8.5 Alkanes, Alkenes, Alkynes, and Aromatic Hydrocarbons: A Comprehensive Guide to Organic Chemistry Substituents and Nomenclature

Introduction to Organic Chemistry and Carbon Properties

• Organic Chemistry Definition: This is the scientific study of compounds that contain carbon-to-hydrogen bonds ($C-H$). This distinguishes it from most of General Chemistry (Inorganic), which focuses on compounds that do not necessarily contain these specific bonds.
• Inorganic Chemistry Definition: The study of chemical compounds that do not necessarily contain carbon-to-hydrogen bonds.
• Unique Properties of Carbon ($C$):
  • Valence Electrons: Carbon has four valence electrons in its outer shell. This allows it to readily form four strong covalent bonds.
  • Atomic Structure: A Carbon-12 atom consists of six protons in the nucleus and a total of six electrons grouped around the nucleus. Two electrons reside in the inner shell, while four electrons reside in the outer (valence) shell.
  • Versatility in Bonding: Carbon forms four covalent bonds not only with other carbon atoms but also with a diverse range of other elements.
  • Structural Diversity: Carbon is unique in its ability to form:
    • Chains: Long, continuous, repeating carbon-to-carbon bonds.
    • Rings: Cyclic structures known as cycloalkanes.
    • Branches: Carbon-to-carbon chains that branch off from a longer main chain.

Hydrocarbons: Classification and General Properties

• Definition: Hydrocarbons are the simplest organic compounds, consisting entirely of carbon ($C$) and hydrogen ($H$).
• Properties: They consist of repeating carbon-to-hydrogen bonds which are highly combustible in the presence of oxygen ($O_2$).
• Common Examples: Propane and butane are frequently studied as smaller hydrocarbons used for fuel.
• Classes of Hydrocarbons:
  • Alkanes: Contain only single bonds.
  • Alkenes: Contain one or more double bonds.
  • Alkynes: Contain one or more triple bonds.
  • Cyclic Hydrocarbons: Include cycloalkanes.
  • Aromatic Hydrocarbons: Derived from benzene structures.

Alkanes: Saturated Hydrocarbons

• Saturated Nature: Alkanes are considered saturated because they contain the maximum number of hydrogen atoms bonded to each carbon atom possible (every bond not involved in a $C-C$ connection is bonded to an $H$).
• Bonding Patterns:
  • Terminal Carbons: The carbons situated at the ends of a chain are bonded to three hydrogens ($CH_3$).
  • Internal Carbons: The carbons in the middle of a chain are bonded to two hydrogens ($CH_2$).
• General Formula for Alkanes: $C_n H_{2n+2}$, where $n$ represents the number of carbon atoms.
  • Example (Pentane): A structure with five carbons has $(2 \times 5) + 2 = 12$ hydrogens ($C_5 H_{12}$).
• Geometry: While drawings often show right angles ($90^\circ$), the actual three-dimensional bond angles are closer to $109.5^\circ$. This results in a "zigzag" geometry where carbons and hydrogens sit at specific angles above and below a central axis.

Structural Representations of Hydrocarbons

• Expanded Formula: A detailed drawing showing every individual bond between carbons and other carbons, as well as between carbons and hydrogens.
• Condensed Formula: A simplified version where the bonds to hydrogens are removed, and the connection of carbons to hydrogens is grouped together (e.g., $CH_3-CH_2-CH_3$), showing primarily carbon-to-carbon bonds.
• Skeletal Structure (Line-Angle Formula): A simplified drawing using only lines and angles to represent the carbon backbone. In this format:
  • The symbols for Carbon ($C$) and Hydrogen ($H$) are removed.
  • Each vertex or end of a line represents a carbon atom.
  • The number of hydrogen atoms is implied; we assume each carbon has enough hydrogens to satisfy its four-bond requirement.

Functional Groups and Substituents

• Functional Group: An atom or group of atoms attached to the main structure of a compound that determines its chemical properties.
• Alkyl Group: A hydrocarbon that is itself the functional group. It is an alkane missing one hydrogen, named with the suffix -yl.
  • Example: A methane ($CH_4$) group branching off a propane chain is called a "methyl" group.
• Halogens: Electronegative atoms like Chlorine ($Cl$) or Bromine ($Br$) that replace a hydrogen on the chain. In IUPAC naming, the "-ide" ending is replaced with "-o" (e.g., Chloride becomes Chloro-, Bromide becomes Bromo-).
• Substitution Reaction: The process by which a functional group is added to an alkane. One hydrogen is replaced by an electronegative atom.
  • Example: Reacting ethane ($C_2 H_6$) with chlorine ($Cl_2$) in the presence of heat or light produces chloroethane and hydrogen chloride ($HCl$).

IUPAC Nomenclature for Alkanes

• Rule 1: Identify the Parent Chain: Find the largest continuous chain of carbon atoms. The prefix determines the base name:
  • $1$ Carbon: Meth-
  • $2$ Carbons: Eth-
  • $3$ Carbons: Prop-
  • $4$ Carbons: But-
  • $5$ Carbons: Pent-
  • $6$ Carbons: Hex-
  • $7$ Carbons: Hept-
  • $8$ Carbons: Oct-
  • $9$ Carbons: Non-
  • $10$ Carbons: Dec-
• Rule 2: Numbering: Number the parent chain starting from the end closest to a substituent (branch or functional group) to ensure the substituent receives the lowest possible number.
• Rule 3: Multiple Substituents:
  • List substituents in alphabetical order (e.g., Bromo before Chloro).
  • Use numeric prefixes for identical groups: Di- ($2$), Tri- ($3$), Tetra- ($4$).
  • If two identical groups are on the same carbon, repeat the number (e.g., $2,2$-dibromo).

Guided Practice: Naming Examples

• Example A: A straight chain of four carbons with no substituents is named Butane.
• Example B: A four-carbon chain with a Chlorine on Carbon $1$ and a Bromine on Carbon $2$ (numbered right-to-left) is named $2$-Bromo-$1$-chlorobutane.
• Example C: A five-carbon chain (Pentane) with an Iodine on Carbon $2$ and two Bromines on Carbon $3$. Alphabetically, Bromine comes before Iodine. Name: $3,3$-Dibromo-$2$-iodopentane.
• Example D (Alkyl Branch): A six-carbon chain (Hexane) with a two-carbon branch (Ethyl) at Carbon $3$. Name: $3$-Ethylhexane.

Alkenes: Unsaturated Hydrocarbons

• Definition: Hydrocarbons containing one or more double bonds ($C=C$).
• Unsaturated Nature: They have fewer hydrogen atoms than the maximum possible due to the double bond. This double bond serves as a functional group and affects molecular geometry.
• Reactivity: Alkenes are more reactive than alkanes.
• Addition Reaction: Functional groups are added by breaking the double bond. Unlike substitution, nothing is removed; new atoms are added to the structure.
  • Example: Ethene ($C_2 H_4$) + Chlorine ($Cl_2$) $\rightarrow$ $1,2$-dichloroethane.
• Naming Alkenes:
  • Use the suffix -ene.
  • The location of the double bond is indicated by the lowest numbered carbon atom in the bond.
  • Practice Case 1: Five carbons with a double bond at the second carbon is $2$-pentene.
  • Practice Case 2: Six carbons with a double bond at the third carbon is $3$-hexene.
  • Practice Case 3: Three carbons with a double bond at Carbon $1$ and a Chlorine at Carbon $2$ is $2$-chloro-$1$-propene.

Alkynes: Triple-Bonded Hydrocarbons

• Definition: Hydrocarbons containing one or more triple bonds ($C \equiv C$).
• Structure: Possess a rigid bond structure that does not allow for rotation around the bond axis.
• Example: Ethyne ($C_2 H_2$), also known as Acetylene, which is used as a combustible component in torch fuel.
• Naming Alkynes:
  • Use the suffix -yne.
  • Example: A four-carbon chain with a triple bond on the second carbon is $2$-butyne.
  • Complex Case (Mixed bonds): A five-carbon chain with a triple bond starting at Carbon $1$ and a double bond starting at Carbon $3$ is named Pent-$3$-ene-$1$-yne.

Cyclic and Aromatic Hydrocarbons

• Cycloalkanes: Alkanes that form a ring structure.
  • Name by adding the prefix cyclo- to the base name (e.g., Hexane becomes Cyclohexane).
  • If substituents are present, the carbon attached to the substituent is designated as Carbon $1$.
  • Example: A five-carbon ring with an Ethyl group is Ethylcyclopentane.
  • Example: A six-carbon ring with methyl groups at carbons $1$ and $2$ is $1,2$-dimethylcyclohexane.
• Aromatic Hydrocarbons: Hydrocarbons containing a Benzene ring.
  • Benzene: A cyclic hexene group ($C_6 H_6$) with three equally distributed double bonds.
  • Resonance: The double bonds are not fixed in one position but are distributed, giving all carbon-to-carbon bonds a character between a single and double bond. This is often represented as a hexagon with a circle inside.
  • Benzene Derivatives:
    • Toluene: Benzene ring with a methyl ($CH_3$) functional group.
    • Xylene: Benzene ring with two methyl groups on separate carbons.
    • Styrene: Benzene ring with an ethene (vinyl) functional group, used in plastic and rubber production.