Alkenes: Structure, Bonding, and Geometrical Isomerism

This detailed overview of alkenes covers their structure, bonding, physical properties, and chemical reactivity based on the provided source material.

1. Introduction and Structure

• Definition: Alkenes, also known as olefins, are hydrocarbons characterized by the presence of a carbon-carbon double bond.
• Classification: They are classified into terminal alkenes (double bond at the end of the chain), internal alkenes (at least one carbon bonded to each end of the double bond), and cycloalkenes (double bond within a ring).
• Hybridization and Geometry: Each carbon in the double bond is $sp^2$ hybridized with a trigonal planar geometry and bond angles of approximately 120°.
• Bonding: The double bond consists of one strong $\sigma$ (sigma) bond and one weaker $\pi$ (pi) bond. Because the $\pi$ bond is much weaker and more easily broken than the $\sigma$ bond, alkenes are more reactive than alkanes.

2. Geometrical Isomerism

• Cause: Isomerism occurs due to the restricted rotation of the C=C double bond.
• Cis/Trans Terminology:
  • Cis (Z): Substituent groups are on the same side of the double bond.
  • Trans (E): Substituent groups are on opposite sides across the double bond.
• E/Z System: Used when there are three or four different groups attached to the double bond. It is based on priority rules; if the higher-priority atoms (based on atomic number) are on the same side, it is the Z isomer; if they are on opposite sides, it is the E isomer.
• Requirement: Stereoisomers only exist if each carbon of the double bond is attached to two different groups. For example, terminal alkenes (ending in $C=CH_2$) do not have cis/trans isomers.

3. Relative Stability

• Substitution Rule: The stability of an alkene increases as the number of R groups (alkyl groups) attached to the double bond increases. The order is: Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted.
• Trans vs. Cis: Trans alkenes are generally more stable than cis alkenes because there is less steric strain between the substituent groups.
• Internal vs. Terminal: Internal alkenes are more stable than terminal alkenes.

4. Physical Properties

• Interactions: Alkenes exhibit weak van der Waals interactions, making their properties similar to alkanes of comparable weight.
• Melting and Boiling Points: These points are low but increase with the number of carbons due to increased surface area.
• Polarity: A cis alkene is more polar than a trans alkene because its bond dipoles reinforce each other, whereas they cancel out in the trans isomer. This results in cis isomers having higher boiling points and higher solubility in polar solvents.

5. Methods of Preparation

Alkenes are typically prepared via elimination reactions, where a small molecule is removed from a substrate:

• Dehydrohalogenation: The loss of HX from an alkyl halide, usually by heating with a strong base.
• Dehydration: The loss of $H_2O$ from an alcohol in the presence of a strong acid catalyst like $H_2SO_4$.
• Regioselectivity: These reactions usually follow a trend where the most stable alkene (the more substituted one) is the major product.

6. Addition Reactions

The characteristic reaction of alkenes is addition, where the $\pi$ bond breaks and two new $\sigma$ bonds form.

• Hydrogenation: Addition of $H_2$ using a catalyst (Ni, Pt, or Pd) to form an alkane.
• Hydrohalogenation (HX): Addition of a hydrogen halide.
  • Markovnikov’s Rule: The H atom adds to the less substituted carbon (the one with more H atoms initially) to form a more stable carbocation intermediate.
  • Exception: Addition of HBr in the presence of peroxides results in anti-Markovnikov orientation.
• Hydration: Addition of water ($H_2O$) in the presence of $H_2SO_4$ to form an alcohol.
• Halogenation: Addition of $X_2$ (Cl or Br) to form a vicinal dihalide.
• Hydroboration-Oxidation: A two-step sequence that converts an alkene into an alcohol.

7. Oxidation and Polymerization

• Oxidation without Cleavage:
  • Reaction with cold alkaline $KMnO_4$ forms a diol (glycol). This serves as a test for unsaturation as the purple $KMnO_4$ turns into a brown $MnO_2$ precipitate.
  • Epoxidation: Reaction with peroxybenzoic acid to form an epoxide.
• Oxidation with Cleavage:
  • Ozonolysis ($O_3$): Cleaves the double bond to form carbonyl compounds; used for identifying alkene structures.
  • Vigorous Oxidation: Hot $KMnO_4$ can cleave alkenes into $CO_2$, carboxylic acids, or ketones depending on the substitution of the double bond.
• Polymerization: A process where many monomers join to form large molecules called polymers (e.g., ethylene becoming polyethylene). Common addition polymers include PVC, polystyrene, and Teflon.