Organic Chemistry

1
Organic Chemistry
William H. Brown Christopher S. Foote
2

• Carboxylic Acids
• Chapter 17

3
Structure

• The functional group of a carboxylic acid is a
  carboxyl group
• the general formula of an aliphatic carboxylic
  acid is RCOOH that of an aromatic carboxylic
  acid is ArCOOH

4
Nomenclature - IUPAC

• IUPAC names drop the -e from the parent alkane
  and add the suffix -oic acid
• if the compound contains a carbon-carbon double
  bond, change the infix -an- to -en-

5
Nomenclature - IUPAC

• The carboxyl group takes precedence over most
  other functional groups

6
Nomenclature - IUPAC

• dicarboxylic acids add the suffix -dioic acid to
  the name of the parent alkane containing both
  carboxyl groups

7
Nomenclature - IUPAC

• if the carboxyl group is attached to a ring, name
  the ring compound and add the suffix -carboxylic
  acid

8
Nomenclature - IUPAC

• benzoic acid is the simplest aromatic carboxylic
  acid
• use numbers to show the location of substituents

9
Nomenclature-Common

• when common names are used, the letters
  ????????????etc. are often used to locate
  substituents

10
Physical Properties

• In the liquid and solid states, carboxylic acids
  are associated by hydrogen bonding into dimeric
  structures

11
Physical Properties

• Carboxylic acids have significantly higher
  boiling points than other types of organic
  compounds of comparable molecular weight
• they are polar compounds and form very strong
  intermolecular hydrogen bonds
• Carboxylic acids are more soluble in water than
  alcohols, ethers, aldehydes, and ketones of
  comparable molecular weight
• they form hydrogen bonds with water molecules
  through their CO and OH groups

12
Physical Properties

• water solubility decreases as the relative size
  of the hydrophobic portion of the molecule
  increases

13
Acidity

• Carboxylic acids are weak acids
• values of pKa for most aliphatic and aromatic
  carboxylic acids fall within the range 4 to 5
• The greater acidity of carboxylic acids relative
  to alcohols, both compounds containing an OH
  group is due to resonance stabilization of the
  carboxylate anion

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Acidity

• electron-withdrawing substituents near the
  carboxyl group increase acidity through their
  inductive effect

15
Reaction with Bases

• Carboxylic acids, whether soluble or insoluble in
  water, react with NaOH, KOH, and other strong
  bases to give water-soluble salts
• They also form water-soluble salts with ammonia
  and amines

16
Reaction with Bases

• Carboxylic acids react with sodium bicarbonate
  and sodium carbonate to form water-soluble salts
  and carbonic acid
• carbonic acid, in turn, breaks down to carbon
  dioxide and water

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Reaction with Bases
18
Preparation

• Carbonation of Grignard reagents
• treatment of a Grignard reagent with carbon
  dioxide followed by acidification gives a
  carboxylic acid

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Methanol to Acetic Acid

• Acetic acid is synthesized by carbonylation of
  methanol
• the carbonylation is exothermic
• the Monsanto process uses a soluble rhodium(III)
  salt and HI to catalyze the reaction

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Methanol to Acetic Acid

• Steps 1 and 2 preparation of the catalyst
• Steps 3 and 4 the catalytic cycle

21
Reduction

• The carboxyl group is very resistant to reduction
• it is not affected by catalytic hydrogenation
  under conditions that easily reduce aldehydes and
  ketones to alcohols, and reduce alkenes and
  alkynes to alkanes
• it is not reduced by NaBH4

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Reduction by LiAlH4

• Lithium aluminum hydride reduces a carboxyl group
  to a 1 alcohol
• reduction is carried out in diethyl ether, THF,
  or other nonreactive, aprotic solvent

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Selective Reduction

• carboxyl groups are not affected by catalytic
  reduction under conditions that reduce aldehydes
  and ketones

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Selective Reduction

• using the less reactive NaBH4, it is possible to
  reduce the carbonyl group of an aldehyde or
  ketone without affecting a carboxyl group

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Fischer Esterification

• Esters can be prepared by treatment of a
  carboxylic acid with an alcohol in the presence
  of an acid catalyst, commonly H2SO4 or gaseous HCl

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Fischer Esterification

• Fischer esterification is an equilibrium reaction
• by careful control of experimental conditions, it
  is possible to prepare esters in high yield
• if the alcohol is inexpensive relative to the
  carboxylic acid, it can be used in excess to
  drive the equilibrium to the right
• alternatively, water can be removed by azeotropic
  distillation and a Dean-Stark trap

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Fischer Esterification

• a key intermediate in Fischer esterification is
  the tetrahedral carbonyl addition intermediate
  formed by addition of ROH to the CO group

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Diazomethane

• Diazomethane, CH2N2
• a potentially explosive, toxic yellow gas, is
  best drawn as a hybrid of two contributing
  structures
• treatment of a carboxylic acid with diazomethane
  gives a methyl ester

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Diazomethane

• Esterification occurs in two steps
• Step 1 proton transfer to diazomethane
• Step 2 nucleophilic displacement of N2

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Acid Chlorides

• The functional group of an acid halide is a
  carbonyl group bonded to a halogen atom
• among the acid halides, acid chlorides are by far
  the most common and the most widely used

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Acid Chlorides

• acid chlorides are most often prepared by
  treatment of a carboxylic acid with thionyl
  chloride

32
Acid Chlorides

• The mechanism for this reaction is divided into
  two steps.
• Step 1 OH-, a poor leaving group, is transformed
  into a chlorosulfite group, a good leaving group

33
Acid Halides

• Step 2 attack of chloride ion gives a
  tetrahedral carbonyl addition intermediate, which
  collapses to give the acid chloride

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Decarboxylation

• Decarboxylation loss of CO2 from a carboxyl
  group
• most carboxylic acids, if heated to a very high
  temperature, undergo thermal decarboxylation
• most carboxylic acids, however, are quite
  resistant to moderate heat and melt or even boil
  without decarboxylation

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Decarboxylation

• Exceptions are carboxylic acids that have a
  carbonyl group beta to the carboxyl group
• this type of carboxylic acid undergoes
  decarboxylation on mild heating

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Decarboxylation

• thermal decarboxylation of a ?-ketoacid involves
  rearrangement of six electrons in a cyclic
  six-membered transition state

37
Decarboxylation

• decarboxylation occurs if there is any carbonyl
  group beta to the carboxyl
• malonic acid and substituted malonic acids, for
  example, also undergo thermal decarboxylation

38
Decarboxylation

• thermal decarboxylation of malonic acids also
  involves rearrangement of six electrons in a
  cyclic six-membered transition state

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Prob 17.17

• Each compound shows strong absorption between
  1720 and 1700 cm-1, and strong broad absorption
  over the region 2500-3500 cm-1. Propose a
  structural formula for each compound.

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Prob 17.17 (contd)
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Prob 17.17 (contd)
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Prob 17.18

• Complete these reactions.

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Prob 17.19

• Show how to bring about each conversion.

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Prob 17.21

• Draw a structural formula for each starting
  compound.

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Prob 17.22

• Show reagents to bring about each conversion.

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Prob 17.23

• Show how to synthesize butanedioic acid starting
  with acetylene and formaldehyde.

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Prob 17.24

• Propose a mechanism for the rearrangement of
  benzil to sodium benzilate.

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Prob 17.33

• Show how to convert trans-3-phenyl-2-propenoic
  (cinnamic acid) to each compound.

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Prob 17.34

• Show how to convert 3-oxobutanoic acid to these
  compounds.

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Prob 17.35

• Complete each example of Fischer esterification.

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Prob 17.37

• Name the carboxylic acid and alcohol from which
  each ester is derived.

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Prob 17.39

• Propose a mechanism for this reaction.

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Prob 17.40

• Draw a structural formula for the product of
  thermal decarboxylation of each compound.

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Prob 17.41

• Propose a mechanism for each decarboxylation.
  Compare your mechanisms with the mechanism for
  decarboxylation of a b-ketoacid.

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Prob 17.43

• Show how to convert cyclohexane to
  cyclohexanecarboxylic acid.

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• Carboxylic Acids

End Chapter 17