Hydrocarbons (Alkenes)

Alkenes

---

Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond ($C=C$). They form a homologous series with the general formula $C_nH_{2n}$ for acyclic alkenes with one double bond.

Structure Of Double Bond

Hybridization: Carbon atoms involved in a double bond are $sp^2$ hybridized.

Bond Formation:

• Sigma ($\sigma$) Bond: Formed by the head-on overlap of $sp^2$ hybrid orbitals of the two carbon atoms.
• Pi ($\pi$) Bond: Formed by the lateral overlap of the remaining unhybridized $p$ orbitals of the two carbon atoms. These orbitals are parallel to each other and lie above and below the plane of the $\sigma$ bond.

Geometry: The $sp^2$ hybridization leads to a trigonal planar geometry around each carbon atom involved in the double bond.

Bond Angles: The bond angles ($C-C=C$) are approximately 120°.

Planarity: The atoms directly attached to the doubly bonded carbons lie in the same plane.

Restricted Rotation: The presence of the $\pi$ bond prevents free rotation around the $C=C$ axis. This restricted rotation is responsible for geometric isomerism in alkenes.

Reactivity: The $\pi$ bond, being weaker and more exposed than the $\sigma$ bond, is the site of chemical reactions, primarily addition reactions.

Nomenclature

IUPAC Naming:

1. Parent Chain: Identify the longest continuous carbon chain containing the double bond.
2. Numbering: Number the parent chain starting from the end nearer to the double bond, giving it the lowest possible number.
3. Suffix: Replace the '-e' ending of the parent alkane with '-ene'. Indicate the position of the double bond by the lower number of the two doubly bonded carbon atoms.
4. Substituents: Name and number the substituents as usual and list them alphabetically before the parent name.

Examples:

• $CH_2=CH_2$: Ethene (common name: ethylene)
• $CH_3CH=CH_2$: Propene
• $CH_3CH=CHCH_3$: But-2-ene (double bond starts at C2)
• $CH_2=CHCH_2CH_3$: But-1-ene (double bond starts at C1)
• 3-Methylbut-1-ene: $CH_2=CHCH(CH_3)CH_3$

Isomerism

1. Structural Isomerism: Alkenes exhibit chain and position isomerism.

• Chain Isomerism: Different arrangements of the carbon skeleton (e.g., butene isomers: but-1-ene and 2-methylpropene).
• Position Isomerism: Different positions of the double bond in the same carbon skeleton (e.g., but-1-ene and but-2-ene).
• Functional Isomerism: Alkenes can be functional isomers with cycloalkanes (same molecular formula, different functional groups, e.g., propene $C_3H_6$ and cyclopropane $C_3H_6$).

2. Geometric Isomerism (Cis-Trans Isomerism):

• Cause: Restricted rotation around the $C=C$ double bond.
• Requirement: Each carbon atom of the double bond must be attached to two different groups.
• Cis Isomer: Similar groups are on the same side of the double bond.
• Trans Isomer: Similar groups are on opposite sides of the double bond.

Example: But-2-ene exists as cis-but-2-ene and trans-but-2-ene.

Preparation

Alkenes are typically prepared by methods that involve the formation of a double bond:

1. Dehydrohalogenation of Alkyl Halides: Elimination of a hydrogen atom and a halogen atom from adjacent carbon atoms. This is usually done by heating the alkyl halide with an alcoholic solution of a strong base like potassium hydroxide ($KOH$).

$CH_3CH_2Br + KOH \xrightarrow[Alcohol]{heat} CH_3CH=CH_2 + KBr + H_2O$

Saytzeff's Rule: In cases where dehydrohalogenation can produce more than one alkene, the major product is the one with the more substituted double bond (more alkyl groups attached to the doubly bonded carbons).

2. Dehydration of Alcohols: Elimination of a water molecule from adjacent carbon atoms by heating the alcohol with a dehydrating agent like concentrated sulfuric acid ($H_2SO_4$) or anhydrous aluminum oxide ($Al_2O_3$) at high temperatures.

$CH_3CH_2OH \xrightarrow[Conc. H_2SO_4, 443K]{\Delta} CH_2=CH_2 + H_2O$

$CH_3CH_2CH_2OH \xrightarrow[Al_2O_3, 623K]{\Delta} CH_3CH=CH_2 + H_2O$ (Major product)

$CH_3CH(OH)CH_3 \xrightarrow[Conc. H_2SO_4, 373K]{\Delta} CH_3CH=CH_2 + H_2O$

3. From Alkynes by Reduction: Partial reduction of alkynes yields alkenes.
• Syn addition of $H_2$ with poisoned catalyst (Lindlar's catalyst): Produces cis-alkenes.

$R-C \equiv C-R' \xrightarrow{H_2, \text{Lindlar's catalyst}} cis-RCH=CHR'$

• Dissolving metal reduction (Sodium in liquid ammonia): Produces trans-alkenes.

$R-C \equiv C-R' \xrightarrow{Na/liq. NH_3} trans-RCH=CHR'$

Properties

Physical Properties:

• Alkenes ($C_2H_4$ to $C_4H_8$) are gases at room temperature.
• $C_5H_{10}$ to $C_{17}H_{34}$ are liquids.
• Higher members ($C_{18}$ onwards) are solids.
• They are generally insoluble in water but soluble in nonpolar organic solvents.
• Boiling points increase with increasing molecular weight. Branching lowers the boiling point.

Chemical Properties: Alkenes are more reactive than alkanes due to the presence of the double bond, particularly the $\pi$ bond.

1. Addition Reactions: The $\pi$ bond readily undergoes addition reactions where atoms are added across the double bond.

• Addition of Hydrogen (Hydrogenation): Addition of $H_2$ in the presence of Ni, Pt, or Pd catalyst.

$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3CH_3$

• Addition of Halogens (Halogenation): Addition of $Cl_2$, $Br_2$. Bromine addition is used as a test for unsaturation (decolorization of bromine water).

$CH_2=CH_2 + Br_2 \rightarrow CH_2BrCH_2Br$

• Addition of Hydrogen Halides (Hydrohalogenation): Addition of $HX$ ($HCl$, $HBr$, $HI$). Follows Markovnikov's rule.

Markovnikov's Rule: In the addition of $HX$ to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom of the double bond that has the greater number of hydrogen atoms already attached.

$CH_3CH=CH_2 + HBr \rightarrow CH_3CHBrCH_3$ (Major product, Propan-2-yl bromide)

• Addition of Water (Hydration): Addition of water in the presence of an acid catalyst (like $H_2SO_4$) forms alcohols. Also follows Markovnikov's rule.

$CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$

• Addition of $H_2O_2$ (Hydroboration-Oxidation): Anti-Markovnikov addition of water, yielding primary alcohols.

$3CH_3CH=CH_2 + 2B_2H_6 \rightarrow 2(CH_3CH_2CH_2)_3B \xrightarrow{H_2O_2, OH^-} 3CH_3CH_2CH_2OH$

• Oxidation: Alkenes react with oxidizing agents.
• Reaction with cold, dilute, alkaline $KMnO_4$ (Baeyer's Reagent): Forms vicinal glycols (1,2-diols).

$3CH_2=CH_2 + 2KMnO_4 + 4H_2O \rightarrow 3CH_2(OH)CH_2OH + 2MnO_2 + 2KOH$

• Reaction with Ozone ($O_3$): Forms ozonides, which upon reductive or oxidative cleavage yield aldehydes, ketones, or carboxylic acids.

$CH_3CH=CH_2 + O_3 \rightarrow \text{Ozonide intermediate} \xrightarrow{H_2O, Zn} CH_3CHO + HCHO$

• Ozonolysis: Reaction with ozone followed by reductive or oxidative workup to cleave the double bond.
• Polymerization: Alkenes can join together to form long polymer chains, e.g., polymerization of ethene to polythene.

$nCH_2=CH_2 \rightarrow -(CH_2-CH_2)_n-$