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Organic Chemistry
Table of Contents
Unit 3 Overview: Alkanes: Structure and Stereochemistry
3.1 Functional Groups
3.2 Alkanes and Alkane Isomers
3.3 Alkyl Groups
3.4 Naming Alkanes
3.5 Properties of Alkanes
3.6 Conformations of Ethane
3.7 Conformations of Other Alkanes

Alkanes are the simplest hydrocarbons, consisting of only carbon and hydrogen atoms. They can be straight-chain or branched, with different arrangements leading to constitutional isomers. Understanding their structure is key to grasping more complex organic molecules.

Naming alkanes follows specific rules set by IUPAC. These guidelines help chemists communicate molecule structures clearly. Mastering alkane nomenclature builds a foundation for naming more complex organic compounds and understanding their properties.

Alkanes

Straight-chain vs branched-chain alkanes

  • Straight-chain alkanes have carbon atoms connected in a single continuous chain following the general formula $C_nH_{2n+2}$ (ethane C2H6, propane C3H8, butane C4H10)
  • Branched-chain alkanes have one or more carbon atoms attached to the main chain as branches called alkyl groups such as methyl (CH3-), ethyl (CH3CH2-), and propyl (CH3CH2CH2-) (isobutane 2-methylpropane, isopentane 2-methylbutane)
  • Alkanes exhibit tetrahedral geometry around each carbon atom

Constitutional isomers of alkanes

  • Constitutional isomers have the same molecular formula but different connectivity of atoms
  • Alkanes with 1-3 carbon atoms have no constitutional isomers
  • Butane (C4H10) has two constitutional isomers: n-butane (straight-chain) and isobutane (2-methylpropane, branched)
  • Pentane (C5H12) has three constitutional isomers: n-pentane (straight-chain), isopentane (2-methylbutane, branched), and neopentane (2,2-dimethylpropane, branched)
  • Hexane (C6H14) has five constitutional isomers: n-hexane (straight-chain), 2-methylpentane (branched), 3-methylpentane (branched), 2,2-dimethylbutane (branched), and 2,3-dimethylbutane (branched)

Structural characteristics of alkanes

  • Alkanes exhibit sp3 hybridization for all carbon atoms
  • The bond angles between carbon atoms in alkanes are approximately 109.5°
  • Alkanes can adopt different conformations through rotation around single bonds

Alkane Nomenclature

IUPAC nomenclature for alkanes

  • Identify the longest continuous chain of carbon atoms which determines the base name of the alkane (meth-, eth-, prop-, but-, pent-, hex-, etc.)
  • Count the number of carbon atoms in the longest chain and add the appropriate suffix (-ane) to the base name (methane CH4, ethane C2H6, propane C3H8, butane C4H10)
  • For branched alkanes, identify and name alkyl substituents:
    1. Determine the longest chain and use it as the base name
    2. Number the carbon atoms in the longest chain starting from the end closest to the first branch
    3. Name the alkyl groups and indicate their positions using the corresponding numbers (2-methylpentane, 3-ethyl-2-methylbutane)
  • If there are multiple identical alkyl groups, use prefixes (di-, tri-, tetra-, etc.) to indicate the number of groups (2,2-dimethylbutane)

Key Terms to Review (39)

Urethane: Urethane, also known as ethyl carbamate, is a compound formed from the reaction between an alcohol and an isocyanate group as part of the step-growth polymerization process. It is a key building block in the production of polyurethanes, which are versatile synthetic polymers used in a wide range of applications from foams to elastomers.
Methylene group: A methylene group is a functional group consisting of two hydrogen atoms bound to a carbon atom, which is then connected to other parts of a molecule. In the context of alkenes, it often refers to the CH2 unit that can be part of the alkene's structure.
Constitutional isomers: Constitutional isomers are compounds that have the same molecular formula but differ in the sequence in which their atoms are connected. These variations lead to molecules with distinct physical and chemical properties, despite having the same numbers of each type of atom.
Branched-chain alkanes: Branched-chain alkanes are hydrocarbons that contain a main carbon chain with one or more alkyl groups attached to it as branches. These compounds are a type of alkane that have the same molecular formula but differ in the structure of the carbon skeleton.
Straight-chain alkanes: Straight-chain alkanes are hydrocarbons that consist of carbon atoms connected in a continuous, linear chain without any branches or cycles. They follow the general formula CnH2n+2, where n represents the number of carbon atoms.
Aliphatic: Aliphatic compounds are organic molecules consisting of carbon and hydrogen arranged in straight chains, branched chains, or non-aromatic rings. They can be saturated (alkanes) or unsaturated (alkenes and alkynes).
Normal alkanes: Normal alkanes are saturated hydrocarbons with a straight-chain structure, containing only single bonds between carbon atoms. They follow the general formula CnH2n+2, where n represents the number of carbon atoms.
Conformations: Conformations refer to the three-dimensional arrangements that molecules can adopt due to the rotation around single bonds. This concept is essential in understanding the behavior and properties of organic compounds, including alkanes, cycloalkanes, and their substituted derivatives.
Sp3 Hybridization: sp3 hybridization refers to the formation of four equivalent hybrid orbitals in an atom, typically observed in carbon compounds. These hybridized orbitals are essential in understanding the structure and bonding patterns of various organic molecules, including alkanes, alkyl halides, and molecules containing nitrogen, oxygen, phosphorus, and sulfur.
Bond Angles: Bond angles refer to the geometric arrangement of atoms around a central atom in a molecule, determined by the number and type of bonds formed. This concept is crucial in understanding the structures and properties of various organic compounds.
Methane: Methane is the simplest and most abundant alkane, with a chemical formula of CH4. It is a colorless, odorless, and flammable gas that serves as the primary component of natural gas and plays a crucial role in organic chemistry and the study of alkanes.
Tetrahedral Geometry: Tetrahedral geometry refers to the three-dimensional spatial arrangement of atoms or groups of atoms in a molecule, where the central atom is bonded to four other atoms or groups in a symmetrical tetrahedral configuration. This geometric structure is a fundamental concept in understanding the structure and properties of various organic and inorganic compounds.
Ethane: Ethane is the simplest alkane, a saturated hydrocarbon with the chemical formula C2H6. It is a key term in understanding the concepts of sp3 hybrid orbitals, the structure of alkanes, and their properties.
IUPAC Nomenclature: IUPAC nomenclature is a standardized system for naming organic compounds, developed by the International Union of Pure and Applied Chemistry (IUPAC). It provides a consistent and unambiguous way to identify and communicate the structure of organic molecules.
Alkanes: Alkanes are a class of saturated hydrocarbons composed entirely of single-bonded carbon and hydrogen atoms. They are the simplest organic compounds and serve as the foundation for many other organic molecules and reactions.
3-methylpentane: 3-methylpentane is a branched-chain alkane with a methyl group (CH3) attached to the third carbon of a five-carbon chain. It is an isomer of the straight-chain alkane pentane, demonstrating the concept of alkane isomerism.
Pentane: Pentane is a straight-chain alkane with the molecular formula C$_{5}$H$_{12}$. It is a colorless, volatile, and flammable hydrocarbon that is widely used in various industrial and commercial applications.
Methyl: The methyl group is a simple alkyl group consisting of a single carbon atom bonded to three hydrogen atoms. It is denoted by the formula -CH3 and is the most basic and common alkyl group found in organic chemistry. The methyl group plays a crucial role in various organic reactions and structural features across several key topics in this course.
Propyl: Propyl is a three-carbon alkyl group with the chemical formula -CH2CH2CH3. It is an important functional group in organic chemistry, particularly in the context of alkanes, alkyl groups, and the naming of organic compounds.
2-methylbutane: 2-methylbutane is a branched-chain alkane with a methyl group (CH3) attached to the second carbon of the main carbon chain. It is an isomer of the straight-chain alkane pentane, with the same molecular formula of C5H12.
Alkyl Groups: Alkyl groups are hydrocarbon substituents derived from alkanes by the removal of one hydrogen atom. They are non-polar, saturated, and can be straight-chain or branched. Alkyl groups play a crucial role in understanding the properties and behavior of various organic compounds, including alkanes, alkenes, and benzene derivatives.
Isopentane: Isopentane, also known as 2-methylbutane, is a branched-chain alkane with the molecular formula C5H12. It is an isomer of the straight-chain alkane pentane, and its unique structure and properties make it an important compound in the context of alkanes and their isomers, as well as the general properties of alkanes.
2,3-dimethylbutane: 2,3-dimethylbutane is an alkane with the chemical formula C6H14. It is a branched-chain isomer of the straight-chain alkane hexane, featuring two methyl (CH3) groups attached to the second and third carbon atoms of the four-carbon backbone.
2,2-dimethylbutane: 2,2-dimethylbutane is a branched-chain alkane with the molecular formula C$_{6}$H$_{14}$. It is an isomer of the straight-chain alkane hexane, and is characterized by the presence of two methyl (CH$_{3}$) groups attached to the second carbon atom of the main carbon chain.
2,2-dimethylpropane: 2,2-dimethylpropane, also known as neopentane, is a branched-chain alkane with the molecular formula C5H12. It is a structural isomer of pentane and is classified as a branched alkane under the topic of Alkanes and Alkane Isomers.
N-butane: n-Butane is a straight-chain alkane with the chemical formula C4H10. It is a colorless, odorless gas that is the second simplest alkane after methane. n-Butane is a key component in the study of alkanes and alkane isomers.
N-hexane: n-Hexane is a straight-chain alkane with the chemical formula C$_{6}$H$_{14}$. It is a colorless, flammable liquid that is commonly used as a solvent and as a fuel component.
Butane: Butane is a simple alkane hydrocarbon with the chemical formula C4H10. It is a colorless, odorless gas that is commonly used as a fuel source and in the production of other chemicals. Butane is an important compound in the context of organic chemistry, particularly in the study of alkanes and alkane isomers, as well as the application of 13C NMR spectroscopy.
Ethyl: Ethyl is a two-carbon alkyl group with the chemical formula -CH2CH3. It is a common substituent group in organic chemistry and plays a crucial role in understanding various topics, including alkanes, alkyl groups, naming conventions, and carbocation rearrangements.
Constitutional Isomers: Constitutional isomers are a type of structural isomerism where molecules have the same molecular formula but differ in the connectivity or arrangement of their atoms. This concept is essential in understanding the properties and behavior of organic compounds across various topics in chemistry.
N-pentane: n-pentane is a straight-chain alkane hydrocarbon with the chemical formula C5H12. It is a colorless, flammable liquid that is widely used as a solvent and in the production of gasoline.
2-methylpentane: 2-methylpentane is a branched-chain alkane with a methyl group (CH3) attached to the second carbon of the pentane (C5H12) backbone. It is an isomer of n-hexane and is classified as an alkane within the broader category of hydrocarbons.
Neopentane: Neopentane, also known as 2,2-dimethylpropane, is a branched-chain alkane with the molecular formula C$_{5}$H$_{12}$. It is a highly symmetrical and stable hydrocarbon that is an important concept in the study of alkanes and alkane isomers.
Isobutane: Isobutane is a branched-chain alkane with the chemical formula C4H10. It is an isomer of the straight-chain alkane, n-butane, and is a key component in the production of various fuels and chemical feedstocks.
2-methylpropane: 2-methylpropane, also known as isobutane, is a branched-chain alkane with the molecular formula C₄H₁₀. It is an isomer of n-butane, where the methyl group (-CH₃) is attached to the second carbon atom of the main chain, creating a branched structure.
Straight-chain Alkanes: Straight-chain alkanes are a class of saturated hydrocarbons with a linear, unbranched structure. They are the simplest and most basic form of alkanes, consisting solely of carbon and hydrogen atoms arranged in a continuous, uninterrupted chain.
Branched-Chain Alkanes: Branched-chain alkanes are a class of saturated hydrocarbons where the carbon backbone contains one or more side chains, or branches, attached to the main carbon chain. These structural variations create unique physical and chemical properties compared to their linear counterparts.
Hexane: Hexane is a straight-chain alkane with the molecular formula C6H14. It is a colorless, volatile, and flammable liquid that is widely used as a solvent and in the production of various chemicals and fuels.
Propane: Propane is a simple alkane hydrocarbon with the chemical formula C3H8. It is a colorless, odorless, and flammable gas that is widely used as a fuel source for heating, cooking, and transportation. Propane's properties and behavior are important in understanding the topics of alkanes, alkane isomers, and the interpretation of 1H NMR spectra.