3\text{CHBrCH}_3$

Predicting Products: Apply Markovnikov’s rule to determine the major product.

Isomers of Alkenes

Structural and Geometric Isomers

Alkenes can exhibit both structural and geometric (cis/trans) isomerism.

• Structural Isomers: Same molecular formula, different connectivity.

• Geometric Isomers: Due to restricted rotation, alkenes can have cis (same side) and trans (opposite side) isomers.

• Naming: Use prefixes cis- and trans- to distinguish isomers.

• Counting Carbons: Start numbering from the end closest to the double bond.

Example: cis-2-butene vs. trans-2-butene

Structure and Physical Properties of Alkynes

Structural Features and Nomenclature of Alkynes

Alkynes are hydrocarbons with at least one carbon-carbon triple bond.

• Alkyne Structure: General formula CnH2n-2. The triple bond is linear, and each carbon is sp hybridized.

• Nomenclature: The suffix -yne is used (e.g., ethyne, propyne).

• Geometry: The triple bond creates a linear geometry (180° bond angle).

Example: Ethyne (acetylene): HC≡CH

Reactions Involving Alkenes and Alkynes

Addition Reactions and Product Prediction

Both alkenes and alkynes undergo addition reactions, where reagents add across the multiple bonds.

• Alkenes: Addition of H2, HX, X2, H2O.

• Alkynes: Addition reactions can occur twice, converting alkynes to alkenes and then to alkanes.

• Product Prediction: Use Markovnikov’s rule for unsymmetrical reagents.

$\text{RC} \equiv \text{CH} + \text{H}_2 \rightarrow \text{RC}=\text{CH}_2 \rightarrow \text{RCH}_2\text{CH}_3$

Example: Addition of Br2 to ethyne yields dibromoethene, then tetrabromoethane.

Structure and Properties of Aromatic Compounds

Structural Features and Nomenclature of Aromatic Molecules

Aromatic compounds contain a benzene ring, characterized by resonance and stability.

• Benzene Structure: Six-membered ring with alternating double bonds, but actual structure is a resonance hybrid.

• Resonance: Electrons are delocalized, providing extra stability.

• Nomenclature: Common derivatives include toluene (methylbenzene), phenol (hydroxybenzene), and aniline (aminobenzene).

• Naming: Use "benzene" as the parent name; substituents are named with their position (ortho-, meta-, para-).

Example: para-dichlorobenzene (1,4-dichlorobenzene)

Resonance and Stability of Aromatic Compounds

Benzene’s resonance leads to unique stability, making it less reactive in addition reactions and more likely to undergo substitution.

• Resonance Structures: Benzene can be represented by two equivalent structures, but the actual molecule is a hybrid.

• Stability: Aromatic compounds are more stable than expected due to resonance energy.

$\text{Benzene:} \quad \text{C}_6\text{H}_6$

Substitution Reactions of Aromatic Compounds

Aromatic compounds typically undergo substitution reactions, where a hydrogen is replaced by another group.

• Electrophilic Aromatic Substitution: Common reactions include nitration, halogenation, and sulfonation.

• Catalysts: Often require catalysts such as FeCl3 or AlCl3.

• Product Prediction: The position of substitution depends on existing substituents (activating or deactivating groups).

Example: Nitration of benzene with HNO3 and H2SO4 yields nitrobenzene.

Common Benzene Derivatives Table

Compound	Structure	Common Name
Methylbenzene	C6H5CH3	Toluene
Hydroxybenzene	C6H5OH	Phenol
Aminobenzene	C6H5NH2	Aniline
1,2-dimethylbenzene	C6H4(CH3)2	o-xylene
1,3-dimethylbenzene	C6H4(CH3)2	m-xylene
1,4-dimethylbenzene	C6H4(CH3)2	p-xylene

Additional info: Table entries inferred for completeness.