Reaction of carboxylic acid

it deals with all the reaction with carboxylic acid

1. Reaction of carboxylic acid

2. Salt Formation They react readily with inorganic bases such as hydroxide ion or ammonia to form salts.

3. Most acids are acidic enough to react with sodium bicarbonate accompanied by evolution of CO2

4. Carboxylic acids with ammonia

5. Conversion into acid chloride Carboxylic acid is often converted to acid chloride than into any other of its functional derivatives. Acid chloride is prepared by substitution of chloride for the OH of COOH.

6. Three reagents are commonly used for this purpose Thionyl chloride- SOCl2 Phosphorous trichloride-PCl3 Phosphorous pentachloride-PCl5

7. Thionyl chloride Thionyl chloride is convenient since the products formed besides the acid chloride are gases and thus easily separated from acid chloride; any excess of low boiling thionyl chloride [79C] is easily removed by distillation .

8. Conversion into ester

9. A carboxylic acid is converted directly into an ester when heated with an alcohol in the presence of little mineral acid [ usually H2SO4 or Dry HCl ] This reaction is reversible. The reversibility is the disadvantage.

10. The preference of acid chloride from acid and preparation of ester from acid chloride are essentially irreversible and go to completion. Direct esterification has the advantage of being a single step synthesis. If either the acid or alcohol is cheaper or readily available. It can be used in large excess to shift the equilibrium towards the product and thus increase the yield of ester.

11. Sometimes the equilibrium is shifted by removing one of the product. The presence of bulky group near the site of reaction, whether in the alcohol or in the acid , slows down esterification [ steric hindrance ]

12. Conversion into amides Amides are the compounds in which OH of COOH is replaced by NH2.This are generally prepared by reaction of ammonia with acid chloride.

13. Reduction of acids to alcohols Lithium Aluminium hydride is one of the two reagents that can reduce an acid to an alcohol; the initial product is an alkoxide from where alcohol is liberated by hydrolysis.

14. LiAlH4 is widely used because of its excellent yield. It is used for reduction of not only acid but also many other class of compound. It is expensive used only in industry. An alternative to direct reduction acids are often converted into alcohols by a two step process esterification and reduction of ester.

15. Halogenation of aliphatic acid Hell- Vohard - zelinsky reaction Aliphatic carboxylic acid react smoothly with chlorine or bromine to yield a compound in the presence of a small amount of phosphorous ,aliphatic carboxylic acid . In this reaction alpha hydrogen has been replaced by halogen. Regioselectivity -only alpha halogenation. It is of considerable importance in synthesis.

16. The function of phosphorous is to convert a little of acid in acid halide. In this form each molecule of acid sooner or later undergoes alpha halogenation. Halogenation is the first step in the conversion of a COOH into many substituted COOH.

17. Important reaction of benzoic acid

18. Salt formation- Benzoic acid reacts with sodium hydroxide to form sodium benzoate

19. Salt formation- Benzoic acid reacts with sodium bi carbonate to form sodium benzoate

20. Esterification : benzoic acid with ethanol in the presence of concentrated sulphuric acid to form ester

21. Benzoic acid reacts with phosphorous penta chloride or thionyl chloride to form benzoyl chloride

22. Benzoic acid undergoes reduction with lithium aluminium hydride to give benzyl alcohol.

23. Electrophilic substitution reaction

24. Acidity of carboxylic acid

25. Acidity of carboxylic acid Carboxylic acid are polar. It form hydrogen bond with each other and with other kinds of molecule. The aliphatic acid shows same solubility behavior as the alcohol. The first four are miscible with water and five carbon is partly soluble and the higher acids are virtually insoluble.

26. Acidity of carboxylic acid Water solubility is due to hydrogen bonding between the carboxylic acid and water. Lower aliphatic acids –sharp odour Formic and acetic acid – irritating odour Butyric , valeric acid and caproic acid- unpleasant odour Higher acid – less odour because of low volatility.

27. Acidity is chiefly determined by the difference in stability between the acid and its anion. Carboxylic acid are acid at all but it readily gives H+ from OH of carboxylic acid.

28. Although both acids and anion are stabilized by resonance, stabilisation is greater for the anion than for acids. Equilibrium is shifted in the direction of ionisation . Resonance is less important for the acid because the contributing are of different stability, whereas the equivalent structure for the ion must necessarily be of equal stability.

29. In structure II two atoms of similar electronegativity carry opposite charges since energy must be supplied to separate opposite charges. Structure II should contain more energy and hence less stable than structure I. Consideration of separation of charge is one of the rules of thumb.

30. Structure of carboxylate Carbon is joined by each oxygen by a one and half bond. The negative charge is eventually distributed over both the oxygen atoms.

31. Anion is supported by the evidence of bond length. In formic acid C= O bond and C-O single bond have different length. sodium formate on other hand if it is resoance hybrid ought to contain two equivalent C-O bond. X-ray and electron diffraction studies show the expectations are correct.

32. Effect of substituent on acidity Changes in the structure of the group bearing COOH affect the acidity. Any factor which stabilizes the anion more than it stabilizes the acid should increases the acidity; any factor which makes anion less stable should decrease acidity. Electron withdrawing substituent should disperse the negative charge , stabilize the anion and thus increase acidity. Electron releasing/donating substituent should intensify the negative charge, destabilize the anion, and thus decrease acidity.

33. Electron withdrawing group Electron donating group

34. Electron withdrawing group strengthen acids Chloro acetic acid is 100 times more strong as acetic acid. Dichloro acetic acid is still stronger. Trichloro acetic acid is more than 10000 times as strong as the unsubstituted acid. Alpha chloro acetic acid is strong as chloroacetic acid. As the chlorine is moved away from the COOH.

35. Beta chloro butyric acid is six times as strong as butyric acid Gamma butyic acid is only twice as strong. Inductive effect decreases rapidly with distance and seldom important when acting through more than four atoms.

36. The aromatic acid are similarly effected by substituents like CH3, OH and NH2 make benzoic acid weaker. Substituents Cl and NO2 make benzoic acid stronger. All ortho substituents exert an effect on the same kind. The ortho effect undoubtedly has to do with the nearness of the group involved, but it is more than just steric hindrance arising from their bulk. Like inductive effect and resonance effect, rate of reaction also useful in dealing with equilibria.

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