Haloalkanes - Basic Organic Chemistry

Haloalkanes

Lessons

Notes:
In this lesson, we will learn:
  • The definition of a haloalkane, their general formulae and some examples of them.
  • The major properties of haloalkanes and their differences to other types of organic compounds.
  • The different types of haloalkane, how they are produced and the reactions they are used in.
  • How to name haloalkanes according to IUPAC organic nomenclature.

Notes:
  • Haloalkanes (A.K.A alkyl halides or halogenoalkanes) are saturated organic compounds containing one or more carbon-halogen bonds. Because the halogens only make one covalent bond to other atoms, like hydrogen does, they have the same structure as alkanes, only except one (or more) hydrogen atoms have been replaced by halogens.
  • Because halogen atoms have the same valence as hydrogen, the general formula of haloalkanes is related to that of alkanes, only depending on the number of halogen atoms the compound has.
    • In haloalkanes with one halogen atom, it is CnH2n+1X, where X = F, Cl, Br, I.
    • In dihaloalkanes which have two halogen atoms it is CnH2nX2
    • In any haloalkane the total number of X and H atoms will combine to give 2n+2 like in an alkane.
  • The properties of haloalkanes depends on what halogen atom they contain, but generally they are:
    • Very slightly soluble in water – because the carbon-halogen bond is normally polar, they are more soluble than alkanes but much less soluble than alcohols.
    • More reactive than alkanes, except fluoroalkanes which are very unreactive. This is because haloalkanes react by breaking the carbon-halogen bond - the weaker this is, the more reactive the chemical is going to be. Carbon-fluorine bonds are amongst the strongest chemical bonds while the carbon-iodine bond is quite weak.
    • Relatively higher boiling and melting point compared to their alkane analogue. This is due to the stronger dipole-dipole interactive forces between haloalkane molecules than the London dispersion forces present in simple alkanes. In the examples in this lesson, I talked about intermolecular forces being broken up when discussing chemicals dissolving in one another – this is important when talking about melting and boiling points too!
  • As with alcohols, haloalkanes can be sub-categorized into primary, secondary and tertiary haloalkanes.
    • In primary haloalkanes, the halogen is bonded to a carbon with only one carbon-carbon bond. This would be the case when the halogen is bonded at the end of a carbon chain.
    • In secondary haloalkanes, the halogen is bonded to a carbon atom with two carbon-carbon bonds.
    • In tertiary haloalkanes, the halogen is bonded to a carbon atom with three carbon-carbon bonds – this would be the case when the halogen is bonded to the molecule at a branch in the carbon chain.e.
  • Haloalkanes, in particular chlorofluorocarbons, used to be very useful as refrigerants but today they are only rarely used because research showed they destroy ozone in the upper atmosphere.
  • Haloalkanes can be prepared by reacting alcohols with strong "hydrohalic" acids (HX, where X is a halogen). You can think of the alcohol group on the molecule being "swapped" for the halide on the acid reacting with it See below for examples:
    • CH3CH2OH + HClCH3CH2Cl + H2O
    • CH3CH2CH2OH + HBrCH3CH2CH2Br + H2O
  • You can perform a simple laboratory test on haloalkanes to test for halides in your organic compounds – this is done by heating gently with sodium hydroxide, then adding acidified silver nitrate to the mixture. The colour of the precipitate (insoluble solid) formed will show the halide present:

    Ion present

    Test observation

    F-

    No precipitate

    Cl-

    White precipitate

    Br-

    Pale cream precipitate

    I-

    Yellow precipitate


    An additional step may be necessary if the precipitate colours are too similar – adding ammonia solution to the mixture will lead to different outcomes depending on the halide present:

  • Ion present

    Test observation

    Cl-

    Precipitate redissolves – colourless solution.

    Br-

    Precipitate remains after adding dilute ammonia - redissolves with concentrated/excess ammonia.

    I-

    Precipitate remains after adding concentrated/excess ammonia.


  • Haloalkanes can be reacted to produce a number of other chemicals including:
    • Amines, which are made by a substitution reaction in the following equation (using 1-chloropropane as an example)
      CH3CH2CH2Cl + 2NH3 CH3CH2CH2NH2 + NH4Cl
    • Alcohols in a substitution reaction with sodium or potassium hydroxide in the following equation:
      CH3CH2CH2Cl + NaOH CH3CH2CH2OH + NaCL
    • Alkenes in an elimination reaction with sodium or potassium hydroxide in the following equation:
      CH3CH2CH2Cl + NaOH CH3CH=CH2 + NaCL + H2
      You should notice that the reaction with sodium or potassium hydroxide can produce alkenes and alcohols. Which product forms normally depends on a few conditions and the type of haloalkane used – this will be explored in another lesson!
  • Haloalkanes can be named according to IUPAC systematic rules already learned, where halogens have equal priority to alkyl branches. This has some basic consequences when naming organic compounds with halogen atoms in them (see the examples below):
    • Alcohol groups and alkene double bonds will be prioritized over the halogens when numbering/ordering the carbon chain.
    • When naming the alkyl and halogen substituents in a molecule, name them in alphabetical order, even if this leads to 'dipping' between numbers or naming alkyl and halogen substituents.
  • Intro Lesson
    Haloalkanes: Introduction
  • 1.
    Compare and explain the properties of the haloalkanes compared to alkanes and alcohols.
  • 2.
    Apply the rules of IUPAC organic nomenclature to draw structural and skeletal formula.
    Draw the structures of the following molecules given by their IUPAC names.
  • 3.
    Apply knowledge of IUPAC organic nomenclature to identify and correct systematic naming.
    There are mistakes in the IUPAC systematic names of the chemicals below. Identify and correct the mistakes and give the true IUPAC name of the compounds.
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Haloalkanes

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