Carbon Bonding Fundamentals

Carbon is fundamental to organic chemistry due to its exceptional capacity to form stable covalent bonds with a wide range of elements, including itself. This ability leads to the creation of complex and versatile compounds essential for both life and industrial applications.

Tetravalence of Carbon

• Tetravalence: Refers to carbon's potential to form four covalent bonds, allowing the construction of a diverse range of structures.
• Enables the development of various compounds and materials, e.g., elongated chain plastics and aromatic rings like benzene.

Hybridisation

Definition

• Hybridisation: The merging of orbitals results in unique molecular geometries.
  • Example: Orbital mixing allows carbon to assume various shapes, similar to arranging LEGO pieces into different configurations.

Types of Hybridisation

sp³ Hybridisation

• Tetrahedral Shape: Consists of four equivalent sp³ hybrid orbitals with bond angles of $109.5^{\circ}$.
• Example: Methane ($\text{CH}_4$), which features a three-dimensional, tetrahedral configuration.

sp² Hybridisation

• Trigonal Planar Shape: Involves three equivalent sp² orbitals and one unhybridised p orbital, resulting in bond angles around $120^{\circ}$.
• Example: Ethene ($\text{C}_2\text{H}_4$).

sp Hybridisation

• Linear Shape: Consists of two sp hybrid orbitals created by combining one s orbital with one p orbital, resulting in bond angles of $180^{\circ}$.
• Example: Ethyne ($\text{C}_2\text{H}_2$).

Importance of Bond Angles

• The molecular shape significantly affects chemical reactivity and interactions.
• Bond angles, dictated by hybridisation, influence molecular orientation and reactivity.
• Example: In pharmaceuticals, the specific shape and angle of molecular bonds are crucial for drug efficacy.

Covalent Bonding and Intermolecular Forces

Covalent Bonds

• Covalent Bonds: Involve the sharing of electron pairs between atoms, forming stable molecular structures akin to alkanes.

Van der Waals Forces

• Alkanes exhibit weak dispersion forces, essential for determining physical properties like boiling points.

• These forces increase with chain length, affecting boiling and melting points.
• Molecular branching modifies boiling points by changing surface area contact.

Alkanes: A Homologous Series

• Homologous Series: A group of compounds with a similar general formula, differing by CH$_2$ units.
• Examples: Methane (CH$_4$), Ethane (C$_2$H$_6$), Propane (C$_3$H$_8$).

Functional Groups in Alkenes and Alkynes

Effect on Reactivity

• Functional groups can modify chemical reactivity and physical properties.
• Example: Hydroxyl Group (-OH) can engage in hydrogen bonds, enhancing reactivity.

Geometric Isomerism

• Restricted rotation around double bonds leads to cis-trans isomers, which are significant for both biological and material properties.

Worked Examples

Example 1: Determining Bond Angles in Propane Propane has a molecular formula of C₃H₈. Each carbon atom in propane exhibits sp³ hybridisation, resulting in tetrahedral geometry with bond angles of approximately 109.5°.

Example 2: Comparing Molecular Rotation In ethene (C₂H₄), the sp² hybridised carbon atoms form a rigid double bond that prevents rotation. This restriction creates distinct cis and trans isomers. In contrast, ethyne (C₂H₂) with sp hybridisation has a linear structure with no possibility for cis-trans isomerism.

Example 3: Predicting Boiling Points When comparing straight-chain and branched alkanes:

• n-butane (straight chain, C₄H₁₀): boiling point = -0.5°C
• iso-butane (branched, C₄H₁₀): boiling point = -11.7°C

The straight-chain molecule has a higher boiling point because it has greater surface area for van der Waals interactions.

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Practice Questions with Solutions

1. Question: Determine the bond angles for propane using knowledge of sp³ hybridisation.

   Solution: In propane (C₃H₈), all carbon atoms are sp³ hybridised. This creates a tetrahedral arrangement around each carbon atom with bond angles of approximately 109.5°. The C-C-C bond angle may deviate slightly from this ideal value due to steric interactions between hydrogen atoms on adjacent carbon atoms.

2. Question: Compare the potential for molecular rotation in Ethene and Ethyne, and explain how this impacts their chemical properties.

   Solution: In ethene (C₂H₄), the carbon atoms are sp² hybridised, forming a double bond that restricts rotation around the C=C axis. This restriction leads to geometric isomerism (cis-trans). In ethyne (C₂H₂), the carbon atoms are sp hybridised with a triple bond and linear geometry. The triple bond also prevents rotation, but due to its linear geometry, ethyne cannot exhibit geometric isomerism. These rotational restrictions affect reactivity patterns: ethene can undergo addition reactions at specific spatial orientations, while ethyne's linear structure allows for symmetrical addition from multiple directions.