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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Synthetic Methods Based on Activating the Reactant

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.

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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Synthetic Methods Based on Activating the Reactant

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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2.5

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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Synthetic Methods Based on Activating the Reactant

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

Synthetic Strategies in Chemistry

2.7

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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Synthetic Methods Based on Activating the Reactant

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).

Synthetic Strategies in Chemistry

2.9

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

6070

OCH3

F-C Acylation 510

9095

NO2

Nitration

9095

CH3

Nitration

5565

3545

CH3

Sulfonation

3035

510

6065

CH3

F-C Acylation 1015

8590

Nitration

3545

5565

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

Activating Substituents

meta-Orientation

ortho & para-Orientation

O()

CO2H

NH2

NO2

NR3(+)

CO2R

NR2

PR3(+)

NHCOCH3

CONH2

SR2(+)

OC6H5

CHO

CH2Cl

OCOCH3

SO3H

C6H5

COR

SO2R

CH=CHNO2

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Synthetic Methods Based on Activating the Reactant

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.

Synthetic Strategies in Chemistry

2.11

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

2.12

Synthetic Methods Based on Activating the Reactant

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

involved

the

rate-determining

step.

one

Synthetic Strategies in Chemistry

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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Synthetic Methods Based on Activating the Reactant

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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2.15

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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Synthetic Methods Based on Activating the Reactant

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

Synthetic Strategies in Chemistry

2.17

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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Synthetic Methods Based on Activating the Reactant

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.

Synthetic Strategies in Chemistry

2.19

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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Synthetic Methods Based on Activating the Reactant

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.

Synthetic Strategies in Chemistry

2.21

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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Synthetic Methods Based on Activating the Reactant

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.

Synthetic Strategies in Chemistry

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

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.

2.24

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

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