KIMOR 2 : asam karboksilat

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Sovia Aprina Basuki KIMIA ORGANIK II FARMASI UMM 2013

Transcript of KIMOR 2 : asam karboksilat

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Sovia Aprina Basuki

KIMIA ORGANIK II

FARMASI UMM

2013

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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

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Organic Chemistry, 7th edition, John

McMurry

Organic Chemistry, T. W. Graham

Solomons

Organic Chemistry, Fessenden and

Fessenden

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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).

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• 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

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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.

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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

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5 4 3 2 1

C — C — C — C — C = O

δ γ β α used in common names

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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

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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)

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COOH

COOH COOH COOH

CH3

CH3

CH3

benzoic acid

o-toluic acid m-toluic acid p-toluic acid

special names

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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

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Carboxylic 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)

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Fluoroacetic, chloroacetic, bromoacetic, and iodoacetic acids are stronger acids than acetic acid

Multiple electronegative substituents have synergistic effects on acidity

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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

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The Inductive Effect in Aliphatic Carboxylic Acids

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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.

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[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.

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Figure 19.8How common substituents

affect the reactivity of a

benzene ring towards

electrophiles and the acidity of

substituted benzoic acids

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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.

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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.

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3. Kelarutan

Carboxylic acids are proton donors toward weak and strong bases, producing metal carboxylate salts, RCO2

+ M

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+

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[1] Oxidation of 1° alcohols

[2] Oxidation of alkyl benzenes

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[3] Oxidative cleavage of alkynes

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1. Reaksi dengan Basa (penyabunan)

R-COOH + NaOH → R-COONa + H2O

2. Reaksi esterifikasi

R-COOH + R’-OH → R-COOR’ + H2OH2SO4

sabun

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

Amida5. Reaksi dengan Cl2

CH3-CH2-COOH + Cl2 → R-CHCl-COOH + HClAsam 2-monokloropropanoat

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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.

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• 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.

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• 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.

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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.

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• An 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.

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• 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).

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• 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.

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• 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 relationship

between acidity and conjugate

base stability for acetic acid,

phenol, and ethanol

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• 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

starting acid H—A more acidic.

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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

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CO2H

CO2H

CO2H

CO2H

CO2H

CO2H

phthalic acid isophthalic acidterephthalic acid

CCOOHH

CCOOHH

CCOOHH

CHHOOC

maleic acid fumaric acid