Alkenes

Alkenes: hydrocarbons with double bonds

Alkenes are unsaturated hydrocarbons that contain a double bond between two carbon atoms. Their general formula is C$_n$H$_{2n}$, where n represents the number of carbon atoms. The functional group in alkenes is represented as C=C, indicating the presence of the double bond. Alkenes are also known to share the same formula as cycloalkanes, making them fractional isomers of each other.

General formula and hybridization

1. General formula for alkenes: C$_n$H$_{2n}$
2. Alkenes exhibit sp$^2$ hybridization because of the double bond, which results in a trigonal planar shape.

Nomenclature of alkenes

The naming of alkenes follows a systematic approach in line with the IUPAC rules. The first member in the alkene series is Ethene, as a double bond cannot exist in a single-carbon compound.

1. C$_2$H$_4$ – Ethene
2. C$_3$H$_6$ – Propene

3-methylbut-1-ene

3-propylhex-2-ene

3-methylhex-3-ene

2,3-dimethylhex-2-ene

5-methylhepta-1,3,6-triene

Types of isomerism in alkenes

Alkenes exhibit four types of isomerism:

1. Chain Isomerism (Skeletal Isomerism): This arises from differences in the structure of the carbon chain.
2. Positional Isomerism: Occurs when the position of the double bond differs within the carbon chain.
3. Geometrical Isomerism: Due to differences in the spatial arrangement of atoms or groups around the double bond.
4. Functional Isomerism: Alkenes and cycloalkanes, despite having the same general formula, differ in their functional groups.

Laboratory preparation of alkenes

There are several methods to prepare alkenes in the laboratory:

1. Dehydrohalogenation of Haloalkanes: Alkyl halides undergo elimination reactions with sodium or potassium hydroxide in alcoholic solutions to form alkenes.
2. Dehydration of Alcohols: Alcohols are heated with concentrated sulfuric acid at temperatures of 175–180°C to form alkenes.
3. Dehalogenation of Vicinal Dihalides: Vicinal dihalides undergo elimination to form alkenes.
4. Hydrogenation of Alkynes: Alkynes can be partially hydrogenated to alkenes using Lindlar's catalyst (a poisoned palladium catalyst).

Saytzeff's rule

Saytzeff's Rule states that during an elimination reaction, the hydrogen atom is removed from the carbon atom with fewer hydrogen atoms, favoring the formation of the more substituted alkene.

Chemical properties of alkenes

Alkenes are more reactive than alkanes due to the presence of the pi bond, which is weaker and more easily broken.

Important reactions

1. Addition of Hydrogen Halides (HX): Alkenes undergo electrophilic addition reactions, where HX adds to the carbon atoms of the double bond. This follows Markovnikov's rule.
2. Hydration of Alkenes: Alkenes react with water in the presence of an acid (e.g., concentrated H$_2$SO$_4$) to form alcohols.
3. Markovnikov's Rule: In the addition of HX, the electrophile (hydrogen) adds to the carbon with more hydrogen atoms.
4. Anti-Markovnikov's Rule: When organic peroxides (R-O-O-R) are used, the addition of HX follows the opposite rule (hydrogen adds to the carbon with fewer hydrogen atoms).
5. Hydrogenation: Addition of hydrogen (H$_2$) in the presence of a platinum or nickel catalyst converts alkenes to alkanes.
6. Halogenation: Halogens (Br$_2$, Cl$_2$) add to alkenes, often in inert solvents like carbon tetrachloride, and the reaction can be used to test for double bonds.
7. Oxidation: Alkenes can react with oxidizing agents like KMnO$_4$ or K$_2$Cr$_2$O$_4$ to form diols, carboxylic acids, or ketones depending on the conditions.
8. Ozonolysis: Ozonolysis involves breaking the double bond of alkenes using ozone (O$_3$) to produce aldehydes or ketones.

Homework assignment

1. Explore Anti-Markovnikov's rule using organic peroxides (HBr with R-O-O-R).
2. Study the reactions of alkenes with bromine water and concentrated sulfuric acid.
3. Describe the mechanism of hydration of alkenes and explain how Markovnikov's rule applies.

Example reaction (ozonolysis)

Given an unsaturated hydrocarbon with molecular formula C$_6$H$_{12}$, ozonolysis produces two compounds (C$_3$H$_6$O), both ketones. Identify the structure of the original compound and the products.

Solution: Compound A is likely 2-hexene, with ozonolysis breaking it into acetone.