1.1 What are ALKANES?

Their molecular structure and nomenclature - naming of alkanes

Part 1. ALKANES and the PETROCHEMICAL INDUSTRY - Doc Brown's Advanced A Level Organic Chemistry Revision Notes

This is quite a long page because, via alkanes, its a very important introduction to most aspects of organic chemistry nomenclature in general

Lots of examples of alkanes nomenclature structure All you need to know about naming alkanes How do you name alkanes? Nomenclature of substituted alkanes - examples of the structure of alkanes, naming alkanes, acceptable names, displayed formula of alkane molecules with names, graphic formula of alkanes, molecular formula of alkanes, skeletal formula of alkanes, structural formulae of the homologous series of alkanes, linear alkanes, branched alkanes, alkyl substituted cycloalkanes etc. Everything you need on the naming and structure of alkanes pre-university!

IUPAC = International Union of Pure and Applied Chemistry, the body that decides on chemistry nomenclature.

A basic introduction to the structure and chemistry of alkanes

(including some diagrams which haven't been repeated on this page).

A Level Organic Chemistry Part 1 sub-index for this page only:

1.1.1 Alkane nomenclature explained and isomerism

1.1.2 Different ways of representing molecules and classes of formula

1.1.3 Comparison of aliphatic/alicyclic/aromatic compounds - definition & examples

1.1.4 Alkane isomerism explained

1.1.5 Homologous series of alkane examples, general formula C_nH_2n+2 including isomers of formulae up to C₇H₁₆ (n = 7)

1.1.6 Examples of cycloalkanes 'alicyclic' compounds of general formula C_nH_2n

1.1.7 All 18 alkane isomers of C₈H₁₈

1.1.8 Examples of alkane isomers of C₉H₂₀

1.1.9 Examples of alkane isomers of C₁₀H₂₂

Important notes on how to deduce empirical and molecular formula including alkane examples

Calculation of empirical formula, deducing molecular formula of a compound starting with % mass composition)

and also

Combined multiple choice and type in name quiz on the structure and naming of alkanes

Simple matching pair quiz on hydrocarbon structure

1.1.1 A brief guide to the structure and nomenclature of non-cyclic alkane hydrocarbons

• Most alkanes you will come across are relatively small molecules in which all the chemical bonds are covalent bonds.
• All the bonds in alkane molecules are single covalent bonds i.e. C–C carbon – carbon or single C–H carbon – hydrogen bonds.
• Each carbon atom forms four single sigma bonds and hydrogen atoms form one single sigma bond.
• All single covalent bonds are formed by sharing a pair of electrons e.g. one from each of a carbon atom and a hydrogen atom, or two carbon atoms contributing (sharing) an electron each to the covalent bond.
• These single covalent C-C and C-H bonds are known as sigma bonds (σ).
• Four  hydrogen atoms (1 outer electron) and one  carbon atom (four outer electrons) combine to form methane so that the hydrogen atoms are electronically like helium (full outer shell of 2 electrons) and the carbon atom becomes like neon (with a full outer shell of 8 electrons, the two inner electrons of carbon are not shown).
  • In advanced hybridisation theory, the 1s orbitals of the hydrogen atom (1s¹) overlap with the four 2sp³ hybrid orbitals of carbon (1s²2s²2p²) to form the four sigma covalent bonds.
• In terms of 'dot and cross' Lewis diagrams the structures of alkanes would be drawn as ...
• or the alkane methane
• Similarly six hydrogen atoms combine with two carbon atoms to form the ethane molecule.
• or the alkane ethane
• BUT, these do not show the 3D shape of alkane molecules, but see later.
• General formula and structural formula of alkanes and alkane nomenclature (naming alkane compounds)
• The first 10 of the homologous series of alkanes are tabulated below.
• Members of a homologous series have a similar molecular structure and a common general formula.
  • Members of a homologous series have similar physical and chemical properties.
• In their naming (nomenclature), the suffix ...ane, tells you the molecule belongs to the homologous series of alkanes.
• Other than alkanes, all other homologous series of organic compounds have a functional group e.g. >C=O or -COOH which gives this set of molecules particular chemical properties (see homologous series and functional groups).
• Reminders:
  • The molecular formula sums up ALL the atoms in the molecule e.g. an alkane hydrocarbon molecule.
  • The empirical formula gives you the simplest whole number (integer) ratio of the atoms in the molecule.
  • In the table below for non-cyclic alkanes (linear, but the same for branched alkanes):
    • when n, the number of carbon atoms is odd, the empirical formula and molecular formula are identical.
    • when n, the number of carbon atoms is even, the empirical formula and molecular formula are different.

• The structural formula can be written simply as e.g. CH₃CH₃ (ethane), CH₃CH₂CH₂CH₂CH₃ or CH₃(CH₂)₃CH₃ pentane

  • CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃ or CH₃(CH₂)₆CH₃ for octane etc.

    • More on molecular representations in 1.1.2., but the style of molecular representation above would be described as an shortened, abbreviated or condensed structural formulae.

    • It is an unambiguous representation, as long as you can translate it correctly into e.g. the full displayed formula showing every atom and bond.

    • e.g. for butane, the abbreviated structural formula CH₃CH₂CH₂CH₃ becomes  

    • This would be described as a linear alkane because there are no side-chains or branches.

  • The structural formula of the alkyl groups are written in a similar manner to the above minus a H e.g.

  • CH₃CH₂ ethyl, CH₃CH₂CH₂ propyl, CH₃CH₂CH₂CH₂CH₂ butyl or CH₃(CH₂)₃CH₂ pentyl

    • Note the use of (brackets) to shorten the octane molecule and pentyl group even further.

    • Further note that no individual C-C or C-H bond is shown in the abbreviate structure.

    • Watch out when using abbreviations of alkyl groups!

    • e.g. -C₂H₅ can only be -CH₂CH₃, no problem, no ambiguity

    • BUT from -C₃H₇ onwards there at are least 2 possibilities!

    • e.g. -C₃H₇ might be -CH₂CH₂CH₃  OR  -CH(CH₃)₂, so take care in ANY organic formula with an alkyl grouping of over two carbon atoms.

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How to name alkanes (introduction to the nomenclature of alkanes)

• 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 'historic' preserved 'old trivial' names, the name is 'numerically' systematic according to IUPAC

    • e.g. C₅ carbon chain pentane; C₆ chain hexane, C₇ chain heptane, C₈ chain octane, C₉ chain nonane, C₁₀ chain decane etc.

    • The IUPAC acronym stands for the International Union of Pure and Applied Chemistry, and one responsibility of this organisation is the development and maintenance of a systematic method of naming compounds that is recognised by chemists from all around the world - an truly international nomenclature.

  • The table above listed the molecular formula and names of the first ten linear alkanes (the term linear applies to butane onwards, i.e. from whence structural chain isomerism is possible.

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

  • e.g. pentane is linear or unbranched, the C-C bonds are shown here.

• If another chain of carbon atoms starts out of the main carbon chain, it is referred to as branching, giving rise to 'branched' alkane, one with a side-chain - so this is how to name branched alkanes.

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

  • The longest continuous chain of 5 carbon atoms forms the basis of the name - more on this below.

  • Note that you need a minimum of 4 carbon atoms in the alkane molecule to create a side chain branch as in the case of methylpropane. Here the three C-C bonds are shown, but just one of the C-H bonds.

  • You must always use the longest continuous carbon chain as the basis of the IUPAC systematic name for alkanes and the lowest possible substituent numbers.

    • For example it would be wrong to name the above molecules trimethylmethane, and

    • would not be named 2-propylpropane or 4-methylpentane, but 2-methylpentane,

    • would not be named 2-ethylbutane or 3-ethylbutane, but 3-methylpentane

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

• 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 and can't have an alky substituent, since that merely elongates the longest carbon chain.

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

  • 3-ethyl-2,4-dimethylhexane , is good example!

  • Note the use of the lowest number 2 as the basis for working out all the other numbers!

  • This is example is quoted from near the end of this page, but there are numerous simple examples before that!

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

• The alkane names in bold are the preferred IUPAC names.

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1.1.2 More on ways of representing the structure of molecules and types 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 C₃H₈ it is C₃H₈, (same as molecular formula)

  • and for butane or 2-methylpropane, both C₄H₁₀, it is C₂H₅. (different from the molecular formula, ÷2).

  • If the number of carbon atoms is an even number, the empirical formula is 'half' the molecular formula.

  • If the number of carbon atoms is an odd number, the empirical formula is the same as the molecular formula.

  • For empirical formula and molecular formula calculations see ....

    • Empirical formula and formula mass of a compound from reacting masses (simple start)

    • Using moles to calculate empirical formula and deduce molecular formula of a compound

• 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 examples of the more advanced IUPAC naming system that accompanies the unambiguous representation and naming of molecular structures.

• An abbreviated, condensed 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 but it doesn't always show all the bonds.

  • propane  or , abbreviated and more detailed styles of alkane structural formulae.

  • The abbreviated structural formula is the minimum possible unambiguous representation of a molecule AND essential to distinguish different structures of the same molecular formula i.e. structural isomers like methylpropane and butane.

  • methylpropane

  • butane  or

  • Note that methylpropane and butane can now be distinguished, BUT, you must be able to envisage these correctly into a full displayed formula 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 , unfortunately it does give the impression that the bond angles are 90^o, rather than the true angles, many of which are ~109^o based on the tetrahedrally arrangement of four single bonds around the carbon atoms in alkane molecules.

  • methylbutane

  • butane

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

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

  •  

• A stereochemical formula is the full displayed formula in terms of a 3D structural or spatial arrangement of the atoms (albeit on a '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 out of the plane of the screen/paper.

  • All the C-C-C, C-C-H or H-C-H bond angles are 109^o in this case, and similarly for all other non-cyclic alkanes.

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

  • This kind of '3D' molecular representation of alkanes, emphasises the tetrahedral arrangement of the four bonds from any carbon atom to other carbon or hydrogen atoms - in fact all the C-C or C-H bonds are usually ~109^o.

  • An important point to appreciate from a 3D representation is that for sigma bonds (single C-C bonds here) there is free rotation of any alkyl group about any C-C bond.

  • Ethane is also represented by a ball-and-stick model molecular diagram AND a space-filling model molecular diagram.

• is the skeletal formula for propane. Wow, that simple!

  • Skeletal formula (e.g. of alkanes) are very simplified organic formulae shown by removing hydrogen atoms from alkyl chains leaving just lines for the carbon skeleton bonds and associated functional groups (the latter does not apply to alkanes.

    • Skeletal formula should be drawn in such way as to give an impression of the shape of the molecule, at least an idea of the spatial arrangement of the carbon chain - the bond angles of the 'backbone' of the molecule.

  • 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. (Note: = means a double bond, ≡ a triple bond)

  • The symbols for carbon atoms and hydrogen atoms are NOT usually shown in a skeletal formula.

  • However, bond lines should be drawn for C-X bonds, where X is not a hydrogen atom, and the symbol for the X atom should be shown too (see e.g. see halogenoalkanes or 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', but actually zig-zag C-C-C-C chain of 4 carbon atoms with no branching.

  • represents 3-methylhexane. A zig-zag line of 5 C-C bonds (but 6 C atoms) and a dash from the 3rd carbon atom indicating the C-C bond to the methyl group.

  • is 3-ethylpentane, you need an extra to indicate ethyl.

• See also ball and stick/ball and spring models and space-filling models for ethane.

• A general formula C_nH_2n+2 for non-cyclic alkanes, represents a member of a homologous series when n is designated an integer value for the number of carbon atoms (in the alkane molecule)

  • e.g. if n = 5, it gives the molecular formula of pentane C₅H₁₂.

  • A homologous series is a series of compounds with the same functional group 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.

  • A functional group is a group of atoms responsible for the characteristic reactions of a compound - but alkanes don't have a functional group like the rest of the homologous series of organic molecules.

  • BUT, alkanes do not have a specific functional group, but they do form a homologous series of compounds with the same sort of molecular structure and similar chemistry, namely chains of carbon atoms combined with the maximum number of hydrogen atoms - so they are saturated hydrocarbon molecules - no double bonds like alkenes or triple bonds like alkynes.

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1.1.3 A simplified structural comparison of aliphatic, alicyclic and aromatic HYDROCARBON compounds

• Reminder - a hydrocarbon is a molecule composed of ONLY carbon and hydrogen atoms.

• You often study the details of alkanes when dealing with the fractional distillation of crude oil, uses of the fractions and cracking and reforming to make other hydrocarbon products.

• You will therefore encounter four types of hydrocarbon, whose basic structures you should make yourself familiar with.

• The names in bold are the preferred IUPAC names.

• 1. ALIPHATIC - have no benzene ring in their structure (see 3.) and can have an open linear carbon chain, or branched carbon chain structure or a carbon ring structure - they are saturated hydrocarbons, no >C=C< double bonds or benzene ring.

  • If they have a cyclic structure, they may be termed alicyclic, but they are still aliphatic (see 2.).

  • Examples of linear and branched aliphatic alkane compounds.

  • heptane, or    (a linear non-branched alkane)

  • 2-methylbutane, or   (a branched alkane)

  • and lots more examples on this page!

  • Note the general formula for non-cyclic alkanes is C_nH_2n+2  irrespective of whether they are branched or unbranched.

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

  • Cyclobutane, cyclopentane, cyclohexane

  • ,     and    ,     and    , , no branches here

  • Methylcyclohexane, or , a branched example of an alicyclic alkane.

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

  • NOTE that later in your course you will meet lots of other homologous series of aliphatic compounds e.g. alkenes, alcohols and halogenoalkanes etc. some of which may have an aliphatic/alicyclic ring structure.

  • Note the general formula for cyclic alkanes (alicyclic) is C_nH_2n  (two hydrogens less than non-cyclic alkanes), take care, this is the same general formula for non-cyclic alkenes with one double bond, so this is a case of isomerism - isomers.

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

  • benzene C₆H₆, or

  • methylbenzene C₇H₈, C₆H₅CH₃ or or or

  • Aromatic hydrocarbons are called arenes (C atoms combined with H atoms only and an aromatic ring).

  • These are manufactured by reforming processes on fractions for crude oil.

  • and lots more on the structure of 'Aromatic molecules' page whether they be aromatic hydrocarbons or phenols etc.

  • and the naming and structure of alkenes is on another page.

• 4. ALKENES - these are also aliphatic hydrocarbon molecules contain at least one carbon-carbon double bond (>C=C<), they are referred to as unsaturated hydrocarbons.

  • These are manufactured by cracking various fractions for crude oil.

  • Examples of alkenes.

  • propene

  • ,

    2-methylbut-1-ene.

  • Again, like alkanes, you can have aliphatic cyclic (alicyclic) alkenes.

  • The structure of cyclohexene is shown on the right.

• 5. ALKYNES - these are aliphatic unsaturated hydrocarbons with a triple C≡C bond.

  • e.g. ethyne   and  propyne

• 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 definition classifications e.g.

    • aliphatic: 2-chloropropane, or

    • alicyclic: fluorocyclopropane, ,

     

    • aromatic: 2,3-dichloromethylbenzene,

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1.1.4 A brief guide to working out isomers of non-cycloalkanes C_nH_2n+2

The alkane names in bold are the preferred IUPAC names.

• Isomers are molecules with the same molecular formula but different in some way in their molecular structure.

• With alkanes you get structural isomerism because you can vary the arrangement of atoms in the carbon chain.

• These can be referred to as structural chain isomers of alkanes.

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

• However, from C₄H₁₀ onwards 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.

  • 

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

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

   

• For C₅H₁₂ you can make 3 structural chain isomers - same molecular formula, but atoms arranged-connected differently:

  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.

• The structural chain isomerism of alkanes is fully described on another page, including the differences in physical properties.

• You can extend these ideas from C₆H₁₄ onwards, working out by trial and error all the possible branchings in terms of methyl, dimethyl, trimethyl or ethyl groupings etc. I've described lots of alkane examples on this page.

  • Make sure you don't:

    • (i) do the same molecule bent into different shapes on your 2D paper, if all the bond connections are the same its not a different structural chain isomer!

    • (ii) draw mirror images of exactly the same structural chain isomer.

• The 18 structural chain isomers of C₈H₁₈ are worked out for you in section 1.1.7 and anything else from C₆-C₇ or C₉ onwards, you should be able to eventually work out for yourself!

• Optical (R/S) isomerism occurs from molecular formula C₇H₁₆ onwards.

  • R/S isomerism occurs when you have a chiral carbon atom or asymmetric carbon atom in the molecule, that is a carbon atom with 4 different atoms/groups attached to it.

  • It is then possible to have two non-superimposable mirror image forms (R/S isomers or enantiomers) - two molecular versions with the same non-cyclic alkane molecular formula.

  • 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 attached to the central asymmetric/chiral carbon atom.

  • The mirror-image forms (enantiomers) would be extremely difficult to separate e.g they would have identical boiling points.

  • Other examples and explanation of Optical (R/S) Isomerism.

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

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

   • The H-C-H bond angle is 109.5^o giving a perfect tetrahedral shape so 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.

   • is a 3D ball and stick representation of the methane molecule.

   • or the dot and cross diagram for methane, just showing the outer valency electrons.

   • The tetrahedral shape of methane arises from the mutual repulsion of four bonding pairs of electrons around the central carbon atoms.

     • Although only methane can have a tetrahedral shape, the same electron pair repulsion theory predicts, correctly, that, apart from cyclopropane, cyclobutane and cyclopentane, all the C-C-C, C-C-H and H-C-H bond angles will be approximately 109^o in linear alkanes, branched alkanes and cyclic alkane molecules from cyclohexane onwards.

     • For more see dot & cross diagrams and molecule shapes derived from electron pair repulsion theory

    

2. structural formulae of ethane, (a) or (b)

   1. or displayed formulae (c) or (d)

   • 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!

   •   dot and cross diagram for ethane, just showing the outer valency electrons.

   • The shape is essentially derived from two tetrahedral bond networks linked by the common C-C bond.

   • ball-and-stick model of ethane showing the typical bond angles of approximately 109^o for H-C-H and C-C-H bond angles in non-cyclic saturated hydrocarbons i.e. alkanes.

   • space-filling model of ethane

    

3. (a) or (b) or

   (c) or (d)

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

   • (c) and (d) full displayed 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)

   • All the C-C-C, C-C-H or H-C-H bond angles are ~109^o. and this applies to all the rest of the linear or branched alkanes which are NOT cyclic.

    

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

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

   • Full displayed formula (ignoring bond angles!)

    

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.

   • This molecule is isomeric with butane above.

   • This is the first instant of the existence of structural isomers, (carbon chain isomers), for an alkane molecular formula (C₄H₁₀) - different carbon chain arrangements, not possible with methane, ethane or propane

   • The structural chain isomerism of alkanes is fully described on another page, including the differences in physical properties.

    

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),

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

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

   • Molecules 6-8 are isomeric with each other.

   • The structural chain isomerism of alkanes is fully described on another page, including the differences in physical properties.

    

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 definitely 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)

    • Molecules 9-13 are isomeric with each other - this is a good example of working out all the structural chain isomers of the alkane with a particular molecular formula, in this case C₆H₁₄. Start with the linear molecule (hexane), then one methyl branch (pentanes) and the two methyl branches (butanes) and you derive the five possible structural chain isomers of C₆H₁₄. Note that you cannot make an ethylpropane - its actually a methylpentane! check it out for yourself!

    • The structural chain isomerism of alkanes is fully described on another page, including the differences in physical properties.

     

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) , has an asymmetric carbon atom so will exhibit optical (R/S) isomerism

     

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)  , has an asymmetric carbon atom so will exhibit optical (R/S) isomerism

     

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, C₃H₆ , , note C-C-C bond angle is 60^o

  because all three carbon atoms are in the same plane, the only cycloalkane in which all the carbon atoms are in the same plane, The H-C-H bond angle is 115^o. It means the C-C-C bond system is highly strained and easily broken unlike the majority of C-C-C bonds in alkane molecules. The C-C bond is so strained the cyclopropane molecule is more reactive than propane and quite readily isomerises to the more stable alkene propene.

  2. methylcyclopropane, C₄H₈ , ,

      

  3. 1,1-dimethylcyclopropane, C₅H₁₀ , ,

      

  4. 1,2-dimethylcyclopropane, C₅H₁₀ , ,

      

  5. ethylcyclopropane, C₅H₁₀ , ,

      

  6. cyclobutane, C₄H₈ , , note the C-C-C bond angle may look as if it is 90^o

  but there are two slightly angled 'butterfly' conformations which oscillate from one form to another with a C-C-C bond angle of ~88^o. In other words the carbon atoms are NOT quite in the same plane, which is the case for every cycloalkane except cyclopropane (above).  The H-C-H bond angles are ~107^o in cyclobutane. The shape means the C-C-C-C bond system is highly strained and easily broken unlike the majority of C-C-C-C bonds in alkane molecules, so the cyclobutane molecule tends to be more reactive than butane.

  7. methylcyclobutane, C₅H₁₀ , ,

      

  8. 1,1-dimethylcyclobutane, C₆H₁₂ , ,

      

  9. 1,2-dimethylcyclobutane, C₆H₁₂ , ,

      

  10. 1,3-dimethylcyclobutane, C₆H₁₂ , ,

       

  11. ethylcyclobutane, C₆H₁₂ , ,

       

  12. cyclopentane, C₅H₁₀ , , , all bond angles ~108^o, so it is NOT quite planar, the pentagonal ring is slightly bent, but all the C-C-C, C-C-H and H-C-H bond angles are close to the 'tetrahedral' angle of ~109^o.

       

  13. methylcyclopentane, C₆H₁₂ , ,

       

  14. 1,1-dimethylcyclopentane, C₇H₁₄ , ,

       

  15. 1,2-dimethylcyclopentane, C₇H₁₄ , ,

       

  16. 1,3-dimethylcyclopentane, C₇H₁₄ , ,

       

  17. ethylcyclopentane, C₇H₁₄ , ,

       

  18. propylcyclopentane, C₈H₁₆ , ,

       

  19. butylcyclopentane, C₉H₁₈ , ,

       

  20. pentylcyclopentane, C₁₀H₂₀ , ,

       

  21. cyclohexane, C₆H₁₂ , , , all bond angles ~109^o NOT 120^o, the hexagon is bent into its most stable double pointed chair form - its most stable 3D conformation (skeletal formula diagram shown on the right).

        ball-and-stick diagram of cyclohexane

       

  22. methylcyclohexane, C₇H₁₄ , , =

       

  23. 1,1-dimethylcyclohexane, C₈H₁₆ , ,

       

  24. 1,2-dimethylcyclohexane, C₈H₁₆ , ,

       

  25. 1,3-dimethylcyclohexane, C₈H₁₆ , ,

       

  26. 1,4-dimethylcyclohexane, C₈H₁₆ , ,

       

  27. ethylcyclohexane, C₈H₁₆ , ,

       

  28. propylcyclohexane, C₉H₁₈ , ,

       

  29. butylcyclohexane, C₁₀H₂₀ , ,

       

  30. pentylcyclohexane, C₁₁H₂₂ , ,

1.1.7 to 1.1.9 Larger Alkanes with 8-10 carbon atoms C_nH_2n+2 continued where n = 8, 9 or 10

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

The alkane names in bold are the preferred IUPAC names.

(1) Octane, C₈H₁₈, 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

(1) to (4) isomers of C₈H₁₈ are further illustrated above, together with the appropriate carbon atom numbers.

.... 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, ,  

Note both the alphabetical AND numerical order in the systematic IUPAC name, the 1st branch has the lowest number (methyl in this case).

(13) 3-ethyl-3-methylpentane,  ,

Again, note the alphabetical, but both have the same branch number in the systematic IUPAC name ...

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

(14) 2,2,4-trimethylpentane (isooctane), (CH₃)₃CCH₂CH(CH₃)₂, _,

(15) 2,2,3-trimethylpentane, (CH₃)₃CCH(CH₃)CH₂CH₃

(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) 2,2,3,3-tetramethylbutane, (CH₃)₃CC(CH₃)₃

(14) to (18) are further illustrated above, together with the appropriate carbon atom chain numbers.

TOP OF PAGE and sub-index

1.1.8 Examples of isomers of C₉H₂₀  (some have an asymmetric (chiral) carbon atom so will exhibit optical (R/S) isomerism)

The alkane names in bold are the preferred IUPAC names.

Note both the alphabetical AND numerical order in the systematic IUPAC name, the 1st branch has the lowest number.

nonane, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃  or  CH₃(CH₂)₇CH₃

2-methyloctane, CH₃CH₂CH₂CH₂CH₂CH₂CH(CH₃)₂

3-methyloctane, CH₃CH₂CH₂CH₂CH₂CH(CH₃)CH₂CH₃

4-methyloctane, CH₃CH₂CH₂CH₂CH(CH₃)CH₂CH₂CH₃

There are lots of highly branched structural isomers of C₉H₂₀ e.g.

3-ethyl-2,2-dimethylpentane, ,

3-ethyl-2,3-dimethylpentane, ,

3-ethyl-2,4-dimethylpentane, , 

3-ethyl-2-methylhexane , ,

TOP OF PAGE and sub-index

1.1.9 Examples of isomers of C₁₀H₂₂  (many have an asymmetric (chiral) carbon atom so will exhibit optical (R/S) isomerism)

Note both the alphabetical AND numerical order in the systematic IUPAC name, the 1st branch has the lowest number.

The alkane names in bold are the preferred IUPAC names.

decane, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃  or  CH₃(CH₂)₈CH₃

2-methylnonane, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH(CH₃)₂

3-methylnonane, CH₃CH₂CH₂CH₂CH₂CH₂CH(CH₃)CH₂CH₃

4-methylnonane, CH₃CH₂CH₂CH₂CH₂CH(CH₃)CH₂CH₂CH₃

5-methylnonane, CH₃CH₂CH₂CH₂CH(CH₃)CH₂CH₂CH₂CH₃

There are lots of highly branched structural isomers of C₁₀H₂₂ e.g.

3-ethyl-2,2-dimethylhexane , ,

3-ethyl-2,3-dimethylhexane , ,

3-ethyl-2,5-dimethylhexane , ,

3-ethyl-2,4-dimethylhexane , ,

formula keywords: how to name naming nomenclature empirical molecular formula graphic formula displayed formula skeletal formula structural isomers isomerism  CH4 C2H6 CH3CH3 CH3-CH3 C3H8 CH3CH2CH3 CH3-CH2-CH2-CH3 C4H10 CH3CH2CH2CH3 CH3-CH2-CH2-CH3 C4H8 C5H12 C5H10 C6H14 C6H12 C7H16 C8H18 C9H20 C10H22 chemistry revision notes structure of alkanes AS AQA GCE A level chemistry how do you name alkanes? AS Edexcel GCE A level chemistry alkane nomenclature rules AS OCR GCE A level chemistry what is the molecular structure of alkanes? AS Salters GCE A level chemistry how to work out isomers of alkanes US grades 11 & 12 chemistry IUPAC naming of alkanes notes for revising the structure and naming of linear and cyclic alkanes how to name alkanes for AQA AS chemistry, how to name alkanes for Edexcel A level AS chemistry, how to name alkanes for A level OCR AS chemistry A, how to name alkanes for OCR Salters AS chemistry B, how to name alkanes for AQA A level chemistry, how to name alkanes for A level Edexcel A level chemistry, how to name alkanes for OCR A level chemistry A, how to name alkanes for A level OCR Salters A level chemistry B how to name alkanes for US Honours grade 11 grade 12 how to name alkanes for pre-university chemistry courses pre-university A level revision notes for how to name alkanes  A level guide notes on how to name alkanes for schools colleges academies science course tutors images pictures diagrams for how to name alkanes A level chemistry revision notes on how to name alkanes for revising module topics notes to help on understanding of how to name alkanes university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 11 grade 11 AQA A level chemistry notes on how to name alkanes Edexcel A level chemistry notes on how to name alkanes for OCR A level chemistry notes WJEC A level chemistry notes on how to name alkanes CCEA/CEA A level chemistry notes on how to name alkanes These detailed notes on the structure and naming of alkanes include the general formula of alkane molecules, empirical formula of alkane molecules, structural formula of alkane molecules, skeletal formula of alkane molecules, displayed formula of alkane molecules, shapes of alkane molecules, isomers of alkane molecules IUPAC rules for alkane nomenclature. Students should be able to draw structural formula of alkane, displayed and skeletal formulas for alkane organic compounds apply IUPAC rules for nomenclature to name alkane acid organic compounds including chains and rings and be able to apply IUPAC rules for nomenclature to draw the structural, displayed or skeletal structure of alkane organic compounds from the alkane IUPAC name from the homologous series of alkanes

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