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

Revision Notes PART 2 ALKENES and other unsaturated hydrocarbons

Nomenclature of alkenes names and structure of alkenes How do you name alkenes? The naming of linear alkenes and the nomenclature of branched and substituted alkenes - examples of acceptable names, displayed formula of alkene molecules, graphic formula, molecular formula, skeletal formula, structural formula of this homologous series to illustrate how to name alkenes including cycloalkene hydrocarbons and isomers of the same molecular formula (including geometrical isomers)

Part 2 sub-index for ALKENE structures on this page:

2.1.1 Nomenclature introduction and styles of representation and isomerism

2.1.2 General formula on series of unsaturated hydrocarbons and styles of formulae

2.1.3 Examples of alkenes

2.1.4 Formation and naming of poly(alkenes)

2.1.5 Alkynes

2.1.6 Naming chloroalkenes

Advanced Level ALKENES INDEX

Other advanced level organic chemistry links

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• How do you name alkenes and substituted alkenes?
• The primary suffix name, ..ene for C=C bond, is based on the longest carbon chain: 2 carbons, ethene; 3 carbons, propene; 4 carbons, butene. After these 4 preserved 'old trivial' names, the name is 'numerically' systematic e.g. 5 carbons, pentene; 6 hexene, 7 heptene etc.
  • Note the prefix change from a to e change based on parent alkane name.
  • e.g. propene (no need to say prop-1-ene),
  •  and pent-1-ene where a positional number is needed, which is the lowest carbon number possible for the 1st carbon 'start' of the double bond
    • Note this molecule is also known legitimately as 1-pentene, IUPAC allowed and typically used in the US/USA etc., BUT the pent-2-ene 'style' is preferred by the IUPAC.
    • I'm afraid several variations of nomenclature are allowed by the IUPAC which can make naming a bit confusing at times!

    • (1) , pent-1-ene

    • (2) , pent-2-ene

    • Molecules (1) and (2) above illustrate structural isomers - position isomers from different positions of the alkene functional group (>C=C<), based on the molecular formula C₅H₁₀.

      • Note the -1- and -2- are needed in the name, its the same for butene onwards with increase in carbon number.

      • You can deduce other structural isomers by varying the carbon chain i.e. branching it e.g.

      • ,

        2-methylbut-1-ene is also a carbon chain isomer of C₅H₁₀.

    • See structural isomerism including positional isomerism in alkene compounds for more on this topic, including comments on their different physical properties (also separate notes on E/Z isomerism in alkenes)

    • All the alkenes illustrated above are isomeric with cycloalkanes (saturated)

    • e.g. unsaturated is isomeric with cyclopentane

• If the molecule has a ring of carbon atoms including the double bond, the name is prefixed by cyclo…
  • e.g. cyclohexene and ,
  cyclopenta-1,3-diene (1,3-cyclopentadiene)
• For the non-cyclic alkenes, beyond propene, number(s) (e.g. x and y) are needed to indicate the position of the double bond (e.g. …..-x-ene, like pent-1-ene shown above) or more than one double bond (e.g. …..-x,y-diene, like penta-1,3-diene above and buta-1,3-diene below).
  • These numbers take precedence over substituent numbers and they indicate, via lowest possible number, the first carbon of each C=C double bond which is a higher ranking group than most substituent groups you will encounter.
  • e.g. hex-3-ene (e.g. in UK, 3-hexene in US)
  • or buta-1,3-diene (e.g. in UK, 1,3-butadiene in US)
• The positions of the substituent(s), denoted with a prefix, e.g. halo… for chloro etc. or alkyl groups like methyl, ethyl etc., are denoted by using the lowest possible numbers for the associated carbon atoms in the main chain BUT these 'lowest' substituent numbers are determined by the number assigned to the ....ene group, so they can end up seeming a bit high because of the higher ranking ...ene functional group.
  • e.g.
  • 5-methylhex-1-ene, the alkene functional group (C1) is higher ranking than the substituent methyl group (on C5),
  • or   3-chlorobut-1-ene
    • which is NOT 2-chloro-but-1-ene or 2-chloro-but-2-ene or 2-chloro-but-3-ene etc.
• If there is more than one 'type' of substituent e.g. 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).
• There is a brief note on the formation and structure of poly(alkenes) and the name of the poly(alkene) is readily derived e.g. poly(ethene) [old/everyday names 'polyethylene'/'polythene'] or poly(propene) [old/everyday names 'polypropylene'/'polypropene'] 
• 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 IUPAC nomenclature for naming cis/trans geometrical isomers, or more correctly now, E/Z isomerism notation, is fully explained with examples in Isomerism Section 2. Stereoisomerism, but here I have pointed out where E/Z isomerism exists and the 'old' cis/trans notation. Where appropriate E = trans and Z = cis, so take care 'oldies'!

• The open chain alkenes with one 'ene' group have the general formula C_nH_2n (n = 2, 3, 4 etc.), they are isomeric with cycloalkanes from C₃ onwards.
  • n must be >1 to give a C=C double bond.
  • The empirical formula of open chain alkenes with one C=C double bond is always CH₂.
• The open chain alkenes with two 'ene' groups, i.e. dienes, have the general formula C_nH_2n-2 (n = 3, 4, 5 etc.)
  • n must be >2 to give two carbon-carbon double bonds.
• The cycloalkenes with one 'ene' group in the ring also have the general formula C_nH_2n-2 (n = 3, 4, 5 etc.)
  • n must be >3 to give a ring, although cyclopropene is very unstable due to the C-C-C bond angle strain.
• The cycloalkenes with two 'ene' groups i.e. cyclodienes, in the ring have the general formula C_nH_2n-4 (n = 4, 5, 6 etc.)
  • Cyclopropadiene does not exist (n = 3) because there is two much strain on the C-C-C bond angle if two of the three carbon-carbon bonds are doubles.
• Non-cyclic alkynes have the general formula C_nH_n where n = 2, 3, 4 etc.
• There are many structural isomers in all the above series, either of the form of chain, positional or functional group isomerism e.g. cycloalkane/alkene.
• 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 names in bold are the preferred IUPAC alkene name.

Styles of representing the molecular structure of alkenes

Reminders: Using the alkene propene as an example

, the molecular formula, summation of all the atoms in the molecule, but no structural detail.

, , abbreviated structural formula

  , a sort of intermediate structure, partly displayed and partly structural !!!

, the full displayed formula, showing every atom and every bond - the full picture how everything is connected in the molecule.

, the skeletal formula, no carbon atoms or hydrogen atoms are shown, just a - for a single C-C bond and a = for C=C double bond.

However, if the atom is NOT a C or a H, then it must be shown with the appropriate bond line.

e.g. comparing the skeletal formula of ...

but-1-ene C₄H₈ with 2-chlorobut-1-ene C₄H₇Cl ...

... where you need the extra dash from the 2nd carbon atom to the chlorine atom substituent.

Note the use of the lowest number for the C=C double bond and for the halogen atom too - BUT the C=C bond is the higher ranking functional group.

It isn't a but-2-ene or a 3-chloro ... compound.

In other words you can also write, illustrated with very abbreviated structural formulae derived from C₄H₇Cl:

3-chlorobut-1-ene (NOT 2-chlorobut-3-ene)  CH₃CHClCH=CH₂

and 4-chlorobut-1-ene (NOT 1-chlorobut-3-ene)  ClCH₂CH₂CH=CH₂

however, if the C=C double bond is in the middle of a four carbon chain, then it becomes a but-2-ene.

e.g CH₃CH=CHCH₂Cl is 1-chlorobut-2-ene  and  CH₃CH=CClCH₃ 2-chlorobut-2-ene

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The simplest alkene is ethene (ethylene), molecular formula of (empirical formula CH₂)

•  bonding diagram of ethene ,
• structural formula of ethene
• or the full displayed formula of ethene and the skeletal formula is only
  • All the bond angles (H-C-C or H-C-H) are 120^o because it is a symmetrical planar molecule
  • The hydrocarbon ethene, is the simplest symmetric alkene - meaning the same structural arrangements attached to each of the carbon atoms of the double bond i.e. H & H and H & H.
  • This also means on adding HX, there is only one structural product i.e. CH₃-CH₂X.
• a substituted ethene: chloroethene (old name 'vinyl chloride'), C₂H₃Cl,  CH₂=CHCl

• phenylethene ('styrene'), C₈H₈, is usually named as a derivative of ethene, even though it is also technically an aromatic compound with a benzene ring, the C₆H₅- aromatic ring grouping is called a phenyl group when quoted as a substituent  prefix. So phenylethene is named as a derivative of ethene.

   

The next open chain alkene is propene (propylene),

• , , , , ,
  • H-C-H bond angle is 109^o in the CH₃- group, but the H-C=C, C-C=C, C=C-H and =CH₂ bond angles are all 120^o
  • The hydrocarbon propene, is the simplest asymmetric alkene - meaning different structural arrangements attached to each of the carbon atoms of the double bond i.e. CH₃ & H and H & H.
  • This also means on adding HX, there are two structural isomeric products i.e. CH₃-CHX-CH₃ and CH₃-CH₂-CH₂X.
  • There are three monochloro substituted propenes
    • 1-chloropropene:  CH₃-CH=CHCl, will exhibit E/Z isomerism
    • 2-chloropropene:  CH₃-CCl=CH₂, cannot exhibit E/Z isomerism
    • 3-chloropropene:  ClCH₂-CH=CH₂, cannot exhibit E/Z isomerism

Methylpropene or 2-methylpropene, but 2- is not really needed here, (isobutene, isobutylene), is the simplest branched open chain alkene, , , , , cannot exhibit E/Z isomerism

The C-C=C, C=C-H and =CH₂ bond angles are ~120^o. The H-C-C and H-C-H of the methyl group are ~109^o.

Propadiene, (propa-1,2-diene, but the numbers NOT needed), is the simplest possible open chain 'diene', that is, with two C=C double bonds in the molecule,

• , , ,
• The H-C=C and H₂C= bond angles are ~120^o.  The C=C=C bond angle is 180^o.

The simplest cycloalkene is cyclopropene, , , ,

But-1-ene (1-butene) is the first alkene, without substituent groups, where a positional number is definitely needed,

• , , , cannot exhibit E/Z isomerism
• The hydrocarbon but-1-ene is an asymmetric alkene - meaning different structural arrangements attached to each of the carbon atoms of the double bond i.e. CH₃CH₂CH & H and H & H.
• This also means on adding HX, there are two structural isomeric products based on C₄H₉X
  • i.e. CH₂-CH₃-CHX-CH₃ and CH₃-CH₂-CH₂-CH₂X.
• There four substituted monochloro but-1-enes C₄H₇Cl 
• 1-chlorobut-1-ene:  CH₃-CH₂-CH=CHCl, (1-chloro-1-butene), can exhibit E/Z isomerism

  2-chlorobut-1-ene:  CH₃-CH₂-CCl=CH_2, (2-chloro-1-butene, cannot exhibit E/Z isomerism

  3-chlorobut-1-ene:  CH₃-CHCl-CH=CH_2, 3-chloro-1-butene), cannot exhibit E/Z isomerism

  4-chlorobut-1-ene:  ClCH₂-CH₂-CH=CH_2, (4-chloro-1-butene), cannot exhibit E/Z isomerism

   

2-methylbut-1-ene, , (2-methyl-1-butene), cannot exhibit E/Z isomerism

3-methylbut-1-ene, , , (3-methyl-1-butene), cannot exhibit E/Z isomerism

2,3-dimethylbut-1-ene, , (2,3-dimethyl-1-butene), cannot exhibit E/Z isomerism

3,3-dimethylbut-1-ene, , (3,3-dimethyl-1-butene), cannot exhibit E/Z isomerism

2,3,3-trimethylbut-1-ene, , (2,3,3-trimethyl-1-butene), cannot exhibit E/Z isomerism

But-2-ene (2-butene) is the first alkene (with no substituent groups) to have geometrical isomers (now correctly termed E/Z isomerism E and Z isomers). In old notation the cis isomer is now the Z isomer and the trans isomers is now the E isomer.

•  , structural formula of but-2-ene , but the latter structural formula doesn't show the two different spatial arrangements possible due to a high energy barrier to rotation about the double bond. However, the structural formulae below do show the two possible spatial arrangements of the atoms/groups bonded to the carbon atoms of the C=C double bond.

   

• displayed formula of Z-but-2-ene or is Z-but-2-ene, (cis-2-butene, cis-but-2-ene)

   

•  or is E-but-2-ene, (trans-2-butene, trans-but-2-ene)
• But-2-ene is a symmetric alkene - meaning the same structural arrangements attached to each of the carbon atoms of the double bond i.e. H & CH₃ and H & CH₃.
• This also means on adding HX, there is only one saturated structural product i.e. CH₃-CHX-CH₂-CH₃.
• In simple cases, with two identical/similar groups, the E/trans isomer has these groups 'diagonally' opposite each other across the double bond and the Z/cis isomer has the groups at a 'right angle' to each other or on the same side of the plane of the double bond.
• For a full explanation with examples see ....
• STEREOISOMERISM general definition, E/Z (geometric/geometrical cis/trans) isomerism
• The are two monochloro substituted but-2-enes, both of which would exhibit E/Z isomerism.
  • 1-chlorobut-2-ene:  CH₃-CH=CH-CH₂Cl, (1-chloro-2-butene)

  • 2-chlorobut-2-ene:  CH₃-CH=CCl-CH₃

  , (2-chloro-2-butene)

2-methylbut-2-ene, does not have E/Z isomers because there are two identical groups (CH₃) attached to the same carbon of the double bond,

• ,

2,3-dimethylbut-2-ene, , , 2,3-dimethyl-2-butene, does not have E/Z isomers because there are two identical groups attached to the same carbon of the double bond.

buta-1,2-diene (note the a after the but), is the next simplest diene after propadiene, i.e. with two C=C double bonds,

•  , , ,

, 1,2-butadiene

 

buta-1,3-diene (note the optional a after the but or 1,3-butadiene), is the next diene i.e. 2 C=C double bonds and isomeric with buta-1,2-diene (above),

• , , and has two E/Z isomers,

   

•  , cis/Z-buta-1,3-diene,

   

• , trans/E-buta-1,3-diene

• 2-methylbuta-1,3-diene is the synthetic rubber monomer 'isoprene'.

Cyclobutene is the next simplest cyclo-alkene after cyclopropene

• , , ,

Cyclobuta-1,3-diene is the simplest cyclo-diene that exists, , ,

Pent-1-ene, , , (1-pentene)

• A selection of methyl substituted methylpent-1-enes and an ethylpent-1-ene

• 2-methylpent-1-ene, ,

(2-methyl-1-pentene)

• 3-methylpent-1-ene, ,

 (3-methyl-1-pentene)

• 4-methylpent-1-ene, ,

 (4-methyl-1-pentene)

• 2,3-dimethylpent-1-ene, ,

  (2,3-dimethyl-1-pentene)

• 2,4-dimethylpent-1-ene, ,

  (2,4-dimethyl-1-pentene)

• 3,3-dimethylpent-1-ene, ,

  (3,3-dimethyl-1-pentene)

• 3,4-dimethylpent-1-ene, ,

  (3,4-dimethyl-1-pentene)

 

• 4,4-dimethylpent-1-ene, ,

  (4,4-dimethyl-1-pentene)

• 3-ethylpent-1-ene, ,

(3-ethyl-1-pentene)

Pent-2-ene, , (2-pentene two E/Z isomers)

• Z/cis- and E/trans-

 

• A selection of methyl substituted pent-2-enes and an ethylpent-2-ene

• 2-methylpent-2-ene, ,

(2-methyl-2-pentene) no E/Z isomers

• 3-methylpent-2-ene, (3-methyl-2-pentene) has two E/Z isomers:

  •  , , E-3-methylpent-2-ene

  • and   ,

  Z-3-methylpent-2-ene

• 4-methylpent-2-ene, , has two E/Z isomers:

  • Z/cis- , , E/trans-

  ,

• 2,3-dimethylpent-2-ene,

(2,3-dimethyl-2-pentene), no E/Z isomers

• 2,4-dimethylpent-2-ene,

(2,4-dimethyl-2-pentene), no E/Z isomers

• 3,4-dimethylpent-2-ene, (3,4-dimethyl-2-pentene)

  • has two E/Z isomers: E- , and Z-

  3,4-dimethylpent-2-ene

   

• 4,4-dimethylpent-2-ene,   (4,4-dimethyl-2-pentene) has two E/Z isomers:

  • Z/cis- , and E/trans-

• 3-ethylpent-2-ene, , (3-ethyl-2-pentene)

  • no E/Z isomers because there are two identical groups attached to the same carbon of the double bond

cyclopentene, , ,

cyclopenta-1,3-diene, , (1,3-cyclopentadiene)

hex-1-ene, , ,   (1-hexene)

2-methylhex-1-ene, ,  (2-methyl-1-hexene)

• There are no E/Z isomers of -1-enes because there are two identical groups (H) attached to the same carbon of the double bond

.

 

• You can also put a methyl substituent (or anything else) on carbons 3, 4 and 5

• 3-methylhex-1-ene, ,

  (3-methyl-1-hexene)

• 4-methylhex-1-ene, ,

  (4-methyl-1-hexene)

• 5-methylhex-1-ene, ,

(5-methyl-1-hexene)

Hex-2-ene, , 2-hexene has two E/Z isomers:

• Z/cis- , , and

• E/ trans-, ,

 

• There are four monosubstituted methylhex-2-enes

• 2-methylhex-2-ene, ,

(2-methyl-2-hexene)

• 3-methylhex-2-ene, ,

  • has two E/Z isomers: E-and Z-

  (3-methyl-2-hexene)

• 4-methylhex-2-ene, ,

  • has two E/Z isomers: Z/cis- , and E/trans-

• 5-methylhex-2-ene, , has two E/Z isomers:

   Z/cis- and E/trans-

Hex-3-ene, , 3-hexene has two E/Z isomers:

• Z/cis- , E/trans-

 

• and two mono substituted methylhex-3-enes

• 2-methylhex-3-ene, , has E/Z isomers:

  •  Z- and E-

  (cis and trans 2-methyl-3-hexene)

• 3-methylhex-3-ene, has two E/Z isomers:

  • Z- and E-

   (cis and trans 3-methyl-3-hexene)

cyclohexene, , ,

cyclohexa-1,3-diene, ,  (1,3-cyclohexadiene)

cyclohexa-1,4-diene, , , (1,4-cyclohexadiene)

hept-1-ene, , , (1-heptene)

 has no E/Z isomers

hept-2-ene, , ,

• has two E/Z isomers: Z/cis- and E/trans-

(cis and trans 2-heptene)

hept-3-ene, ,

• has two E/Z isomers: Z/cis- and E/trans- (cis and trans 3-heptene)

1. C₂H₃Cl,   chloroethene, no E/Z isomers

2. C₃H₅Cl,   1-chloropropene, has E/Z isomers

3. C₃H₅Cl,   2-chloropropene, no E/Z isomers

4. C₃H₅Cl,   3-chloropropene, no E/Z isomers

5. C₄H₇Cl,   1-chlorobut-1-ene (1-chloro-1-butene), has E/Z isomers

6. C₄H₇Cl,   2-chlorobut-1-ene (2-chloro-1-butene), no E/Z isomers

7. C₄H₇Cl,   3-chlorobut-1-ene (3-chloro-1-butene), no E/Z isomers

8. C₄H₇Cl,   4-chlorobut-1-ene (4-chloro-1-butene), no E/Z isomers

9. C₄H₇Cl, 4-chlorobut-2-ene (4-chloro-2-butene), has E/Z isomers

10. C₄H₇Cl,   2-chlorobut-2-ene (2-chloro-2-butene), has E/Z isomers

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LINKS TO ASSOCIATED ADVANCED ORGANIC CHEMISTRY PAGES

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formula keywords: how to name naming nomenclature empirical molecular formula graphic formula displayed formula skeletal formula structural isomers E/Z cis/trans isomerism 2.1.1 Nomenclature introduction * 2.1.2 General formula note on various series of unsaturated hydrocarbons * 2.1.3 Examples of alkenes * 2.1.4 Formation and naming of poly(alkenes) * 2.1.5 Alkynes * 2.1.6 Naming chloroalkenes C2H2 C2H4 CH2=CH2 C3H4 C3H6 CH3CH=CH2 CH3-CH=CH2 C4H4 C4H6 C4H8 CH2=CHCH2CH3 CH3-CH2-CH=CH2 CH3CH=CHCH3 CH3-CH2-CH2-CH3 C5H8 C5H10 C6H12 C2H3Cl CH2=CHCl C3H5Cl CH3CH=CHCl CH3CCl=CH2 C4H7Cl CH3CH2CH=CHCl CH3CHClCH=CH2 CH3CH2CCl=CH2 C5H10 C6H12 C7H14 chemistry revision notes structure of alkenes AS AQA GCE A level chemistry how do you name alkenes? AS Edexcel GCE A level chemistry alkene nomenclature rules AS OCR GCE A level chemistry what is the molecular structure of alkenes? AS Salters GCE A level chemistry how to work out isomers of alkenes US grades 11 & 12 chemistry IUPAC naming of alkenes notes for revising the structure and naming of linear and cyclic alkenes These detailed notes on the structure and naming of alkenes include the general formula of alkene molecules, empirical formula of alkene molecules, structural formula of alkene molecules, skeletal formula of alkene molecules, displayed formula of alkene molecules, shapes of alkene molecules, isomers of alkene molecules IUPAC rules for alkene nomenclature. Students should be able to draw structural formula of alkene, displayed and skeletal formulas for alkene organic compounds apply IUPAC rules for nomenclature to name alkene 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 alkene organic compounds from the alkene IUPAC name from the homologous series of alkenes

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