ALKENE

Ethylene and Alkenes

Introduction to Ethylene

• Ethylene (H₂C=CH₂): A gaseous compound produced by many fruits and vegetables that accelerates ripening. Removing ethylene can double the shelf life of produce.
• Absorption Method: Ethylene can be efficiently removed through absorption using alumina impregnated with potassium permanganate.

Definition and Classification of Alkenes

• Alkenes: Hydrocarbons characterized by the presence of a carbon-carbon double bond (C=C) in their structures.
• General Formula: The general structural formula of alkenes is $C_nH_{2n}$, indicating that they contain two fewer hydrogen atoms than their corresponding alkanes, which have the formula $C_nH_{2n+2}$.
• Unsaturation: Alkenes are classified as unsaturated hydrocarbons since they contain double bonds.
• Olefines: A common name for alkenes derived from Latin, meaning oily, due to the oily products formed when lower members react with chlorine or bromine.

Structural Characteristics of Alkenes

Ethylene and Propylene Representation

1. Ethylene (Ethene) - H₂C=CH₂
2. Propylene (Propene) - H₂C=CHCH₃
3. Butenes:
   • 1-Butene: H₂C=CH-CH₂-CH₃
   • 2-Butene: CH₃-CH=CH-CH₃

Orbital Structure of Alkenes

• Hybridization: In ethylene, each carbon atom is sp² hybridized. The double bond comprises:
  • σ (sigma) bond: Formed from the overlap of two sp² hybrid orbitals.
  • π (pi) bond: Formed from the lateral overlap of unhybridized p orbitals.
• Molecular Geometry: Ethylene is a planar molecule with bond angles approximately 120° due to sp² hybridization. The bond lengths are approximately 1.34 Å for the C-H bond and 1.09 Å for the C=C bond.

Nomenclature of Alkenes

Common Name System

• First four members are named by modifying the alkane ending -ane to -ylene.
  • Examples: Ethylene (from ethane), propylene (from propane).
• Greek letters (α, β) designate the position of the double bond.
  • α-Butylene: $CH_2=CH-CH_2-CH_3$
  • β-Butylene: $CH_3-CH=CH-CH_3$

IUPAC Nomenclature System

1. The name is based on the parent alkene with the longest chain containing the double bond.
2. Number the chain from the end closest to the double bond and indicate the bond's position with a number.
3. Combine with substituents prefixed to the parent name.
   • Example: 1-Butene and 2-Methyl-1-butene.
4. For alkenes with two or three double bonds, replace -ane with -adiene or -atriene.
   • Example: 1,3-Butadiene.

Alkenyl Groups

• Monovalent groups formed by removing one hydrogen from alkenes, named by replacing -e with -yl.
  • Example: Ethenyl (Vinyl) = $CH_2=CH-$;
  • Example: 2-Propenyl (Allyl) = $CH_2=CH-CH_2-$.

Degree of Unsaturation

• To determine the degree of unsaturation in a molecular formula, use: n = C + 1 - rac{1}{2}(H-N+X) Where:
  • $n$ = Degree of unsaturation
  • $C$ = Number of carbon atoms
  • $H$ = Number of hydrogen atoms
  • $N$ = Number of nitrogen atoms
  • $X$ = Number of halogen atoms

Degrees of Unsaturation Interpretation

1. $n = 0$: Saturated compound.
2. $n = 1$: One double bond or one ring present.
3. $n = 2$: Two double bonds, or one double bond plus one ring, or one triple bond present.
4. $n = 4$: Example of aromatic compounds (one ring plus three double bonds).

Methods of Preparation of Alkenes

1. Dehydration of Alcohols

• Alcohols lose water in the presence of sulfuric acid to form alkenes.
  • Example: Dehydration of 1-propanol gives propene.
  • General trend for ease of dehydration: 3° > 2° > 1° alcohols.

2. Dehydrohalogenation of Alkyl Halides

• Heating alkyl halides with alcoholic KOH leads to alkene formation by eliminating HX, often following Saytzeff's rule.

3. Dihalides Dehalogenation

• Heating vicinal dihalides with zinc dust can yield alkenes.

4. Controlled Hydrogenation of Alkynes

• Reaction with hydrogen in the presence of Lindlar's catalyst can produce alkenes from alkynes.

5. Cracking of Alkanes

• Heating alkanes in the absence of air to produce lower molecular weight alkenes.

Physical Properties of Alkenes

1. State: Ethylene, propylene, and butenes are gases; larger alkene molecules are liquids or solids.
2. Solubility: Slightly soluble in water, freely soluble in organic solvents.
3. Boiling Points and Melting Points: Generally increase with molecular weight.

Melting and Boiling Points of 1-Alkenes

Chemical Properties of Alkenes

1. Reactivity: Alkenes are more reactive than alkanes due to less stable π bonds.
2. Addition Reactions: Primary reactions, where the π bond in alkenes breaks to form new σ bonds:
   • Addition of Hydrogen Halides: E.g., HBr to alkenes resulting in alkyl halides; follows Markovnikov's rule.
   • Hydration: Alkenes react with water in acid to yield alcohols.
   • Halogenation: Alkenes react with halogens to form vic-dihalides.
   • Ozonolysis: Cleavage of alkenes into carbonyls or acids.
   • Diels-Alder Reaction: Reaction with dienophiles to form cyclic products.

Markovnikov's Rule

• In the addition of HX to unsymmetrical alkenes, the hydrogen atom adds to the carbon atom with more hydrogen atoms already attached.
  • Example: $CH_3-CH=CH_2 + HBr <br /> ightarrow 2-Bromopropane$.

Peroxide Effect

• The presence of organic peroxides can lead to anti-Markovnikov addition, where the bromine adds to the less substituted carbon atom.

Conclusion

• Alkenes are essential in organic chemistry with significant industrial importance, from polymer synthesis to as precursors for numerous chemicals. Understanding their properties, reactivity, and preparation methods is fundamental for both academic study and practical applications in chemistry.