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Organic Chemistry
Table of Contents
Unit 7 Overview: Alkenes
7.1 Industrial Preparation and Use of Alkenes
7.2 Calculating the Degree of Unsaturation
7.3 Naming Alkenes
7.4 Cis–Trans Isomerism in Alkenes
7.5 Alkene Stereochemistry and the E,Z Designation
7.6 Stability of Alkenes
7.7 Electrophilic Addition Reactions of Alkenes
7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule
7.9 Carbocation Structure and Stability
7.10 The Hammond Postulate
7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

Alkenes are hydrocarbons with at least one carbon-carbon double bond. Naming them involves identifying the longest carbon chain with the double bond, numbering it from the end closest to the bond, and using the -ene suffix.

Substituents are named and numbered based on their position. The double bond's location is specified before the -ene suffix. Cis-trans isomerism and E-Z nomenclature are used to describe the arrangement of groups around the double bond.

Alkene Nomenclature

Naming alkenes with -ene suffix

  • Alkenes are unsaturated hydrocarbons contain at least one carbon-carbon double bond ($\ce{C=C}$)
  • Determine the parent chain by identifying the longest continuous chain of carbon atoms that includes the double bond
    • Establish the root name based on the number of carbons in the longest chain
      • 2 carbons (ethene), 3 carbons (propene), 4 carbons (butene), 5 carbons (pentene), 6 carbons (hexene)
    • Replace the -ane suffix of the corresponding alkane with -ene to denote the presence of a double bond
  • Follow IUPAC rules for systematic naming of alkenes

Numbering alkene carbon chains

  • Assign numbers to the carbon atoms in the longest chain
    • Begin numbering from the end of the chain that gives the lowest number for the double bond position
  • Specify the position of the double bond by its lower-numbered carbon
    • Include the double bond position in the name before the -ene suffix
      • 1-butene $\ce{CH2=CHCH2CH3}$ has the double bond between C1 and C2
      • 2-pentene $\ce{CH3CH=CHCH2CH3}$ has the double bond between C2 and C3
  • Use structural formula to clearly show the arrangement of atoms and bonds in alkenes

Alkene nomenclature conventions

  • Name any substituents (branches) attached to the parent chain
    • Use prefixes like methyl-, ethyl-, propyl- for alkyl groups
    • Indicate the number of the carbon to which each substituent is attached
      • Number the carbons based on the previously determined double bond position
  • Construct the complete alkene name
    • Arrange the substituents in alphabetical order before the parent chain name
    • Specify the double bond position before the -ene suffix
      • 4-methyl-2-pentene $\ce{CH3CH=CHCH(CH3)CH3}$ has a methyl group on C4 and double bond at C2-C3
      • 3-ethyl-1-hexene $\ce{CH2=CHCH(CH2CH3)CH2CH2CH3}$ has an ethyl group on C3 and double bond at C1-C2

Alkene Isomerism and Advanced Nomenclature

  • Cis-trans isomerism occurs in alkenes due to restricted rotation around the double bond
  • E-Z nomenclature is used to unambiguously describe the configuration of substituents around the double bond
  • Molecular formula represents the types and numbers of atoms in an alkene molecule

Key Terms to Review (32)

Vinyl group: A vinyl group is an alkene with the formula CH₂=CH−, where two carbon atoms are double-bonded to each other, and one of these carbons is bonded to another group or element. It acts as a fundamental unit in organic chemistry for forming more complex molecules through reactions at its double bond.
Gibbs free-energy change (ΔG): Gibbs free-energy change (ΔG) is a thermodynamic property indicating the amount of available energy to perform work during a reaction at constant temperature and pressure. It predicts the direction and spontaneity of chemical reactions, with negative values indicating spontaneous processes.
Hydrocarbon: Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon atoms. They form the framework for alkanes and vary in size, structure, and properties based on the arrangement of their atoms.
Allyl group: An allyl group is a functional group with the structure H2C=CH-CH2-. It is derived from propylene by removing a hydrogen atom from the methylene group.
Carbon-Carbon Double Bond: A carbon-carbon double bond is a covalent chemical bond in which two carbon atoms share four electrons, with two pairs of electrons forming the bond. This type of bond is a fundamental structural feature in many organic compounds and is central to understanding the properties and reactivity of alkenes, a class of unsaturated hydrocarbons.
Molecular Formula: The molecular formula is a concise representation of the composition of a chemical compound, showing the types and number of atoms present in the molecule. It is a fundamental concept in organic chemistry that is closely related to the structure and properties of compounds.
-ene: The suffix '-ene' is used in organic chemistry to indicate the presence of a carbon-carbon double bond within a molecule. It is a functional group that is a key characteristic of alkenes, a class of unsaturated hydrocarbons.
Parent Chain: The parent chain is the longest continuous carbon chain in a molecule, which serves as the basis for naming organic compounds. This key term is central to the systematic nomenclature used across various classes of organic compounds, including alkanes, alkenes, aromatic compounds, alcohols, phenols, and carboxylic acid derivatives.
Substituents: Substituents are atoms or functional groups that replace hydrogen atoms in a molecule's structure. They are an essential concept in organic chemistry, as they play a crucial role in determining the properties, reactivity, and naming of various organic compounds.
Hydrocarbon: A hydrocarbon is a chemical compound consisting entirely of hydrogen and carbon atoms. These molecules form the basis for organic chemistry and are the simplest organic compounds, serving as the building blocks for more complex organic molecules.
Structural Formula: The structural formula, also known as the line-angle formula or skeletal formula, is a graphical representation of the arrangement of atoms and bonds within a molecule. It provides a clear and concise way to depict the connectivity and spatial arrangement of the atoms, allowing for the identification and understanding of the chemical structure of a compound.
Alphabetical Order: Alphabetical order refers to the arrangement of items, such as names, words, or chemical compounds, in a sequence based on the letters of the alphabet. This organizational method is widely used to provide a standardized and easily navigable system for presenting and accessing information.
Cis-Trans Isomerism: Cis-trans isomerism is a type of stereoisomerism that occurs when two identical substituents are either on the same side (cis) or on opposite sides (trans) of a carbon-carbon double bond or a ring structure. This concept is crucial in understanding the properties and behaviors of cycloalkanes and alkenes.
Lowest Number: The lowest number is the smallest integer value that satisfies a particular condition or requirement, often used in the context of naming organic compounds such as alkenes.
Hex-: The prefix 'hex-' is a numerical prefix used in organic chemistry to indicate the presence of six carbon atoms in a compound. It is commonly used in the context of naming alkenes, which are unsaturated hydrocarbons with a carbon-carbon double bond.
Double Bond Position: The double bond position refers to the location of the carbon-carbon double bond within an alkene molecule. This is an important consideration in the systematic naming of alkenes according to the IUPAC (International Union of Pure and Applied Chemistry) nomenclature rules.
Unsaturated: Unsaturated refers to a molecule that contains one or more carbon-carbon double or triple bonds, resulting in fewer hydrogen atoms than the fully saturated counterpart. This feature has important implications in the context of organic chemistry and biochemistry.
Ethene: Ethene, also known as ethylene, is a simple unsaturated hydrocarbon with the chemical formula C₂H₄. It is the simplest alkene and plays a crucial role in various topics within organic chemistry, including calculating the degree of unsaturation, naming alkenes, understanding cis-trans isomerism, and evaluating the stability of alkenes.
But-: The prefix 'but-' is used in the systematic naming of alkenes, which are unsaturated hydrocarbons containing at least one carbon-carbon double bond. This prefix indicates the position of the double bond within the carbon chain of the alkene molecule.
Prop-: The prefix 'prop-' is used in organic chemistry to indicate the presence of a propyl group, which is a three-carbon alkyl group with the formula -CH2CH2CH3. This prefix is particularly relevant in the context of naming alkenes, as it helps to specify the position and orientation of the propyl substituent on the alkene backbone.
Pent-: The prefix 'pent-' refers to the presence of five carbon atoms in a compound. It is commonly used in the context of naming alkenes, which are unsaturated hydrocarbons with a carbon-carbon double bond.
Eth-: The prefix 'eth-' is a common organic chemistry term used to denote the presence of an ethyl group (CH3CH2-) in a compound. It is particularly relevant in the context of naming alkenes, as the 'eth-' prefix is used to indicate the position of the ethyl substituent on the alkene carbon chain.
Locants: Locants are the numbers used to indicate the position of substituents or functional groups on the parent chain or ring in organic chemistry. They help provide a precise and unambiguous way to name and identify the structure of organic compounds.
Methyl-: The methyl group is a fundamental organic functional group consisting of a single carbon atom bonded to three hydrogen atoms. It is represented by the formula -CH3 and is a key component in the naming and structure of many organic compounds, particularly in the context of alkene nomenclature.
Ethyl-: The ethyl group, represented by the formula -CH2CH3, is a two-carbon alkyl substituent that is commonly encountered in organic chemistry. This term is particularly relevant in the context of naming alkenes, as it is one of the key substituents that must be considered when applying the IUPAC naming system.
Hexene: Hexene is a type of alkene, a class of organic compounds characterized by the presence of a carbon-carbon double bond. As the name suggests, hexene has a six-carbon chain with a double bond between two of the carbon atoms.
Pentene: Pentene is an unsaturated hydrocarbon with the molecular formula C5H10. It is an alkene, a class of organic compounds characterized by the presence of a carbon-carbon double bond. Pentene is an important intermediate in the petrochemical industry and has various applications in organic synthesis.
E-Z Nomenclature: E-Z nomenclature is a system used to describe the stereochemistry of alkenes, where E (from the German 'entgegen') indicates that the two substituents with the higher priority are on opposite sides of the double bond, while Z (from the German 'zusammen') indicates that the two substituents with the higher priority are on the same side of the double bond.
IUPAC Rules: IUPAC rules are a standardized set of guidelines established by the International Union of Pure and Applied Chemistry (IUPAC) for the systematic naming and identification of organic compounds. These rules provide a consistent and unambiguous way to name molecules, which is essential for clear communication in the field of organic chemistry.
Propyl-: The propyl group is a three-carbon alkyl substituent with the chemical formula -CH2CH2CH3. It is a common functional group in organic chemistry and is often used in the naming and identification of organic compounds.
Propene: Propene, also known as propylene, is a simple unsaturated hydrocarbon with the molecular formula C3H6. It is the second member of the alkene family and is an important industrial chemical used in the production of various petrochemicals and plastics.
Butene: Butene, also known as 1-butene or butylene, is an unsaturated hydrocarbon with the molecular formula C4H8. It is a simple alkene, a class of organic compounds characterized by the presence of a carbon-carbon double bond.