Alkenes Knowledge-Based Study

Study notes

1. What Are Alkenes?

   Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond ($C=C$). Because they possess fewer hydrogen atoms than corresponding alkanes with the same number of carbon atoms, they are termed unsaturated. Alkenes containing one double bond form an aliphatic homologous series with the general molecular formula:

   $C_nH_{2n}$ (where $n \ge 2$)

   Bonding and Geometry

   The double bond in an alkene consists of two distinct types of covalent bonds:

   • One $\sigma$ (sigma) bond: Formed by the end-to-end (axial) overlap of $sp^2$ hybridized orbitals of the two carbon atoms.
   • One $\pi$ (pi) bond: Formed by the lateral (side-by-side) overlap of unhybridized $2p$ orbitals perpendicular to the molecular plane.

   Each double-bonded carbon atom is $sp^2$ hybridized, resulting in a trigonal planar geometry around each carbon center with bond angles of approximately 120°. The presence of the $\pi$ bond prevents free rotation around the carbon-carbon double bond at ambient temperatures.

   IUPAC Naming Rules for Alkenes

   Alkenes are systematically named following standard IUPAC conventions by modifying the suffix of the corresponding alkane from -ane to -ene.

   Key Nomenclature Steps

   1. Identify the longest continuous carbon chain: The selected parent chain must contain both carbon atoms of the double bond, even if a longer saturated chain exists.
   2. Number the parent chain: Begin numbering from the end closest to the double bond, ensuring the double-bonded carbons receive the lowest possible locants. If the double bond is equidistant from both ends, number from the end closer to the first substituent.
   3. Indicate the position of the double bond: Designate the position using the lower-numbered carbon atom of the double bond (for example, but-1-ene or 1-butene).
   4. Assemble substituents: List alkyl groups and halogens in alphabetical order with their corresponding locants preceding the parent alkene name.

   Alkenes with Multiple Double Bonds

   Compounds containing two or three double bonds are designated as alkadienes or alkatrienes (for example, buta-1,3-diene). A prefix indicating the total number of carbon atoms retains the letter 'a' before the multiplying prefix.

   Types of Isomerism Observed in Alkenes

   Alkenes exhibit both constitutional (structural) isomerism and stereoisomerism due to their unique structural features.

   1. Constitutional Isomers

   • Chain Isomerism: Arises from differences in the carbon skeleton arrangement (e.g., but-1-ene and 2-methylprop-1-ene).
   • Positional Isomerism: Arises from differing locations of the double bond along the same carbon chain (e.g., but-1-ene and but-2-ene).

   2. Geometric (Cis-Trans) Isomerism

   Because the $\pi$ bond restricts rotation around the $C=C$ axis, substituents attached to the double-bonded carbons maintain fixed spatial orientations. This gives rise to geometric isomerism (diastereomerism).

   Conditions for Geometric Isomerism

   Geometric isomerism can only occur if neither carbon atom of the double bond carries two identical groups. In a general alkene $R_1R_2C=CR_3R_4$, it is required that $R_1 \ne R_2$ and $R_3 \ne R_4$.

   • cis-isomer: Similar or identical high-priority groups reside on the same side of the double bond.
   • trans-isomer: Similar or identical high-priority groups reside on opposite sides of the double bond.

   Because of differences in symmetry and molecular dipole moments, cis and trans isomers possess distinct physical properties such as boiling points, melting points, and polarities.

   Characteristic Addition Reactions

   The electron-rich $\pi$ bond in alkenes makes them significantly more reactive than alkanes. The characteristic reactions of alkenes are electrophilic addition reactions, in which the relatively weak $\pi$ bond is broken to form two new stronger $\sigma$ bonds.

   Major Addition Reactions

   • Hydrogenation (Addition of $H_2$): In the presence of a metal catalyst such as $Ni$, $Pt$, or $Pd$, alkenes add hydrogen to yield alkanes:
     $C_nH_{2n} + H_2 \xrightarrow{Ni} C_nH_{2n+2}$
   • Halogenation (Addition of $X_2$): Rapid addition of halogens (typically $Br_2$ or $Cl_2$) produces vicinal dihalides. The decolorization of reddish-brown bromine water ($Br_2$ in $CCl_4$) serves as an analytical test for unsaturation.
   • Hydrohalogenation (Addition of $HX$): Reaction with hydrogen halides ($HCl$, $HBr$, $HI$) yields haloalkanes.
   • Hydration (Addition of $H_2O$): Acid-catalyzed addition of water yields alcohols.

   Markovnikov's Rule

   When an unsymmetrical reagent (such as $HX$ or $H_2O$) adds to an unsymmetrical alkene, the acidic hydrogen atom attaches preferentially to the double-bonded carbon that already bears the greater number of hydrogen atoms, yielding the more stable carbocation intermediate and forming the major product.