Carbonyl Compounds

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20.9-20.11, 20.13-20.14
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Carbonyl Compounds
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Fischer Esterification

• Acid alcohol yields ester water.
• Acid catalyzed for weak nucleophile.
• All steps are reversible.
• Reaction reaches equilibrium.

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Fischer Esterification
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Mechanism of Fischer Esterification
Protonate, and go to town!
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Mechanism of Fischer Esterification Going to
town. . .
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Carbonyl Compounds
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Acid Chlorides

• Chloride is a good leaving group, so undergoes
  acyl substitution easily.
• To synthesize acid chlorides use thionyl chloride
  or oxalyl chloride with the acid.

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Esters from Acid Chlorides

• Acid chlorides react with alcohols to give esters
  in good yield.
• Mechanism is attack by alcohol, electrons go up,
  back down, and chloride leaves. Clean up by
  deprotonation.

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Carbonyl Compounds
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Amides from Acid Chlorides

• Acid chlorides react with ammonia and amines to
  give amides.
• A base (NaOH or pyridine) is added to remove HCl
  by-product.

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Mechanism

• Animation

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Amides from Acids

• Amine (base) removes a proton from the carboxylic
  acid to form a salt.
• Heating the salt above 100?C drives off steam
  (H2O) and forms the amide.

Can think of it as loss of H2O to form the amide.
Kind of like how peptides are made from amino
acids!
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Reduction to 1? Alcohols

• Use strong reducing agent, LiAlH4.
• Borane, BH3 in THF, reduces carboxylic acid to
  alcohol, but does not reduce ketone.

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Reduction to Aldehyde

• Difficult to stop reduction at aldehyde.
• Use a more reactive form of the acid (an acid
  chloride) and a weaker reducing agent, lithium
  aluminum tri(t-butoxy)hydride.

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Chapter 24Amino Acids, Peptides, and Proteins
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Structure of Proteins
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Proteins

• Biopolymers of ?-amino acids.
• Amino acids are joined by peptide bond.
• Variety of functions
• structure
• enzymes
• transport
• protection
• hormones

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Zwitterion

• Amino acid exists as a dipolar ion.
• -COOH loses H, -NH2 gains H.

• Actual structure depends on pH.
  
  

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Structure and pH
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Strecker Synthesis

• First known synthesis of amino acid, 1850.
• Aldehyde reaction with NH3 yields imine.
• Cyanide ion attacks the protonated imine.
• Resulting ?-amino nitrile is hydrolyzed to a
  carboxylic acid.

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Strecker Mechanism
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Structure of Proteins
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Structure of Peptide

• The peptide bond is an amide bond.
• Amides are very stable and neutral.

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Peptide Bond Formation

• The amino group of one molecule condenses with
  the acid group of another.
• Polypeptides usually have molecular weight less
  than 5000.
• Protein molecular weight 6000-40,000,000.

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Bradykinin

• An oligopeptide, four to ten amino acids.
• Peptide structures are drawn with the N-terminal
  end at the left.
• Peptides are named from left to right
  arginylprolylprolylarginine.

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Disulfide Linkages

• Cysteine can form disulfide bridges.

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Disulfide Cleavage