Hydrocarbons

Last updated Jul 29, 2023 Edit Source

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Read Lucarelli Chapter 12

• Hydrocarbons are molecules that contain hydrogen and carbon only
• Hydrocarbons are broken down into different categories based around their structure
  • e.g. alkanes, alkenes, alkynes, cycloalkanes, cycloalkanes
• Saturated means the bonds between carbon atoms are only single bonds
  • Saturated hydrocarbons are a form of alkanes/cycloalkanes

• Structural formula (all bonds, all atoms)
  • example: StructuralFormula
• General formula
  • e.g. $C_nH_{2n + 2}$ for alkanes
• Molecular formula and empirical formula
  • e.g. $C_6H_{14}$
• Condensed formula (sequence of atoms)
  • e.g. $CH_3CH(CH_3)CH_2CH_2CH_2CH_3$
  • For cyclo- hydrocarbons, you cannot write condensed formula in a line and must show the bonds between carbons
• Skeletal formula
  • This is only used for very big compounds
  • Example of skeletal formula: SkeletalFormula

• Alkanes → $C_nH_{2n+2}$
• Cycloalkanes → $C_nH_{2n}$
• Alkenes → $C_nH_{2n}$

Isomers

Structural isomers are variations of organic compounds with the same molecular formula with different structures.

• Geometrical isomerism is a form of stereoisomerism when two or more compounds differ from each other in the arrangement of groups with respect to a double bond, ring, or other rigid structure
  • This occurs when there is a double bond as the carbon cannot rotate around that double bond 450
• If geometrical isomerism can be present, we must use the prefix cis- or trans- to note the arrangement of the compound
  • cis- → carbon is on the same line
  • trans- → carbon is across
• Criteria for geometrical isomerism:
  • Double bond present
  • Two different groups attached to $C^2$
    • $C^n$ is the $n^{th}$ carbon in the longest chain
  • Two different groups attached to $C^3$

• Alkanes are typically unreactive except for certain types of reactions
• Alkanes react with oxygen in combustion reactions
  • Two types of combustion:
    • Complete combustion (plenty of $O_2$)
      • Carbon dioxide as a product
      • Product: $CO_2 + H_2O$
    • Incomplete combustion (limited supply of $O_2$)
      • Carbon OR carbon monoxide as a product
      • Products:
        • $CO + H_2O$
          • Carbon monoxide is a colourless, odourless toxic gas
        • $C + H_2O$
          • Carbon is in the form of soot
  • Energy is released in a combustion reaction
    • Bonds are formed releasing energy
      • The more bonds that form, the greater the energy released
      • Therefore, complete combustion creates the most energy
• Substitution Reactions
  • Halogenation (reaction with halogen $Cl_2,\\ Br_2,\\ I_2$)
    • A hydrogen atom is substituted by a halogen
      • Examples:
        • $CH_{4\\ (g)} + Cl_{2\\ (g)} → CH_3Cl_{\\ (g)} + HCl_{\\ (g)}$
        • $CH_3Cl_{\\ (g)} + Cl_{2\\ (g)} → CH_2Cl_{2\\ (g)} + HCl_{\\ (g)}$
        • $CH_2Cl_{2\\ (g)} + Cl_{2\\ (g)} → CHCl_{3\\ (g)} + HCl_{\\ (g)}$
        • $CHCl_{3\\ (g)} + Cl_{2\\ (g)} → CCl_{4\\ (g)} + HCl_{\\ (g)}$
    • Halogenation means a reaction with a halogen, it is a general term that does not necessarily refer to hydrocarbon reactions
      • Substitution reaction is the proper name for when referring to a hydrocarbon halogenation reaction
    • The reaction is uncontrollable because molecules are colliding erratically
      • This method is never used to manufacture these products even though they are very useful solvents

• Addition Reactions
  • Two reactants → one product
  • Hydrogenation (Saturation, $+\\ H_2$)
    • Addition of hydrogen
    • Unsaturated hydrocarbon → saturated hydrocarbon
      • ALKENE $+$ HYDROGEN $→$ ALKANE
    • Conditions for Reaction:
      • Nickel catalysts
      • High temperature/pressure
  • Halogenation ($+\\ Cl_2$ or $Br_{2\\ {(aq)}}$ or $I_2$)
    • Addition of a halogen
      • Added to carbon with double bond; double bond removed
      • ALKENE $+$ HALOGEN $→$ HALOALKANE
    • Reaction is a test for alkene double bond
      • Happens readily
        • Happens without light
    • $Br_{2\\ (aq)}$ → bromine water
      • Bromine water is an orange solution
      • Observations for bromine water halogenation:
        • Colourless gas bubbled through orange solution → orange solution becomes colourless
      • This reaction is a good test for an alkene, the solution should decolourise
  • Hydrogen Halide Reaction ($HCl_{\\ (g)}$ or $HBr$ or $HI$)
    • Combination of hydrogenation and halogenation
      • ALKENE $+$ HYDROGEN HALIDE → HALOALKANE
      • Product gains hydrogen and halogen
  • Hydration ($+\\ H_2O$)
    • Reaction of alkene and water
      • ALKENE $+$ WATER $→$ ALCOHOL
      • Product gains hydrogen and hydroxide functional group
    • Conditions for Reaction:
      • Phosphoric acid catalyst
      • High temperature and pressure

• Classification Diagram: 600