Alkenes: Nomenclature and Overview

Alkenes - Overview

• Alkenes are aliphatic hydrocarbons.
• They have a molecular formula of CnH2n where n = 1, 2, 3, 4, etc.
• Examples of naturally occurring alkenes include beta-carotene (orange pigment in carrots), zingiberene (oil of ginger), limonene (from lemons and oranges), and alpha-farnesene (found in the waxy coating on apple skins).

Nomenclature

• Common Names
• The name ends with "-ylene" instead of "-ane" (alkane).
• Carbons are numbered using Greek letters (α, β, γ, etc.).
• Examples include ethylene, propylene, α-butylene, and β-butylene and iso-butylene.
• IUPAC Names
• The longest carbon chain with the double bond is the parent hydrocarbon.
• The name ends with "-ene" instead of "-ane" (alkane).
• Numbering starts from the side closest to the double bond.
• Examples include ethene, propene, 1-butene, 2-butene, 2-chloro-2-butene, and 3-chloropropene.
• When numbering the carbon chain, prioritize the multiple bond for the lowest possible number.
• If the multiple bond is equidistant from both ends, number the chain from the end nearest the first branch point.

Preparation Methods for Alkenes

• Dehydrohalogenation of Alkyl Halides
• Involves the elimination of HX from an alkyl halide using alcoholic KOH.
• Saytzeff's Rule: During dehydrohalogenation, the hydrogen atom is eliminated from the neighboring carbon atom attached to the halogen atom, favoring the carbon with the fewest hydrogen atoms.
• Dehydration of Alcohols
• Involves the elimination of a water molecule (OH and H from adjacent carbon atoms) in the presence of concentrated acids and heat.
• Dehalogenation of Dihalogenated Alkanes
• Eliminates X2 from a dihalo compound in the presence of zinc dust.
• From Alkynes
• Alkynes can be hydrogenated to form alkenes using H2 with a platinum catalyst.

Chemical Reactions of Alkenes

• Addition Reactions
• Breaking the double bond and adding atoms of the reagent to each carbon atom.
• Examples include reactions with Br2, H2, HCl, HOCl, H2O, and H2SO4.
• Test for Unsaturation
• Addition of bromine.
• Markovnikov's Rule
• When an asymmetric reagent is added to an asymmetric double bond, the negative part of the reagent adds to the carbon with the least number of hydrogen atoms.
• Hydrohalogenation
• Adding hydrogen and a halogen atom.
• The reaction is regioselective.
• The carbon bonded to more hydrogen atoms receives another hydrogen.
• Anti-Markovnikov's Rule
• Kharasch and Mayo in 1933 discovered the peroxide effect.
• Oxidation with KMnO4
• The oxidizing agent attacks the double bond, forming a corresponding glycol.
• Oxidation with Ozone
• Ozone reacts with alkenes to form ozonides, which decompose via hydrolysis to produce two carbonyl compounds.
• Polymerization of Alkenes
• Many alkene molecules (n) combine under high temperature and pressure to form a polyethylene polymer.

Alkynes - Overview

• Alkynes are aliphatic hydrocarbons.
• Molecular formula is CnH2n-2 where n = 1, 2, 3, 4, etc.

Nomenclature of Alkynes

• Common Names
• Alkynes are named as derivatives of acetylene (the simplest alkyne).
• IUPAC Names
• Alkynes are named similarly to alkenes, but the ending "-ene" is replaced with "-yne".
• The longest continuous carbon chain containing the carbon-carbon triple bond is chosen.
• The chain is numbered to give the triple bond the lowest possible number, even if it means other substituents get higher numbers.
• Examples
• 1-Butyne, 2-Butyne, 3-Chloro-1-butyne

Methods for Preparing Alkynes

• From Gem- and Vic-Dihalogeno Compounds
• Alkynes can be prepared from geminal (gem) and vicinal (vic) dihalogeno compounds by elimination reactions using alcoholic KOH.
• Dehalogenation of Tetrahalogeno Compounds
• Alkynes Result from the dehalogenation of tetrahalogeno compounds using zinc dust and heat.
• Hydration of Calcium Carbide
• Calcium carbide reacts with water to produce ethyne (acetylene) and calcium hydroxide.

Chemical Reactions of Alkynes

• Addition of Hydrogen
• Alkynes can undergo addition of hydrogen (H2) with a platinum (Pt) catalyst to form alkenes and further to form alkanes.
• Addition of Halogens (X2)
• Alkynes can undergo addition of halogens (e.g., Cl2) to form dihaloalkenes and tetrahaloalkanes.
• Addition of HX
• Alkynes can undergo addition of hydrogen halides (HX) to form haloalkenes and dihaloalkanes.
• Reaction follows Markovnikov's rule.
• Addition of Water (HgSO4/H2SO4)
• Alkynes react with water in the presence of HgSO4 and sulfuric acid (H2SO4) to form vinyl alcohols, which then rearrange to form aldehydes or ketones.
• Addition of HCN
• Alkynes can undergo addition of hydrogen cyanide (HCN) to form acrylonitrile.
• Polymerization of Alkynes
• Under specific conditions (e.g., Ni catalyst and heat), acetylene can polymerize to form benzene.
• Addition of Oxygen and Ozone
• Alkynes undergo ozonolysis or oxidation with potassium permanganate to form carboxylic acids.
• Acidic Character of Acetylenic H Atom
• Terminal alkynes (those with a hydrogen atom bonded to a triply bonded carbon) exhibit acidic character.
• They can react with strong bases like sodium amide (NaNH2) in liquid ammonia to form acetylides.
• These alkynes react with strong bases to form monosodium or disodium acetylides.
• Internal alkynes don't show reactivity.