Alkenes, Alkynes, and Aromatic Compounds: Structure, Nomenclature, and Reactions

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Alkenes, Alkynes, and Aromatic Compounds

Introduction to Alkenes, Alkynes, and Aromatic Compounds

Alkenes, alkynes, and aromatic compounds are classes of hydrocarbons distinguished by the presence of double, triple, or delocalized bonds, respectively. Their unique bonding leads to distinct chemical and physical properties, as well as specific rules for nomenclature and reactivity.

Alkenes & Alkynes

Definitions and Classification

Saturated hydrocarbons: Molecules whose carbon atoms bond to the maximum number of hydrogen atoms (alkanes, only C–C single bonds).
Unsaturated hydrocarbons: Molecules containing carbon–carbon multiple bonds (alkenes with C=C double bonds, alkynes with C≡C triple bonds).

Models of propane (alkane), propene (alkene), and propyne (alkyne) with unsaturated carbons marked

Bonding and Molecular Geometry

Alkanes: Tetrahedral geometry, bond angles of 109.5°.
Alkenes: Planar geometry, bond angles of 120° due to sp2 hybridization.
Alkynes: Linear geometry, bond angles of 180° due to sp hybridization.

Bond angles and molecular geometry of methane, ethylene, and acetylene

Naming Alkenes and Alkynes

Nomenclature for alkenes and alkynes follows similar rules to alkanes, with modifications to indicate the presence and position of multiple bonds.

Change the suffix: -ane (alkane), -ene (alkene), -yne (alkyne).
Identify the longest chain containing the multiple bond as the parent chain.
Number the chain from the end nearest the multiple bond; if a branch point is present, assign the lowest possible numbers to both the multiple bond and the branch.
For cycloalkenes, the ring must contain the double bond.
Indicate the position of the multiple bond by the number of the first carbon involved.
List substituents alphabetically, using commas between numbers and hyphens between numbers and words.

Shapes and Isomerism in Alkenes and Alkynes

Double and triple bonds restrict rotation, leading to the possibility of geometric (cis-trans) isomerism in alkenes.

Cis-trans isomers: Occur when each carbon of the double bond has two different groups attached.
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.

Diagram showing sides and ends of a double bond for isomerism

Not all alkenes can form cis-trans isomers; if either carbon of the double bond has two identical groups, isomerism is not possible.

2-Methyl-1-butene cannot have cis-trans isomers Cis and trans isomers of 2-pentene

Properties of Alkenes and Alkynes

Physical properties are similar to alkanes (nonpolar, low solubility in water).
Chemically, alkenes and alkynes are more reactive due to the presence of π bonds, which are more easily broken than σ bonds.
They readily undergo addition reactions, converting unsaturated compounds to saturated ones.

General Types of Organic Reactions

Addition Reactions

In an addition reaction, atoms or groups are added to the carbons of a multiple bond, resulting in a saturated product.

General form: (with X and Y added to the carbons)

Elimination Reactions

Elimination reactions are the reverse of addition reactions, where atoms or groups are removed from adjacent carbons, forming a multiple bond.

General elimination reaction: saturated to unsaturated

Substitution Reactions

In substitution reactions, one atom or group in a molecule is replaced by another atom or group.

Substitution of Cl for H in methane

Rearrangement Reactions

Rearrangement reactions involve the reorganization of bonds within a molecule to form an isomer.

Rearrangement reaction: cis-2-butene to trans-2-butene

Reactions of Alkenes and Alkynes

Hydrogenation

Hydrogenation is the addition of hydrogen (H2) across a double or triple bond, converting unsaturated compounds to saturated ones. A metal catalyst such as palladium (Pd) is required.

Example: Conversion of vegetable oil (unsaturated fat) to margarine (saturated fat).

Hydrogenation of an unsaturated oil to a saturated oil

Halogenation

Halogenation is the addition of halogens (Cl2 or Br2) to a double or triple bond, forming dihaloalkanes.

Hydrohalogenation: addition of HBr or HCl to a double bond

Hydrohalogenation

Hydrohalogenation is the addition of hydrogen halides (HCl, HBr) to alkenes or alkynes. The regioselectivity of this reaction is described by Markovnikov's Rule:

Markovnikov's Rule: The hydrogen atom attaches to the carbon with more hydrogens already attached; the halogen attaches to the carbon with fewer hydrogens.
If both carbons have equal hydrogens, two products are formed.

Markovnikov's rule in hydrohalogenation Hydrohalogenation of 2-methylpropene

Hydration

Hydration is the addition of water (H2O) to an alkene, usually in the presence of a strong acid catalyst. It also follows Markovnikov's Rule, with H attaching to the carbon with more hydrogens and OH to the carbon with fewer hydrogens.

Alkene Polymers

Polymerization

Polymerization is the process by which small molecules (monomers) join to form large molecules (polymers). Many alkenes undergo addition polymerization in the presence of a catalyst.

Monomer: The small repeating unit (e.g., ethylene, propylene).
Polymer: The large molecule formed (e.g., polyethylene, polypropylene).

Polymerization of ethylene, propylene, and styrene

Common Alkene Polymers and Their Uses

Monomer Name	Monomer Structure	Polymer Name	Uses
Ethylene	H2C=CH2	Polyethylene	Packaging, bottles
Propylene	H2C=CH–CH3	Polypropylene	Bottles, rope, pails, medical equipment
Vinyl chloride	H2C=CH–Cl	Poly(vinyl chloride)	Insulation, plastic pipe
Styrene		Polystyrene	Foams and molded plastics
Styrene and 1,3-butadiene	H2C=CH–CH=CH2	Styrene-butadiene rubber (SBR)	Synthetic rubber for tires
Acrylonitrile	H2C=CH–CN	Orlon, Acrilan	Fibers, outdoor carpeting
Methyl methacrylate	H2C=C(COCH3)OCH3	Plexiglas, Lucite	Windows, contact lenses, fiber optics
Tetrafluoroethylene	F2C=CF2	Teflon	Nonstick coatings, bearings, replacement heart valves and blood vessels

Table of alkene polymers and their uses

Aromatic Compounds

Structure and Resonance

Aromatic compounds contain benzene-like rings with delocalized electrons. The actual structure is a resonance hybrid, with electrons shared equally among the six carbon atoms, resulting in bond order of 1.5 between each pair of carbons.

Benzene does not undergo typical addition reactions; it is unusually stable due to resonance.

Benzene does not react with H2, Br2, HCl, or H3O+

Naming Aromatic Compounds

Single substitutions on benzene are named as derivatives of benzene (e.g., chlorobenzene, nitrobenzene).
Multiple substituents are indicated by numbering or by using ortho-, meta-, para- prefixes for relative positions.
Some aromatic compounds have common names (e.g., toluene, phenol, aniline, benzoic acid, benzaldehyde, para-xylene).

Structure	Name	Structure	Name
Benzene ring with CH3	Toluene	Benzene ring with two CH3 (para)	para-Xylene
Benzene ring with OH	Phenol	Benzene ring with COOH	Benzoic acid
Benzene ring with NH2	Aniline	Benzene ring with CHO	Benzaldehyde

Table of common aromatic compounds and their names

Reactions of Aromatic Compounds

Aromatic compounds typically undergo substitution reactions rather than addition reactions, preserving the aromatic ring.

Nitration: Substitution of a nitro group (–NO2).
Halogenation: Substitution of a halogen atom.

Substitution reaction on benzene ring

Additional info: The resonance stabilization of benzene is a key reason for its unique reactivity, and the rules for naming aromatic compounds are essential for clear communication in organic chemistry.