Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

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PART 3 HALOGENOALKANES (haloalkanes, alkyl halides)

3.1 The molecular structure and naming-nomenclature of halogenoalkanes once known as 'haloalkanes' ('alkyl halides') - 3.1.1 The homologous series of halogenoalkanes

Sub-index for this page

3.1.1 A brief guide to halogenoalkane nomenclature

3.1.2 Formulae and styles of molecular representation for halogenoalkanes

3.1.3 Sub-classification of halogenoalkanes (primary, secondary and tertiary)

3.1.4 Structural isomerism - positional isomerism in halogenoalkane compounds

3.1.5 Examples of non-cyclo C₁-C₃ Halogenoalkanes

3.1.6 More examples of Halogenoalkanes (C₄ non-cyclo molecules)

3.1.7 Examples of cyclohalogenoalkanes (secondary haloalkanes)

3.1.8 Open chain halogenoalkanes with 5 or more carbon atoms

How do we name haloalkanes? What is the molecular structure of halogenoalkanes?

This page describes the molecular structure and how to name halogenoalkanes of the simplest monosubstituted halogenoalkanes and some polysubstituted haloalkanes of the C1, C2 and C3 carbon chains.

Examples of displayed formula of halogenoalkane molecules (fluoro, chloro, bromo and iodo alkane derivatives), graphic formula, molecular formula, skeletal formula, structural formula of these homologous series, including isomers of halogenoalkane molecules and some of the further halogen substitution products including CFCs, HFCs and HCFCs.

Some 'old' trivial and commercial names of the fluoroalkanes, chloroalkanes, bromoalkanes, and iodoalkanes are quoted in () though their use should be avoided if possible.

• The generic suffix name (..ane) is based on the longest carbon chain.:
  • 1 carbon, methane; 2 carbons, ethane; 3 carbons, propane; 4 carbons, butane.
  • After these 4 preserved 'old trivial' names, the name is 'numerically' systematic eg 5 carbons, pentane; 6 hexane, 7 heptane etc.
• The halogenoalkane structures are based on replacing one or more hydrogen atoms in the alkane structure with a halogen atom X (X = F, Cl, Br or I).
  • Therefore, where X = halogen, the halogenoalkane functional group is C-X i.e. C-F,  C-Cl,  C-Br,  C-I
• This gives rise to a homologous series of halogenoalkanes.
  • A homologous series have a similar molecular structure, a general formula and the same functional group e.g. C-X (where X = halogen) for halogenoalkanes giving the molecules a characteristic set of reactions.
  • The physical properties are similar, but with general tends in e.g. melting point, boiling point, density and solubility.
  • The general formula for non-cyclic halogenoalkanes is C_nH_2n+1X, where X = F, Cl, Br or I.
  • The halogenoalkane general formulas are similar to alkanes, when one H atom is replaced with a single halogen atom - from a monosubstituted alkane - giving a halogenoalkane.
• As usual, in naming halogenoalkanes, the IUPAC nomenclature rules apply:
  • The prefix for each halogenoalkane name becomes: fluoro..., chloro..., bromo..., iodo...
    • e.g. bromoethane,  1-iodopropane
  • You use the lowest possible numbers for the associated carbon atoms in the main chain.
    • 2-chlorobutane, NOT 3-chlorobutane (doesn't matter which end you start counting from)
  • If there is more than one 'type' of substituent eg using the prefixes: bromo…, chloro…, methyl… etc., they are written out in alphabetical order irrespective of carbon atom number (note: di, tri are ignored in using this rule).
  • 1,2-dibromo-1-chlorobutane, but you still use the lowest numbers possible on the carbon chain i.e. you would no name the above 3,4-dibromo-4-chlorobutane.
• The initial examples on this page are based on 1 to 3 carbon atoms, then lots more bigger molecules down the page.
• Some 'old' and commercial names are quoted in () though their use should be avoided if possible [but many still used - just put one into GOOGLE!].

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3.1.2 Formulae and styles of molecular representation for halogenoalkanes (haloalkanes, alkyl halides)

• Full displayed formula compared to the limited information from molecular formula examples
  • The molecular formula tells you the number of atoms of each element in the molecule, a sort of summary formula, but it tells you nothing about how the atoms are arranged and bonded in the molecule which the full displayed formula of these halogenoalkanes does e.g.
  • 1-chlorobutane has the molecular formula C₄H₉Cl
  • 1,3-dibromo-3-chlorobutane has the molecular formula C₄H₇Br₂Cl
• Empirical formula examples of halogenoalkanes
  • The simplest whole number ratio formula found from experimental analysis.
  • e.g. for chloroethane (derived from the parent alkane ethane and reacted with chlorine) CH₃CH₂Cl the empirical formula is C₂H₅Cl, the same as the molecular formula.
  • Because there is only one halogen atom in monosubstituted alkanes, the empirical formula is always the same as the molecular formula.
  • However, for some dihalo derivatives, the empirical formula is 'half' the molecular formula.
    • e.g. for 1,2-dibromoethane, BrCH₂CH₂Br, molecular formula C₂H₄Br₂, CH₂Br is the empirical formula.
• Structural formula examples of halogenoalkanes
  • is 1-chlorobutane, an unambiguous structure (see below)
• Condensed/shortened/abbreviated structural formula examples of halogenoalkanes
  • is also 1-chlorobutane, and also an unambiguous structure if you know how to read it! i.e. you know it is really !
• Displayed formula examples
  • 2-bromopropane, 1,2-dibromo-1-chlorobutane
  • A displayed formula shows all the individual atoms and bonds in the molecule, but makes all the bond angles look like 90^o, when, in this case, the C-C-C, H-C-C, H-C-H, H-C-Br bond angles etc. are all ~109^o.
  • Note in the 2nd example the use of minimum numbers and the alphabetical order rule, where the 'bromo' takes precedence over the 'dibromo'.
• '3D' style of displayed formula of halogenoalkanes
  • chloromethane,  bromoethane, 2-bromopropane
  • Here you have a better idea of the stereochemistry of the molecule which more clearly shows the tetrahedral arrangement of the bonds around each carbon atom,
  • therefore all the bond angles are all ~109^o i.e. H-C-H, H-C-Cl, H-C-C, C-C-Br, H-C-Br, C-C-C
  • The plain lines represent bonds in the plane of the screen/paper, the dotted line a bond pointing away from you and the triangular wedge a bond pointing towards you.
• Skeletal formula examples of halogenoalkanes
  • Carbon and hydrogen atoms are not usually shown in skeletal formula.
  • Here you just use dashes to indicate C-C bonds and (usually) only show bonds to non-carbon atoms, in this case halogen atoms in halogenoalkanes e.g. -F, -Cl, -Br or -I.
  • It should give an idea of the shape of the molecule e.g. the zig-zag pattern of the carbon chain.
  • is 1-chlorobutane, note that you must show the bond to the non-carbon atom chlorine.
  • On the left you have 3 dashes for the C-C-C-C chain and the 4th dash to the chlorine atom.
  • 3-chloro-1,2-difluoro-3-methylpentane ==> ==>
    • is an extreme example! Does it check out in your head?

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3.1.3 Sub-classification of halogenoalkanes (primary, secondary and tertiary)

• A sub-CLASSIFICATION of halogenoalkanes based on structural differences, which can have significant chemical consequences e.g. on the rate of the reaction, mechanism of the reaction or the products formed in the reaction.
  • Halogenoalkanes are classified according to the atoms/groups attached to the carbon of the halogen atom X.
  • Primary halogenoalkanes have the structure R-CH₂-X, R = H, alkyl, aryl etc. i.e. apart from chloromethane they have two hydrogens and one alkyl/aryl group attached to the C of the C-X functional group.
    • e.g. chloroethane CH₃CH₂Cl, and  1-bromopropane,
    • C₆H₅CH₂Cl is an example of an aryl (aromatic) substituent attached to the carbon of the C-X bond.
    • Apart from the methyl haloalkanes (CH₃X), there are two hydrogen atoms on the C of the functional group.
  • Secondary halogenoalkanes have the structure R₂CH-X, R = alkyl or aryl etc. i.e. they have one hydrogen and two alkyl/aryl groups attached to the C of the C-X functional group.
    • e.g. 2-iodobutane CH₃CHICH₂CH₃ , and 2-chloropropane
    • There is one hydrogen atom on the C of the functional group.
  • Tertiary halogenoalkanes have the structure R₃C-X, R = alkyl or aryl etc. i.e. they have three alkyl/aryl groups attached to the C of the C-X group.
    • e.g. 2-chloro-2-methylpropane (CH₃)₃CI
    • There no hydrogen atoms on the C of the C-X functional group.

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3.1.4 Structural isomerism - positional isomerism in halogenoalkane compounds

Reminder: Structural isomers have the same molecular formula but different arrangements of how the atoms in the molecule are connected e.g. two halogenoalkane examples of positional isomerism (position isomerism).

From C₃ compounds onwards the possibility of structural isomers exists where the position of the halogen atom or atoms in the same carbon chain can be varied e.g.

(1) 1-bromopropane, CH₃CH₂CH₂Br, , , primary halogenoalkane,

(2) 2-bromopropane, CH₃CHBrCH₃,  , , secondary halogenoalkane, 

(1) and (2) are structural positional isomers of C₃H₇Br

 

(3)  , , , 1,1-dichlorobutane

(4) , , , 1,2-dichlorobutane

(3) and (4) are two possible structural position isomers of molecular formula C₄H₈Cl₂

 

See structural isomerism including positional isomerism in halogenoalkane compounds for more on this topic, including comments on their different physical properties.

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3.1.5 Examples of non-cyclo C₁-C₃ Halogenoalkanes

NOTE:

• Some 'old' and commercial names are quoted in () though their use should be avoided if possible.
• AND some abbreviations:
  • CFC = chlorofluorocarbon (molecule consists of C, Cl, F atoms)
  • HFC = hydrofluorocarbon (molecule consists of C, H, F atoms)
  • HCFC = hydrochlorofluorocarbon (molecule consists of C, H, Cl, F atoms)

• The shapes and bond angles of simple molecules - section on bond angles in organic molecules

  • Virtually all the bond angles for the molecules on this page are ~109^o e.g. the H-C-H, H-C-Cl, H-C-Br, Cl-C-F etc.

Names in bold are the preferred IUPAC name for the halogenoalkanes

1. The simplest halogenoalkane is exemplified by chloromethane (methyl chloride), whose formula and structured are shown in 4 styles. (a)molecular formula, also the abbreviated structural formula, (b) structural formula,

(c), full displayed structural formula, (d) '3D' representation of the structural formula. The H-C-H and H-C-Cl bond angles will be ~109^o

Watch out for the alphabetical order rule for the prefixes - fluoro, chloro, bromo, iodo, but NOT necessarily in that order!

Abbreviations used: CFC = chlorofluorocarbon;  HCFC = hydrochlorofluorocarbon;  HFC = hydrofluorocarbon

 2. (a) trichlorofluoromethane, (CFC-11, a chlorofluorocarbon or CFC). The Cl-C-Cl and F-C-Cl bond angles will be ~109^o

4. difluoromethane (a hydrofluorocarbon or HFC, methylene difluoride)

7. chlorodifluoromethane, (HCFC-22 a hydrochlorofluorocarbon or HCFC)

8. Bromoethane (ethyl bromide) exemplifies the simplest mono-substituted alkane with 2 carbon atoms.

(a) molecular formula, (b) abbreviated structural formula,

(c) more expanded structural formula, (d) full displayed structural formula. The H-C-H and H-C-Br bond angles will be ~109^o

Extra images of bromoethane: ,  ,  ,  , the latter being the skeletal formula

(e) '3D' structural formula, (f) skeletal formula

and now some polysubstituted halogenoalkane molecules based on ethane

(f) The molecular formula C₂H₄X₂ will give rise to two structural positional isomers i.e. 1,1-di ... and 1,2-di ...

e.g.  1,1-dichloroethane  and  1,2-dichloroethane

e.g. 1,1-dibromoethane  and  1,2-dibromoethane

For more see Structural isomerism in organic compounds

9. to 14. show a series of multi-substituted halogenated ethane molecules (with the possibility of positional isomerism with only 2 carbon atoms) ...

15. (a), , (b) , (c), 1-bromopropane (propyl bromide)

16. (a), (b), (c) , (d), 2-bromopropane (isopropyl bromide)

15 and 16 are structural isomers - positional isomers of C₃H₇Br

See structural isomerism including positional isomerism in halogenoalkane compounds for more on this topic

Similarly for the chlorine compounds, for isomers of C₃H₇Cl we have:

17.(a), (b), (c) , 2-chloropropane (isopropyl chloride)

 and the structural-positional isomer 1-chloropropane ...

(a)   (b)   (c)    (b)

18.(a), (b), (c), (d) , 1-iodopropane (propyl iodide)

19. With 2 like halogen atoms there are four positional isomers e.g. for dichloropropanes

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses) formulae keywords: how to name naming nomenclature empirical molecular formula graphic formula displayed formula skeletal formula structural isomers isomerism  haloalkanes halogenoalkanes alkyl halides chloroalkanes bromoalkanes iodoalkanes fluoro alkanes CFC HFC HCFC CFCs HFCs HCFCs CH3Cl CCl3F CCl2F2 CH2F2 CHBr3 CCl4 CHClF2 C2H5Br CH3CH2Br CH3-CH2 -Br CH2FCClF2 CF3CH2Cl CHCl FCH2F CH2F2CH2Cl CHF2CHClF CH3CHClF CH2ClCH2F CH3CClF2 CCl2FCClF2 CF3CHCl2 CH3CHF2 CH3CCl2F CH3CF3 CH2FCHF2 CH2FCF3 CHF2CHF2 C3H7Br CH3CH2CH2Br CH3-CH2-CH2-Br CH3CHClCH3 C3H7I CH3CH2CH2Br CH3-CH2-CH2-Br CH3CHBRCH3 CH3CHClCH3 C3H7I CH3-CH2-CH2-I CH3CH2CH2I C3H6Cl2 C3H6F3

 Doc Brown's Chemistry 

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