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Alkanes: The Building Blocks of Organic Chemistry
Dive into the world of alkanes, the simplest organic compounds. Explore their structures, formulas, and naming conventions. Gain essential knowledge for advanced organic chemistry concepts and real-world applications.

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Now Playing:Alkanes – Example 0a
Intros
  1. Alkanes - Introduction
  2. Alkanes - Introduction
    Alkane definition, general formula and examples.
  3. Alkanes - Introduction
    Reactions and properties of alkanes.
Examples
  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
    Introduction to organic chemistry
    Jump to:NotesConceptFAQsPrerequisites
    Notes
    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.
    Concept

    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.

    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.

    Prerequisites

    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.