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Brown's Chemistry Clinic
The structure and naming-nomenclature of ALKENES
GCE-AS-A2-IB ADVANCED LEVEL ORGANIC CHEMISTRY
including cycloalkene
hydrocarbons and isomers of the same molecular formula (including geometrical isomers) and
a brief
appendix on
poly(alkenes),
ALKYNES
and chloroalkenes
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A brief guide to alkene nomenclature
- 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.
- e.g.
propene, and
pent-1-ene
- 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
Beyond propene, numbers
(e.g. x and y) are needed to indicate the position of the double bond (e.g.
…..-x-ene, like pent-1-ene 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 the substituent groups.
- e.g.
hex-3-ene
- or
buta-1,3-diene
The positions of the substituent(s), denoted with a prefix,
e.g. halo… for chloro etc. or alky 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 () though their
use should be avoided if possible [but many still used - just put one into
GOGGLE!].
Note on the homologous series of Alkenes
- The open chain alkenes with one 'ene' group have the general formula
CnH2n (see examples 1, 2, 6), they are isomeric with cycloalkanes
from C3 onwards
- The open chain alkenes with two 'ene' groups,
i.e. dienes, have the general formula CnH2n-2 (see examples 4 and 17)
- The cycloalkenes with one 'ene' group in the ring also have the general formula
CnH2n-2 (see examples 5 and
19)
- The cycloalkenes with two 'ene' groups i.e.
cyclodienes, in the ring have the general formula
CnH2n-4 (see example
20)
- There are many structural isomers in all the
above groups, either of the form of chain, positional or functional group
isomerism.
- Some 'old' names are quoted in () though their
use should be avoided if possible [but many still used - just put one into
GOGGLE!].

Examples of Alkenes
- The simplest alkene
is ethene (ethylene), molecular formula of
, structural formulae are
-
or
and the skeletal formula is only
!
- a substituted ethene: chloroethene (old
name 'vinyl chloride'), C2H3Cl, CH2=CHCl
- The next open chain alkene
is propene (propylene),
- Methylpropene
or
2-methylpropene, but 2- is not really needed here, (isobutene,
isobutylene), is the simplest branched open chain alkene,
,
,
- 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 simplest cycloalkene
is cyclopropene,
,
,
,
- But-1-ene
(1-butene) is the first alkene, without substituent groups, where a positional number is needed,
-
,
- 1-chlorobut-1-ene: CH3-CH2-CH=CHCl
2-chlorobut-1-ene: CH3-CH2-CCl=CH2
3-chlorobut-1-ene: CH3-CHCl-CH=CH2
4-chlorobut-1-ene: ClCH2-CH2-CH=CH2
- 2-methylbut-1-ene
,
,
- 3-methylbut-1-ene
,
,
,
- 2,3-dimethylbut-1-ene
,
,
- 3,3-dimethylbut-1-ene
,
,
 - 2,3,3-trimethylbut-1-ene
,
,
- But-2-ene
(2-butene) is the first alkene (with no substituent groups)
to have geometrical isomers of
-
,
, 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 cis-but-2-ene,
-
or
is trans-but-2-ene,
- in simple cases, with two identical/similar groups, the
trans isomer has these groups 'diagonally' opposite and the cis isomer has the groups at a 'right angle' to each other)
-
1-chlorobut-2-ene: CH3-CH=CH-CH2Cl
-
2-chlorobut-2-ene: CH3-CH=CCl-CH3
-
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,
,
 , 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,
-
buta-1,3-diene
(note the optional a after the but), is the next diene
i.e. 2 C=C double bonds and isomeric with buta-1,2-diene (above),
-
, , and has
two geometrical isomers,
-
cis-buta-1,3-diene
-
,
and
-
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 ,
,
,
-
2-methylpent-1-ene ,
,
-
3-methylpent-1-ene ,
,
-
4-methylpent-1-ene ,
,
-
2,3-dimethylpent-1-ene ,
,
-
2,4-dimethylpent-1-ene ,
,
-
3,3-dimethylpent-1-ene ,
,
-
3,4-dimethylpent-1-ene ,
,
-
4,4-dimethylpent-1-ene ,
,
-
3-ethylpent-1-ene ,
,
 -
Pent-2-ene ,
,
, has geometrical
isomers:
-
cis-
and trans-

-
2-methylpent-2-ene,
,
 , has no geometrical isomers.-
3-methylpent-2-ene ,
, has two geometrical
isomers:
-
4-methylpent-2-ene,
, has two geometrical isomers:
-
2,3-dimethylpent-2-ene,

-
2,4-dimethylpent-2-ene ,
-
3,4-dimethylpent-2-ene ,
,
-
4,4-dimethylpent-2-ene,
, has two geometrical isomers:
-
cis-
, and trans-

-
3-ethylpent-2-ene,
,
,
-
Cyclopentene,
,
,

 -
Cyclopenta-1,3-diene ,
,
-
Hex-1-ene ,
,
,
-
2-methylhex-1-ene ,
,
-
3-methylhex-1-ene ,
,
-
4-methylhex-1-ene ,
,
-
5-methylhex-1-ene ,
,
-
Hex-2-ene ,
, two geometrical isomers:
-
2-methylhex-2-ene,
,

-
3-methylhex-2-ene ,
,
-
4-methylhex-2-ene,
,
-
5-methylhex-2-ene,
, has two geometrical isomers:
cis-
and trans-

-
Hex-3-ene ,
, has two geometrical isomers:
-
cis-
, trans-

-
2-methylhex-3-ene,
, has geometrical isomers:
-
cis-
and trans-

-
3-methylhex-3-ene,
has geometrical isomers:
-
cis-
and trans-

-
Cyclohexene,
,
,

-
Cyclohexa-1,3-diene ,
,
-
Cyclohexa-1,4-diene ,
,
,
-
hept-1-ene ,
,
,
, no geometrical isomers
-
hept-2-ene ,
,
,
-
hept-3-ene,
,


APPENDIX 1.
The
general equation for the formation of a
poly(alkene)


APPENDIX 2.
Short note on Alkyne structure and
naming

APPENDIX
3. The naming and structure of some simple chloroalkenes
-
chloroethene
-
1-chloropropene
-
2-chloropropene
-
3-chloropropene
-
1-chlorobut-1-ene
-
2-chlorobut-1-ene
-
3-chlorobut-1-ene
-
4-chlorobut-1-ene
-
1-chlorobut-2-ene
-
2-chlorobut-2-ene

GENERAL
REVISION
NOTES

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