60 Organic Compounds: Alkanes and Their Stereochemistry

3-1 Functional Groups

Definition:

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. These groups are fundamental in categorizing organic compounds into families with similar properties and behaviors. Understanding functional groups is essential for predicting the chemical behavior of molecules and their reactivity in various chemical reactions.

Functional Group Behavior:

Once a functional group is established, it will exhibit consistent behavior across different molecules, enabling chemists to anticipate reactions and mechanisms. For instance, carbon-carbon double bonds (C=C) behave similarly in various contexts, regardless of the neighboring atoms, facilitating the prediction of chemical behavior in reactions such as electrophilic additions.

Examples:

• Ethylene (C₂H₄): A simple alkene containing a double bond that participates in various addition reactions, such as those with bromine, to form dibromoethane.

• Menthene: An example of a more complex alkene that also contains a C=C double bond and reacts in a manner similar to ethylene with halogens, showcasing the concept of functional groups in organic reactions.

3-2 Alkanes and Alkane Isomers

Simplest Family of Molecules:

Alkanes are known as saturated hydrocarbons, characterized by the exclusive presence of single bonds (sigma bonds) between carbon atoms. This characteristic structure makes them the simplest class of organic compounds available. They serve as fundamental building blocks in organic chemistry and are often used as a reference in various organic reactions.

General Formula:

Alkanes follow the general molecular formula of CₙH₂ₙ₊₂, where 'n' represents the number of carbon atoms in the chain. For example, if n=1, the formula becomes CH₄ (methane), which is the simplest alkane and consists of one carbon atom bonded to four hydrogen atoms.

Isomerism:

Isomerism is a phenomenon occurring when compounds share the same molecular formula but differ in their structure or spatial arrangement of atoms. Alkanes often exhibit this characteristic. For instance, the molecular formula C₄H₁₀ represents two distinct isomers:

• Butane: A straight-chain alkane with all four carbon atoms connected in a linear fashion.

• Isobutane (or methylpropane): A branched alkane where three carbon atoms form a chain with a fourth carbon attached to the second carbon in the chain.These distinctions can significantly influence their physical and chemical properties.

Types of Isomers:

1. Constitutional Isomers: These isomers differ in their connectivity. The arrangement of atoms varies among the compounds, leading to distinct properties, such as boiling points and reactivity.

2. Stereoisomers: These compounds share the same connectivity but differ in their spatial orientation. Among stereoisomers, geometric isomers are of particular importance, which occur in alkenes due to the restricted rotation around the carbon-carbon double bond.

Functional Groups with Carbon-Carbon Multiple Bonds

Alkenes:

• Reactivity: Alkenes, which contain a carbon-carbon double bond (C=C), are naturally more reactive compared to alkanes. This increased reactivity stems from the electron-rich nature of the double bond, making them susceptible to electrophilic attack.

• Name Ending: Alkenes are named with the suffix “-ene.”

• Example: Ethene (C₂H₄) is a foundational alkene and serves as a precursor in various chemical syntheses, illustrating the versatility and importance of alkenes in organic chemistry.

Alkynes:

• Reactivity: Alkynes are characterized by the presence of a carbon-carbon triple bond (C≡C), which increases their reactivity compared to both alkanes and alkenes due to the higher energy associated with the triple bond.

• Name Ending: Alkynes are identified by the suffix “-yne.”

• Example: Ethyne (C₂H₂), commonly known as acetylene, is the simplest alkyne and serves as a key compound in organic synthesis, particularly in the production of plastics and synthetic fibers.

Arenes (Aromatic Compounds):

• Structure: Arenes are characterized by the presence of alternating double bonds within a six-membered carbon ring, which imparts unique stability and reactivity due to resonance stabilization.

• Name Ending: Arenes do not have a specific name ending but are often recognized by their structural formulas or common names.

• Example: Benzene (C₆H₆) is the most well-known arene and plays a vital role in organic chemistry as a precursor for numerous industrial chemicals, including solvents and synthetic fibers.

Key Functional Groups

• Halide:

  • General Structure: C-X, where X signifies a halogen (F, Cl, Br, or I).

  • Example: Chloromethane (CH₃Cl) is an illustrative compound that incorporates a halogen into an organic framework, significantly affecting its chemical behavior and reactivity, and is used in various industrial applications, including refrigerants and solvents.

• Alcohol:

  • Functional Group: The hydroxyl group (-OH) characterizes alcohols.

  • Example: Methanol (C₃H₄O) is an alcohol that highlights the influence of hydroxyl groups on the physical properties of compounds, affecting solubility, boiling point, and chemical reactivity, and is commonly utilized as a solvent and antifreeze.

Properties of Alkanes

Chemical Inertness:

Alkanes are renowned for their chemical inertness, showcasing minimal reactivity with most reagents. This stability arises from the strong sigma bonds found in C-C and C-H connections, making alkanes generally unreactive under standard conditions, and thus suitable as fuels and feedstocks in various chemical processes.

Combustion:

In the presence of sufficient oxygen, alkanes can undergo complete combustion reactions, resulting in the production of carbon dioxide (CO₂) and water (H₂O), while releasing a substantial amount of energy, highlighting their value as energy sources.

• Example: The combustion of methane can be represented in the following reaction:

  • Equation: CH₄ + 2O₂ → CO₂ + 2H₂O + 890 kJ/molThis exothermic reaction underscores the energy content that alkanes provide, making them essential in fuel applications.

Melting and Boiling Points:

As the molecular weight of alkanes increases, both melting and boiling points tend to rise due to enhanced dispersion forces arising from increased surface area. Additionally, the degree of branching significantly impacts boiling points; branched alkanes typically exhibit lower boiling points due to reduced surface area contact among molecules, leading to weaker intermolecular interactions.

3-6 Conformations of Ethane

Conformational Isomers:

Conformational isomers are structural variations that arise due to the different spatial arrangements obtained through free rotational movement around single C-C sigma bonds. The nature of these conformations can substantially influence molecular properties, reactivity, and sterics.

Staggered vs. Eclipsed Conformations:

• Staggered Conformation: In this arrangement, atoms are positioned in such a way that overlap is minimized, yielding lower torsional strain. This conformation is energetically favorable and hence more stable for ethane.

• Eclipsed Conformation: Here, atoms align perfectly, resulting in significant torsional strain, which raises the energy of the conformation and leads to instability; this strain is quantitatively approximated at 12 kJ/mol for ethane.

Key Concept: Newman Projections

• Sawhorse Representation: This technique provides an angular view of a molecule, allowing for an effective visual representation of different conformations and their implications for molecular interactions.

• Newman Projection: By viewing a molecule directly along the bond, chemists can better understand spatial orientation and sterics, facilitating insight into potential steric hindrance and reactivity.

3-7 Conformations of Other Alkanes

Propane:

Although propane is another straightforward alkane, it reveals a higher torsional barrier (14 kJ/mol) than ethane, indicative of increased steric interactions caused by more complex spatial arrangements of its three carbon atoms. Understanding propane’s conformational behavior is crucial for predicting its reactivity in various chemical processes.

Butane:

Butane presents a rich array of conformational behaviors that can greatly influence its chemical and physical properties:

• Anti Conformation: In this stable arrangement, the methyl groups are positioned 180° apart, which minimizes repulsive interactions and provides the lowest energy conformation.

• Gauche Conformation: This conformation positions methyl groups 60° apart, resulting in heightened energy due to steric strain and repulsion between the bulky groups.

• Example: A graphical representation of Butane’s Energy Profile depicts the variations in stability across these conformations:

  • Energy levels vary:

    • Anti: 0 kJ/mol (most stable)

    • Gauche: 3.8 kJ/mol

    • Eclipsed: 16 kJ/mol (highest energy state).

SOMETHING EXTRA: Gasoline

Gasoline represents a complex mixture of hydrocarbons derived from the decomposition of marine organisms over millions of years. The refining processes, such as fractional distillation of crude oil, yield various principal cuts, which include straight-run gasoline, kerosene, and heating oil. While gasoline was once regarded as a waste product, it is now integral to modern transportation and the economy, celebrated for its high energy content and efficiency in combustion engines. Understanding the chemistry behind gasoline is essential for advancements in energy technology and environmental considerations in the use of fossil fuels.