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KIMOR 2 : asam karboksilat

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materi kimia organik 2

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KIMOR 2 : asam karboksilat

  1. 1. Sovia Aprina Basuki KIMIA ORGANIK II FARMASI UMM 2013
  2. 2. Mahasiswa dapat:  Menggambarkan struktur asam karboksilat,  memberi nama asam karboksilat,  menjelaskan sifat keasaman  Menjelaskan konsep penarik dan pendorong elektron  Menjelaskan efek orto pada asam karboksilat aromatik, menjelaskan sifat-sifat fisika asam karboksilat.  Menuliskan reaksi-reaksi pembuatan asam karboksilat  Menuliskan reaksi-reaksi asam karboksilat  Menuliskan rumus umum asam dikarboksilat  Menyebutkan sifat-sifat asam dikarboksilat
  3. 3. Organic Chemistry, 7th edition, John McMurry Organic Chemistry, T. W. Graham Solomons  Organic Chemistry, Fessenden and Fessenden
  4. 4. A general acyl group (blue) as an acylium ion (top centre), as an acyl radical (top right), in a ketone (top left), an aldehyde (bottom left), ester (bottom centre) or amide (bottom right). (R1, R2, R3 = organyl substituents or hydrogen).
  5. 5. Structure and Bonding • Carboxylic acids are compounds containing a carboxy group (COOH). • The structure of carboxylic acids is often abbreviated as RCOOH or RCO2H, but keep in mind that the central carbon atom of the functional group is doubly bonded to one oxygen atom and singly bonded to another.
  6. 6. The two most important features of the carbonyl group are: ·Because oxygen is more electronegative than either carbon or hydrogen, the C—O and O—H bonds are polar.
  7. 7.  Carboxylic Acids, R-COOH  If derived from open-chain alkanes, replace the terminal -e of the alkane name with -oic acid  The carboxyl carbon atom is C1 Common names: IUPAC Common HCO2H methanoic acid formic acid CH3CO2H ethanoic acid acetic acid CH3CH2CO2H propanoic acid propionic acid CH3CH2CH2CO2H butanoic acid butyric acid CH3CH2CH2CH2CO2H pentanoic valeric acid 8
  8. 8. 5 4 3 2 1 C — C — C — C — C = O δ γ β α used in common names
  9. 9. Carboxylic acids, common names: … CH3(CH2)4CO2H caproic acid CH3(CH2)5CO2H --- CH3(CH2)6CO2H caprylic acid CH3(CH2)7CO2H --- CH3(CH2)8CO2H capric acid CH3(CH2)9CO2H --- CH3(CH2)10CO2H lauric acid
  10. 10.  Compounds with CO2H bonded to a ring are named using the suffix -carboxylic acid  The CO2H carbon is not itself numbered in this system  Use common names for formic acid (HCOOH) and acetic acid (CH3COOH) 11
  11. 11. COOH special names COOH COOH COOH CH3 CH3 CH3 benzoic acid o-toluic acid m-toluic acid p-toluic acid
  12. 12. salts of carboxylic acids: name of cation + name of acid: drop –ic acid, add –ate CH3CO2Na sodium acetate or sodium ethanoate CH3CH2CH2CO2NH4 ammonium butyrate ammonium butanoate (CH3CH2COO)2Mg magnesium propionate magnesium propanoate
  13. 13.  Carboxylic acids transfer a proton to water to give H3O+ and carboxylate anions, RCO2 15 , but H3O+ is a much stronger acid  The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid (pKa ~ 5)
  14. 14.  Fluoroacetic, chloroacetic, bromoacetic, and iodoacetic acids are stronger acids than acetic acid  Multiple electronegative substituents have synergistic effects on acidity 17
  15. 15.  If pKa of given acid and the pH of the medium are known, % of dissociated and undissociated forms can be calculated using the Henderson- Hasselbalch eqn 18
  16. 16. 19 The Inductive Effect in Aliphatic Carboxylic Acids
  17. 17. 20
  18. 18. Substituted Benzoic Acids Recall that substituents on a benzene ring either donate or withdraw electron density, depending on the balance of their inductive and resonance effects. These same effects also determine the acidity of substituted benzoic acids. [1] Electron-donor groups destabilize a conjugate base, making an acid less acidic—The conjugate base is destabilized because electron density is being donated to a negatively charged carboxylate anion. 21
  19. 19. [2] Electron-withdrawing groups stabilize a conjugate base, making an acid more acidic. The conjugate base is stabilized because electron density is removed from the negatively charged carboxylate anion. 22
  20. 20. 23 Figure 19.8 How common substituents affect the reactivity of a benzene ring towards electrophiles and the acidity of substituted benzoic acids
  21. 21.  Subtituen posisi orto dari turunan asam benzoat selalu meningkatkan sifat keasaman senyawa tersebut karena subtituen ini mengurangi resonansi luar cincin.  Efek orto pada asam benzoat tidak tergantung pada jenis substituen apakah cenderung menarik atau melepaskan elektron.  Efek resonansi sangat berpengaruh terhadap kekuatan asam. Subtituen yang berada pada posisi orto akan mengurangi resonansi luar cincin sehingga akan meningkatkan kekuatan asam.  Senyawa turunan asam benzoat yang mempunyai kekuatan asam tertinggi adalah senyawa turunan asam benzoate yang subtituennya terletak pada posisi orto.
  22. 22. 1. Wujud Pada temperatur kamar, asam karboksilat yang bersuku rendah adalah zat cair yang encer, suku tengah berupa zat cair yang kental, dan suku tinggi berupa zat padat yang tidak larut dalam air. Rumus Struktur T d H-COOH 101 CH3-COOH 118 CH3-CH2-COOH 141 CH3-CH2-CH2-COOH 163 CH3-CH2-CH2-CH2-COOH 187 2. Titik didih dan titik leleh Asam karboksilat membentuk ikatan hidrogen berupa siklik dimer antarmolekul. Ikatan hidrogen yang kuat ini menyebabkan TD dan TL lebih tinggi dari alkohol yang bersesuaian.
  23. 23. 3. Kelarutan  Carboxylic acids are proton donors toward weak and strong bases, producing metal carboxylate salts, RCO + M 2  Carboxylic acids with more than six carbons are only slightly soluble in water, but their conjugate base salts are water-soluble 4. Daya hantar listrik Asam karboksilat dapat terionisasi sebagian dalam air, sehingga termasuk senyawa elektrolit lemah. R-COOH ⇋ R-COO- + H+
  24. 24. 27 [1] Oxidation of 1° alcohols [2] Oxidation of alkyl benzenes
  25. 25. 28 [3] Oxidative cleavage of alkynes
  26. 26. 1. Reaksi dengan Basa (penyabunan) R-COOH + NaOH → R-COONa + H2O 2. Reaksi esterifikasi sabun H2SO4 R-COOH + R’-OH → R-COOR’ + H2O Asam karboksilat Alkohol Ester 3. Reaksi dengan PCl5 R-COOH + PCl5 → R-CO-Cl + POCl3 + HCl Alkanoilklorida 4. Reaksi dengan NH3 R-COOH + NH3 → R-CONH2 + H2O Amida 5. Reaksi dengan Cl2 CH3-CH2-COOH + Cl2 → R-CHCl-COOH + HCl Asam 2-monokloropropanoat
  27. 27. Reactions of Carboxylic Acids The most important reactive feature of a carboxylic acid is its polar O—H bond, which is readily cleaved with base.
  28. 28. • The nonbonded electron pairs on oxygen create electron-rich sites that can be protonated by strong acids (H—A). • Protonation occurs at the carbonyl oxygen because the resulting conjugate acid is resonance stabilized (Possibility [1]). • The product of protonation at the OH group (Possibility [2]) cannot be resonance stabilized.
  29. 29. • The polar C—O bonds make the carboxy carbon electrophilic. Thus, carboxylic acids react with nucleophiles. • Nucleophilic attack occurs at an sp2 hybridized carbon atom, so it results in the cleavage of the  bond as well.
  30. 30. Carboxylic Acids—Strong Organic BrØnsted-Lowry Acids • Carboxylic acids are strong organic acids, and as such, readily react with BrØnsted-Lowry bases to form carboxylate anions.
  31. 31. • An acid can be deprotonated by a base that has a conjugate 34 acid with a higher pKa. • Because the pKa values of many carboxylic acids are ~5, bases that have conjugate acids with pKa values higher than 5 are strong enough to deprotonate them.
  32. 32. • Carboxylic acids are relatively strong acids because deprotonation forms a resonance-stabilized conjugate base—a carboxylate anion. • The acetate anion has two C—O bonds of equal length (1.27 Å) and intermediate between the length of a C—O single bond (1.36 Å) and C=O (1.21 Å).
  33. 33. • Ethoxide, the conjugate base of ethanol, bears a negative charge on the O atom, but there are no additional factors to further stabilize the anion. Because ethoxide is less stable than acetate, ethanol is a weaker acid than acetic acid. • Phenoxide, the conjugate base of phenol, is more stable than ethoxide, but less stable than acetate because acetate has two electronegative O atoms upon which to delocalize the negative charge, whereas phenoxide has only one.
  34. 34. Figure 19.7 Summary: The relationship between acidity and conjugate base stability for acetic acid, phenol, and ethanol • Note that although resonance stabilization of the conjugate base is important in determining acidity, the absolute number of resonance structures alone is not what is important!
  35. 35. • Resonance stabilization accounts for why carboxylic acids are more acidic than other compounds with O—H bonds—namely alcohols and phenols. • To understand the relative acidity of ethanol, phenol and acetic acid, we must compare the stability of their conjugate bases and use the following rule: - Anything that stabilizes a conjugate base A:¯ makes the 39 starting acid H—A more acidic.
  36. 36. HOOC-COOH oxalic acid HO2C-CH2-CO2H malonic acid HO2C-CH2CH2-CO2H succinic acid HO2C-CH2CH2CH2-CO2H glutaric acid HOOC-(CH2)4-COOH adipic acid HOOC-(CH2)5-COOH pimelic acid
  37. 37. CO2H CO2H CO2H CO2H CO2H CO2H phthalic acid isophthalic acid terephthalic acid H COOH C C H COOH H COOH C C HOOC H maleic acid fumaric acid

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