Advanced Level pre-university Organic Chemistry: Alkenes - their naming and structure

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alkenes structure and naming (c) doc balkenes structure and naming (c) doc b2.1 The molecular structure and naming (nomenclature) of ALKENES (plus short note on alkynes)

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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 E/Z 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


2.1.1 An Introduction Alkene Nomenclature

The names in bold are the preferred IUPAC alkene name.

  • How do you name alkenes and substituted alkenes?
  • The primary suffix name is ..ene for a 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 change from a to e based on the 'parent' alkane name.
    • e.g. alkenes structure and naming (c) doc b propene (no need to say prop-1-ene),
    •  and alkenes structure and naming (c) doc b 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) (c) doc b, pent-1-ene

      • (2) (c) doc b, 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.

        • alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b

        •   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 alkenes structure and naming (c) doc b is isomeric with cyclopentane alkanes structure and naming (c) doc b

 

  • If the molecule has a ring of carbon atoms including the double bond, the name is prefixed by cyclo i.e. a cycloalkene …
    • e.g. alkenes structure and naming (c) doc b cyclohexene and alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b
    • 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.alkenes structure and naming (c) doc b hex-3-ene (e.g. in UK, 3-hexene in US)
    • or alkenes structure and naming (c) doc b 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. alkenes structure and naming (c) doc b
    • 5-methylhex-1-ene, the alkene functional group (C1) is higher ranking than the substituent methyl group (on C5),
    • or alkene (c) doc b  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 again: 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'!


2.1.2 General formula notes on the various associated unsaturated series of hydrocarbons

and the many styles of representing the molecular formula and structure of alkenes

  • The open chain alkenes (non-cyclic) with one 'ene' group (C=C double bond) have the general formula CnH2n (n = 2, 3, 4 etc.), they are isomeric with cycloalkanes from C3 onwards.
    • n must be >1 to give a C=C double bond e.g. alkenes structure and naming (c) doc b, C3H6, n = 3
    • The empirical formula of open chain alkenes with one C=C double bond is always CH2.
  • The open chain alkenes (non-cyclic) with two 'ene' groups, i.e. dienes, have the general formula CnH2n-2 (n = 3, 4, 5 etc.)
    • n must be >2 to give two carbon-carbon double bonds e.g. alkenes structure and naming (c) doc b, C4H6 ,n = 4
  • The cycloalkenes with one 'ene' group in the ring also have the general formula CnH2n-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.
    • Therefore these cycloalkenes are isomeric with open chain dienes (above).
    • Shown on the right is cyclohexane, C6H12, n = 6
  • alkenes structure and naming (c) doc bThe cycloalkenes with two 'ene' groups in the ring have the general formula CnH2n-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 but cyclodienes exist for C4 molecules onwards.
    • Shown on the right is cyclobuta-1,3-diene, C4H4,  = 4, a cyclohexadiene would be C6H8.
  • Non-cyclic alkynes have the general formula CnH2n-2 where n = 2, 3, 4 etc. e.g. propyne CH3-CCH, C3H4, where n = 3.
    • These are isomeric with open-chain dienes and cycloalkenes with one double bond.
  • 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.
  • See also E/Z (cis/trans) isomerism - example of stereoisomerism
  • One example of so-called E/Z isomerism (cis/trans isomers) occurs when you can have two different spatial orientations of the groups attached to the carbon atoms of the double bond of an alkene e.g. there are two possible spatial orientations of but-2-ene, shown in the right-hand diagram, both are isomers of the molecular formula C4H8.
  • I'm not discussing E/Z isomerism of alkenes in further detail here, but link to detailed notes and the priority rules for assigning the E or Z isomer is also explained.
  • However, on first reading this page you might not have encountered the E/Z isomerism of alkenes, however on this alkenes page, I have in places, indicated whether the alkene molecule can/cannot exist as an E/Z isomer and in some cases presented the E and Z molecular structure of the isomers.


Styles of representing the molecular structure of alkenes

Reminders: Using the alkene propene as an example

alkenes structure and naming (c) doc b, the molecular formula, summation of all the atoms in the molecule, but no structural detail.

alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , abbreviated structural formula

 alkenes structure and naming (c) doc b , a sort of intermediate structure, partly displayed and partly structural !!!

alkenes structure and naming (c) doc b , the full displayed formula, showing every atom and every bond - the full picture how all atoms are connected in the molecule.

alkenes structure and naming (c) doc b , the skeletal formula, no carbon atoms or hydrogen atoms are shown, just a - for a single C-C bond and a = for a C=C double bond.

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

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

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

... 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:

1-chlorobut-1-ene CH3CH2CH=CHCl

(NOT 4-chlorobut-1-ene or 4-chlorobut-2-ene or 1-chlorobut-2-ene)

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


TOP OF PAGE and sub-index


dot and cross diagram of the ethene molecule2.1.3 Examples of Alkene hydrocarbons and substituted alkenes

The names in bold are the preferred IUPAC alkene name.

The simplest alkene is ethene (ethylene), molecular formula of alkenes structure and naming (c) doc b (empirical formula CH2)

  •  dot and cross electronic bonding diagram of ethene (c) doc b ,
  • structural formula of ethenealkenes structure and naming (c) doc b
  • or the full displayed formula of ethene alkenes structure and naming (c) doc b and the skeletal formula is only alkenes structure and naming (c) doc b
    • All the bond angles (H-C-C or H-C-H) are ~120o 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,(c) doc b 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),

  • alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b
    • H-C-H bond angle is ~109o ('tetrahedral') in the methyl CH3- group (as is the H-C-C= angle), but the H-C=C, C-C=C, C=C-H and =CH2 bond angles are all ~120o
    • 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 possible structural isomeric products of molecular formula C3H7X.
      • i.e. CH3-CHX-CH3 and CH3-CH2-CH2X.
    • There are three monochloro substituted propenes, of molecular formula C3H5Cl
      • 1-chloropropene:  CH3-CH=CHCl, will exhibit E/Z isomerism (1-chloroprop-1-ene)
      • 2-chloropropene:  CH3-CCl=CH2, cannot exhibit E/Z isomerism (2-chloroprop-1-ene)
      • 3-chloropropene:  ClCH2-CH=CH2, cannot exhibit E/Z isomerism (3-chloroprop-1-ene)

Methylpropene or 2-methylpropene, but 2- is not really needed here, (isobutene, isobutylene), is the simplest branched open chain alkene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , skeletal formula of methylpropene 2-methylpropene advanced organic chemistry , cannot exhibit E/Z isomerism

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

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,

  • alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b
  • The H-C=C and H2C= bond angles are ~120o.  The C=C=C bond angle is 180o.

The simplest cycloalkene is cyclopropene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b 

 

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

  • alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkene , 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 a hydrogen halide 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=CH2, (2-chloro-1-butene, cannot exhibit E/Z isomerism alkene

    3-chlorobut-1-ene:  CH3-CHCl-CH=CH2, 3-chloro-1-butene), cannot exhibit E/Z isomerism

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

     

2-methylbut-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (2-methyl-1-butene), cannot exhibit E/Z isomerism

 

3-methylbut-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (3-methyl-1-butene), cannot exhibit E/Z isomerism

 

2,3-dimethylbut-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (2,3-dimethyl-1-butene), cannot exhibit E/Z isomerism

3,3-dimethylbut-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (3,3-dimethyl-1-butene), cannot exhibit E/Z isomerism

2,3,3-trimethylbut-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (2,3,3-trimethyl-1-butene), cannot exhibit E/Z isomerism

But-2-ene (2-butene) is the first hydrocarbon alkene (with no non-alkyl substituent groups) to have geometrical isomers (geometric isomerism is now correctly termed E/Z isomerism i.e. E and Z isomers).

In old notation the cis isomer is now the Z isomer and the trans isomers is now the E isomer.

E/Z isomerism is an example of stereoisomerism, where isomers of a particular molecular formula can exist in two or more forms of different spatial orientation which are NOT mirror images.

  •  alkenes structure and naming (c) doc b , structural formula of but-2-ene alkenes structure and naming (c) doc b , 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 does 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 alkenes structure and naming (c) doc b or alkenes structure and naming (c) doc b is Z-but-2-ene, (cis-2-butene, cis-but-2-ene)

     

  •  alkenes structure and naming (c) doc b or  alkenes structure and naming (c) doc b is E-but-2-ene, (trans-2-butene, trans-but-2-ene)
  • alkene 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.
  • These isomers, and all other E/Z isomers are also referred to as diastereoisomers.

  • Diastereoisomerism is defined as where stereoisomers exist, of the same molecular formula, they have different spatial arrangements which are not mirror images of each other.

  • You cannot get E/Z isomers if both atoms/groups attached to one of the carbon atoms are identical, as in but-1-ene or methyl propene.

  • 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)
  • But-2-ene is a symmetrical 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.

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,

  • alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b

  •  

 

2,3-dimethylbut-2-ene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b, 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,

  •  alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b

  • , 1,2-butadiene, isomer of molecular formula C4H6.

     

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),

  • alkenes structure and naming (c) doc b,alkenes structure and naming (c) doc b , and has two E/Z isomers, isomers of molecular formula C4H6.

     

  •  alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b cis/Z-buta-1,3-diene, isomer of molecular formula C4H6.

     

  • alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b  trans/E-buta-1,3-diene, isomer of molecular formula C4H6.

  • 2-methylbuta-1,3-diene (2-methyl-1,3-butadiene) is the synthetic rubber monomer 'isoprene' (shown on the right of the diagram below of beta-carotene, a much more complex alkene!).

  • skeletal formula isoprene beta-carotene 2-methylbuta-1,3-diene molecular structure advanced organic chemistry

  • β-Carotene is  strongly coloured red-orange organic pigment found in fungi, plants (e.g. carrots), and fruits. Beta-carotene is a member of the carotenes, which are terpenoids, synthesized biochemically from eight isoprene units and thus has 40 carbons.

  • Technically isoprene is the Z-isomer (cis) of the molecular formula C5H8, hence the molecular formula of beta-carotene with eight times as many carbon atoms is C40H56 and has 11 double bonds (all in the E (trans) configuration), 9 in the linear chain section and 1 in each of the end cyclic section - a much more interesting naturally occurring molecular structure than synthetic ethene and propene, but you can't make alcohols and plastics from it!

Cyclobutene is the next simplest cyclo-alkene after cyclopropene, molecular formula C4H6.

  • alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

Cyclobuta-1,3-diene is the simplest cyclo-diene that exists, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , molecular formula C4H4.

Pent-1-ene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (1-pentene)

 

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

  • 2-methylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  • (2-methyl-1-pentene)

  • 3-methylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •  (3-methyl-1-pentene)

  • 4-methylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •  (4-methyl-1-pentene)

  • 2,3-dimethylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •   (2,3-dimethyl-1-pentene)

  • 2,4-dimethylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •   (2,4-dimethyl-1-pentene)

  • 3,3-dimethylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •   (3,3-dimethyl-1-pentene)

  • 3,4-dimethylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •   (3,4-dimethyl-1-pentene)

     

  • 4,4-dimethylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •   (4,4-dimethyl-1-pentene)

  • 3-ethylpent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  • (3-ethyl-1-pentene)

Pent-2-ene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b (2-pentene two E/Z isomers)

  • Z/cis- alkenes structure and naming (c) doc b , structural formula Z-pent-2-ene cis pent-2-ene Z-2-pentene cis 2-pentene molecular structure and E/trans- alkenes structure and naming (c) doc b

  •   , structural formula E-pent-2-ene trans pent-2-ene E-2-pentene trans 2-pentene molecular structure

     

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

  • 2-methylpent-2-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

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

  • 3-methylpent-2-ene, alkenes structure and naming (c) doc b (3-methyl-2-pentene) has two E/Z isomers:

    •  alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b, E-3-methylpent-2-ene

    • and   alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

    • Z-3-methylpent-2-ene

  • 4-methylpent-2-ene, alkenes structure and naming (c) doc b , has two E/Z isomers:

    • Z/cis- alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , E/trans- alkenes structure and naming (c) doc b

    • , alkene

  • 2,3-dimethylpent-2-ene, alkenes structure and naming (c) doc b

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

  • 2,4-dimethylpent-2-ene, alkenes structure and naming (c) doc b

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

  • 3,4-dimethylpent-2-ene, alkenes structure and naming (c) doc b (3,4-dimethyl-2-pentene)

    • has two E/Z isomers: E-alkenes structure and naming (c) doc b , and Z- alkenes structure and naming (c) doc b

    • 3,4-dimethylpent-2-ene

       

  • 4,4-dimethylpent-2-ene, alkenes structure and naming (c) doc b  (4,4-dimethyl-2-pentene) has two E/Z isomers:

    • Z/cis- alkenes structure and naming (c) doc b , and E/trans- alkenes structure and naming (c) doc b

  • 3-ethylpent-2-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (3-ethyl-2-pentene)

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

cyclopentene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b 

 

cyclopenta-1,3-diene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (1,3-cyclopentadiene)

hex-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b  (1-hexene)

2-methylhex-1-ene, alkenes structure and naming (c) doc balkenes structure and naming (c) doc b (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, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •   (3-methyl-1-hexene)

  • 4-methylhex-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •   (4-methyl-1-hexene)

  • 5-methylhex-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  • (5-methyl-1-hexene)

Hex-2-ene, alkenes structure and naming (c) doc b , 2-hexene has two E/Z isomers:

  • Z/cis- alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b, and

  • E/ trans-, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  •  

     

  • There are four monosubstituted methylhex-2-enes e.g.

  • 2-methylhex-2-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  • (2-methyl-2-hexene)

  • 3-methylhex-2-ene, alkenes structure and naming (c) doc b

    • has two E/Z isomers: E-alkenes structure and naming (c) doc b and Z-alkenes structure and naming (c) doc b

    • (3-methyl-2-hexene)

  • 4-methylhex-2-ene, alkenes structure and naming (c) doc b ,

    • has two E/Z isomers: Z/cis- alkenes structure and naming (c) doc b , and E/trans- alkenes structure and naming (c) doc b

  • 5-methylhex-2-ene, alkenes structure and naming (c) doc b , has two E/Z isomers:

    •  Z/cis- alkenes structure and naming (c) doc b and E/trans- alkenes structure and naming (c) doc b

Hex-3-ene, alkenes structure and naming (c) doc b , 3-hexene has two E/Z isomers:

  • Z/cis- alkenes structure and naming (c) doc b , E/trans- alkenes structure and naming (c) doc b

  •  

     

  • and two mono substituted methylhex-3-enes

  • 2-methylhex-3-ene, alkenes structure and naming (c) doc b , has E/Z isomers:

    •  Z- alkenes structure and naming (c) doc band E- alkenes structure and naming (c) doc b

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

  • 3-methylhex-3-ene, alkenes structure and naming (c) doc b has two E/Z isomers:

    • Z- alkenes structure and naming (c) doc b and E- alkenes structure and naming (c) doc b

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

cyclohexene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b ,   , alkenes structure and naming (c) doc b

 

cyclohexa-1,3-diene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b  , , alkene (1,3-cyclohexadiene)

cyclohexa-1,4-diene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (1,4-cyclohexadiene)

hept-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (1-heptene)

 has no E/Z isomers

hept-2-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

  • has two E/Z isomers: Z/cis- alkenes structure and naming (c) doc b and E/trans- (c) doc b

  • (cis and trans 2-heptene)

hept-3-ene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b

  • has two E/Z isomers: Z/cis- alkenes structure and naming (c) doc band E/trans- alkenes structure and naming (c) doc b (cis and trans 3-heptene)


TOP OF PAGE and sub-index


2.1.4 The general equation for the formation of a poly(alkene)

alkenes structure and naming (c) doc b

The polymers so formed are name on the basis of the monomer i.e. poly(monomer)

e.g. poly(ethene), poly(chloroethene), poly(phenylethene)


2.1.5 Short note on Alkyne structure and naming

The names in bold are the preferred IUPAC alkene name.

  • ALKYNES are unsaturated hydrocarbons with a Calkene (c) doc bC triple bond

    • Examples: C2H2, alkene (c) doc b ethyne (acetylene)

      • The H-C(c) doc bC bond angle is180o, the ethyne molecule has a linear shape

    • C3H4, alkene (c) doc b propyne (methylacetylene)

      •  the H-C(c) doc bC and C(c) doc bC-C bond angles are 180o and the (c) doc bC-C-H and H-C-H in -CH3 are ~109o


2.1.6 Summary of the naming and structure of some simple chloroalkenes

The names in bold are the preferred IUPAC alkene name.

A little practice in working out the isomers of alkene molecular formulae of C3H5Cl and C4H7Cl and whether they exhibit E/Z isomerism.

  1. C2H3Cl, alkene (c) doc b  chloroethene, no E/Z isomers

  2. C3H5Cl, alkene (c) doc b  1-chloropropene, has E/Z isomers

  3. C3H5Cl, alkene (c) doc b  2-chloropropene, no E/Z isomers

  4. C3H5Cl, alkene (c) doc b  3-chloropropene, no E/Z isomers

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

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

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

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

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

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


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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