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Asam karboksilatSovia Aprina BasukiKIMIA ORGANIK IIFARMASI UMM2013TUJUAN INSTRUKSIONAL KHUSUSMahasiswa dapat: Menggambarkan struktur asam karboksilat,memberi nama asam karboksilat, menjelaskan sifat keasamanMenjelaskan konsep penarik dan pendorong elektronMenjelaskan efek orto pada asam karboksilat aromatik, menjelaskan sifat-sifat fisika asam karboksilat. Menuliskan reaksi-reaksi pembuatan asam karboksilatMenuliskan reaksi-reaksi asam karboksilatMenuliskan rumus umum asam dikarboksilatMenyebutkan sifat-sifat asam dikarboksilatpustakaOrganic Chemistry, 7th edition, John McMurryOrganic Chemistry, T. W. Graham SolomonsOrganic Chemistry, Fessenden and FessendenACYL GROUP

ACYL GROUP

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).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.Structure and Bonding

I. struktur asam karboksilatThe two most important features of the carbonyl group are:

Structure and BONDINGBecause oxygen is more electronegative than either carbon or hydrogen, the CO and OH bonds are polar.78II. TATA NAMA asam karboksilatCarboxylic Acids, R-COOHIf derived from open-chain alkanes, replace the terminal -e of the alkane name with -oic acidThe carboxyl carbon atom is C1Common names:IUPACCommonHCO2Hmethanoic acid formic acidCH3CO2Hethanoic acidacetic acidCH3CH2CO2Hpropanoic acidpropionic acidCH3CH2CH2CO2Hbutanoic acidbutyric acidCH3CH2CH2CH2CO2Hpentanoicvaleric acid

5 4 3 2 1 C C C C C = O used in common names

Carboxylic acids, common names:CH3(CH2)4CO2Hcaproic acidCH3(CH2)5CO2H---CH3(CH2)6CO2Hcaprylic acidCH3(CH2)7CO2H---CH3(CH2)8CO2Hcapric acidCH3(CH2)9CO2H---CH3(CH2)10CO2Hlauric acid11Alternative Names Compounds with CO2H bonded to a ring are named using the suffix -carboxylic acidThe CO2H carbon is not itself numbered in this systemUse common names for formic acid (HCOOH) and acetic acid (CH3COOH)

special namessalts of carboxylic acids:name of cation + name of acid: drop ic acid, add ate

CH3CO2Nasodium acetate or sodium ethanoate

CH3CH2CH2CO2NH4 ammonium butyrate ammonium butanoate

(CH3CH2COO)2Mg magnesium propionate magnesium propanoate

15III. Keasaman ASAM KARBOKSILATCarboxylic acids transfer a proton to water to give H3O+ and carboxylate anions, RCO2, but H3O+ is a much stronger acid The acidity constant, Ka,, is about 10-5 for a typical carboxylic acid (pKa ~ 5)

16Substituent Effects on AcidityElectronegative substituents promote formation of the carboxylate ion

17Inductive Effects on AcidityFluoroacetic, chloroacetic, bromoacetic, and iodoacetic acids are stronger acids than acetic acid Multiple electronegative substituents have synergistic effects on acidity

18Biological Acids and the Henderson-Hasselbalch EquationIf 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

19The Inductive Effect in Aliphatic Carboxylic Acids

IV. PENGARUH GUGUS penarik dan pendorong elektron20

21Substituted Benzoic AcidsRecall 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 acidicThe conjugate base is destabilized because electron density is being donated to a negatively charged carboxylate anion.

v. efek orto pada asam karboksilat aromatik22[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.

23Figure 19.8How common substituentsaffect the reactivity of abenzene ring towardselectrophiles and the acidity ofsubstituted benzoic acids

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.vI. SIFAT-SIFAT FISIK asam karboksilat1. WujudPada 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 StrukturT dH-COOH101CH3-COOH118CH3-CH2-COOH141CH3-CH2-CH2-COOH163CH3-CH2-CH2-CH2-COOH1872. Titik didih dan titik lelehAsam karboksilat membentuk ikatan hidrogen berupa siklik dimer antarmolekul. Ikatan hidrogen yang kuat ini menyebabkan TD dan TL lebih tinggi dari alkohol yang bersesuaian.

3. KelarutanCarboxylic acids are proton donors toward weak and strong bases, producing metal carboxylate salts, RCO2 + MCarboxylic acids with more than six carbons are only slightly soluble in water, but their conjugate base salts are water-soluble

4. Daya hantar listrikAsam karboksilat dapat terionisasi sebagian dalam air, sehingga termasuk senyawa elektrolit lemah.R-COOH R-COO- + H+

27[1]Oxidation of 1 alcohols

[2]Oxidation of alkyl benzenes

viI. CARA pembuatan asam karboksilat28[3]Oxidative cleavage of alkynes

vIIi. reaksi-reaksi asam karboksilat1. Reaksi dengan Basa (penyabunan)R-COOH + NaOH R-COONa + H2O 2. Reaksi esterifikasiR-COOH + R-OH R-COOR + H2O H2SO4sabunAsam karboksilatAlkoholEster3. Reaksi dengan PCl5R-COOH + PCl5 R-CO-Cl + POCl3 + HClAlkanoilklorida4. Reaksi dengan NH3R-COOH + NH3 R-CONH2 + H2OAmida5. Reaksi dengan Cl2CH3-CH2-COOH + Cl2 R-CHCl-COOH + HClAsam 2-monokloropropanoatReactions of Carboxylic AcidsThe most important reactive feature of a carboxylic acid is its polar OH bond, which is readily cleaved with base.

The nonbonded electron pairs on oxygen create electron-rich sites that can be protonated by strong acids (HA).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.

The polar CO 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.

Carboxylic AcidsStrong Organic Brnsted-Lowry AcidsCarboxylic acids are strong organic acids, and as such, readily react with Brnsted-Lowry bases to form carboxylate anions.

34An acid can be deprotonated by a base that has a conjugate 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.

Carboxylic acids are relatively strong acids because deprotonation forms a resonance-stabilized conjugate basea carboxylate anion.

The acetate anion has two CO bonds of equal length (1.27 ) and intermediate between the length of a CO single bond (1.36 ) and C=O (1.21 ).

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.

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!Figure 19.7Summary: The relationshipbetween acidity and conjugatebase stability for acetic acid,phenol, and ethanol

39Resonance stabilization accounts for why carboxylic acids are more acidic than other compounds with OH bondsnamely 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 starting acid HA more acidic.HOOC-COOHoxalic acidHO2C-CH2-CO2Hmalonic acidHO2C-CH2CH2-CO2Hsuccinic acidHO2C-CH2CH2CH2-CO2Hglutaric acidHOOC-(CH2)4-COOHadipic acidHOOC-(CH2)5-COOHpimelic acidIX. asam dikarboksilat