 
2.1
The molecular structure and naming (nomenclature) of
ALKENES
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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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, ..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
-
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'!
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 with one 'ene' group 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.
- 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 CnH2n-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
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.
- The cycloalkenes with two 'ene' groups i.e.
cyclodienes, 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.
- Non-cyclic alkynes have the general
formula CnHn 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
TOP OF PAGE and
sub-index
2.1.3 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
(empirical
formula CH2)
The next open chain alkene
is propene (propylene),
-
,
,
,
,
,
-
H-C-H
bond angle is 109o in the CH3- group, 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 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 ~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,
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=CH2,
(2-chloro-1-butene, cannot exhibit E/Z isomerism

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,
,
(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.
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,
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:
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)
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)
cyclopentene,
,
,
cyclopenta-1,3-diene,
,
(1,3-cyclopentadiene)
hex-1-ene,
,
,
(1-hexene)
2-methylhex-1-ene,
, (2-methyl-1-hexene)
(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 ,
,
-
4-methylhex-2-ene,
,
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-

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,
,
,
hept-3-ene,
,

TOP OF PAGE and
sub-index
2.1.4
The general equation for the formation of a
poly(alkene)
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.
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.
-
C2H3Cl,
chloroethene, no E/Z isomers
-
C3H5Cl,
1-chloropropene, has E/Z isomers
-
C3H5Cl,
2-chloropropene, no E/Z isomers
-
C3H5Cl,
3-chloropropene, no E/Z isomers
-
C4H7Cl,
1-chlorobut-1-ene (1-chloro-1-butene), has E/Z isomers
-
C4H7Cl,
2-chlorobut-1-ene (2-chloro-1-butene), no E/Z isomers
-
C4H7Cl,
3-chlorobut-1-ene (3-chloro-1-butene), no E/Z isomers
-
C4H7Cl,
4-chlorobut-1-ene (4-chloro-1-butene), no E/Z isomers
-
C4H7Cl,
4-chlorobut-2-ene (4-chloro-2-butene), has E/Z isomers
-
C4H7Cl,
2-chlorobut-2-ene (2-chloro-2-butene), has E/Z isomers
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LINKS TO ASSOCIATED ADVANCED ORGANIC CHEMISTRY PAGES
What
next?
Advanced
Level ALKENES INDEX
All Advanced Level Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
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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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