Organic Chemistry - Chemistry of Unsaturated Hydrocarbons

Synthesis of Alkenes

1. From Dehydrohalogenation of Alkyl Halides

• Dehydrohalogenation involves the removal of a hydrogen and a halogen from an alkyl halide.
• It's a common method for synthesizing alkenes via elimination.
• Alkyl halides, when heated with a strong base such as alcoholic $NaOH$ or $KOH$, undergo elimination to form alkenes.
• Zaitsev's rule is followed during this reaction, which means the most substituted alkene (the alkene with the most alkyl groups attached to the double-bonded carbons) is the major product.

E2 Mechanism

• The E2 mechanism is a concerted reaction where bond breaking and bond forming occur simultaneously.
• Rate Law: $Rate = k[Alkyl Halide][Base]$

E1 Mechanism

• The E1 mechanism proceeds through a carbocation intermediate.
• Step 1: Ionization - The alkyl halide ionizes to form a carbocation.
• Step 2: Deprotonation - A base removes a proton from a carbon adjacent to the carbocation, forming the alkene.
• Rate Law: $Rate = k[Alkyl Halide]$

2. Catalytic Cracking of Alkanes

• Catalytic cracking is a cost-effective industrial method for large-scale alkene production.
• It involves heating a mixture of alkanes in the presence of a catalyst, typically aluminosilicates.
• Alkenes are formed by cleaving bonds in alkanes, resulting in an alkene and a shorter alkane.
• Cracking primarily produces smaller alkenes, generally with up to six carbon atoms.
• The process's economic viability depends on the demand for the various alkenes and alkanes produced.
• The average molecular weight and the relative amounts of alkanes and alkenes can be controlled by adjusting temperature, catalyst, and hydrogen concentration.
• The resulting mixture is separated into pure components through careful distillation in large columns.

3. From Controlled Hydrogenation of Alkynes

• Alkynes can be selectively hydrogenated to alkenes using Lindlar's catalyst.
• Lindlar’s catalyst is palladium poisoned with calcium carbonate ($CaCO_3$) and quinoline to prevent complete reduction to an alkane.

Mode of Action of Alkenes

General Characteristics

• Alkenes contain one or more carbon-carbon double bonds, consisting of one sigma ($\sigma$) bond and one pi ($\pi$) bond.
• Doubly bonded carbon atoms in alkenes are $sp^2$ hybridized.
• Each $sp^2$ hybridized carbon has an unhybridized $2p_z$ orbital that overlaps with its neighbor, forming a pi bond.
• The pi electrons are loosely held and exposed, making them weaker than sigma bonds.
• Alkenes are electron-rich due to the double bond and act as nucleophiles, attacking electron-deficient species (electrophiles).
• Nucleophiles form covalent bonds by sharing their electron pair with an electrophile.

Addition Reactions - General Mechanism

• The chemical behavior of alkenes is largely dictated by the reaction of the electron-rich carbon-carbon double bond (nucleophile) with electron-deficient species (electrophiles).
• These reactions are termed Addition Reactions.
• General Equation: Not provided in the text.
• Carbocations are common reaction intermediates in these reactions.
• Step 1: Pi electrons attack the electrophile, forming a carbocation.
• Step 2: The nucleophile attacks the carbocation.

Types of Additions to Alkenes

\begin{itemize}
\item Hydration: Addition of water ($H<em>2O$) to form an alcohol; adds H and OH across the double bond. \item Hydrogenation: Addition of hydrogen ($H</em>2$) to form an alkane; a reduction reaction; adds H and H across the double bond.
\item Hydroxylation: Addition of $[HOOH]$ to form a diol (glycol); an oxidation reaction; adds OH and OH across the double bond.
\item Halogenation: Addition of a halogen ($X<em>2$) to form a 1,2-dihalide; an oxidation reaction; adds X and X across the double bond. \item Halohydrin Formation: Addition of a halogen and water ($HOX$) to form a halohydrin; an oxidation reaction; adds X and OH across the double bond. \item HX Addition (Hydrohalogenation): Addition of a hydrogen halide ($HX$) to form an alkyl halide; adds H and X across the double bond. \item Oxidative Cleavage: Reaction with ozone ($O</em>3$) to cleave the double bond and form carbonyl compounds (aldehydes, ketones, or carboxylic acids); an oxidation reaction.
\item Epoxidation: Reaction with an oxidizing agent to form an epoxide; an oxidation reaction; adds an oxygen atom across the double bond to form a three-membered ring.
\item Cyclopropanation: Addition of a carbene ($CH_2$) to form a cyclopropane ring.
\end{itemize}

1. Addition of Hydrogen Halides (HX i.e. HBr)

• Hydrogen halides are polar substances that add to alkenes, yielding alkyl halides.
• In the case of unsymmetrically substituted alkenes, Markovnikov's rule is followed.
• The reactivity of hydrogen halides is related to their acidity: HI > HBr > HCl > HF. HI reacts fastest.

Mechanism

• Step 1: Attack of pi electrons on the electrophilic proton of HBr, forming a carbocation.
• Step 2: Nucleophilic attack of the halide ion on the carbocation.

Regioselectivity - Markovnikov's Rule

• Markovnikov’s rule states that in the addition of $HX$ to an unsymmetrical alkene, the hydrogen atom adds to the carbon atom with the greater number of hydrogen substituents, and the halogen atom adds to the carbon atom with the fewer hydrogen substituents.
• The rule is based on the formation of the most stable carbocation intermediate (tertiary > secondary > primary).

2. Addition of Halogen ($X_2$)

• The addition of halogens (e.g., $Cl<em>2$, $Br</em>2$, $I_2$) to alkenes yields 1,2-dihalides (vicinal dihalides).
• The addition occurs trans, meaning the two halogen atoms add to opposite faces of the double bond.
• Rearrangements do not occur.
• The reaction proceeds through a cyclic halonium ion intermediate (e.g., bromonium ion).

3. Addition of Hydrogen (Hydrogenation)

• Alkenes can be reduced to alkanes by the addition of hydrogen ($H_2$) in the presence of a metal catalyst (e.g., Pt, Pd, Ni).
• The reaction occurs on the surface of the metal catalyst.
• Hydrogen molecules adsorb onto the surface of the catalyst and react with the metal atoms.
• The alkene approaches the surface of the catalyst.
• The pi bond between the two carbon atoms is broken, and two new C-H sigma bonds are formed.
• Syn addition is observed, where both hydrogen atoms add to the same face of the double bond.
• Anti addition refers to the addition of atoms or groups to opposite faces of the double bond. Catalytic hydrogenation typically proceeds via syn addition. Often hydrogenation is stereospecific, leading to specific isomers.