CHEM10007 Lecture Notes - Lecture 31: Benzene, Substituent, Alkene
12 Jun 2018
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LECTURE 31
POLYENES, BENZENE & ALKYNES
CHAPTER 16 - BIOCHEMISTRY
POLYENES
hPolyenes are a general class of unsaturated
hydrocarbons containing two or more C=C
double bonds.
‣Eg. dienes, trienes.
hTheir nomenclature is analogous to that of
alkenes.
hSome common polyenes are on the right.
SYSTEMATIC NOMENCLATURE OF POLYENES
•Rule 1. Identify the parent chain - the longest chain which
contains the C=C double bonds, and add the suffix -diene, -
triene, etc.
•Rule 2. Number the carbons in the chain, beginning at the
end nearest to the double bond. Number the first carbon of
the double bond as an immediate prefix to the parent name.
•Rule 3. Write the full name, numbering the substituents
according to their position in the chain, and list
alphabetically.
STRUCTURES OF POLYENES
•Multiple bonds have various relationships within
the chain. There are 3 different types of
relationships;
•Cumulated (e.g. 1,2 pentadiene)
•Conjugated (e.g. 1,3 pentadiene)
•Non or unconjugated (e.g. 1,4 pentadiene)
•These different relationships result in differences in
reactivity of the compounds.
•1,4-pentadiene has a fully saturated carbon centre.
•Unconjugated dienes are separated by one or more
saturated carbons.
CONJUGATED POLYENES
•Conjugated polyenes are recognised by their alternate
sequence of C=C-C=C-C=C.
•Conjugated dienes are significantly more stable than
corresponding non-conjugated diene (Eg. 1,3
pentadiene is more stable than 1,4 pentadiene by
14Kj/mol) (above, right).
‣Pi (π) bonding between C1, C2 & between
C3, C4 is an additional pi bonding overlap
between the p orbitals on C2 and C3.
‣This extra overlap results in 4 pi (π) electrons
being delocalised over 4 carbon nuclei.
POLYENES IN BIOLOGICAL MOLECULES
•Examples of Polyenes in biological molecules include;
CYCLOALKENES
hCycloalkenes are cyclic alkenes. Their
nomenclature is analogous to that of
alkenes, however we add the prefix
‘cyclo’ to the parent alkene name (Eg.
cyclobutene)
hThey are numbered with C=C between
C1 and C2, and the first substituent has
as low a number as possible.
ADDITION REACTIONS OF CYCLOALKENES
hHalogens (Eg. bromine) can add to an alkene C=C
bond. This is a diagnostic test to detect the presence
of a C=C bond in a molecule.
hA characteristic reaction of alkenes is the addition
of bromine (or any halogen) to the double bond to
give a product containing two bromine substituents
in place of the double bond. This is an example of
an addition reaction. (right).
CYCLOHEXATRIENE - DIVERGENCE FROM
ALKENE BEHAVIOUR
hStructure of 1,3,5-cyclohexatriene;
h1,3,5-Cyclohexatriene with its alternating
double and single bonds might be expected to
have a geometry with single bond distances of
ca. 1.5 Å and double bond distances of 1.33 Å
(predicted).
hIn the structure of ‘cyclohexatriene’ all the C-
C bond distances were 1.39 Å which was
somewhere between ‘normal’ single and double
bond distances.
h‘Cyclohexatriene’ showed a
completely different reaction with
bromine than did cyclohexene and
cyclohexadiene.
hCyclohexene contains a C=C double
bond which has a bond distance of
1.33 Å (similar to any alkene), Adding
a second double bond into cyclohexene
gives two isomers; 1,3-cyclohexadiene, and 1,4 cyclohexadiene.
h1,3-Cyclohexadiene is a conjugated diene, whereas 1,4-cyclohexadiene is non-conjugated.
hReaction with bromine (addition reaction not observed);
hReaction with bromine (substitution reaction observed);
hThe new 1,3,5-cyclohexatriene molecule is more stable than
predicted (150 kJ/mol).
hThis would suggest that it is actually a benzene, a different class of
Document Summary
Polyenes hpolyenes are a general class of unsaturated hydrocarbons containing two or more c=c double bonds. Eg. dienes, trienes. htheir nomenclature is analogous to that of alkenes. hsome common polyenes are on the right. Identify the parent chain - the longest chain which contains the c=c double bonds, and add the suffix -diene, - triene, etc: rule 2. Number the carbons in the chain, beginning at the end nearest to the double bond. Number the first carbon of the double bond as an immediate prefix to the parent name: rule 3. Write the full name, numbering the substituents according to their position in the chain, and list alphabetically. Structures of polyenes: multiple bonds have various relationships within the chain. Conjugated polyenes: conjugated polyenes are recognised by their alternate sequence of c=c-c=c-c=c, conjugated dienes are significantly more stable than corresponding non-conjugated diene (eg. 1,3 pentadiene is more stable than 1,4 pentadiene by.
