9.2. Alkanes, Alkenes, Alkynes (Hydrocarbons)

These are the simplest organic compounds, composed solely of carbon and hydrogen atoms, and are collectively known as hydrocarbons.

Hydrocarbons are organic compounds featuring only carbon and hydrogen atoms. They are categorized into three main types: alkanes, alkenes, and alkynes, based on the type of bonding between carbon atoms (single, double, or triple bonds). Understanding hydrocarbons is fundamental as they form the basis of many organic molecules.

Think of hydrocarbons as the building blocks of nature, similar to how Lego blocks create various structures. Each type of block (alkanes, alkenes, alkynes) connects differently, leading to diverse configurations and properties.

● General Formula: CnH₂n+₂ (for acyclic, saturated hydrocarbons). ● Bonding: All carbon atoms are sp³ hybridized, forming only carbon-carbon single (σ) bonds and carbon-hydrogen single (σ) bonds. This means they are saturated – they contain the maximum possible number of hydrogen atoms for a given number of carbon atoms. The bond angles around each carbon are approximately 109.5 degrees (tetrahedral geometry). ● Nomenclature: Named using the root corresponding to the number of carbons, followed by the suffix '-ane'. ○ Examples: Methane (CH₄), Ethane (CH₃CH₃), Propane (CH₃CH₂CH₃), Butane (CH₃(CH₂)₂CH₃). ● Structure: Alkanes can exist as straight chains, branched chains, or cyclic structures (cycloalkanes, with general formula CnH₂n). ● Properties: ○ Reactivity: Alkanes are relatively unreactive due to the strong, nonpolar C-C and C-H sigma bonds. They do not react with acids, bases, or most oxidizing/reducing agents. ○ Reactions: Their primary reactions are combustion (burning in oxygen to produce carbon dioxide and water, releasing significant energy) and free radical substitution reactions (e.g., with halogens in the presence of ultraviolet (UV) light or high temperatures, where a hydrogen atom is replaced by a halogen atom). ○ Physical Properties: Non-polar molecules. Insoluble in water but soluble in non-polar organic solvents. Boiling points and melting points generally increase with increasing molecular mass (due to stronger London dispersion forces). Branching decreases the boiling point due to reduced surface area for intermolecular contact.

Alkanes are saturated hydrocarbons characterized by a general formula of CnH₂n+₂ and only single bonds between carbon atoms. They have tetrahedral geometry due to sp³ hybridization, which influences their bond angles and structure. Alkanes can be linear or branched and are generally unreactive due to their stable bonds. They are primarily known for combustion reactions and are non-polar, making them insoluble in water but soluble in organic solvents.

Consider alkanes like a stable relationship in a family where everyone gets along. They are safe and dependable, reacting in predictable ways, just as family members support one another during gatherings (combustion) or occasionally change roles (substitution reactions) when necessary. Their stability ensures they remain undisturbed in water, similar to how a calm family dinner is unaffected by outside noise.

● General Formula: CnH₂n (for acyclic hydrocarbons with one carbon-carbon double bond). They are unsaturated because they contain fewer hydrogen atoms than the corresponding alkane. ● Bonding: Contain at least one carbon-carbon double bond (C=C). Each carbon atom involved in the double bond is sp² hybridized, forming a planar trigonal arrangement with bond angles of approximately 120 degrees. The double bond consists of one strong sigma (σ) bond and one weaker pi (π) bond. The π bond limits rotation around the C=C axis. ● Nomenclature: Named using the root, followed by the suffix '-ene'. The position of the double bond is indicated by the lowest possible number, placed before the suffix (e.g., ethene, propene, but-1-ene, but-2-ene). ● Structure: The restricted rotation around the double bond leads to geometric isomerism (cis-trans isomerism) if each carbon of the double bond is attached to two different groups. ● Properties: ○ Reactivity: More reactive than alkanes due to the presence of the relatively weaker and exposed pi (π) bond. The electron-rich double bond acts as a nucleophile. ○ Reactions: Undergo characteristic addition reactions, where the π bond breaks and new single bonds form with incoming atoms/groups. ■ Hydrogenation: Addition of H₂ (with Ni, Pt, or Pd catalyst) to form alkanes. ■ Halogenation: Addition of X₂ (e.g., Br₂) to form dihaloalkanes. ■ Hydrohalogenation: Addition of HX (e.g., HBr) to form haloalkanes (often follows Markovnikov's rule). ■ Hydration: Addition of H₂O (steam, with H₃PO₄ catalyst) to form alcohols. ■ Polymerization: Alkenes can add to each other to form long polymer chains. ○ Test: Rapidly decolorize bromine water (a solution of Br2 in water), indicating unsaturation.

Alkenes are unsaturated hydrocarbons featuring at least one double bond (C=C) between carbon atoms, represented by the general formula CnH₂n. This double bond introduces new reactivity due to its weaker π bond. Alkenes have distinct planar geometry due to sp² hybridization, allowing for the possibility of geometric isomerism. They primarily participate in addition reactions, where new atoms attach to the carbons involved in the double bond.

Imagine alkenes as flexible rubber bands that can stretch and adjust their shape. The double bond represents a part of the rubber band that can twist (like the restricted rotation), allowing it to change direction easily. When you apply new elements (like hydrogen during hydrogenation), the rubber band can expand into a new form, similar to how alkenes react to become more saturated species.

● General Formula: CnH₂n-₂ (for acyclic hydrocarbons with one carbon-carbon triple bond). They are even more unsaturated than alkenes. ● Bonding: Contain at least one carbon-carbon triple bond (C≡C). Each carbon atom involved in the triple bond is sp hybridized, resulting in a linear geometry with bond angles of 180 degrees. The triple bond consists of one sigma (σ) bond and two pi (π) bonds. ● Nomenclature: Named using the root, followed by the suffix '-yne'. The position of the triple bond is indicated by the lowest possible number (e.g., ethyne, propyne, but-1-yne). ● Structure: The linear geometry around the triple bond means they do not exhibit geometric isomerism. ● Properties: ○ Reactivity: Highly reactive due to the presence of two π bonds. ○ Reactions: Undergo addition reactions, similar to alkenes, but can add two moles of reagent across the triple bond. ■ Can be hydrogenated, halogenated, or hydrohalogenated to form saturated products (after two additions) or unsaturated products (after one addition). ○ Test: Rapidly decolorize bromine water, similar to alkenes.

Alkynes are the most unsaturated hydrocarbons, characterized by a carbon-carbon triple bond and a general formula of CnH₂n-₂. The sp hybridization of the carbon atoms involved leads to a linear arrangement, impacting their chemical behavior. With two π bonds, alkynes are highly reactive and can undergo multiple addition reactions, similar to alkenes but with the ability to add more reagents.

Think of alkynes as tightly coiled springs. Their triple bond acts like the stiff connection at the center of the spring, providing tension. When you apply force (add a reagent), the spring can release energy and extend in new ways, similar to how alkynes can interact with other chemicals to change their structure and reactivity.

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