Study Notes on Alkenes, Alkynes, and Aromatic Compounds

12.5 Alkenes and Alkynes

  • Ethyne:

    • Commonly known as acetylene.

    • Used in welding, reacts with oxygen to produce flames with temperatures above 3300 °C.

  • Learning Goal: Identify structural formulas as alkenes, cycloalkenes, and alkynes, and write their IUPAC names.

Functional Groups

  • Definition:

    • Functional groups are a characteristic feature of organic molecules that behave in a predictable way.

    • Composed of an atom or group of atoms.

    • Groups that replace a hydrogen atom in the corresponding alkane.

    • A way to classify families of organic compounds.

Alkenes and Alkynes

  • Definition:

    • Alkenes: Hydrocarbons that contain double bonds.

    • Alkynes: Hydrocarbons that contain triple bonds.

    • Both families are classified as unsaturated hydrocarbons because they do not contain the maximum number of hydrogen atoms.

    • Important note: Carbon atoms always form four covalent bonds.

Alkenes

  • Characteristics:

    • Contain a carbon–carbon double bond that forms when adjacent carbon atoms share two pairs of valence electrons.

    • Flat structure, as the atoms in a double bond lie in the same plane.

    • Trigonal planar arrangement around the double bonded carbon atoms.

Alkynes

  • Characteristics:

    • Contain a triple bond that forms when two carbon atoms share three pairs of valence electrons.

    • Linear geometry with a 180° bond angle.

Naming Alkanes, Alkenes, and Alkynes

  • IUPAC Naming:

    • Names for alkenes and alkynes are similar to those of alkanes.

    • Use the alkane name with the same number of carbon atoms, replacing the suffix „-ane“ with „-ene“ for alkenes or „-yne“ for alkynes.

    • Cyclic alkenes are named as cycloalkenes.

Guide to Naming Alkenes and Alkynes

  • Steps:

    1. Identify the longest carbon chain that contains the double or triple bond.

    2. Number the carbon chain, starting from the end nearer the double or triple bond.

    3. Identify and name each substituent (in alphabetical order) and add as a prefix to the alkene or alkyne name.

Cycloalkenes

  • Characteristics of Cycloalkenes:

    • Contain a double bond within a ring structure.

    • Named by assigning the double bond to be between carbon 1 and carbon 2 when a substituent is on the ring.

    • Do not need to include numbering for the double bond if there is a substituent present.

    • Example: For the compound 3-methylcyclopentene, it is understood the double bond is between carbon 1 and carbon 2.

Chemistry Link to Environment - Fragrant Alkenes

  • Context:

    • Odors of lemons, oranges, roses, and lavender result from volatile compounds synthesized by plants.

    • Many such compounds contain double or triple bonds or ring structures, making them unsaturated.

12.6 Cis–Trans Isomers

  • Definition:

    • Carbon–carbon double bonds do not rotate as carbon–carbon single bonds do.

    • Atoms or groups attached to the carbon atoms around the double bond can form two different structures termed cis-trans isomers.

  • Learning Goal: Draw the condensed and line-angle structural formulas and provide the names for the cis-trans isomers of alkenes.

Characteristics of Cis–Trans Isomers

  • In alkenes:

    • The double bond is rigid and does not rotate, keeping atoms in fixed positions.

    • Cis-trans isomerism (geometric isomerism) occurs when two different groups are attached to the carbon atoms on each side of the double bond.

    • Requires prefixes cis or trans to reflect arrangements of the groups across the double bond.

  • Distinct Physical Properties:

    • Cis and trans isomers differ in physical and chemical properties.

    • Cannot occur when both groups on one carbon are identical.

Examples of Cis-Trans Isomers

  • 1,2-Dichloroethane:

    • In the cis isomer, the chloro group is on the same side of the double bond.

    • In the trans isomer, the chloro group is on opposite sides of the double bond.

  • Hexene:

    • In the cis isomer, the alkyl groups are on the same side of the double bond, with H atoms on the opposite side.

    • In the trans isomer, alkyl groups and H atoms are on opposite sides of the double bond.

Naming Cis–Trans Isomers

  • The prefix cis or trans is placed before the alkene name for the respective structure.

    • Example:

      • cis-1,2-dibromoethene

      • trans-1,2-dibromoethene

Chemistry Link to Environment - Pheromones

  • Definition:

    • Insects emit pheromones to convey messages to members of the same species, e.g., warn of danger, mark trails, and attract mates.

  • Effect of Configuration:

    • The effectiveness of many pheromones relies on the cis or trans configuration of double bonds.

    • Example: Bombykol, a sex pheromone produced by female silkworm moths, contains one cis double bond and one trans double bond.

12.7 Addition Reactions for Alkenes

  • Definition:

    • Alkenes are very reactive and undergo addition reactions.

    • Atoms are added to the double bond, converting it from a double bond to a single bond.

  • Learning Goal: Draw the condensed or line-angle structural formulas and name organic products of addition reactions of alkenes.

Types of Addition Reactions

  • Hydrogenation:

    • Addition of H2 to the double bond.

  • Hydration:

    • Addition of H2O (H-OH) to form an alcohol.

  • Polymerization:

    • Involves the addition of alkenes at high temperatures and pressures.

Hydrogenation

  • Process:

    • In hydrogenation, hydrogen atoms add to carbon atoms of a double bond, usually with a catalyst such as Pt, Ni, or Pd to speed up the reaction.

  • Example:

    • Write an equation for the hydrogenation of 1-butene using a platinum catalyst:
      PtCH<em>2=CH−CH</em>2−CH<em>3+H</em>2→CH<em>3−CH</em>2−CH<em>2−CH</em>3\text{Pt} \text{CH}<em>2=\text{CH−CH}</em>2−\text{CH}<em>3 + \text{H}</em>2 → \text{CH}<em>3−\text{CH}</em>2−\text{CH}<em>2−\text{CH}</em>3

Hydrogenation of Unsaturated Fats

  • Vegetable oils:

    • Composed of fatty acids containing double bonds.

    • Hydrogenation converts double bonds into single bonds, producing compounds with higher melting points suitable for margarine, shortening, etc.

Hydration Reactions

  • Definition:

    • In hydration reactions, alkenes react with water (H-OH) to form alcohols.

    • Catalyzed by strong acids such as H2SO4.

Markovnikov’s Rule

  • Rule Definition:

    • When H2O adds to a double bond in an asymmetrical alkene, two products can form.

    • States that the H (from H-OH) attaches to the carbon with more hydrogens, and —OH adds to the more substituted carbon.

Addition of Alkenes: Polymerization

  • Definition:

    • A polymer consists of large molecules made of small, repeating units (monomers).

  • Industrial Relevance:

    • Synthetic polymers, e.g., plastics, are created using addition reactions of alkene monomers under high temperature, pressure, and sometimes catalysts.

Example of Alkenes and Their Polymers

  • Table 12.9: Indicates various alkenes and their corresponding synthetic polymers, uses, and recycling codes.

  • Significant Polymer Examples:

    • Polyethylene (PE): Made from ethene; common uses include plastic bottles, films, insulation.

    • Polyvinyl Chloride (PVC): Made from chloroethene; used in plastic pipes and tubing, medical containers.

    • Polystyrene (PS): Made from phenylethene; used for plastic coffee cups, cartons.

  • Environmental Impact:

    • Synthetic polymers are unreactive and non-biodegradable, contributing to pollution.

12.8 Aromatic Compounds

  • History:

    • In 1825, Michael Faraday isolated benzene, a hydrocarbon with a molecular formula of C6H6.

  • Learning Goal: Describe bonding in benzene, name aromatic compounds, and draw their structural formulas.

  • Benzene Structure:

    • Ring of six C atoms bonded to one H atom each, displayed with alternating double bonds or a singe circle to represent aromatic bonding.

Naming Aromatic Compounds

  • Phenyl Group:

    • When a benzene ring acts as a substituent, it is referred to as a phenyl group (—C6H5).

  • Benzene Derivatives:

    • Aromatic compounds containing a benzene with a single substituent are benzene derivatives and are not normally numbered. Some common names are accepted by IUPAC (e.g., toluene, aniline, phenol).

  • Substituent Arrangement:

    • For two substituents, the benzene ring is numbered based on the lowest number distribution:

    • 1,2 (Ortho - o)

    • 1,3 (Meta - m)

    • 1,4 (Para - p)

  • Example with Two Different Substituents:

    • Numbered to give the lowest numbers to the substituents in alphabetical order. Example: 1-bromo-4-chlorobenzene.

Chemistry Link to Health - Common Aromatic Compounds

  • The benzene ring can be found in pain relievers like aspirin, acetaminophen, ibuprofen, and flavorings like vanillin.

Properties of Aromatic Compounds

  • Structure:

    • The flat, symmetrical structure of benzene contributes to higher melting and boiling points of benzene derivatives.

  • Solubility:

    • Aromatic hydrocarbons are insoluble in water and are used as solvents for organic compounds.

    • Only aromatic compounds containing strongly polar functional groups (e.g., —OH, —COOH) exhibit some solubility in water.

  • Chemical Stability:

    • Aromatic compounds are resistant to reactions that disrupt the aromatic system, although they may still be flammable.

Visual Resources

  • Concept Map - Chapter 12:

    • A visual representation encompassing the discussed topics on alkenes, alkynes, functional groups, addition reactions, polymerization, and aromatic compounds.