Organic Chemistry I
8. Delocalized Electrons
8.19 The Diels-Alder Reaction is a 1,4-Addition Reaction
Diels-Alder reaction = a conjugated diene reacts with a
compound containing a C-C double bond that “loves a diene”
(called dienophile)
The Diels-Alder reaction creates two new C-C bonds and forms a cyclic compound in the process; it is a
pericyclic reaction that is a [4 + 2] cycloaddition reaction.
Mechanism for the Diels-Alder reaction
This reaction is the concerted 1,4-addition of an electrophile and a
nucleophile to a conjugated diene; the electrophile and the
nucleophile are the adjacent sp2 carbons of a double bond.
The double bond in the product is between the 2- and 3- positions of
the diene’s conjugated system.
The reactivity of the dienophile is increased if an electron-withdrawing group, e.g.
a carbonyl (C=O) or cyano (C≡N) group, is attached to one of its sp2 carbons.
This puts a partial positive charge on the sp2 carbon that the π electrons of the
conjugated diene add to; makes the dienophile a better electrophile.
A wide variety of cyclic compounds can be obtained by varying the structures
of the conjugated diene and the dienophile.
Compounds containing C-C triple bonds can also be used as
dienophiles; the product is a compound with two isolated double
bonds.
If the dienophile has two C-C double bonds, two successive
Diels-Alder reactions can occur if excess diene is available.
When one or both of the reacting molecules is symmetrical,
only one product is formed (disregarding stereoisomers).
If both the diene and the dienophile are
unsymmetrically substituted, two products
(constitutional isomers) are possible; the reactants
can align in two different ways.
To determine the structure of the second product, don’t change the
position of one of the reactants and turn the other upside down.
The product formed in greater yield depends on the charge
distribution in each of the reactants; the partially positively
charged carbon of the dienophile will bond preferentially to
the partially negatively charged carbon of the diene.
1
,s-cis configuration = the double bonds are cis about the single bond (s = single)
s-trans configuration = the double bonds are trans about the single bond; little bit more stable because
less steric strain
A conjugated diene is most stable in one of these two planar
conformations.
The rotational barrier between both conformations is low enough to
allow the conformations to interconvert rapidly at room temperature.
To undergo a Diels-Alder reaction, the conjugated diene must be in
an s-cis conformation; C-1 and C-4 in an s-trans conformation are
too far apart to react with the dienophile in a concerted reaction.
A conjugated diene that is locked in an s-trans conformation cannot
undergo a Diels-Alder reaction.
A conjugated diene that is locked in an s-cis conformation is highly reactive in a Diels-Alder reaction.
fused bicyclic compound = contains two
rings that share two adjacent atoms
bridged bicyclic compound = contains two
rings that share two nonadjacent carbons;
product of the Diels-Alder reaction when
the diene is cyclic
For substituted bicyclic compounds, two configurations are
possible; the substituent can point away from the double bond
(exo), or not point away from the double bond (endo).
The exo product is thermodynamically more stable (less steric
hindrance), but the product is kinetically determined.
The transition state for the formation of the endo product is lower in energy, so it is formed faster when
the dienophile has a substituent with π electrons; interaction between the π electrons of the substituent
and the π electrons of the ring stabilizes the transition state.
If a Diels-Alder reaction creates a product with an
asymmetric center, the product will be a racemic
mixture.
It is a syn addition reaction (one face of the diene adds to one face of the dienophile); if the substituents
of the dienophile are cis, they will be cis in the product and if the substituents of the dienophile are trans,
they will be trans in the product.
When a Diels-Alder reaction forms a product with two new
asymmetric centers, a pair of enantiomers is formed.
8.20 Retrosynthetic Analysis of the Diels-Alder Reaction
To determine the reactants needed to synthesize a Diels-Alder product:
1. Locate the double bond in the product;
the diene used to form the cyclic
product has double bonds on either
side of this bond, so draw in those
double bonds and remove the original double bond
2. The new σ bonds are now on either side of the double bond; delete these σ bonds and put a new
π bond between the two carbons whose σ bonds were deleted.
2
, 13. Radicals - Reactions of Alkanes
alkanes = contain only carbon-carbon single bonds; saturated hydrocarbons
alkenes = contain carbon-carbon double bonds
alkynes = contain carbon-carbon triple bonds
13.1 Alkanes are Unreactive Compounds
Carbon-carbon double and triple bonds are composed of strong σ and weaker π bonds.
Because of their relatively weak π bonds, alkenes and alkynes undergo electrophilic addition reactions.
Alkanes only have strong σ bonds, in which the electrons are shared equally by all bonding atoms, so
none of the atoms has any significant charge.
They are neither nucleophiles nor electrophiles, so they are relatively unreactive compounds.
13.2 The Chlorination and Bromination of Alkanes
The halogenation of alkanes with chlorine or bromine only takes place at
high temperatures or with irradiation of light (hv).
Alkanes only undergo halogenation and combustion without the
assistance of a metal catalyst.
combustion = alkanes react with oxygen at high temperatures to form carbon dioxide and water
heterolytic bond cleavage (heterolysis) = both of the electrons stay
with one of the atoms
homolytic bond cleavage (homolysis) = each of the atoms retains
one of the bonding electrons; single-headed arrows are used
Homolysis results in the formation of (free) radicals, which are neutral species containing an atom with an
unpaired electron; highly reactive because acquiring an electron will complete its octet.
Mechanism for the monochlorination of methane
initiation step = creates radicals from a molecule in which all the electrons are paired
propagation steps = the chlorine radical created in the first step reacts in the second step to produce
again the chlorine radical that participates in the first step; the chain is propagated
termination step = any two radicals in the reaction mixture can combine to form a molecule in which all
the electrons are paired; helps bring the reaction to an end by decreasing the number of radicals
available to propagate the reaction
radical chain reaction = the reaction has radical intermediates and repeating propagation steps
radical substitution reaction = radical chain reaction that substitutes e.g. a chlorine for one of the
hydrogens of the alkane
3
8. Delocalized Electrons
8.19 The Diels-Alder Reaction is a 1,4-Addition Reaction
Diels-Alder reaction = a conjugated diene reacts with a
compound containing a C-C double bond that “loves a diene”
(called dienophile)
The Diels-Alder reaction creates two new C-C bonds and forms a cyclic compound in the process; it is a
pericyclic reaction that is a [4 + 2] cycloaddition reaction.
Mechanism for the Diels-Alder reaction
This reaction is the concerted 1,4-addition of an electrophile and a
nucleophile to a conjugated diene; the electrophile and the
nucleophile are the adjacent sp2 carbons of a double bond.
The double bond in the product is between the 2- and 3- positions of
the diene’s conjugated system.
The reactivity of the dienophile is increased if an electron-withdrawing group, e.g.
a carbonyl (C=O) or cyano (C≡N) group, is attached to one of its sp2 carbons.
This puts a partial positive charge on the sp2 carbon that the π electrons of the
conjugated diene add to; makes the dienophile a better electrophile.
A wide variety of cyclic compounds can be obtained by varying the structures
of the conjugated diene and the dienophile.
Compounds containing C-C triple bonds can also be used as
dienophiles; the product is a compound with two isolated double
bonds.
If the dienophile has two C-C double bonds, two successive
Diels-Alder reactions can occur if excess diene is available.
When one or both of the reacting molecules is symmetrical,
only one product is formed (disregarding stereoisomers).
If both the diene and the dienophile are
unsymmetrically substituted, two products
(constitutional isomers) are possible; the reactants
can align in two different ways.
To determine the structure of the second product, don’t change the
position of one of the reactants and turn the other upside down.
The product formed in greater yield depends on the charge
distribution in each of the reactants; the partially positively
charged carbon of the dienophile will bond preferentially to
the partially negatively charged carbon of the diene.
1
,s-cis configuration = the double bonds are cis about the single bond (s = single)
s-trans configuration = the double bonds are trans about the single bond; little bit more stable because
less steric strain
A conjugated diene is most stable in one of these two planar
conformations.
The rotational barrier between both conformations is low enough to
allow the conformations to interconvert rapidly at room temperature.
To undergo a Diels-Alder reaction, the conjugated diene must be in
an s-cis conformation; C-1 and C-4 in an s-trans conformation are
too far apart to react with the dienophile in a concerted reaction.
A conjugated diene that is locked in an s-trans conformation cannot
undergo a Diels-Alder reaction.
A conjugated diene that is locked in an s-cis conformation is highly reactive in a Diels-Alder reaction.
fused bicyclic compound = contains two
rings that share two adjacent atoms
bridged bicyclic compound = contains two
rings that share two nonadjacent carbons;
product of the Diels-Alder reaction when
the diene is cyclic
For substituted bicyclic compounds, two configurations are
possible; the substituent can point away from the double bond
(exo), or not point away from the double bond (endo).
The exo product is thermodynamically more stable (less steric
hindrance), but the product is kinetically determined.
The transition state for the formation of the endo product is lower in energy, so it is formed faster when
the dienophile has a substituent with π electrons; interaction between the π electrons of the substituent
and the π electrons of the ring stabilizes the transition state.
If a Diels-Alder reaction creates a product with an
asymmetric center, the product will be a racemic
mixture.
It is a syn addition reaction (one face of the diene adds to one face of the dienophile); if the substituents
of the dienophile are cis, they will be cis in the product and if the substituents of the dienophile are trans,
they will be trans in the product.
When a Diels-Alder reaction forms a product with two new
asymmetric centers, a pair of enantiomers is formed.
8.20 Retrosynthetic Analysis of the Diels-Alder Reaction
To determine the reactants needed to synthesize a Diels-Alder product:
1. Locate the double bond in the product;
the diene used to form the cyclic
product has double bonds on either
side of this bond, so draw in those
double bonds and remove the original double bond
2. The new σ bonds are now on either side of the double bond; delete these σ bonds and put a new
π bond between the two carbons whose σ bonds were deleted.
2
, 13. Radicals - Reactions of Alkanes
alkanes = contain only carbon-carbon single bonds; saturated hydrocarbons
alkenes = contain carbon-carbon double bonds
alkynes = contain carbon-carbon triple bonds
13.1 Alkanes are Unreactive Compounds
Carbon-carbon double and triple bonds are composed of strong σ and weaker π bonds.
Because of their relatively weak π bonds, alkenes and alkynes undergo electrophilic addition reactions.
Alkanes only have strong σ bonds, in which the electrons are shared equally by all bonding atoms, so
none of the atoms has any significant charge.
They are neither nucleophiles nor electrophiles, so they are relatively unreactive compounds.
13.2 The Chlorination and Bromination of Alkanes
The halogenation of alkanes with chlorine or bromine only takes place at
high temperatures or with irradiation of light (hv).
Alkanes only undergo halogenation and combustion without the
assistance of a metal catalyst.
combustion = alkanes react with oxygen at high temperatures to form carbon dioxide and water
heterolytic bond cleavage (heterolysis) = both of the electrons stay
with one of the atoms
homolytic bond cleavage (homolysis) = each of the atoms retains
one of the bonding electrons; single-headed arrows are used
Homolysis results in the formation of (free) radicals, which are neutral species containing an atom with an
unpaired electron; highly reactive because acquiring an electron will complete its octet.
Mechanism for the monochlorination of methane
initiation step = creates radicals from a molecule in which all the electrons are paired
propagation steps = the chlorine radical created in the first step reacts in the second step to produce
again the chlorine radical that participates in the first step; the chain is propagated
termination step = any two radicals in the reaction mixture can combine to form a molecule in which all
the electrons are paired; helps bring the reaction to an end by decreasing the number of radicals
available to propagate the reaction
radical chain reaction = the reaction has radical intermediates and repeating propagation steps
radical substitution reaction = radical chain reaction that substitutes e.g. a chlorine for one of the
hydrogens of the alkane
3
