Hydrocarbons Overview and Classification

General Formula for Alkenes

• The general formula for alkenes is CnH2n, where 'n' represents the number of carbon atoms in the molecule.

First Stable Alkene

• Ethene (C2H4), commonly known as ethylene, is the first stable member of the alkene series.

IUPAC Nomenclature for Alkenes

• The suffix 'ane' is replaced with 'ene' when naming alkenes using the IUPAC system.

Numbering the Carbon Chain in Alkenes

• The carbon chain is numbered in such a way that the carbon atoms involved in the double bond get the lowest possible numbers.

Characteristics of Alkenes Compared to Alkanes

• Alkenes are more reactive than alkanes due to the presence of the carbon-carbon double bond.

Reactions of Alkenes

• Alkenes undergo addition reactions, where atoms or groups of atoms are added across the double bond.

Carbene

• Methylene (CH2), also known as carbene, is a very reactive species with a short lifetime.

Chemical Structure of Alkenes

• The chemical structure of alkenes includes a carbon-carbon double bond, which consists of a sigma bond and a pi bond.

Vicinal Dihalides and Zinc Metal Reaction

• The primary reaction that occurs when vicinal dihalides are treated with zinc metal is the formation of an alkene through a dehalogenation process.

Chemical Properties of Alkenes

• Alkenes are generally more reactive than alkanes due to the presence of the double bond, which is a region of high electron density.

Reaction of Alkenes with Dihydrogen Gas

• Alkenes react with dihydrogen gas in the presence of finely divided nickel, palladium, or platinum catalysts to produce alkanes.

Acidic Dehydration of Alcohols

• Acidic dehydration of alcohols results in the formation of alkenes.

Alkenes Under Free Radical Mechanisms

• Alkenes, due to their double bond, can participate in free radical reactions, but they are not as readily involved as alkanes.

Preparation of Alkenes

• Alkenes can be prepared from various methods, such as elimination reactions, dehydrohalogenation, and the Wittig reaction.

Boiling Points of Vicinal Dihalides

• Vicinal dihalides have higher boiling points compared to branched-chain compounds due to the presence of dipole-dipole interactions.

Alkenes and Electrophiles

• Alkenes act as nucleophiles in addition reactions with electrophiles because of the electron-rich nature of the double bond.

Polarity of Trans-But-2-Ene

• Trans-but-2-ene is a non-polar molecule due to the symmetrical arrangement of its substituents.

Melting Point of Trans-But-2-Ene

• The higher melting point of trans-but-2-ene compared to cis-but-2-ene is attributed to the closer packing of molecules in the trans isomer, leading to stronger intermolecular forces.

Lindlar's Catalyst

• Lindlar's catalyst, a palladium catalyst poisoned with lead and quinoline, is specifically used for partially reducing alkynes to produce cis alkenes.

Reduction of Alkynes with Sodium in Liquid Ammonia

• When alkynes are reduced with sodium in liquid ammonia, they yield trans alkenes.

Cis Isomer Structure

• A cis isomer has the same substituents on the same side of the double bond.

Geometry of Geometrical Isomers

• The geometry of geometrical isomers is determined by the arrangement of substituents around the double bond, resulting in either cis or trans isomers.

Melting Point and Structure Misconception

• It is a misconception that the melting point of an alkene is always higher for the cis isomer; it is typically higher for the trans isomer due to better packing.

Partial Reduction of Alkynes to Alkenes

• A necessary condition for the partial reduction of alkynes to alkenes is the presence of a suitable catalyst, such as Lindlar's catalyst, which promotes cis addition.