9.2.2. Alkenes

• 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.

Alkenes have the general molecular formula CnH₂n, which means that for every 'n' carbon atoms, there are '2n' hydrogen atoms. This formula indicates that alkenes are unsaturated hydrocarbons, meaning they have fewer hydrogen atoms than alkanes (which are saturated with hydrogen) because they contain at least one carbon-carbon double bond. This double bond leads to important chemical reactivity and structural variations.

Think of alkenes like a park that has fewer trees (hydrogens) because some tree space is taken up by a double gazebo (the double bond). In comparison, an alkane park has trees laid out all over the park (saturated). Just like a park's layout affects how people can move around, the double bond in alkenes affects how they react chemically.

• 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.

In alkenes, the carbon atoms connected by the double bond exhibit sp² hybridization. This means each of the two carbon atoms in the double bond forms three sigma bonds with two other atoms and one pi bond with the other carbon atom. The result is a planar geometry with bond angles of about 120 degrees. The pi bond forms by the sidewise overlap of p-orbitals, which means that the atoms cannot freely rotate around the double bond, leading to structural rigidity.

Imagine holding two flat books (the carbon atoms) together by a rubber band (the sigma bond). When you try to twist the books (which represents rotation around the double bond), the rubber band restricts the movement, similar to how the pi bond restricts rotation in alkenes.

• 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).

Alkenes are named based on the number of carbon atoms in the longest chain containing the double bond. The suffix '-ene' indicates that the compound contains a double bond. To accurately reflect the location of the double bond in the name, the lowest possible number (indicating its position in the chain) is placed before the suffix. For example, 'but-1-ene' means the double bond starts at the first carbon in the four-carbon chain.

Naming alkenes can be likened to giving addresses to houses on a street. Just as a house number tells you which house to look for on the street (where the double bond is), the number in an alkene name tells you where the double bond is located in the carbon chain.

• 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.

Due to the rigidity caused by the presence of the double bond, alkenes can have different spatial arrangements known as geometric isomers, specifically cis (same side) and trans (opposite side). If each carbon atom in the double bond is attached to different groups, the molecule can exist in two configurations: one where substituents are on the same side of the double bond (cis) and one where they are on opposite sides (trans). This can lead to substances with different physical properties.

Imagine two windows (the groups attached to the carbons) on either side of a door (the double bond). If both windows are opened similarly (cis configuration), you experience a specific airflow, whereas if the windows are opened differently (trans configuration), the airflow changes dramatically, reflecting the physical differences in the isomers.

• 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.

Alkenes are generally more reactive than alkanes because of the pi bond associated with the double bond. This π bond is weaker and more exposed, making it a target for many chemical reactions. Alkenes tend to behave as nucleophiles, meaning they can donate electron pairs to electron-deficient species (electrophiles) to form new bonds, which is a fundamental step in various chemical processes.

Think of alkenes as popular socialites at a party who invite others to join in. The pi bond acts like a warm welcome, making it easier for new guests (reactants) to bond and create new interactions (products) compared to alkanes, which are more reserved and unreactive.

• 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.

Alkenes react through addition reactions, which involve breaking the weak π bond of the double bond and forming new single bonds with atoms or groups. Common reactions include:

• Hydrogenation: The addition of hydrogen (H₂) to produce an alkane (saturation).
• Halogenation: The addition of halogens (like bromine), resulting in dihaloalkanes.
• Hydrohalogenation: The addition of hydrogen halides (like HBr) to create haloalkanes, adhering to Markovnikov's rule, where the hydrogen atom adds to the carbon with the most hydrogen atoms.
• Hydration: The addition of water to form alcohols in the presence of an acid catalyst.
• Polymerization: Alkenes can link together to form large chains called polymers, as seen in many plastics.

These addition reactions are like a cooking class where a base recipe (the alkene) can be modified by adding various ingredients (like hydrogen or halogens). Just as you can add different ingredients to create new recipes (products), alkenes can combine with different atoms or groups to form new substances.

• Test: Rapidly decolorize bromine water (a solution of Br2 in water), indicating unsaturation.

One way to test for the presence of alkenes is by using bromine water, which is orange-brown in color. When bromine reacts with an alkene, the π bond breaks, and bromine adds across the double bond, resulting in a colorless compound. This rapid change in color from brown to colorless is a clear indication of the unsaturation typically found in alkenes.

Imagine using a special dye that changes color when it meets certain objects. If you spilled orange juice (bromine water) on a white shirt (alkene), and it became colorless, you'd know there was an interaction, just like how alkenes change color with bromine, showing their reactivity.

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