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SYNTHETIC METHODS BASED ON ACTIVATING THE REACTANT:HALOGENATION OF BENZENE

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Chapter ­ 2
SYNTHETIC METHODS BASED ON ACTIVATING THE
REACTANT
P. Ramana Murthy
INTRODUCTION:
In organic synthesis, activating the reactant has a crucial role to play. Conventionally, the
activating of the reactant is carried out thermally, photochemically; catalytically, or by
change in the concentration of the reactant, and /or acid and base. The effects of these
parameters on the reactant and also reaction conditions have been studied.
HALOGENATION OF BENZENE:
Substitution Reactions:
Benzene reacts with chlorine or bromine in the presence of a catalyst, replacing one of
the hydrogen atoms of the ring by a chlorine or bromine atom.The reaction take place at
room temperature. The catalyst is either aluminium chloride (or aluminium bromide if
one were to react benzene with bromine) or iron. Strictly speaking iron is not a catalyst,
because it gets permanently changed during the reaction. It reacts with some of the
chlorine or bromine to form iron (III) chloride, FeCl3, or iron (III) bromide, FeBr3.
2FeCl3
2Fe + 3Cl2
2Fe + 3Br2
2FeBr3
These compounds act as catalyst and behave exactly like aluminium chloride, AlCl3, or
aluminium bromide, AlBr3, in these reactions.
The reaction with chlorine:
The reaction between benzene and chlorine in the presence of either aluminium chloride
or iron gives chlorobenzene.
(or) C6H6 + Cl2
C6H5Cl + HCl
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The Reaction with Bromine:
The reaction between benzene and bromine in the presence of either aluminium bromide
or iron gives bromobenzene. Iron is usually used because it is cheaper and more readily
available.
(or) C6H6 + Br2
C6H5Br + HBr
Addition Reactions:
In the presence of ultraviolet light (but without a catalyst), hot benzene will also undergo
an addition reaction with chlorine or bromine. The ring delocalisation is permanently
broken and a chlorine or bromine atom adds on to each carbon atom.
For example, if one were to bubble chlorine gas through hot benzene exposed to UV
light for an hour, one gets 1,2,3,4,5,6-hexachloro-cyclohexane.
ss
Bromine would behave similarly.
One of these isomers was once commonly used as an insecticide known variously as
BHC, HCH and Gammexane. One of the "chlorinated hydrocarbons" caused much
environmental harm.
THE HALOGENATION OF METHYLBENZENE:
Substitution Reactions:
It is possible to get two quite different substitution reactions between methylbenzene and
chlorine or bromine depending on the conditions used. The chlorine or bromine can
substitute into either the ring or the methyl group.
Substitution into the Ring:
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2.3
Substitution in the ring happens in the presence of aluminium chloride (or aluminium
bromide if one were to use bromine) or iron, and in the absence of UV light. The
reactions takes place at room temperature. This is exactly the same as the reaction with
benzene, except that one has to worry about where the halogen atom attaches in the ring
relative to the position of the methyl group
Methyl groups are 2,4-directing, which means that incoming groups will tend to go
into the 2 or 4 positions on the ring - assuming that the methyl group is in the first
position. In other words, the new group will attach to the ring next door to the methyl
group or opposite it. With chlorine, substitution into the ring gives a mixture of 2-
chloromethylbenzene and 4-chloro-methylbenzene.
Substitution into the Methyl group:
If chlorine or bromine react with boiling methylbenzene in the absence of a catalyst but
in the presence of UV light, substitution takes place in the methyl group rather than the
ring.
For example, with chlorine (bromine would be similar):
The organic product is (chloromethyl)benzene or benzyl chloride. The brackets in the
name emphasise that the chlorine is part of the attached methyl group, and is not in the
ring.
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One of the hydrogen atoms in the methyl group has been replaced by a chlorine atom.
However, the reaction does not stop there, and all three hydrogens in the methyl group
can in turn be replaced by chlorine atoms.
That
means
that
one
could
also
get
(dichloromethyl)benzene
and
(trichloromethyl)benzene as the other hydrogen atoms in the methyl group are replaced
one at a time.
SUBSTITUTION REACTIONS OF BENZENE AND OTHER AROMATIC
COMPOUNDS
The remarkable stability of the unsaturated hydrocarbon benzene has been discussed. The
chemical reactivity of benzene contrasts with that of the alkenes in that substitution
reactions occur in preference to addition reactions, as illustrated in the following diagram
(some comparable reactions of cyclohexene are shown in the green box).
Many other substitution reactions of benzene have been observed,
five most useful are listed below in Table 2.1.
Since the reagents and conditions
employed in these reactions are electrophilic, these reactions are commonly referred to as
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Electrophilic Aromatic Substitution. The catalysts and co-reagents serve to generate the
strong electrophilic species needed to effect the initial step of the substitution. The
specific electrophile believed to function in each type of reaction is listed in the right
hand column.
Table 2.1 Substitution reactions of Benzene
Electrophile
Reaction Type
Typical Equation
E(+)
Cl(+) or Br(+)
C6H6
+ Cl2 & heat
C6H5Cl + HCl
Halogenation:
FeCl3
Chlorobenzene
NO2+
Nitration
C6H6
C6H5NO2+ H2O
catalyst
Nitrobenzene
+HNO3 &
heat,
SO3H(+)
Sulfonation:
C6H6
H2SO4
C6H5SO3H + H2O;
Catalyst
Benzene-sulfonic acid
+ H2SO4 +
SO3, & heat
R(+)
C6H5-R + HCl
C6H6
+ R-Cl &
Alkylation:
heat, AlCl3
Arene
Friedel-Crafts
RCO(+)
C6H6
Acylation:
C6H5COR + HCl
+ RCOCl &
Friedel-Crafts
heat, AlCl3
Aryl Ketone
1. Mechanism for Electrophilic Substitution Reactions of Benzene - Nitration:
(or)
The concentrated sulphuric acid is acting as catalyst.
The formation of the electrophile
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The electrophile is the "nitronium ion" or the "nitryl cation", NO2+. This is formed by
reaction between the nitric acid and the sulphuric acid.
The electrophilic substitution mechanism
Stage one
Stage two
A two-step mechanism has been proposed for these electrophilic substitution reactions. In
the first, slow or rate-determining, step the electrophile forms a sigma-bond to the
benzene ring, generating a positively charged benzenonium intermediate. In the second,
fast step, a proton is removed from this intermediate, yielding a substituted benzene ring.
2. Substitution Reactions of Benzene Derivatives
When substituted benzene compounds undergo electrophilic substitution reactions of the
kind discussed above, two related features must be considered:
I. The first is the relative reactivity of the compound compared with benzene itself.
Experiments have shown that substituents on a benzene ring can influence reactivity in a
profound manner. For example, a hydroxy or methoxy substituent increases the rate of
electrophilic Nitration substitution about ten thousand fold, as illustrated by the case of
anisole. In contrast, a nitro substituent decreases the ring's reactivity by roughly a million.
This activation or deactivation of the benzene ring toward electrophilic substitution may
be correlated with the electron donating or electron withdrawing influence of the
substituents, as measured by molecular dipole moments. In the following diagram one
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can see the electron donating substituents activate the benzene ring toward electrophilic
attack, and electron withdrawing substituents deactivate the ring (make it less reactive to
electrophilic attack).
The influence a substituent exerts on the reactivity of a benzene ring may be explained by
the interaction of two effects:
The first is the inductive effect of the substituent. Most elements other than metals and
carbon have a significantly greater electronegativity than hydrogen. Consequently,
substituents in which nitrogen, oxygen and halogen atoms form sigma-bonds to the
aromatic ring exert an inductive electron withdrawal, which deactivates the ring. The
second effect is the result of conjugation of a substituent function with the aromatic ring.
This conjugative interaction facilitates electron pair donation or withdrawal, to or from
the benzene ring, in a manner different from the inductive shift. If the atom bonded to the
ring has one or more non-bonding valence shell electron pairs, as do nitrogen, oxygen
and the halogens, electrons may flow into the aromatic ring by p-š conjugation
(resonance), as in the middle diagram. Finally, polar double and triple bonds conjugated
with the benzene ring may withdraw electrons, as in the right-hand diagram.In both cases,
the charge distribution in the benzene ring is greatest at sites ortho and para to the
substituent.
Electron donation by resonance dominates the inductive effect and these compounds
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show exceptional reactivity in electrophilic substitution reactions. Although halogen
atoms have non-bonding valence electron pairs that participate in p-š conjugation, their
strong inductive effect predominates, and compounds such as chlorobenzene are less
reactive than benzene. The three examples on the left of the bottom row (in the same
diagram) are examples of electron withdrawal by conjugation to polar double or triple
bonds, and in these cases the inductive effect further enhances the deactivation of the
benzene ring. Alkyl substituents such as methyl increase the nucleophilicity of aromatic
rings in the same fashion as they act on double bonds.
II. The second factor that becomes important in reactions of substituted benzenes
concerns the site at which electrophilic substitution occurs. Since a mono-substituted
benzene ring has two equivalent ortho-sites, two equivalent meta-sites and a unique para-
site, three possible constitutional isomers may be formed in such a substitution. If
reaction occurs equally well at all available sites, the expected statistical mixture of
isomeric products would be 40% ortho, 40% meta and 20% para. One can find that the
nature of the substituent influences the product ratio in a dramatic fashion. Bromination
of methoxybenzene (anisole) is
fast and gives mainly the para-bromo isomer,
accompanied by 10% of the ortho-isomer and only a trace of the meta-isomer.
Bromination of nitrobenzene requires strong heating and produces the meta-bromo
isomer as the chief product.
Some additional examples of product isomer distribution in other electrophilic
substitutions are given in Table 2 It is important to note that the reaction conditions for
the substitution reactions are not the same, and must be adjusted to fit the reactivity of the
reactant C6H5-Y. The high reactivity of anisole, for example, requires that the first two
reactions be conducted under mild conditions (low temperature and little or no catalyst).
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The nitrobenzene reactant in the third example is unreactive, so rather harsh reaction
conditions must be used to accomplish that reaction.
Table 2.2. Examples of product isomer distribution in electrophilic substitution
Y in C6H5­Y
Reaction
% Ortho-Product % Meta-Product % Para-Product
­O­CH3
Nitration
30­40
0­2
60­70
­O­CH3
F-C Acylation 5­10
0­5
90­95
­NO2
Nitration
5­8
90­95
0­5
­CH3
Nitration
55­65
1­5
35­45
­CH3
Sulfonation
30­35
5­10
60­65
­CH3
F-C Acylation 10­15
2­8
85­90
­Br
Nitration
35­45
0­4
55­65
­Br
Chlorination
40­45
5­10
50­60
These observations, and many others like them, have led chemists to formulate an
empirical classification of the various substituent groups commonly encountered in
aromatic substitution reactions. Thus, substituents that activate the benzene ring toward
electrophilic attack generally direct substitution to the ortho and para locations. With
some exceptions, such as the halogens, deactivating substituents direct substitution to the
meta location. The following table summarizes this classification.
Table 2.3. Summary of the classification of the substituents
Orientation and Reactivity Effects of Ring Substituents
Deactivating Substituents
Deactivating Substituents
Activating Substituents
meta-Orientation
ortho & para-Orientation
ortho & para-Orientation
­O(­)
­F
­CO2H
­NH2
­NO2
­NR3(+)
­Cl
­CO2R
­OH
­NR2
­PR3(+)
­Br
­NHCOCH3
­CONH2
­OR
­SR2(+)
­I
­OC6H5
­R
­CHO
­CH2Cl
­OCOCH3
­SO3H
­C6H5
­COR
­CN
­SO2R
­CH=CHNO2
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The information summarized in Table 2.3 is useful for rationalizing and predicting the
course of aromatic substitution reactions.
Nucleophilic Substitution Reactions
The carbon-halogen bond in alkyl halides is polarized, placing a partial positive charge
on the carbon, and a partial negative charge on the halogen. The partial positive charged
carbon is therefore electrophilic and will be susceptible to attack by nucleophiles. When a
suitable nucleophile attacks an alkyl halide, it can displace the halogen in a substitution
reaction to release the halide anion and form a new bond to the carbon. The nucleophile
is usually neutral or negatively charged and some examples are HO-, H2O, MeOH, EtO-,
RS .
With simple primary alkyl halides reacting with simple nucleophiles, the rate at which
this substitution reaction proceeds is proportional to both the concentration of the
nucleophile and the concentration of the reactant alkyl halide, making the reaction second
order. This type of second-order, nucleophilic displacement reaction is therefore termed
an "SN2" reaction (substitution, nucleophilic, bimolecular). The mechanism for this
reaction is best described as concerted with the reaction coordinate passing through a
single energy maximum with no distinct intermediate. The transition state for this
reaction is described by the structure shown below in which partial bonds exist between
the central carbon and the attacking nucleophile and departing halogen.
The geometry of this transition state, with the planar carbon in the center, requires
that the central carbon undergo a stereochemical inversion; therefore if the central
carbon is chiral, the absolute configuration of the central carbon must change. In the
example shown R-2-bromobutane reacts with bromide anion to form the enantiomer, S-
2-bromobutane.
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Predicting the product from these types of substitution reactions simply requires that the
bond to the halogen leaving group be broken and a new bond be made between the
nucleophilic atom and the central carbon, inverting the absolute configuration if
appropriate.
SN2 Reaction: Kinetics
Nucleophilic substitution reactions follow different rate laws, depending on the exact
mechanism. The rate law for an SN2 reaction is: Rate = k [RX] [Nuc] where k is the rate
constant, RX is the alkyl halide and Nuc is the nucleophile. The reaction rate is therefore
second order overall. This also tells us that the reaction is bimolecular, i.e. two species
are involved in the rate-determining step.
This proposed mechanism for the SN2 reaction raises an interesting question: If the
substitution occurs at a chiral carbon, does the reaction proceed with retention, inversion
or loss of stereochemistry? The answer to this question lies in the direction of attack of
the incoming nucleophile. Attack on the same side as the halogen would result in
retention of stereochemistry. Attack from the opposite side to the halogen would result in
inversion of stereochemistry. A mixture of these two possibilities would lead to loss of
stereochemical integrity at the chiral carbon. Experimentally, it is found that a purely SN2
reaction at a chiral carbon proceeds with inversion of stereochemistry.
In 1896, German chemist Paul Walden reported the conversion of enantiopure (+)-
(R)-malic acid into the enantiomer (-)-(S)-malic acid, although he did not know at which
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step inversion was occurring. In the 1920s Kenyon and Philips investigated a similar
process with 1-phenyl-2-propanol:
From this and several other such cycles, Kenyon and Phillips concluded that the
nucleophilic substitution reaction of primary and secondary alkyl halides and tosylates
always proceeds with inversion of stereochemistry. In the cycle shown above inversion
takes place in the nucleophilic substitution of tosylate ion by acetate ion.
SN1 Reaction
The SN2 reaction is favoured by basic nucleophiles such as
hydroxide ion and
disfavoured by solvents such as alcohols and water. The reaction also depends on the
nature of the substrate: primary substrates react rapidly, secondary substrates react more
slowly and tertiary substrates are almost inert to SN2 reaction. In protic media with non-
basic nucleophiles under neutral or acidic conditions, tertiary substrates can be orders of
magnitude more reactive than their primary or secondary counterparts. The SN2
mechanism clearly cannot account for this and it can be concluded that a different
mechanism can operate under these circumstances. This mechanism is called SN1 which
denotes Substitution by a nucleophile, unimolecular.
That
is,
only
species
is
involved
in
the
rate-determining
step.
one
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2.13
Kinetics
The SN1 reaction is first order and the rate varies only as the concentration of the alkyl
halide: Rate = k [RX]. The rate of reaction is found to be independent with respect to the
concentration of the nucleophile. In other words, the nucleophile does not take part in the
rate-determining step. Any proposed mechanism for the reaction must therefore have the
alkyl halide undergoing some change without the aid of the nucleophile. The first step
must therefore be cleavage of the C-X bond to form a carbocation, followed by reaction
with the nucleophile to give the substitution product.
This mechanism is clearly different from the SN2 pathway and the stereo-chemical
outcome should also differ. Carbocations are sp2 hybridized, planar species - at first
glance it would appear that the nucleophile, Y- could attack from either face of the
carbocation, with an equal probability. One would predict that this should lead to
complete racemisation, if the starting alkyl halide were optically pure. In practice,
complete racemisation is rarely observed and usually, a minor excess (up to ~20%) of
inversion is observed. One explanation for this was provided by Winstein, an eminent
physical organic chemist. It was proposed that an ion-pair, between the carbocation and
the leaving group X- is present, which partly blocks attack of the nucleophile from one
face. Thus, inversion slightly dominates.
Factors which Influence the Reaction Pathway
SN2
Steric Effects: The transition state in the SN2 reaction involves partial bonding
·
between the nucleophile and the substrate. The bulkier the substrate, the more
difficult it is for the transition state to be reached. The reactivity order is 1o > 2o >
3o.
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The Nucleophile: By definition, a nucleophile must have an unshared pair of
·
electrons, whether it is charged or neutral. Nucleophilicity follows approximately
basicity, so pKa values can be used. Nucleophilicity usually increases going down
a group in the periodic table. The reactivity order of the more common
nucleophiles is: CN- > I- > MeO- > HO- > Cl- > H2O.
The Leaving Group: The leaving group is normally ejected with a negative
·
charge. Therefore the best leaving groups are those which can best stabilise a
negative charge. Weak bases (TsO-, I-, Br-) are generally good leaving groups,
whereas strong bases (F-, HO-, RO-) are generally poor leaving groups.
The Solvent: Polar aprotic solvents are best for SN2 reactions. These include
·
acetonitrile (CH3CN), dimethyl sulphoxide (Me2SO) and N,N-dimethylformamide
(Me2NCHO). Proticsolvents tend to form a 'cage' around the nucleophile,
decreasing its reactivity.
SN1:
The Substrate: Substrates which can form relatively stable carbocation
·
intermediates favour SN1 reactions The order of stability of carbocations is: 3o >
2o > benzyl > allyl > 1o.
The Nucleophile: The nucleophile is not involved in the rate-determining step in
·
an SN1 reaction but the SN1 pathway is more likely to be followed if the
nucleophile is poor, e.g. H2O.
The Leaving Group: The leaving group is also involved in the rate-determining
·
step for an SN1 reaction, so the same reactivity order as for SN2 is followed.
The Solvent: The solvent can have an effect on the rate of the SN1 reaction, but
·
for different reasons. Solvent effects arise from stabilisation of the transition
state and not the reactants themselves. The rate of SN1 reaction is increased in a
polar solvent such as water or aqueous ethanol.
Esterification Reaction:
This is a reaction of
formation of esters from carboxylic acids and alcohols in the
presence of concentrated sulphuric acid acting as the catalyst. It uses the formation of
ethyl ethanoate from ethanoic acid and ethanol as a typical example.
Mechanism
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Ethanoic acid reacts with ethanol in the presence of concentrated sulphuric acid as a
catalyst to produce the ester, ethyl ethanoate. The reaction is slow and reversible. To
reduce the chances of the reverse reaction happening, the ester is distilled off as soon as it
is formed.
Step 1
In the first step, the ethanoic acid takes a proton (a hydrogen ion) from the concentrated
sulphuric acid. The proton becomes attached to one of the lone pairs on the oxygen which
is double-bonded to the carbon.
The transfer of the proton to the oxygen gives it a positive charge.
The positive charge
is delocalised over the whole of the right-hand end of the ion, with a fair amount of
positive charge on the carbon atom. In other words, one can think of an electron pair
shifting to give the structure:
One could also imagine another electron pair shift producing a third structure:
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So which of these is the correct structure of the ion formed? None of them! The truth lies
somewhere in between all of them. One way of writing the delocalised structure of the
ion is:
The double headed arrows means you that each of the individual structures makes a
contribution to the real structure of the ion. They do not mean that the bonds are flipping
back and forth between one structure and another. The various structures are known as
resonance structures or canonical forms.
There will be some degree of positive charge on both of the oxygen atoms, and also
on the carbon atom. Each of the bonds between the carbon and the two oxygens will be
the same - somewhere between a single bond and a double bond.
For the purposes of the rest of this discussion, we are going to use the structure where the
positive charge is on the carbon atom.
Step 2
The positive charge on the carbon atom is attacked by one of the lone pairs on the oxygen
of the ethanol molecule.
Step 3
What happens next is that a proton (a hydrogen ion) gets transferred from the bottom
oxygen atom to one of the others. It gets picked off by one of the other substances in the
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mixture (for example, by attaching to a lone pair on an unreacted ethanol molecule), and
then dumped back onto one of the oxygens more or less at random.
The net effect is:
Step 4
Now a molecule of water is lost from the ion.
The positive charge is actually delocalised all over that end of the ion, and there will also
be contributions from structures where the charge is on the either of the oxygens:
Step 5
The hydrogen is removed from the oxygen by reaction with the hydrogensulphate ion
which was formed way back in the first step.
And there we are! The ester has been formed, and the sulphuric acid catalyst has been
regenerated.
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Reimer-Tiemann Reaction:
The Reimer-Tiemann reaction is a used for the ortho-formylation of phenols. The reaction
was discoverd by Karl Ludwig Reimer and Ferdinand Tiemann.
Reaction mechanism:
(1) reacts with strong base to form the chloroform carbanion (2), which will quickly
alpha-eliminate to give dichlorocarbene (3). Dichlorocarbene will react in the ortho- and
para- position of the phenate (5) to give the dichloromethyl substituted phenol (7). After
basic hydrolysis, the desired product (9) is formed.
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The Aldol Condensation of Aldehydes :
Reaction type : Nucleophilic addition : Summary
Reagents : commonly a base such as NaOH or KOH is added to the aldehyde.
·
The reaction involves an enolate reacting with another molecule of the aldehyde.
·
Remember enolates are good nucleophiles and carbonyl C are electrophiles.
·
Since the pKa of an aldehyde is close to that of NaOH, both enolate and aldehyde
·
are present.
The products of these reactions are hydroxyaldehydes or aldehyde-alcohols =
·
aldols.
The simplest aldol reaction is the condensation of ethanal. This is shown below in
·
2 different representations.
Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic-
hydrogen giving the reactive enolate.
Step 2:
The nucleophilic enolate attacks the aldehyde at the electrophilic carbonyl C in a
nucleophilic addition type process giving an intermediate alkoxide.
Step 3:
An acid-base reaction. The alkoxide deprotonates a water molecule creating hydroxide
and the hydroxyaldehydes or aldol product
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Benzoin Condensation:
The Benzoin Condensation is a coupling reaction between two aldehydes that allows the
preparation of α-hydroxyketones. The first method is only suitable for the conversion of
aromatic aldehydes.
Mechanism
Addition of the cyanide ion to create a cyanohydrin effects an umpolung of the normal
carbonyl charge affinity, and the electrophilic aldehyde carbon becomes nucleophilic
after deprotonation.
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A strong base is now able to deprotonate at the former carbonyl C-atom:
A second equivalent of aldehyde reacts with this carbanion; elimination of the catalyst
regenerates the carbonyl compound at the end of the reaction:
Neighbouring group participation:
The direct interaction of the reaction centre (usually, but not necessarily, an incipient
carbenium centre) with a lone pair of electrons of an atom or with the electrons of a
- or
-bond contained within the parent molecule but not conjugated with the reaction centre.
A distinction is sometimes made between n-,
- and
-participation. When NGP is in
operation it is normal for the reaction rate to be increased.
A rate increase due to neighbouring group participation is known as `anchimeric
assistance'. `Synartetic acceleration' is the special case of anchimeric assistance ascribed
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to participation by electrons binding a substituent to a carbon atom in a
-position
relative to the leaving group attached to the
-carbon atom.
A classic example of NGP is the reaction of a sulfur or nitrogen mustard with a
nucleophile, the rate of reaction is higher for the sulfur mustard and a nucleophile than it
would be for a primary alkyl chloride without a heteroatom.
The š orbitals of an alkene can stabilize a transition state by helping to delocalize the
positive charge of the carbocation. For instance the unsaturated tosylate will react more
quickly with a nucleophile than the saturated tosylate.
The carbo-cationic intermediate will be stabilized by resonance where the positive charge
is spread over several atoms.
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2.23
PHOTOCHEMICAL REACTIONS:
Any chemical reaction can be caused by absorption of light (including visible, ultraviolet,
and infrared). The light excites atoms and molecules (shifts some of their electrons to a
higher energy level) and thus makes them more reactive. In comparison to ordinary
reactions using thermal energy alone, photochemical reactions can follow different routes
and are more likely to produce free radicals, which can trigger and sustain chain
reactions.
Some
photochemical
reactions
like
photoadditions,
photocycloadditions,
photoeliminations, photoenolizations, photo-Fries rearrangements, photoisomerizations,
photooxidations, photoreductions, photosubstitutions, etc.
The Reaction between hydrogen and chlorine:
H2(g) + Cl2(g)
2HCl(g)
A mixture of hydrogen and chlorine gases kept in the dark reacts only very slowly if at
all. Now subject it to a pulse of ultraviolet light and an explosive reaction takes place.
Initiation, Propagation and Termination
The reaction of hydrogen and chlorine is a typical photochemical chain reaction
involving radicals. The reaction involves three stages: initiation, propagation, and
termination. It requires photons of light only to get it started (Initiation of the reaction)
after which it rapidly reaches completion. These photons, absorbed by a few of the
chlorine molecules, cause the Cl-Cl bonds to break homolytically.
2Cl.
Step 1
Cl2 + h
Initiation
The reaction now has to keep going, or propagate itself. The next two steps in the
mechanism involve propagation. A propagation reaction involves the loss of a radical,
but also the formation of another radical. Two propagation steps are required otherwise
the reaction would come to a stop before completion.
Cl. + H2
H. + HCl
Step 2
Propagation
H. + Cl2
HCl + Cl.
Step 3
Propagation
The propagation steps repeat over and over in a chain reaction. Radicals also come
together forming covalent bond in termination steps.
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Synthetic Methods Based on Activating the Reactant
H. + Cl.
Step 4
HCl
Termination
The Chlorination of Methane
The chlorination of methane is another photochemical radical chain reaction.The reaction
is a substitution reaction; a hydrogen atom of methane is swapped for a chlorine atom.
CH4 + Cl2
CH3Cl + HCl
2Cl.
Here is the mechanism for the reaction..Step 1 Cl2 + h
Initiation
CH4 + Cl.
.
Step 2
CH3 + HCl
Propagation
.
CH3Cl + Cl.
Step 3
CH3 + Cl2
Propagation
.
CH3 + Cl.
Step 4
CH3Cl
Termination
In the reaction of methane and chlorine, chloromethane (CH3Cl) is not the only organic
product. A mixture of organic products (also CH2Cl2, CHCl3, CCl4) is obtained,
corresponding to the substitution of each of the hydrogen atoms of methane. The
formation of these arises from steps 2 and 3 above repeating. The formation of the
disubstituted derivative, dichloromethane (CH2Cl2) is shown:
CH3Cl + Cl.
.
CH2Cl + HCl
.
CH2Cl2 + Cl.
CH2Cl + Cl2
Finally, to be more precise about the name of the mechanism for this reaction: it is a
radical substitution reaction.
REFERENCES:
1. O. D. Tyagi, M. Yadav, A Textbook of Organic Reaction Mechanism, Anmol
Publications Pvt. Ltd., 2002.
2. O. P. Agarwal, Unified Course in Chemistry, Volume II, Jai Prakash
Nath & Co.,
(2003).
3. Pure and Applied Chemistry, 1994, 66, 1077.
Glossary of terms used in Physical Organic Chemistry, (IUPAC Recommendations
1994).
4. www.chemguide.co.uk