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 The homologous aliphatic series of ALKANES - The Structure and Nomenclature of Alkane Hydrocarbons

 GCE-AS-A2-IB ADVANCED LEVEL ORGANIC CHEMISTRY

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Examples of displayed formula, graphic formula, molecular formula, skeletal formula, structural formula

Four introductory sections (a) to (d), precede the numerous examples of how to draw and name alkanes.

* (a) Nomenclature explained * (b) Different ways of representing molecules and classes of formulae *

* (c) Comparison of aliphatic/alicyclic/aromatic compounds - definition & examples * (d) Isomers explained  *

EXAMPLES of structures and names

(i) non-cyclic alkanes, general formula C_nH_2n+2

including isomers of molecular formulae up to C₇H₁₆ (up to n = 7)

(ii) examples of cycloalkanes 'alicyclic' compounds of general formula C_nH_2n and

(iii) examples of higher non-cyclo alkanes general formula C_nH_2n+2 where n = 8 to 10

Some other useful organic chemistry pages.

Organic chemical identifying functional group tests * Summary of functional groups

(a) A brief guide to non-cyclic alkane structure-naming-nomenclature 

• The primary suffix name is based on the longest carbon chain and ending in ...ane.

  • 1 carbon, methane; 2 carbons, ethane; 3 carbons in chain, propane; 4 carbons in chain, butane. After these four preserved 'old trivial' names, the name is 'numerically' systematic e.g. C₅ carbon chain pentane; C₆ chain hexane, C₇ chain heptane, C₈ chain octane, C₉ chain nonane, C₁₀ chain decane etc.

• If all the carbon atoms of the molecule are in one continuous chain, it is referred to as unbranched or linear.

  • e.g. pentane is linear or unbranched.

• If another chain of carbon atoms starts out of the main carbon chain, it is referred to as branching, giving rise to 'branched' alkane.

  • e.g. 3-ethylpentane is branched, because it has an 'ethyl branch' from the 3rd carbon atom in the main chain.

  • The longest continuous chain of 5 carbon atoms forms the basis of the name.

  • The 3- denotes the position of the carbon chain branch i.e. the lowest number possible.

• The positions of the substituent alkyl groups (side chains or 'branches') are denoted by using the lowest possible number(s)

  • e.g. 2, 3 etc. for the associated carbon atoms in the main chain, where the 1st carbon atom in the chain is considered as C atom 1.

• If there is more than one 'type' of substituent e.g. using the prefixes: methyl... and ethyl.... etc., they are written out in alphabetical order (di, tri are ignored in using this rule).

• Some 'old' names are quoted in () though their use should be avoided if possible [but many still used - just put one into GOGGLE!].

(b) Ways of representing the structure of molecules and classes of formulae

This is illustrated by looking at the structure of the propane, 2-methyl propane and butane molecules. Follow the sequence of bullet points down, and then back up, so you are quite clear on the relationship between all the structural and formula styles.

• The empirical formula means the simplest possible formula showing the whole number stoichiometric ratio of the different atoms (elements) in the compound. It derives from an elemental analysis of a compound.

  • e.g. for propane it is C₃H₈, and for butane or 2-methylpropane it is C₂H₅.

• The molecular formula summarises all the atoms in the molecule BUT does not show their arrangement at all.

  • C₃H₈ is the of molecular formula for propane and C₄H₁₀ that of butane and 2-methylpropane.

  • Note that for propane, the empirical formula is identical to the molecular formula BUT for 2-methylpropane and butane, they are not identical. In the case of the latter, the molecular formula is 'twice' the empirical formula.

  • Note also, that the molecular formula, does NOT distinguish the two structural isomers butane and 2-methylpropane ('methylpropane'). The 2- is not strictly required, since the branching must occur on the middle carbon as you will see below, where we introduce a more advanced structure notation that allows the unambiguous representation of molecular structures.

• An 'abbreviated' or 'shortened' structural formula, is unambiguous if you know how to interpret it! It shows how groups of atoms are linked or sequenced in a molecule. It is the minimum possible representation to distinguish different structures of the same molecular formula i.e. structural isomers like methylpropane and butane.

  • propane  or

  • methylpropane

  • butane  or

  • Note that methylpropane and butane can now be distinguished, BUT, you must be able to envisage these correctly into a full structure that shows how all the atoms are 'connected', and this is explained next.

• A full structural/graphic/displayed formula gives a '2D' projection-representation of the molecule and must clearly show how all the atoms are connected i.e. in this case all the C-C and C-H covalent bonds, but does it not give the full 3D structural, or spatial arrangement, of the atoms, though for most purposes, this level of detail is quite sufficient.

  • propane

  • methylbutane

  • butane

  • You can use a mixture of styles of the abbreviated/structural/graphic/displayed formulae.

    • e.g. 2-methylbutane but take care!

• A stereochemical formula is the full structural/graphic/displayed formula in terms of the 3D structural or spatial arrangement of the atoms (albeit on '2D' screen or paper). Only e.g. ball and stick models can fully show the 'true' spatial arrangement of all the atoms, more than adequately simulated by modern computer software for 'molecular modelling'. In the 'picture' of propane below, imagine the single thin lines as the C-C bonds lying in the plane of the screen/paper, the dotted C-H bonds point away from you, and the triangular wedge C-H bonds point towards you. All the C-C-C, C-C-H or H-C-H bond angles are 109^o in this case, similarly for all other non-cyclic alkanes.

  • propane This kind of representation is essential for displaying e.g. mirror image optical isomers (enantiomers). See Isomerism Part 2.

• is the skeletal formula for propane. This is derived by drawing a short line to represent a C-C single bond, so the V shape for propane comes from the C-C-C carbon chain skeleton and the C-C-C bond angle of 109^o. No lines are shown for C-H bonds, they are assumed. However, bond lines should be drawn for C-X bonds, where X is not a hydrogen atom (see e.g. alcohols and ethers).

  • methylpropane is showing the main C-C-C chain and a single carbon branch from the middle carbon.

  • butane is showing the 'linear' C-C-C-C chain with no branching.

• A general formula e.g. C_nH_2n+2 for non-cyclic alkanes, represents a member of a homologous series when n is designated an integer value e.g. if n = 5, it gives the molecular formula of pentane.

  • A homologous series is a series of compounds in which each member differs from the next member by a constant amount e.g. for alkanes, the addition of a -CH₂- 'unit' as the series is ascended n = 1, 2, 3 etc. Consequently, they have a very similar molecular structure, very similar physical and chemical properties. However, within a homologous series, the members will show trends in physical properties like increasing boiling point or decreasing solubility, which are a function of intermolecular forces that increase with chain length.

(c) A simplified structural comparison of aliphatic, alicyclic and aromatic hydrocarbon compounds

• 1. ALIPHATIC - have no benzene ring in their structure (see 3.) and can have an open linear or branched structure. If they have a non-benzene ring cyclic structure, they may be termed alicyclic, but they are still aliphatic (see 2.) e.g.

  • heptane, or

  • 2-methylbutane, or

  • and lots more examples on this page!

• 2. ALICYCLIC - these molecules have an aliphatic structure but contain a cyclic or ring structure of at least 3 carbon atoms (can't be less than 3!) BUT not a benzene ring e.g.

  • Methylcyclohexane, or

  • they resemble the aliphatic molecules illustrated in 1. both physically and chemically, and there are lots more of them on this page!

• 3. AROMATIC - These molecules contain a benzene ring based a 'special' cyclic C₆ system, which is an unsaturated ring (BUT not an alkene system) e.g.

  • benzene C₆H₆, or

  • methylbenzene C₇H₈, C₆H₅CH₃ or or or

  • and lots more on the 'Aromatic molecules' page.

• NOTE:

  • (i) Some molecules can be classified in several ways depending on which part of their structure imparts the functional group chemistry you might be interested in e.g.

    • phenylethene ('styrene'), is named as a derivative of ethene, but its chemistry could be that of an aliphatic alkene (C=C group) or an aromatic compound (contains a benzene ring).

  • (ii) Substituted hydrocarbons e.g. a halogen replacing a hydrogen atom on the carbon chain, does not affect these basic definitions e.g.

    • aliphatic: 2-chloropropane, or

    • alicyclic: fluorocyclopropane, ,

    • aromatic: 2,3-dichloromethylbenzene,

(d) A brief guide to working out isomers of non-cycloalkanes C_nH_2n+2

• No structural isomers exist for methane, ethane or propane.

• Structural chain isomers

• However, from C₄H₁₀, structural isomers exist. (for other examples and explanation see Isomerism Part 1)

• C₄H₁₀ can be set out as a linear carbon chain to give butane itself.

  1. 

     • Or you can make a 3 carbon chain with a 'branch in the middle to give 2-methylpropane.

  2. and that's it, two isomers for C₄H₁₀.

• For C₅H₁₂ you can make 3 isomers:

  1. pentane, longest possible linear or 'unbranched chain'.

  2.  , 2-methylbutane, longest chain with a single branch shortening the main chain by 1 carbon.

  3. 2,2-dimethylpropane, shortest possible main chain by double branching and shortening the main chain by 2 carbons.

• You can extend these ideas to C₆H₁₄ onwards, working out by trial and error all the possible branchings in terms of methyl, dimethyl, trimethyl or ethyl groupings etc.

• The 18 structural chain isomers of C₈H₁₈ are worked out for you in section (iii) and anything else from C₆-C₇ or C₉ onwards, you can work out for yourself!

• Optical isomerism occurs from molecular formula C₇H₁₆ onwards.

• The first possible example i.e. with 4 different groups attached to give a chiral carbon is 3-methylhexane.

• 

• The 4 different groups are -H, -CH₃, -CH₂-CH₃ and -CH₂CH₂CH₃

• The mirror-image forms (enantiomers) must be extremely difficult to separate.

• Other examples and explanation of Optical Isomerism.

1. (a)  the molecular formula of methane, two full structural graphic formula representations are

   • (b) and (c) which gives a 3-dimensional (3D) structural formula impression of the molecule.

   • The H-C-H bond angle is 109^o. NOTE: all the C-C-H, H-C-H or H-C-H angles are approximately 109^o in all the non-cyclic alkanes shown below.

2. (a) or (b) or (c) or (d) ethane,

   • molecular formula (e), the skeletal formula is (f), yes! just a dash!

   • NOTE: from (a) to (d) you go from the most abbreviated structural formula representation to the maximum 3D structural graphic formula representation on a 2D format!

3. (a) or (b) or (c) or (d)

   • (a) and (b) are the abbreviated structural formula for propane,

   • (c)/(d) full displayed-graphic structural formula, but (d) is a 3D version to indicate the spatial arrangement of the atoms,

   • for molecular formula (e) and the skeletal formula is (f)

4. (a) or (b) are abbreviated structural formula for butane

   •  (normal or n-butane), molecular formula (c) and the skeletal formula is (d)

5. (a) or (b) are the abbreviated structural formula for methylpropane (isobutane), the prefix 2- isn't strictly needed. BUT can be added for clarity, especially for beginners,

   • the molecular formula is (c) and

   • the skeletal formula is (d) and is the simplest branched hydrocarbon.

6. (a) or (b) are abbreviated structural formula

   •  for pentane (normal or n-pentane), molecular formula (c) and the skeletal formula is (d)

7. (a) (b)abbreviated structural formula for methylbutane (isopentane),

   • (prefix 2- isn't strictly needed, but 2-methybutane helps at the beginning of studying organic nomenclature), the molecular formula is (c) and the skeletal formula is (d)

8. (a) or (b) are the abbreviated structural formula for dimethylpropane (neopentane),

   • (the prefix 2,2- isn't strictly needed but can help initially),

   • molecular formula (c) and the skeletal formula is (d)

9. (a) or (b)

   • are abbreviated structural formula for hexane (normal or n-hexane),

   • molecular formula (c) and the skeletal formula is (d)

10. (a) or (b) are abbreviated structural formula for

    •  2-methylpentane (NOTE: prefix numbers needed from now on),

    • molecular formula and the skeletal formula is (d)

11. (a) or (b) are abbreviated structural formula

    •  for 3-methylpentane, the molecular formula is (c) and the skeletal formula is (d)

12. (a) or (b) are abbreviated structural formula for

    •  2,2-dimethylbutane (NOTE: numbers needed here, cross-check with 7.),

    • molecular formula (c) and the skeletal formula is (d)

13. (a) or (b) are abbreviated structural formula for

    • 2,3-dimethylbutane, molecular formula (c) and the skeletal formula is (d)

14. (a) or (b)

    • are the abbreviated structural formula for heptane, molecular formula (c)

    • and the skeletal formula is (d)

15. (a) or (b) 

    • are the abbreviated structural formula for 2-methylhexane,

    • molecular formula (c) and the skeletal formula is (d)

16. (a)or (b) 

    • are abbreviated structural formula for 3-methylhexane, molecular formula (c)

    • and the skeletal formula is (d)

17. (a) or (b) are abbreviated structural formula

    • for 3-ethylpentane, molecular formula (c) and the skeletal formula is (d)

18. (a) or (b) are abbreviated structural formula

    • for 2,2-dimethylpentane, molecular formula (c) and the skeletal formula is (d)

19. (a) or (b)  are abbreviated structural formula

    • for 2,3-dimethylpentane, molecular formula (c) and the skeletal formula is (d)

20. (a) or (b)  are abbreviated structural formula

    • for 2,4-dimethylpentane, molecular formula (c) and the skeletal formula is (d)

21. (a) or (b)  are abbreviated structural formula

    •  for 3,3-dimethylpentane, molecular formula (c) and the skeletal formula is (d)

22. (a) or (b) are the abbreviated structural formula for

    • 2,2,3-trimethylbutane, molecular formula and the skeletal formula is (d)

• Cycloalkanes are named according to the rules previously described, but the name is based on the number of carbon atoms in the ring itself.

• The 'smallest' cycloalkanes must have at least 3 carbon atoms in the ring.

• The structures are shown as abbreviated structural formulae and skeletal formulae.

• They are sometimes referred to as examples of alicyclic hydrocarbons, that is, aliphatic in nature, but with a ring i.e. cyclic-aliphatic compounds, as opposed to an aromatic ring compound based on a benzene ring.

  1. Cyclopropane , , note C-C-C bond angle is 60^o

  2. Methylcyclopropane , ,

  3. 1,1-dimethylcyclopropane , ,

  4. 1,2-dimethylcyclopropane , ,

  5. Ethylcyclopropane , ,

  6. Cyclobutane , , note C-C-C bond angle is 90^o

  7. Methylcyclobutane , ,

  8. 1,1-dimethylcyclobutane , ,

  9. 1,2-dimethylcyclobutane , ,

  10. 1,3-dimethylcyclobutane , ,

  11. Ethylcyclobutane , ,

  12. Cyclopentane , ,

  13. Methylcyclopentane , ,

  14. 1,1-dimethylcyclopentane , ,

  15. 1,2-dimethylcyclopentane , ,

  16. 1,3-dimethylcyclopentane , ,

  17. Ethylcyclopentane , ,

  18. Propylcyclopentane , ,

  19. Butylcyclopentane , ,

  20. Pentylcyclopentane , ,

  21. Cyclohexane , ,

  22. Methylcyclohexane , ,

  23. 1,1-dimethylcyclohexane , ,

  24. 1,2-dimethylcyclohexane , ,

  25. 1,3-dimethylcyclohexane , ,

  26. 1,4-dimethylcyclohexane , ,

  27. Ethylcyclohexane , ,

  28. Propylcyclohexane , ,

  29. Butylcyclohexane , ,

  30. Pentylcyclohexane , ,

  

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  (iii) Larger Alkanes with 8-10 carbon atoms C_nH_2n+2 continued

  Illustrated as brief guide to working out the 18 isomers of non-cycloalkanes C₈H₁₈ !!!!!

  1. Octane , CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃

     • Start with the linear (unbranched) carbon chain, then make the next longest chain with a single, but shortest, carbon branch (-CH₃), to give three methylheptanes ...

  2. 2-methylheptane, (CH₃)₂CHCH₂CH₂CH₂CH₂CH₃

  3. 3-methylheptane, CH₃CH₂CH(CH₃)CH₂CH₂CH₂CH₃

  4. 4-methylheptane, CH₃CH₂CH₂CH(CH₃)CH₂CH₂CH₃

     • then do double methyl branching permutations to make 6 dimethylhexanes ...

  5. 2,2-dimethylhexane, ,

  6. 2,3-dimethylhexane, ,

  7. 2,4-dimethylhexane, ,

  8. 2,5-dimethylhexane, ,

  9. 3,3-dimethylhexane, ,

  10. 3,4-dimethylhexane, ,

      • then you can make one ethylhexane ...

  11. 3-ethylhexane, ,

      • and don't try 2-ethylhexane, because its actually 3-methylheptane using the nomenclature rules correctly.

      • Now you can do a double branching again to make two ethylmethylpentanes ...

  12. 3-ethyl-2-methylpentane, ,

  13. 3-ethyl-3-methylpentane, ,

      • and you can do a triple branching to give four trimethylpentanes ...

  14. 2,2,3-trimethylpentane, (CH₃)₃CCH(CH₃)CH₂CH₃

  15. 2,2,4-trimethylpentane, (CH₃)₃CCH₂CH(CH₃)₂

  16. 2,3,3-trimethylpentane, (CH₃)₂CHC(CH₃)₂CH₂CH₃

  17. 2,3,4-trimethylpentane, (CH₃)₂CHCH(CH₃)CH(CH₃)₂

      • Then finally, the most branched isomer is the single tetramethylbutane (shortest possible main chain).

  18. 2,2,4,4-tetramethylbutane, (CH₃)₃CC(CH₃)₃

  Examples of isomers of C₉H₂₀

• 3-ethyl-2,2-dimethylpentane, ,

• 3-ethyl-2,3-dimethylpentane, ,

• 3-ethyl-2,4-dimethylpentane, ,

• 3-ethyl-2-methylhexane , ,

• 3-ethyl-3-methylhexane , ,

• 3-ethyl-4-methylhexane , ,

• 4-ethyl-2-methylhexane , ,

• 3-ethyl-2,2-dimethylhexane , ,

• 3-ethyl-2,3-dimethylhexane , ,

• 3-ethyl-2,5-dimethylhexane , ,

• 3-ethyl-2,4-dimethylhexane , ,

• 4-ethyl-2,2-dimethylhexane , ,

• 4-ethyl-2,3-dimethylhexane , ,

• 4-ethyl-3,3-dimethylhexane ,

   

  

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