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Alkanes

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  1. Alkanes - Introduction
  2. Alkane definition, general formula and examples.
  3. Reactions and properties of alkanes.
  4. Unbranched alkanes.
  5. Branched alkanes.
  6. Cyclic alkanes.
  7. Isomerism in alkanes.
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Examples
Lessons
  1. Recall the difference between alkanes and other hydrocarbons (alkenes and alkynes).
    Look at the following molecular formulae and identify which formulae show alkanes.
    i) C2_2H4_4
    ii) C4_4H10_{10}
    iii) C3_3H6_6
    iv) C10_{10}H22_{22}
    v) CH4_4
    vi) C2_2H2_2
    1. Apply the rules of organic nomenclature to branched and unbranched alkanes.
      From the descriptions, suggest the IUPAC systematic name of the following compounds:
      1. An alkane with 6 carbons in the main chain, and a 1 carbon branch at the 3rd carbon.
      2. An alkane with 4 carbons in the main chain, and a 1 carbon branch at the 2nd carbon.
      3. An alkane with 8 carbons in the main chain, and a 2 carbon branch at the 3rd carbon.
      4. An alkane with 5 carbons in the main chain with the two end carbons bonded together.
    2. Draw the structural formula of simple branched, unbranched and cyclic alkanes.
      Use the IUPAC systematic names of the compounds below to draw their structural formula.
      1. 2-methylhexane.
      2. Heptane.
      3. Methylpropane.
      4. 3-ethyloctane.
      5. Methylcyclohexane.
      6. 1-ethyl-2-methylcyclohexane.
    Topic Notes
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    Introduction to Alkanes

    Alkanes are fundamental hydrocarbons in organic chemistry, playing a crucial role in various industries and everyday life. This introduction video provides an essential overview of these important compounds. Alkanes are the simplest class of organic molecules, consisting solely of carbon and hydrogen atoms bonded together with single covalent bonds. Their general formula is CnH2n+2, where n represents the number of carbon atoms. We'll explore different types of alkanes, including straight-chain, branched, and cyclic structures. Understanding alkanes is vital for grasping more complex organic chemistry concepts. This lesson covers key topics such as the definition of alkanes, their general formula CnH2n+2, various types, and the IUPAC naming conventions. By mastering alkanes, students lay a solid foundation for further studies in organic chemistry and gain insights into the molecular world that shapes our environment and technology.

    Definition and General Formula of Alkanes

    Alkanes are a fundamental class of organic compounds known as saturated hydrocarbons. These molecules consist entirely of carbon and hydrogen atoms, with all carbon-carbon bonds being single covalent bonds. The term "saturated" refers to the fact that each carbon atom in an alkane is bonded to the maximum number of hydrogen atoms possible, leaving no room for additional bonds.

    The general formula for alkanes is CnH2n+2, where n represents the number of carbon atoms in the molecule. This formula is significant because it allows us to predict the number of hydrogen atoms in any alkane molecule based on the number of carbon atoms. For example, if an alkane has 5 carbon atoms, it will have 12 hydrogen atoms (5 × 2 + 2 = 12).

    Simple alkanes include methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12), and hexane (C6H14). These compounds form the beginning of what is known as a homologous series, which is a family of compounds with similar chemical properties and a general formula that differs by a constant unit.

    The structural formulas of these simple alkanes are as follows:

    • Methane: CH4
    • Ethane: CH3-CH3
    • Propane: CH3-CH2-CH3
    • Butane: CH3-CH2-CH2-CH3
    • Pentane: CH3-CH2-CH2-CH2-CH3
    • Hexane: CH3-CH2-CH2-CH2-CH2-CH3

    As we progress through this homologous series, each subsequent member differs from the previous one by a -CH2- group. This consistent difference in structure leads to predictable trends in physical properties of alkanes such as boiling point, melting point, and density.

    The concept of a homologous series is crucial in organic chemistry foundation as it allows chemists to predict the properties and reactions of compounds based on their structural similarities. In the case of alkanes, each member of the series shares similar chemical properties due to their saturated nature, but their physical properties change gradually as the carbon chain length increases.

    Understanding the general formula and structural characteristics of alkanes is essential for grasping more complex organic chemistry foundation concepts. Alkanes serve as the simplest examples of hydrocarbons and form the backbone for more complex organic molecules. Their study provides a foundation for understanding the behavior of carbon-based compounds, which are fundamental to life and numerous industrial applications.

    In summary, alkanes are saturated hydrocarbons with the general formula CnH2n+2, forming a homologous series where each member differs by a -CH2- group. Their simple structure and predictable properties make them an ideal starting point for the study of organic chemistry and the vast world of carbon-based compounds.

    Properties and Reactions of Alkanes

    Alkanes are a fundamental class of organic compounds characterized by their unique properties and behavior. These hydrocarbons, consisting solely of carbon and hydrogen atoms connected by single bonds, exhibit several distinctive characteristics that set them apart from other organic molecules. Among the most notable properties of alkanes are their unreactivity, insolubility in water, and hydrophobicity.

    The unreactive nature of alkanes is a result of their strong, stable carbon-carbon and carbon-hydrogen single bonds. This stability makes alkanes relatively inert under normal conditions, earning them the nickname "paraffins," which means "little affinity" in Latin. Their lack of reactivity is both a blessing and a challenge in various applications, as it makes them resistant to many chemical reactions but also difficult to manipulate in certain industrial processes.

    Another key property of alkanes is their insolubility in water. This characteristic is closely related to the concept of hydrophobicity, which literally means "fear of water." Alkanes are hydrophobic due to their non-polar nature. The electrons in the C-C and C-H bonds are shared equally, resulting in no significant charge separation. Consequently, alkane molecules cannot form hydrogen bonds with water molecules, leading to their insolubility in aqueous solutions. This property is crucial in many biological and industrial processes, influencing everything from cell membrane formation to oil spill behavior.

    Despite their general unreactivity, alkanes do participate in one significant type of reaction: combustion. The combustion of alkanes is an exothermic process that releases a substantial amount of energy, making these compounds valuable as fuels. There are two main types of combustion reactions for alkanes: complete combustion and incomplete combustion.

    Complete combustion occurs when an alkane reacts with an excess of oxygen, producing only carbon dioxide and water as products. This reaction releases the maximum amount of energy possible from the alkane. Let's use pentane (CH) as an example to illustrate complete combustion:

    CH + 8O 5CO + 6HO

    In this balanced equation, one molecule of pentane reacts with eight molecules of oxygen to produce five molecules of carbon dioxide and six molecules of water. The reaction is highly exothermic, releasing heat and light, which is why alkanes are excellent fuels for heating and combustion engines.

    Incomplete combustion, on the other hand, occurs when there is an insufficient supply of oxygen for the reaction. This results in the formation of carbon monoxide (CO) instead of carbon dioxide, and sometimes even elemental carbon (soot). Incomplete combustion is less efficient and can be dangerous due to the production of toxic carbon monoxide. Using pentane again as an example, one possible incomplete combustion reaction could be:

    2CH + 11O 10CO + 12HO

    In this case, two molecules of pentane react with eleven molecules of oxygen, producing ten molecules of carbon monoxide and twelve molecules of water. The exact products can vary depending on the oxygen availability, potentially including a mix of CO, CO, and carbon.

    Understanding these properties and reactions of alkanes is crucial in various fields, from environmental science to energy production. Their hydrophobicity influences their behavior in ecosystems, while their combustion reactions are fundamental to our energy infrastructure. As we continue to grapple with energy needs and environmental concerns, a deep understanding of alkane chemistry remains essential for developing cleaner, more efficient fuel technologies and addressing issues like oil spills and air pollution.

    Naming Straight-Chain and Branched Alkanes

    The IUPAC (International Union of Pure and Applied Chemistry) nomenclature system provides a standardized method for naming organic compounds, including alkanes. This system ensures that each compound has a unique name that accurately reflects its structure. Understanding IUPAC nomenclature is crucial for chemists and students alike, as it allows for clear communication about chemical structures.

    Let's begin with straight-chain alkanes, which are the simplest form of alkanes. These compounds consist of carbon atoms bonded in a linear chain with hydrogen atoms attached. The naming of straight-chain alkanes follows a simple pattern based on the number of carbon atoms in the chain. The prefix indicates the number of carbons (e.g., meth- for one, eth- for two, prop- for three), and the suffix "-ane" denotes that it's an alkane. For example, CH is methane, CHCH is ethane, and CHCHCH is propane.

    As we move to longer chains, we continue with butane (4 carbons), pentane (5 carbons), hexane (6 carbons), and so on. This systematic approach allows for the naming of alkanes with any number of carbon atoms in a straight chain.

    Branched alkanes are more complex and require additional rules for proper naming. The first step in naming a branched alkane is to identify the longest continuous carbon chain, which becomes the parent chain. This parent chain determines the base name of the alkane. For example, if the longest chain has six carbon atoms, the base name would be hexane.

    Next, we need to identify and name the substituents, which are the branches attached to the main chain. Common substituents include methyl (CH-), ethyl (CHCH-), and propyl (CHCHCH-) groups. The position of these substituents on the parent chain is crucial for naming.

    To indicate the position of substituents, we number the carbon atoms in the parent chain. The numbering should start from the end that gives the substituents the lowest possible numbers. This is a key rule in IUPAC nomenclature and ensures consistency in naming.

    When naming the compound, we list the substituents in alphabetical order, preceded by their position numbers. If there are multiple identical substituents, we use prefixes like di-, tri-, tetra-, etc., to indicate the number of occurrences. The parent chain name comes at the end.

    Let's look at some examples to illustrate these rules. Consider the compound CHCH(CH)CHCH. The longest chain has four carbons, so the parent name is butane. There's a methyl group attached to the second carbon, so the full name is 2-methylbutane.

    For a more complex example, take (CH)CHCH(CH)CHCH. The longest chain has five carbons (pentane), with methyl groups at the second and third positions. Following the alphabetical rule, we name it 3,3-dimethylpentane.

    An even more intricate example is CHCH(CHCH)CH(CH)CHCHCH. Here, the longest chain has six carbons (hexane). There's an ethyl group at the second carbon and a methyl group at the third carbon. The correct IUPAC name for this compound is 3-ethyl-2-methylhexane.

    It's important to note that when there are multiple substituents, the one that appears first alphabetically gets the lower number if there's a choice. For instance, in (CHCH)(CH)CHCHCH, we would name it 3-ethyl-2-methylpentane, not 2-ethyl-3-methylpentane.

    Mastering IUPAC nomenclature for alkanes provides a solid foundation for naming more complex organic compounds. This system's logical approach allows chemists to communicate precise structural information efficiently. As you practice naming various alkanes, you'll fin

    Cyclic Alkanes and Their Naming

    Cyclic alkanes, also known as cycloalkanes, are a unique class of hydrocarbons characterized by their ring structure. Unlike their linear counterparts, these compounds form closed loops of carbon atoms, creating a distinctive cyclic arrangement. The general formula for cycloalkanes is CnH2n, where n represents the number of carbon atoms in the ring. This formula reflects the fact that cycloalkanes have two fewer hydrogen atoms than their corresponding straight-chain alkanes due to the ring formation.

    Naming cycloalkanes follows a systematic approach that takes into account the ring size and any substituents attached to it. The base name of a cycloalkane is derived from the number of carbon atoms in the ring, with the prefix "cyclo-" added to indicate its cyclic nature. For example, a three-carbon ring is called cyclopropane, a five-carbon ring is cyclopentane, and a six-carbon ring is cyclohexane. These simple cycloalkanes form the foundation for more complex structures.

    When substituents are present on the cycloalkane ring, the naming process becomes more intricate. Substituents are additional atoms or groups attached to the main ring, and they must be named and numbered correctly. The numbering system for cycloalkanes starts at one of the carbon atoms in the ring and proceeds around the ring in the direction that gives the lowest possible numbers to the substituents. This is crucial for providing an unambiguous and standardized name for each compound.

    Choosing the correct numbering system is of utmost importance in organic chemistry nomenclature. The goal is to assign the lowest possible numbers to the substituents while following the rules of organic chemistry nomenclature. This ensures that each compound has a unique and universally recognized name. In cases where there are multiple substituents, the one that appears first alphabetically is given priority in determining the numbering direction.

    Let's explore some examples to illustrate these naming principles. A simple cycloalkane like cyclopentane has no substituents and is named based solely on its five-carbon ring structure. However, if we add a methyl group to one of the carbon atoms in cyclopentane, we get methylcyclopentane. In this case, no number is needed since there's only one possible position for the methyl group on the symmetrical ring.

    For more complex structures, such as a cyclohexane ring with two methyl groups and one ethyl group, we would name it something like 1-ethyl-2,4-dimethylcyclohexane. Here, the numbering starts at the carbon with the ethyl group (giving it position 1) and proceeds around the ring to give the methyl groups the lowest possible numbers (2 and 4). The prefix "di-" is used to indicate two methyl groups.

    Understanding cycloalkanes and their naming conventions is essential in organic chemistry. These compounds play significant roles in various fields, including pharmaceuticals, petrochemicals, and materials science. Cyclopentane and cyclohexane, in particular, are important building blocks in many organic syntheses and natural products. Their unique ring structures contribute to specific physical and chemical properties that distinguish them from linear alkanes.

    In conclusion, mastering the nomenclature of cycloalkanes involves recognizing their general formula, understanding the importance of the ring structure, and applying the rules for naming substituents correctly. By following these guidelines, chemists can communicate clearly about these important compounds, facilitating research, education, and industrial applications in the field of organic chemistry.

    Isomerism in Alkanes

    Isomers are compounds that share the same molecular formula but have different structural arrangements of atoms. In organic chemistry, isomers play a crucial role in understanding the diversity of compounds and their properties. The concept of isomerism is particularly significant in alkanes, where various types of isomers can exist, including chain isomers, position isomers, and functional group isomers.

    Chain isomers are compounds with the same molecular formula but different arrangements of carbon atoms in the main chain. For example, hexane (C6H14) can exist as several chain isomers. The straight-chain isomer is n-hexane, while branched isomers include 2-methylpentane and 3-methylpentane. These chain isomers have distinct physical properties, such as boiling points and melting points, despite sharing the same molecular formula.

    Position isomers occur when functional groups or substituents are attached to different carbon atoms along the main chain. In the case of hexane derivatives, we can observe position isomers in compounds like 2-hexene and 3-hexene. These isomers have the same molecular formula but differ in the position of the double bond along the carbon chain. Position isomers often exhibit similar chemical properties but may have slight variations in their physical characteristics.

    Functional group isomers are compounds with the same molecular formula but different functional groups. While alkanes themselves do not exhibit functional group isomerism, their derivatives can showcase this type of isomerism. For instance, the molecular formula C4H8O can represent both butanal (an aldehyde) and butanone (a ketone). These functional group isomers have distinct chemical properties due to the different nature of their functional groups.

    To illustrate these concepts using hexane isomers, let's consider the following examples:

    1. Chain isomers of hexane (C6H14): - n-hexane (straight chain) - 2-methylpentane (branched) - 3-methylpentane (branched) - 2,2-dimethylbutane (highly branched) - 2,3-dimethylbutane (branched)

    2. Position isomers of hexene (C6H12): - 1-hexene - 2-hexene - 3-hexene

    3. Functional group isomers (C6H12O): - Hexanal (aldehyde) - 2-hexanone (ketone) - 3-hexanone (ketone)

    Understanding isomerism is essential for predicting and explaining the properties of organic compounds. Isomers can have significantly different physical and chemical characteristics, despite sharing the same molecular formula. This concept is fundamental in various fields, including pharmaceuticals, where different isomers of a drug may have distinct biological activities.

    In conclusion, isomerism in alkanes and their derivatives demonstrates the complexity and diversity of organic compounds. Chain isomers, position isomers, and functional group isomers all contribute to the rich tapestry of organic chemistry. By studying these different types of isomers, chemists can better understand the relationship between molecular structure and properties, leading to advancements in various applications, from materials science to drug design.

    Conclusion

    Alkanes are fundamental hydrocarbons in organic chemistry, consisting of single-bonded carbon and hydrogen atoms. These saturated compounds exhibit unique properties, including low reactivity and nonpolarity. Understanding alkane naming conventions, following IUPAC rules, is crucial for effective communication in chemistry. Isomerism, particularly structural isomers, plays a significant role in alkane diversity. Mastering alkanes provides a solid foundation for exploring more complex organic compounds. Their importance extends beyond academic study, as alkanes are prevalent in fossil fuels and various industrial applications. By grasping these key concepts, students can confidently progress to more advanced topics in organic chemistry. We encourage further exploration of functional groups, reaction mechanisms, and synthesis techniques, building upon this essential knowledge of alkanes. Remember, alkanes are just the beginning of an exciting journey into the vast world of organic molecules and their transformations.

    Alkanes - Introduction

    Alkane definition, general formula and examples.

    Step 1: Understanding the Definition of Alkanes

    Alkanes are a fundamental group in organic chemistry. They are defined as saturated hydrocarbons, which means they consist solely of carbon and hydrogen atoms with single carbon-carbon bonds. The term "saturated" in chemistry is akin to a sponge soaked with water; it cannot absorb any more. Similarly, in alkanes, the carbon atoms are fully saturated with hydrogen atoms, meaning they cannot form additional bonds. This is because all the bonds are single bonds, and there are no double or triple bonds that could potentially open up to form new bonds. This characteristic makes alkanes relatively stable and less reactive compared to other hydrocarbons.

    Step 2: General Formula of Alkanes

    The general formula for alkanes is CnH2n+2. This formula applies to acyclic (non-cyclic) alkanes, where 'n' represents the number of carbon atoms. For example, if an alkane has three carbon atoms (n=3), the number of hydrogen atoms would be calculated as 2(3) + 2 = 8, giving the molecular formula C3H8. This formula helps in determining the molecular structure of any alkane by simply knowing the number of carbon atoms it contains.

    Step 3: Examples of Alkanes

    Let's look at some specific examples to understand how the general formula is applied:

    • Methane (CH4): The simplest alkane with one carbon atom. According to the formula, C1H2(1)+2 = CH4.
    • Ethane (C2H6): An alkane with two carbon atoms. Using the formula, C2H2(2)+2 = C2H6.
    • Propane (C3H8): An alkane with three carbon atoms. Applying the formula, C3H2(3)+2 = C3H8.
    • Butane (C4H10): An alkane with four carbon atoms. According to the formula, C4H2(4)+2 = C4H10.

    As we can see, each subsequent alkane in the series differs by a CH2 unit, which is a characteristic of a homologous series. This systematic addition of CH2 units allows for the creation of longer and more complex alkanes while maintaining the same general formula.

    Step 4: Homologous Series and Naming Conventions

    Alkanes belong to a homologous series, which means they follow a specific pattern where each member differs from the next by a CH2 unit. This pattern is crucial in organic chemistry as it helps in predicting the properties and reactivity of the compounds. The naming of alkanes follows a systematic approach based on the number of carbon atoms:

    • Methane: One carbon atom (CH4).
    • Ethane: Two carbon atoms (C2H6).
    • Propane: Three carbon atoms (C3H8).
    • Butane: Four carbon atoms (C4H10).
    • Pentane: Five carbon atoms (C5H12).
    • Hexane: Six carbon atoms (C6H14).

    The suffix "-ane" is used to denote that the compound is an alkane, and the prefix (meth-, eth-, prop-, but-, pent-, hex-) indicates the number of carbon atoms in the molecule. This systematic naming helps in easily identifying and categorizing different alkanes.

    Step 5: Structural Representation of Alkanes

    Alkanes can be represented using structural formulas, which show the arrangement of atoms within the molecule. For example, the structural formula of propane (C3H8) can be written as:

            H   H   H
        |   |   |
        H-C-C-C-H
        |   |   |
        H   H   H

    This representation helps in visualizing the molecule's structure and understanding the bonding between atoms. Similarly, butane (C4H10) can be represented as:

            H   H   H   H
        |   |   |   |
        H-C-C-C-C-H
        |   |   |   |
        H   H   H   H

    These structural formulas are essential for understanding the physical and chemical properties of alkanes, as well as their reactivity and interactions with other compounds.

    Step 6: Properties and Applications of Alkanes

    Alkanes are known for their relatively low reactivity due to the presence of only single bonds. This makes them useful as fuels and lubricants. For example, methane is a primary component of natural gas, while propane and butane are used in liquefied petroleum gas (LPG). Additionally, alkanes serve as starting materials for the synthesis of various chemicals and are used in the production of polymers, solvents, and other industrial products.

    Understanding the basic properties and structure of alkanes is crucial for further studies in organic chemistry and their practical applications in various industries.

    FAQs

    1. What is the general formula for alkanes?

      The general formula for alkanes is CnH2n+2, where n represents the number of carbon atoms in the molecule. This formula allows us to determine the number of hydrogen atoms in any alkane based on its carbon count. For example, an alkane with 5 carbon atoms (pentane) would have 12 hydrogen atoms (5 × 2 + 2 = 12).

    2. How are straight-chain alkanes named?

      Straight-chain alkanes are named using a prefix that indicates the number of carbon atoms, followed by the suffix "-ane". For example:
      1 carbon: methane
      2 carbons: ethane
      3 carbons: propane
      4 carbons: butane
      5 carbons: pentane
      6 carbons: hexane, and so on.

    3. What are the main types of isomerism in alkanes?

      The main types of isomerism in alkanes are:
      1. Structural isomerism: Compounds with the same molecular formula but different structural arrangements.
      2. Chain isomerism: Isomers with different carbon chain arrangements (e.g., n-butane and isobutane).
      3. Position isomerism: Isomers where substituents are attached at different positions on the carbon chain.
      4. Conformational isomerism: Different spatial arrangements of atoms that can be interconverted by rotation around single bonds.

    4. How do cycloalkanes differ from straight-chain alkanes?

      Cycloalkanes differ from straight-chain alkanes in several ways:
      1. Structure: Cycloalkanes form closed rings, while straight-chain alkanes are linear.
      2. Formula: Cycloalkanes have the general formula CnH2n, while straight-chain alkanes follow CnH2n+2.
      3. Naming: Cycloalkanes use the prefix "cyclo-" in their names (e.g., cyclohexane).
      4. Properties: Cycloalkanes often have higher boiling points and different reactivity compared to their straight-chain counterparts.

    5. What are the main reactions of alkanes?

      Alkanes are generally unreactive, but they can undergo a few important reactions:
      1. Combustion: Reaction with oxygen to produce carbon dioxide and water (complete combustion) or carbon monoxide and water (incomplete combustion).
      2. Halogenation: Substitution reactions with halogens (e.g., chlorine or bromine) to form alkyl halides.
      3. Cracking: Breaking of larger alkanes into smaller hydrocarbons at high temperatures.
      4. Isomerization: Conversion between different isomers under specific conditions.
      5. Dehydrogenation: Removal of hydrogen to form alkenes or alkynes.

    Prerequisite Topics for Understanding Alkanes

    When delving into the study of alkanes, a fundamental class of organic compounds, it's crucial to have a solid foundation in certain prerequisite topics. These foundational concepts not only enhance your understanding of alkanes but also provide a broader context for organic chemistry as a whole.

    One essential prerequisite is arrow pushing (curly arrows) in organic chemistry. This concept is vital for understanding the mechanisms of reactions involving alkanes. Arrow pushing helps visualize the movement of electrons during chemical reactions, which is particularly important when studying isomerism in organic chemistry. Isomerism plays a significant role in alkane structures, as these compounds can exist in various structural arrangements despite having the same molecular formula.

    Another crucial prerequisite topic is the properties of elements in the periodic table. This knowledge is fundamental to grasping the physical properties of alkanes. Understanding the characteristics of carbon and hydrogen, the primary elements in alkanes, provides insight into why alkanes behave the way they do. For instance, the electron configuration and bonding properties of carbon directly influence the structure and reactivity of alkanes.

    Mastering these prerequisite topics lays a strong foundation for studying alkanes. The concept of arrow pushing helps in comprehending reaction mechanisms and structural changes in alkanes, while knowledge of elemental properties explains their physical and chemical behaviors. For example, the tetrahedral arrangement of carbon atoms in alkanes, which is crucial for understanding their three-dimensional structure, stems directly from carbon's electron configuration and bonding capabilities.

    Moreover, these prerequisites are not isolated concepts but interconnected aspects of chemistry that continually resurface in the study of alkanes and beyond. The ability to visualize electron movement using arrow pushing becomes invaluable when exploring more complex organic reactions involving alkanes. Similarly, understanding the periodic trends helps predict and explain the gradual changes in physical properties observed across the homologous series of alkanes.

    In conclusion, a thorough grasp of arrow pushing in organic chemistry and the properties of elements in the periodic table significantly enhances your ability to understand and work with alkanes. These prerequisite topics provide the necessary tools to analyze, predict, and explain the behavior of alkanes in various chemical contexts. As you progress in your study of organic chemistry, you'll find that these foundational concepts continue to be relevant, forming the basis for more advanced topics and complex molecular interactions.

    In this lesson, we will learn:
    • The definition of an alkane, their general formula and the major types of them.
    • The major properties of alkanes and differences to other hydrocarbons.
    • How to name branched, unbranched and cyclic alkanes and draw their structural formula.

    Notes:

    • An alkane is a saturated hydrocarbon with only single bonds between carbon atoms.
      • They are the simplest hydrocarbon compounds compounds that only contain carbon and hydrogen.
      • Saturated means the compound contains online single bonds. Saturated compounds cannot hold any more hydrogen atoms: the maximum number of hydrogens is the CnH2n+2 that alkanes have.

    • Alkanes are an example of a homologous series in organic chemistry.
      • A homologous series is a set of compounds with the same general formula. Each compound in the set differs from the next by a -CH2- unit.
      • Other examples of a homologous series are alkenes (general formula CnH2n), alkynes (CnH2n-2) and alcohols (CnH2n+2O).
      • We saw in Organic chemistry introduction that carbon atoms can make strong bonds to other carbon atoms and form long chains in the process. You can have an alkane with a carbon chain length of three, five, twenty or fifty carbon atoms. The same is true of alcohols or alkenes.

      Alkanes, alkenes and alcohols are also families of compounds with different functional groups. The functional group is the most reactive part(s) of a molecule, so it usually identifies a substance in chemical reactions. It is the group that makes the compound function (react) the way it does.
      • For example, the functional group in an alkene is the C=C double bond, which alkanes do not have. This C=C bond makes alkenes react in different ways to alkanes, which have only C-C single bonds. This is different to the O-H group in alcohols, which reacts differently to both.

    • Alkanes have the general formula: CnH2n+2. This means that in an alkane, the number of hydrogen atoms is double the number of carbon atoms plus two. If a compound has a molecular formula that matches CnH2+2, it is an alkane.
      • We get this general formula because every carbon atom in the chain is bonded to two other carbons in the chain and two hydrogen atoms each. The exception is the beginning and end carbons, which have three hydrogens each.

    • Alkanes are unreactive compared to other hydrocarbons because they have no double or triple bonds – for them to react requires breaking a strong C-C bond which is difficult.

    • Most alkanes are insoluble in water – they don't mix! They are more lipophilic (fat loving) than hydrophilic, so they are more easily dissolved in oils and fats instead of water.

    • Hydrocarbons are flammable – methane, the simplest hydrocarbon, is also known as natural gas, and all of the shorter chain alkanes are volatile, flammable gases or liquids. Many of them are used as fuels!

    • Alkanes will react with oxygen in a combustion reaction to produce carbon dioxide and water. For example, the reaction of methane and oxygen is given below:

      • C5H12 + 8 O2   \, →  \, 5 CO2 + 6 H2O

      This is an example of complete combustion where CO2 and H2O are produced, but there is also the possibility of incomplete combustion, where not enough oxygen is present to react the alkane. Below is the incomplete combustion of methane to produce carbon monoxide:

      C5H12 + 512\frac{1}{2} O2   \, →  \, 5 CO2 + 6 H2O

      Incomplete combustion can produce carbon particulates (soot) when even less oxygen is available:

      C5H12 + 3 O2   \, →  \, 5 C + 6 H2O

      Which reaction occurs depends on how much oxygen is available. Look at the equations above; the complete combustion requires the most oxygen per mole of the alkane.

      Incomplete combustion is undesirable for safety reasons, as the carbon monoxide released is highly toxic. It is also inefficient for engine performance, because incomplete combustion releases less energy overall than complete combustion does.

    • Straight chain alkanes are also called unbranched alkanes as there are no side chains. Each carbon is only bonded to two other carbon atoms in the middle of the chain, while the two chain ends are only bonded to one. They are named using the simple rules we saw in Organic chemistry introduction. See the table and examples below. Notice they all fit the alkane general formula: CnH2n+2.

      • Length of main chain

      Root name

      Alkane name

      Molecular formula

      1

      Meth-

      Methane

      CH4

      2

      Eth-

      Ethane

      C2H6

      3

      Prop-

      Propane

      C3H8

      4

      But-

      Butane

      C4H10

      5

      Pent-

      Pentane

      C5H12

      6

      Hex-

      Hexane

      C6H14


      Pentane, C5H12


      Octane, C8H18


    • Branched alkanes or substituted alkanes contain carbon atoms in the middle of the chain bonded to carbon atoms that branch off of the main chain. This can create more than one possible 'chain lengths' – the longest chain is always considered the main chain, and the shorter ones are considered only branches. Naming branches uses the root words for the main chain, but ending in –yl to show it is a branch.
      • Note one H atom is missing from the alkyl branch formulae. This because what would have been a C-H bond is instead a bond to the main carbon chain. See the table below:
      • You must also add a number before this prefix, to show which carbon atom in the main chain the branch is found on. The numbering should always be the lowest possible.

      Chain length of branch

      Alkyl root

      Formula of alkyl branch

      1

      Methyl-

      -CH3

      2

      Ethyl-

      -C2H5

      3

      Propyl-

      -C3H7

      4

      Butyl-

      -C4H9

      5

      Pentyl-

      -C5H11

      6

      Hexyl-

      -C6H13


    • Using these rules we will look at two examples of hydrocarbons.
      • In the first example, there are two branches of one carbon each – they are equal so it doesn't matter which you choose; one can be the main chain and the other the branch. So the longest chain is 5 carbons, with a single branch of 1 carbon. The 1 carbon branch is on the second carbon in the chain, because we always number using the lowest values possible (it could be on the 4th carbon if we counted the other way). So here have:
        • A branch on the second carbon in the main chain: 2-
        • A branch of 1 carbon length: methyl-
        • A 5 carbon main chain alkane: pentane
        • Combining these we have a compound named 2-methylpentane.

        2-methylpentane, C6H14


      • In the second example, there are two end branches of two carbons each – again they are equal so we can pick one as the main chain and the other can be considered a branch. Again, number the branch off of the main chain with the lowest value possible, we will choose 3 instead of 4. We now have:
        • A branch on the third carbon in the main chain: 3-
        • A branch of 2 carbon atoms in length: ethyl-
        • A 6 carbon main chain alkane: hexane
        • Combining these, we have a compound named 3-ethylhexane.

        3-ethylhexane, C8H18


      • If branched alkanes have more than one type of branch, branches must be named in alphabetical order. For example, 'e' comes before 'm' in the alphabet, so ethyl chains should be named before methyl chains. See the example below:

        • 3-Ethyl-2-methylhexane, C9H20


      • If you have an organic compound with more than one of the same type of branch or substituent, you need to say how many of these branches there are, and at which carbons in the chain they're found. This is done systematically:
        • Two of the same substituents: di-
        • Three of the same substituents: tri-
        • Four: tetra-
        • Five: penta-
        • Six: hexa-
        See the example below:
        2,3 - dimethyl Pentane, C7H16


    • Some hydrocarbons have the beginning and end of the chain bonded together, making a closed cyclic ring. These are unique chemical compounds with different properties to the open chain of the same carbon length. Because the ends of the chain are bonded together, cyclic alkanes have a different general formula to the rest of the alkanes. The general formula for cyclic alkanes is Cn_nH2n_{2n}, like an alkene.
      • To name cyclic compounds, the same basic rule of 'chain' length applies, with slight changes:
      • Unbranched rings: Find the number of carbon atoms in the ring. This would be your original chain length. Then add 'cyclo-' in front. See the example:

      Cyclopentane, C5H10


      • Rings with one branch or substituent: Start numbering from the ring carbon bonded to the branch. Because the ring has no 'start' or 'end', you can just pick the carbon bonded to the branch as number 1. Doing this is implied, we don't need to say 1-ethylcyclooctane, just ethylcyclooctane. See below:

      Ethylcyclooctane, C10H20


      • Rings with more than one branch or substituent: Start numbering in alphabetical order, and count from there following the lowest number rule. See the example below: ethyl comes before methyl in A-Z order, therefore it is named and numbered first. The rest of the ring is then numbered from here to give the lowest numbering possible (so 1-ethyl-3-methyl instead of going the other way around, which would give 1-ethyl-5-methyl).

      1-ethyl-3-methylCyClohexane, C9H18


    • Notice that these branched alkanes and cycloalkanes can have the same molecular formula as some unbranched, straight chain alkanes and alkenes. These are examples of isomers – where the molecular formulae are the same but the structural formulae are different. This is very important as they are not the same chemical, they are unique and have different properties. This is another major reason why we use structural or skeletal formula when describing chemicals.