organic chemistry: June 2008

ELECTROPHYLIC ADDITION REACTIONS

# Electophylic addition reactions are those addition reactions in which E+ initiates the reaction.

# This reaction involves two step mechanism.

# This is characteristic reaction of olifiens whose unsaturated double bond e- density is good enough for attach.

MECHANISM AND STEREO CHEMISTRY:

#This reaction's first step involves attach of electrophyl in order to result an cyclic cationic intermediate.

#In the second step the anionic part of reagent attaches on one of the two carbons of cyclic cation in trans manner.

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#Hence this reaction is stereo selective.

REACTIVITY OF ALKYNES TOWARDS E+ ADDITION:

#Though this reaction very much favorable for olifiens , the alkynes are less reactive towards this reaction. This is due to the pi electrons in the triple bond are tightly held by SP hybridized C atoms. Hence they are not available for the attach of electrophyl.

#How ever this reactivity of alkynes increased by Hg2+ like metal ions which can attack on that triple bond.

The mechanism as follow.

ENERGY-PROFILE DIAGRAM:

#Since this reaction involves two types of transitions there will be two peaks in energy-profile diagram.

REACTIVITY OF ALKENES:

#Aalkenes which can capable to form stable carbo cation are more reactive towards this reaction.

For example consider the following alkanes,

Reactivity is in the order 1>2>3>4.
This is because of decreasing of stability of intermediate cation in the given order , because of inductive and hyper conjugation effects.

SOME EXAMPLE REACTIONS:

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ALDOL CONDENSATION

--->Condensation of aldehydes and ketones having α hydrogens under go this type of condensation in presence of base

BASE CONDITIONS TO BE MAINTAINED:

--->This reaction is feasible in presence of bases like dil NaoH, K2CO3 , etc....

--->It is very important to remember that base conc. must be low. with increase in base conc. repeated condensation occurs and resultant products will be resins(polymers formed due to repeat condensation.)

DEHYDRATION OF PRODUCT:

--->If second α hydrogen is there on heating dehydration occurs. Resonance of new formed double bond with that carbonyl already exist is the driving force for dehydration.

FORMATION OF INTRA MOLECULAR ACETALS IN PRODUCT:

--->Generally the aldol condensation product has both alcoholic and carbonyl groups. So the products in sevaral conditions are going to be acetals.
--->So products has to isolate carefully in presence of catalysts like dil K2CO3.

REACTION
Aldol condensation of an aldehdye

loss of water gives a C=C in a conjugated aldehyde

SIMPLY........

MECHANISM

REACTIVITY OF CARBONYLS TOWARDS ALDOL CONDENSATION:

--->-I EFFECT i.e electron with drawing groups increases the reaction rate by making the carbane ion much stable, while the electron donating groups decreases reactivity.

--->Hence aldehydes are more reactive than ketones.

CROSSED ALDOL CONDENSATION BETWEEN ALDEHYDES AND KETONES:

--->If you want to condensate an aldehyde with ketone, the major product will be condesation product between aldehydes due to their reactivity.

---> So in order to get max. cross product yield add aldehyde slowly to the stirred mixture of ketone and base.

SOME EXAMPLES:

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Introduction Synthesis of Porphyrins

Porphyrin an intruduction:

==>Porphyrins are macro-cyclic molecules.
==>They are highly Florasent molecules. 
==>they are formed in the process of condensation of pyrrole with aldehydes in presence
of acid,followed by tretment of oxidizing agent like DDQ.
==>They have applications in medicinal chemistry also.

SYNTHESIS:
Porphyrin is a macro organic molecule result of condensation of pyrrole(4eq) and aldehydes (4eq), in presence of lewis acid like BF3. Generally BF3 exists as BF3.Et2O adduct.
DDQ(2,3-dichloro-5,6-dicyanobenzoquinone) used as oxidizing agent(dehydrogenation).

Generally we can varii the groups are brought at meso position by using different aldehydes in condensation.
If we used benzaldehyde we get phenyl groups at meso position.
If we use tolaldehyde we get tolyl groups at meso position.

Here H+ is BF3,
DDQ used as dehydrogenation agent.

MECHANISM INVOLVED:

(porphyrin 1*)

1. Let the alde hyde used in condensation is para 'X' benzaldehyde , then,
if,
X=CH3-, aldehyde is p-tolaldehyde, porphyin will be H2TTP(TETRA TOLYL PORPHYRIN)
X=H-,aldehyde is benzaldehyde,porphyin will be H2TPP(TETRA PHENYL PORPHYRIN)

synthesis of mono substituted porphyrins:

Take 3 eq of one type of aldehyde and 1 eq of other type aldehyde and condensate with 4 eq of pyrrole, then you are going to get 5 types of products, separate them by column chrometography, then you can isolete one porphyrin having the aldehyde residue in 3:1 ratio(that is product will have 3 'X=R', 1X=Y in the above porphyrin1*) .

the structure will be as follow,

This product will be obtained by condensation of p-tolaldehyde and p-X aldehyde(where X can be any group).

For example if we have taken 3 eq tolaldehyde, 1eq p-nitro benzaldehyde the we get 5 products.
1.H2TTP ( that is product will have 4X=CH3, in the above porphyrin1*)
2.Mono Nitro H2TTP( that is product will have 3 'X=CH3', 1X=NO2 in the above porphyrin1*)
3.DI Nitro H2TTP ( that is product will have 2 'X=CH3', 2=NO2 in the above porphyrin1*)
4.Tri Nitro H2TTP ( that is product will have 1 'X=CH3', 3=NO2 in the above porphyrin1*)
5.Tetra Nitro H2TTP ( that is product will have 4X=NO2 in the above porphyrin1*)

SEPARATION OF PORPHYRINS BASING ON POLARITY(COLUMN CHROMATOGRAPHY):
We can separate them by column chromatography, basing on polarities.
polarity of above porphyrins increases with increase in nitro groups.
Here we are running column with mixture of pet ether(nonpolar) and DCM(polar).
so as percentage of DCM increases polarity of eluting solvent increases and polar porphyrin starts coming.

so H2TTP comes out first,
mono nitro H2TTP next,
di nitro H2TTP next,
tri nitro H2TTP next,
tetra nitro H2TTP comes last through silica gel column.
We can find out separating sovent mixture by TLC(Thin Layer Chromatography)

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CLAISEN CONDENSATION

--->This is a characteristics reaction of aldehydes with α-hydrogens.

--->This reaction takes place in presence of bases like NaOEt , NaOtBu etc......In this mechanism intermediate anion is stabilized by resonance, hence this is one of the driving force of reaction.

--->If two different types of esters taken then, the one which can capable of forming more stable anion stabilized through resonance will form carbane ion another one will be electron pair acceptor from carbane ion and that product will be more .There will be four different products.

--->Rate of reaction will be dependent of conc. of both ester and base, so we can consider this reaction as SN2 type.(order in the initiation step, total 3 in whole reaction. With respect to ester 2, and with respect to base1.)

--->This SN2 manner can be supported by the fact that, the use of stronger bases, e.g. sodium amide or sodium hydride instead of sodium ethoxide, often increases the yield.

--->Here the carbane ion formed acts as nucliophyl and substitution will be on the other ester's carbonyl group.

--->Simply it is an addition and elimination type reaction, elimination of small components like EtOH taking place hence this is a condensation reaction.

Example reaction(Formetion of aceto acetic ester):
Claisen condensation of esters