Organic Chemistry Part 2 sub-index: 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 * Other advanced level organic chemistry links

• 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.)
    • I'm afraid several variations of nomenclature are allowed by the IUPAC which can make naming a bit confusing at times!
• 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 GOGGLE!].
• 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.

• 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 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 GOGGLE!].

The simplest alkene is ethene (ethylene), molecular formula of , structural formulae are

•  or 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
• a substituted ethene: chloroethene (old name 'vinyl chloride'), C₂H₃Cl,  CH₂=CHCl

   

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
  • There are three monochloro substituted propenes
    • 1-chloropropene or 1-chloroprop-1-ene:  CH₃-CH=CHCl
    • 2-chloropropene or 2-chloroprop-1-ene:  CH₃-CCl=CH₂
    • 3-chloropropene or 3-chloroprop-1-ene:  ClCH₂-CH=CH₂

Methylpropene or 2-methylpropene, but 2- is not really needed here, (isobutene, isobutylene), is the simplest branched open chain alkene, , ,

The C-C=C, C=C-H and =CH₂ bond angles in ~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 needed,

• ,
• There four substituted monochloro but-1-enes
• 1-chlorobut-1-ene:  CH₃-CH₂-CH=CHCl, 1-chloro-1-butene

  2-chlorobut-1-ene:  CH₃-CH₂-CCl=CH_2, 2-chloro-1-butene

  3-chlorobut-1-ene:  CH₃-CHCl-CH=CH_2, 3-chloro-1-butene

  4-chlorobut-1-ene:  ClCH₂-CH₂-CH=CH_2, 4-chloro-1-butene

   

2-methylbut-1-ene, , 2-methyl-1-butene

3-methylbut-1-ene, , , 3-methyl-1-butene

2,3-dimethylbut-1-ene, , 2,3-dimethyl-1-butene

3,3-dimethylbut-1-ene, , 3,3-dimethyl-1-butene

2,3,3-trimethylbut-1-ene, , 2,3,3-trimethyl-1-butene

But-2-ene (2-butene) is the first alkene (with no substituent groups) to have geometrical isomers (now correctly termed E/Z isomerism)

•  , , but the latter doesn't show the two different spatial arrangements possible due to a high energy barrier to rotation about the double bond

   

• or is Z-but-2-ene, cis-2-butene etc.

   

•  or is E-but-2-ene, trans-2-butene etc.

   

• 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 Isomerism Section 2. Stereoisomerism.
  • The are two monochlorosubstituted but-2-enes
  • 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 geometrical isomers because there are two identical groups (CH₃) attached to the same carbon of the double bond,

• ,

• does not have geometrical 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 geometrical 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 geometrical isomers,

   

•  , Z/cis-buta-1,3-diene,

   

• , E/trans-buta-1,3-diene

• 2-methylbuta-1,3-diene is the synthetic rubber '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 geometrical 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 has no geometrical isomers

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

  •  , , Z-3-methylpent-2-ene

  • and   ,

  E-3-methylpent-2-ene

• 4-methylpent-2-ene, , has two geometrical isomers:

  • Z/cis- , , E/trans-

• 2,3-dimethylpent-2-ene,

(2,3-dimethyl-2-pentene)

• 2,4-dimethylpent-2-ene,

(2,4-dimethyl-2-pentene)

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

  • has two geometrical isomers: E- , and Z-

  3,4-dimethylpent-2-ene

   

• 4,4-dimethylpent-2-ene,   (4,4-dimethyl-2-pentene) has two geometrical isomers:

  • Z/cis- , and E/trans-

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

  • no geometrical 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 geometrical 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 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 geometrical 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 geometrical isomers: E-and Z-

  (3-methyl-2-hexene)

• 4-methylhex-2-ene, ,

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

  

• 5-methylhex-2-ene, , has two geometrical isomers:

   Z/cis- and E/trans-

Hex-3-ene, , 3-hexene has two geometrical isomers:

• Z/cis- , E/trans-

 

• and two mono substituted methylhex-3-enes

• 2-methylhex-3-ene, , has geometrical isomers:

  •  Z- and E-

  (2-methyl-3-hexenes)

• 3-methylhex-3-ene, has geometrical isomers:

  • Z- and E-

   3-methyl-3-hexene

cyclohexene, , ,

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

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

hept-1-ene, , ,

1-heptene has no geometrical isomers

hept-2-ene, , ,

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

(2-heptene)

hept-3-ene, ,

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

1. C₂H₃Cl,   chloroethene

2. C₃H₅Cl,   1-chloropropene

3.   2-chloropropene

4.   3-chloropropene

5. C₄H₇Cl,   1-chlorobut-1-ene (1-chloro-1-butene)

6.   2-chlorobut-1-ene (2-chloro-1-butene)

7.   3-chlorobut-1-ene (3-chloro-1-butene)

8.   4-chlorobut-1-ene (4-chloro-1-butene)

9. 4-chlorobut-2-ene (4-chloro-2-butene)

10.   2-chlorobut-2-ene (2-chloro-2-butene)