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Carboxylic Acid Derivatives - Organic Chemistry - Lecture Notes, Study notes for Organic Chemistry. Birla Institute of Technology and Science.
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- Supporting Materials
- Lecture Notes | Organic Chemistry II | Chemistry | MIT OpenCourseWare
It can catalyze the hydration of 3. Carbonic acid is a weaker acid than acetic acid. This indicates that the hydroxyl group is actually an electron donating group EDG. Remember that there are two effects operating here in opposition to each other. This resonance donation is stronger than the electron withdrawing inductive effect and so the overall effect of the oxygen is to release electrons to the electron deficient carbonyl.
The second pKa, as we would expect, is higher than the first one, since the oxygen anion is an even better donating group. Potassium permanganate or chromic acid will oxidize any aryl side chain down to the carboxylic acid as long as there is at least one benzylic hydrogen. A variety of reagents will oxidize aldehydes to carboxylic acids, including chromic acid. Recall that alkenes can be cleaved using ozonolysis and by using an oxidative work-up with a peroxide reagent we can form carboxylic acids. O New Methods for Synthesis of Carboxylic Acids 5 Carbonylation of Grignard Reagents of Organolithium Reagents Grignard reagents, RMgX, or organolithium reagents, RLi, both react with carbon dioxide to give, after acidic work-up, a carboxylic acid with the addition of one extra carbon.
As we saw earlier, Grignards and organolithium reagents react with carbonyls and carbon dioxide, as a dicarbonyl is an excellent electrophile. As we know, we can make alkyl nitriles by reaction of alkyl halides with a nitrile salt. This is a simple SN2 reaction that forms a new C-C bond in the process. Then we can hydrolyze the nitrile to the carboxylic acid derivative. This method is complementary to the carbonylation of Grignard or organolithium reagents and can be useful with molecules that have an acidic proton and so that would not be compatible with the strongly basic conditions of the Grignard or organolithium intermediate.
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Acidic proton would destory a Grignard intermidiate but is not deprotonated by the weakly basic nitrile. We can also hydrolyze cyanohydrins, which can be made from ketones or aldehydes by nucleophilic attack of the nitrile on the carbonyl carbon. Reactions of Carboxylic Acids 1 Carboxylic acids can be converted to acid chlorides by heating with thionyl chloride, SOCl2.
The first steps are the same as the acetal formation. First we protonate the carbonyl carbon to make it more electrophilic and then the alcohol attacks to make, after proton transfer back to the ethanol solvent, the hemi-acetal. This is also called the tetrahedral intermediate. As we know, hemi-acetals are not stable. Now, we can shorten the writing of this mechanism by combining some of the proton transfer steps.
A shortened form of this is shown below.
Notice that in the first step the carboxylic acid is protonated on the carbonyl oxygen rather than the hydroxyl group. This oxygen is more electron rich due to resonance. No resonance donation possible from the carbonyl oxygen since this would make a pentavalent carbon. Note also that these reactions are completely reversible and as we shall see in the next chapter, we can also cleave esters in dilute aqueous acid.
This is called hydrolysis. We drive the equilibrium in favor of ester formation by using an excess of the alcohol under anhydrous conditions and we drive the reaction in favor of ester hydrolysis by using an excess of water. Lactone Formation We can form esters intramolecularly to form cyclic esters. These are called lactones. These are favorable when we have a 4- or 5-hydroxy carboxylic acid that can cyclic to form a five- or six-membered ring.
This is called a decarboxylation reaction and as we will see in a future chapter it is very useful in synthesis. The mechanism involves a six-membered ring transition state in which a C-C bond is broken. If you don't receive any email, please check your Junk Mail box. If it is not there too, then contact us to info docsity.
If even this does not goes as it should, we need to start praying! This is only a preview. Search in the document preview. Microsoft Word - Chapter The negative charge is distributed equally over the two oxygens each has a charge docsity. Taking the log of both sides: If there are 12 — 18 carbons in the carboxylic acid salt then the behavior is unusual. Note also that part of the increased acidity of oxalic acid as compared to docsity. Resonance effect donates eltrons to the carbonyl carbon through the oxygen lone pair. Overall the resonance effect is stronger than the donating effect.
NaOH, H2O, heat 2. H H Note also that these reactions are completely reversible and as we shall see in the next chapter, we can also cleave esters in dilute aqueous acid. Access your Docsity account. Sign in via social Sign up with Facebook. Sign up with Google. In acidic conditions there are additional proton transfer steps but these are very fast. All species are already hydrogen-bonded to protons and the proton transfer steps are essentially instantaneous. The slow step is still the attack of the nucleophile on the carbonyl. As we will see, there. Since the slow step is the attack of the nucleophiles on the carbonyl carbon, what determines the rate of this reaction is the degree of electron deficiency at the carbonyl carbon.
As we know, the carbonyl carbon has a partial positive charge so we can also say, that the greater the positive charge on the carbonyl carbon, the more reactive it will be to nucleophiles, which by definition, has to have a partial negative charge and a lone pair. As we will see, there are two opposing trends: The chlorine is a relatively strong electron withdrawing group and the carbonyl carbon — chlorine bond is relatively long because chlorine is in the second row of the periodic table and larger than carbon. Cl In acid chlorides, the inductive effect of electron withdrawal is stronger than the resonance effect of electron donation.
But since there are two carbonyl carbons competing for the oxygen lone pair, the lone pair donation resonance effect is diluted. Sulfur is a third row element, like chlorine, and so it is considerably larger than oxygen. But thioesters are less reactive than acid chlorides and anhydrides due to the fact that the sulfur is considerably less electronegative than oxygen and chlorine. The inductive electron withdrawing effect of the sulfur is less than that of oxygen or chlorine due to the decreased electronegativity of sulfur versus oxygen 2.
The oxygen substituent of esters is an overall electron-donating group. There is the electron withdrawing effect due to the greater electronegativity of oxygen as compared to carbon but this is outweighed by the electron donating effect of the oxygen lone pair due to resonance. In esters, the electron donation of the oxygen lone pair is stronger than the eelctron withdrawal due to the greater electronegativity of oxygen as compared to carbon.
Nitrogen is less electronegative than oxygen 3. The strong resonance donation gives the carbon-nitrogen bond in amides lots of double bond character. The C-N bond in amides is much shorter than a normal C-N bond.
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There is a considerable barrier to rotation around the C-N bond since there is a lot of sp2 character for the amide nitrogen. All of the three bonds of the amide lie in the same plane. This is also stabilized by resonance. The negatively charged oxygen is a powerful electron donor and so a Carboxylate anion behaves very differently from the other carboxylic acid derivatives under discussion. The carbonyl carbon is not electrophilic and is not attacked by nucleophiles. Again, to convert one carboxylic acid derivative into another one, the reaction is feasible only if the new derivative lies BELOW it in reactivity or, in other words, only if the conversion is from a less stable carbonyl to a more stable one.
A very useful way to remember the reactivity order is to consider the leaving group ability of the X group. As we discussed above, the slow step rate determining step of the reaction is the attack on the carbonyl and loss of the leaving group is fast but the leaving group ability of X does correlate with the overall rate of the reaction.
And so we can remember the reactivity order by considering which is the better leaving group. As we know, the more stable the anion — i. This is simply a mnemonic, a useful way to remember the reactivity order but you can see that there is good correlation between the reactivity and leaving group ability. Acid chlorides Preparation Acid chlorides are extremely reactive and are generally prepared in situ from carboxylic acids by heating in a solution of thionyl chloride. The thionyl chloride is generally used in excess as the solvent and when the reaction is finished the excess is removed by distillation, leaving behind the moisture sensitive and highly reactive acid chloride.
Aqueous workups are to be avoided since the acid chlorides react rapidly with water to reform the carboxylic acid. Reactions of Acid chlorides Acid chlorides are the most reactive of the carboxylic acid derivatives and can therefore be used to prepare all of the other derivatives: Usually a mild, non-nucleophilic base such as pyridine is added to the reaction mixture so as to neutralize the HCl that is produced. The by-product is NaCl, rather than HCl.
Tertiary amines give unstable products that cannot be isolated.
Since amines are fairly strong bases and good nucleophiles we 1 don not need to add a second base such as pyridine and we simply use an excess of the amine to neutralize the HCl that is produced provided that our amine is relatively inexpensive. And 2 we do not need to use an amine anion since the neutral amine is an excellent nucleophile.
This is not a useful reaction synthetically since acid chlorides are produced from carboxylic acids but it is a reaction that we must be aware of and usually try to avoid. Anhydrides After acid chlorides, the next most reactive derivatives are the anhydrides. They can be used to form the esters and amides and are also subject to hydrolysis.
It is best to think of acid chlorides and anhydrides as reagents used for the preparation of the more stable end products, the esters and the amides.
Preparation In the laboratory anhydrides are usually prepared from acid chlorides, as we have just seen. Other common derivatives are prepared by special methods on an industrial scale. Reactions of Anhydrides Since a nucleophile can attack either carbonyl, symmetrical anhydrides are usually used so as to give one product.
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With the neutral alcohols we usually use acid catalysis to activate the carbonyl carbon of the anhydride to nucleophilic attack since the neutral alcohol is a relatively weak nucleophile. Acid catalysis increases the rate of formation of the tetrahedral intermediate.
Lecture Notes | Organic Chemistry II | Chemistry | MIT OpenCourseWare
No catalysis is needed, since amines are basic and good nucleophiles. Use of acid would protonate the amine rather than the carbonyl carbon, making the amine non-nucleophilic. The reaction is generally slow at room temperature and is accelerated by heat. An alkyl or aryl group migrates from carbon to oxygen. This reaction is very useful for cyclic symmetric ketones. The product is a lactone cyclic ester that contains one more atom, so it is a ring-expansion reaction. Physical Properties of Esters Esters are moderately polar compounds that have higher boiling points than hydrocarbons but lower than alcohols and much lower than carboxylic acids.
Esters cannot donate hydrogen-bonds but they can accept them, so they have some solubility in water. Reactions of Esters 1 Review Reduction with lithium aluminum hydride to a primary alcohol. Amides are below esters in the reactivity scale and so can be converted to amides. We drive the reaction to the right in favor of hydrolysis by using an excess of water.
This reaction is called saponification from the Latin sapon for soap because the basic hydrolysis of animal fat was a traditional way of making soap. One advantage of the basic hydrolysis is the reaction is irreversible. The initial carboxylic acid formed is irreversibly deprotonated by the basic hydroxide solution.
To isolate the neutral carboxylic a final protonation step is required. The pH is made acidic by the addition of aqueous HCl. Final step is irreversible and this drives the equilibrium to the right in favor of hydrolysis. Note that cleavage always occurs between the carbonyl carbon and the oxygen, not between the alcohol carbon and the oxygen. Cleavage is always of this bond Therefore cleavage of esters with optically active alcohols results in retention of configuration of the alcohol moiety. S-configuration 4 Reaction of Esters with Grignard and organolithium reagents.
Grignard and organolithium reagents react twice with esters to give tertiary alcohols in which two of the substituents are the same. Thioesters can be prepared from acid chlorides or anhydrides using the same conditions as discussed above for esters. R Reactions Thioesters will react with alcohols, alkoxides and amides.
They see limited use in laboratory synthesis but are very important in biological systems.