PPT 102 ORGANIC CHEMISTRY 1SEM 1 (2012/2013)

Carboxylic Acid

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Outline

• Nomenclature• Physical Properties• General Mechanism for Nucleophile

Addition-Elimination Reaction• Acid Catalyzed Esterification• Reaction of Carboxylic acids• Reaction of Amides

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Compound that containing carbonyl groups –called carbonyl compound.

An acyl group consists of a carbonyl group attached to an alkyl group ( R) or an aryl group (Ar)

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Carbonyl compound can be divide into 2 classes:

Class 1: carbonyl compounds are those in which the acyl group is attach to a group (or atom) that can be replace by another group.

Which compound belong to this class?

• Carboxylic acid• Acyl halides• Acid anhydride• Ester• Amides

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Class I Carbonyl Compounds

Carboxylic Acid Deritives

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Why it called as carboxylic acid derivatives ?

Because they differ from carboxylic acid only thenature of the group or atom that has replace theOH group of the carboxylic acid.

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Class II : Carbonyl compounds are those in which the acyl is attached to a group that cannot be readily replaced by another group.

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The functional group of a carboxylic acid is called acarboxyl group

Carboxylic Acid Nomenclature

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In IUPAC nomenclature, a carboxylic acid is named by replacing the terminal “e” of the alkane with “oic acid”

Carboxylic Acid Nomenclature

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In systematic nomenclature, the carbonyl carbon is always C-1

In common nomenclature, the carbon next to the carbonyl is the a-carbon

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Carboxylic Acid Nomenclature

Carboxylic acid in which a carboxyl group is attached to a ring are named by adding “carboxylic acid” to the name of the cyclic compound.

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Salts of Carboxylic Acids

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Acyl Halides

Acyl halides have a Cl or Br in place of OH.

Acyl halides are named by replacing “ic acid” with the “yl chloride”

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Acid Anhydrides

Loss of water from 2 molecules of a carboxylic results of acid anhydride.

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Symmetrical anhydride when R1 the same as R2

Mixed (unsymmetrical anhydride) when R1 the not the same as R2

There are 2 types of acid anhydride:

Ethanoic AnhydrideAcetic Anhydride

Ethanoic Methanoic AnhydrideAcetic Formic Anhydride

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Esters

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Amides

An amide has an NH2, NHR, or NHR2 group in place of OH group.

Amides are named by replacing “oic acid”, “ic acid” or “ylic acid” of the acid name with “amide”

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Nitriles

Nitriles are compound that contain C≡N functional group, called a cyno group.

In common nomenclature, nitriles are named by replacing “ ic acid” with the “onitrile”

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Physical Properties

• The carboxyl group contains three polar covalent bonds; C=O, C-O, and O-H– the polarity of these bonds determines the

major physical properties of carboxylic acids

δ-O – Hδ+ δ+C=Oδ-

R

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Physical Properties– The carbonyl group has a large dipole– The hydroxy group is capable of hydrogen

bonding.– The molecules can H-bond to each other

H3C C

O

O

H

CH3C

O

O

H- +

+ -

hydrogen bondingbetween two molecules

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H3CC

O H O

OHO

H

H

H

Solubility in Water• Carboxylic acids are similar to alcohols in respect to their solubility in water• Form hydrogen bonds to water

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Physical Properties

– Carboxylic acids are more soluble in water than are

alcohols, ethers, aldehydes, and ketones of

comparable molecular weight

– Sharp and or sour odor/taste

Vinegar, rancid butter, sweat, sauerkraut.

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Carboxylic acids have relatively high boiling points because…

Amides have the highest boiling points:

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How the carbonyl compound react?

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The reactivity of the carbonyl compound is due to the polarity of the carbonyl group that result from oxygen being more electronegative than carbon.So the carbonyl carbon is therefore electron deficient (electrophile)

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The tetrahedral intermediate is a transient species that eliminates the leaving group Y– or the nucleophile Z–:

This is a nucleophilic acyl substitution reaction

When nucleophile adds to the carbonyl carbon, the weakest bond in the molecule- the carbon-oxygen π bond- breaks =tetrahedral intermediate

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Z– will be expelled if it is a much weaker base than Y–; that is, Z– is a better leaving group than Y– (k–1

>> k2):

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Y– will be expelled if it is a weaker base than Z–; that is, Y– is a better leaving group than Z– (k2 >> k–1):

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The reactivity of a carboxylic acid derivative depends on the basicity of the substituent attached to the acyl group:

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General Mechanism for Nucleophile Addition-Elimination Reaction

• All carboxylic acids derivatives undergo nucleophilic addition-elimination reaction by the same mechanism.

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Mechanism: Negatively charge nucleophile

1. The nucleophile adds to the carbonyl carbon, forming a tetrahedral intermediates.2. The tetrahedral intermediates collapse, eliminating the weaker base

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If the nucleophile is neutral…

Where :B represent any spesies in the solution that is capable of removing a proton, and HB+ Represent any spesies in solution that is capable of donation a proton.

1. The nucleophile adds to the carbonyl carbon, forming a tetrahedral intermediate.

2. A proton is loss from the tetrahedral intermediate, resulting in a tetrahedral intermediate equivalent to the one formed by a negatively charge nucleophile.

3. The π bond re-form and the weaker of the two base is eliminated.

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Reaction of Carboxylic Acid

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Nucleophililic Acyl Substitutution Reactions

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1. Preparation of Acyl Chloride

2. Preparation of Acid Anhydride

3. Preparation of ester

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4. Preparation of Amide

5.Reduction Of Carboxylic Acid

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Reactions of Carboxylic Acids

• Carboxylic acid can undergo nucleophilic acyl substitution reactions only when they are in their acidic form.

• The basic form of a carboxylic acid cannot undergo nucleophilic acyl substitution reactions because the negatively charge carboxylate ion is resistant to nucleophilic reaction

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Reactions of Carboxylic Acids

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Carboxylic acid react with alcohol to form esters.

The reaction must carried out in an acidic solution, not only to catalyze the reaction but also to keep the carboxylic acid in its acidic form so that the nucleophilic will react with it.

Since the tetrahedral intermediate formed in this reaction has two potential leaving groups of aproximately the same basicity, the reaction must carried out with excess alcohol to drive it towards product.

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• Emil Fischer was the first to discover that an ester could be prepared by treating a carboxylic acids with excess alcohol in the presence of an acid catalyst.

• The reaction is called, FISCHER ESTERIFICATION.

• Its mechanism is the exact reverse of the mechanism for the acid catalyzed hydrolysis of an ester.

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Carboxylic acids do not undergo nucleophilic acylsubstitution reactions with amines at room temperature

Because a carboxylic acid is an acid and an amine is base, the carboxylic acid immediately donates a proton to the amine.

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Reactions of Amides

Amides are very unreactive carboxylative derivatives.

Amides do not react with halide ions, carboxylate ions, alcohols, or water because in each case, the incoming nucleophile is a weaker base than the leaving group of the amide ( Table 17.1)

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Amides can react with water and alcohols if an acid catalyst is present:

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Dehydration of an Amide

Dehydration reagents commonly used are SOCl2, P2O5,or POCl3

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Acid catalyzed Esterification

• Ester hydrolyzed slowly because water is a poor nucleophile and ester have very basic leaving groups.

• However the rate of hydrolysis can be increased by either acid or hydroxide ion.

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• When you examine the mechanisms for these reactions, notice the two following features that hold for all organic reactions:

• 1. All organic intermediates and products in acidic solution are positively charge or neutral

• 2. All organic intermediates and products in basic solutions are negatively charge.

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Hydrolysis of an ester with primary or secondary alkyl groups can be catalyzed by an acid

The carbonyl oxygen is first protonated,

MECHANISM FOR ACID CATALYZED ESTER HYDROLYSIS

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There are no negatively charged species in the reaction:

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Excess water will force the equilibrium to the rightExcess alcohol will force the equilibrium to the left

Because tetrahedral intermediate I and III are equally likely to collapse, both ester and carboxylic acid will present in approximately equal amounts when the reaction reach equilibrium

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Esters with tertiary alkyl groups undergo hydrolysis much more rapidly than do others:

1. An acid protonates the carbonyl oxygen2. The leaving group departs, forming the tertiary

carbocation3. A nucleophile react with the carbocation4. A base removes a proton fronm the strongly

acidic protonated alcohol

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Transesterification is also catalyzed by acid:

TRANSESTERIFICATION

The reaction of an ester with an alcohol.The mechanism is identical to the mechanism for acid catalyzed ester hydrolysis (for ester with primary or secondary alkyl groups), except that the nucleophile is ROH rather than H20

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• Biodiesel Production-Transesterification.

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Rudolf Diesel (1893)

“The use of vegetable oils for engine fuels may seem insignificant today,” he argued, “but such oils may become, in the course of time, as important as petroleum and the coal-tar products of the present time."

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Biodiesel

• Biodiesel is not the same thing as raw vegetable oil. It is produced by a chemical process which removes the glycerol from the oil.

• Biodiesel– Domestic – Renewable – For diesel engines – Derived from oils and fats

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Biodiesel production

• Biodiesel is typically produced by a reaction of a vegetable oil or animal fat with an alcohol such as methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters and glycerol, which is removed.

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How?

• Tranesterification: alcohol + ester → different alcohol + different ester – Base of acid as a catalyst – The oil is mixed with an alcohol, usually

methanol or ethanol, and separated into methyl esters (biodiesel) and glycerol.

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Transesterification

Methyl esters

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Biodiesel technology

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Possible sources

• Vegetable oil (soy, canola, palm, rapeseed,

coconut etc.)• Non food plants (jatropha)• Recycled oil (McDonald’s fryer)• Animal fats (fish oil)• Algae