E-StoreChemical Reactions.
Addition reactions are essentially of two types. Electrophilic and Nucleophilic addition reactions.
1. Addition across >C=C< or -C
C- are usually initiated by either an electrophile or a radical. The p -electrons shield the molecule from nucleophilic attack.
2. The second type of addition is across >C=O or -Cº N and is initiated by a nucleophile. The p -electron density is slightly shifted towards the more electronegative elements O and N, thus the carbon atom acquires a slightly positive charge and is preferentially attacked by nucleophilic reagents. Also systems that are negatively charged are more stable than the positively charged ones, so the addition at carbon with a slightly positive charged center proceeds that of Oxygen or Nitrogen with a negatively charged center. These reactions are of great synthetic application because carbon-carbon bonds are formed. Some examples of named reactions are:
i. Grignard reagent
ii.. Addition of Acetylide ion
iii. Aldol condensation
iv. Perkin reaction
v. Wittig reaction
vi. Claisen ester condensation
vii. Dieckman reaction
viii. Acid decomposition of b -diketones
ix. Benzoin condensation
x. Mannich reaction
Grignard Reagent - Grignard reagents are organo-metalic compounds, prepared by the reaction of metallic magnesium with the appropriate organic halide. The actual structure of Grignard reagents is still a matter of some dispute.
2R.Mg.Hal 
2R
+ 2Mg
Hal
2R.Mg.Hal 
2R· + 2MgHal
2R.Mg.Hal R2Mg + MgHal2
They however behave as though they are polarized as a source negative carbon. Rd - . Mgd + . Hal. In their reaction with carbonyl groups two molecules of Grignard reagent are involved. One molecule acts as Lewis acid while the second molecule stabilize addition by completing the cyclic transition sate.
To justify this mechanism, it is expected that a better Lewis acid than Grignard reagent should promote this reaction because it would coordinate preferentially to yield an even more positive carbonyl carbon atom. To this end, when MgBr2 is introduced, a double yield of the tertiary alcohol was observed in the reaction ketones with Grignard reagents.
Acetylide ion - The addition reaction of acetylide is usually carried out in Liquid ammonia in the presence of sodamide (NaNH2) to convert acetylene into its carbanion.
The hydrogenation of the resultant acetylenic carbinol in the presence of Lindar catalyst yields the olefine. Lindar catalyst is partially poisoned palladium.
Aldol Condensation - Under the influence of dilute base or dilute acid two molecules of an aldehyde or a ketone may combine to form b-hydroxaldehyde or b-hydroxyketone. This reaction is called aldol condensation.
The corresponding reaction of acetone to diacetone alcohol proceeds much more slowly and, when carried out in D2O, deuterium is incorporated into the methyl group; this is the result of a less rapid attack of the carbanion on a carbonyl carbon atom which is markedly less positive than that of an aldehyde.
Aldehydes having no a-hydrogen atoms cannot form carbanions and they therefore, undergo the cannizaro reaction with concentrated alkali. The cannizaro reaction is very slow and as such an advantage is taken of the time lag to synthesis polyhydroxy compounds using such aldehydes in cross-aldol condensation as carbanion acceptors.
Note: Cross-Aldol condensation is aldol condensation between two different carbonyl compounds, e.g., Formaldehyde in excess, reacting with acetaldehyde.
In the last stage the aldehyde formed (HO.H2C)3 C.CHO can no longer form a carbanion hence it undergoes cannizaro reaction with the formaldehyde to yield penta-erythrtol and formate ion. To initiate cannizaro reaction, the OH
is preferentially added to formaldehyde because its carbonyl carbon is more positive than that of penta-erythraldehyde (HO.H2C)3 C.CHO. This then leads to the transfer of the hydride (H) ion.
Another type of cross-aldol condensation is the base-catalyzed addition of aliphatic nitro compound to carbonyl groups. In this case the carbonyl compounds act as the carbanion acceptor because the carbanion formed by the nitro compound tends to be the more stable as it has a more effective delocalisation of its charge.
Usually when adol compounds are formed, it is possible for water molecule to be eliminated. The dehydration method depends on the concentration of the base-catalyst and on the ease of forming another carbanion by the adol compound.
The formation of adol compounds followed by elimination of water in presence of a strong base, has resulted in the formation of low molecular weight polymer from simple aliphatic aldehydes.
If the process is to be halted at the simple aldol, a weak base such as K2CO3 is used. A controlled reaction of Aldol formation followed by elimination of water is called the Claisen-Schmidt condensation of aromatic aldehydes with aliphatic aldehydes or ketones in the presence of 10% mineral alkali.
The presence of electron-donating group in the aromatic nucleus will reduce the positive nature of the carbonyl carbon atom, e.g., p-MeOC6H4CHO is found to react at only about 14% the rate of benzaldehyde while self-condensation of aliphatic aldehydes also constitute an important side reaction.
Perkin Reaction - The Perkin reaction is a type of aldol condensation of aromatic aldehydes and the anhydride of an aliphatic acid in the presence of sodium salt of the same acid, to give on heating an a,b-unsaturated acid.
The carboxylate anion abstracts a proton from the a-carbon of the anhydride to form carbanion I. This carbanion undergoes nucleophilic addition to carbonyl carbon of the aldehyde. The anion II so formed takes up a proton to form a hydroxy compound III which first undergoes dehydration as an anhydride before it is hydrolyzed to the a,b-unsaturated acid.
Wittig Reaction - Wittig reaction is used to synthesis alkenes from carbonyl compounds. The reaction is simply the replacement of the carbonyl oxygen (=O) by the group =CR1R2. This is brought about by a nucleophilic attack on carbonyl carbon by a ylide to form a betaine which undergoes elimination to yield the product.
The carbonyl compounds may contain a wide variety of substituents, and so may the ylide. The phosphorous ylides have hybrid structures, and it is the negative charge on carbon (the carbanion in the ylide) that is responsible for their nucleophilic attack on carbonyl carbon.
Claisen Condensation - This is the exact counterpart of aldol condensation for esters instead of aldehyde. It leads to the formation of b-keto esters. In aldol condensation, nucleophilic attack leads to addition, which is a typical reaction of aldehydes and ketones, whereas in claisen condensation, the nucleophilic attack leads to substitution, which is also the typical reaction of acyl compounds. Also at the end of the reaction, it is not the b -keto ester that is produced but its sodium salt. This is because the a-hydrogen of b-keto ester are located a to two carbonyl groups and hence ionization yields a particularly stable carbanion in which two carbonyl groups help to accommodate the charge. In fact b-keto esters are much stronger acids than ordinary esters or other compounds containing a single carbonyl group. Normally a gram equivalent of sodium is employed as source of sodium ethoxide catalyst but only a little ethanol is needed as more will be liberated as soon as the reaction starts with ethyl acetate. It is an essential feature of this reaction because I is not highly stabilized as so II drive the equilibrium to the right.
Crossed Claisen Condensation - Like a crossed adol condensation, a crossed claisen condensation is generally the reaction of esters that are incapable of undergoing self-condensation because they don’t have a-hydrogens or their type I cabanaion are not stabilized like in the case of aldehydes, e.g., 
CH2-CHO. In such case the reaction is catalyzed with strong bases such as Ph3C
Na
so that the initial carbanion formation becomes irreversible.
Dieckman Reaction - When two ester groups are part of the same molecule, cyclisation can result. This type of condensation is known as Dieckman reaction.
Acid Decomposition of b-diketones - The decomposition of b-diketones follows the backward reaction of claisen condensation.
Benzoin Condensation - The product of Benzoin condensation is also a b-hydroxyaldehyde like that in aldol condensation but the mechanisms of the reactions are different. Lapworth was the first to show that the reaction is not a simple aldol type condensation. The cyanide ion is the specific catalyst for this reaction and not just any base. Its effectiveness depends on the resonance stabilized carbanion II of cyanohydrine.
The mechanism is as follows:
The observed kinetics of the reaction is Rate 
[Ph.CHO]2[CN] and the rate determining step is believed to be the combination of the carbanion II with a second molecule of aldehyde to yield III.
Mannich Reaction - This is nucleophilic addition reaction of an aldehyde and at least a secondary amine to produce what is known as a schiff base on protonation and elimination of a water molecule. The Schiff base is often stabilized by resonance. The addition of a cabanaion to the schiff base gives another base called the Mannich base. The Mannich base formed can readily eliminate the secondary amine to give the synthetic usefulness of the reaction, but when primary amines or ammonia are used the hydrogen on nitrogen atom can participate in a further reaction to give more complex products.
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