Transcript Physical Properties - Winthrop University

Aldehydes, Ketones, Carboxylic Acids
and Amides
The carbonyl group >C=O is one of the most
biologically important chemical entities in Organic
Chemistry
Aldehydes, Ketones, Carboxylic Acids
and Amides
O
H
Aldehydes
C
R
O
Four families of
compounds contain
the carbonyl group:
R
C
Ketones
R
O
R
Carboxylic Acids
C
OH
O
R
Amides
C
HN
R
O
H
Aldehydes
C
R
•We replace the –e ending of compounds with –al for
aldehydes
•We know that aldehyde has to be at the end of the molecule.
•Why?
O
H
Methanol
or
Formaldehyde
C
H
O
H3C
Ethanol
or
Acetaldehyde
C
H
Aldehydes
are found in
many oils
and natural
extrcts
O
R
C
Ketones
R
We replace the –e ending with -one
O
H3C
C
Propanone
CH3
The simplest
Ketone.
Why?
Carboxylic Acid
O
O
R
R
C
C
+
H+
O-
OH
•Weak Acids
•They have an acidic proton because of the electron
withdrawing effect of the carbonyl oxygen and resonance
stabilization of the resultant carboxylate anion
O
Formaldehyde
H
H
O
Formic Acid
H
OH
Carboxylic Acids
Named by replacing the –e with –oic acid
OH
Benzoic Acid
O
O
H3C
H
Acetaldehyde
H3C
OH
Acetic Acid
From Alcohols to Acids
O
H3C
OH
C
H2
H
C
O
H3C
CH3
C
OH
Progressively Oxidizing (adding Oxygen) the alcohol
allows us to go from an alcohol to a carboxylic acid
Amides
An Amide is a compound containing an amine bound
to a carboxyl group
R
O
C
An Amide
N
R
H
A peptide bond is an example of an amide
Note:
When drawing organic structures, keep in
mind the hybridization of the carbons
• Alkanes have sp3 hybridized carbons
– Geometry?
• Carbonyl carbons have sp2 hybridized
carbons
– Geometry?
Esters
O
• The fatty acids in our
bodies are examples of
esters
• Esters are formed from
the condensation of a
carboxylic acid and an
alcohol
HO
O
H2C
OH
HC
OH
H2C
OH
HO
O
HO
Amines
H
CH3
CH3
CH3
N
N
N
N
H
H
Amine
H
H
H
CH3
H3C
CH3
Methylamine
Dimethylamine
Trimethylamine
1° Amine
2° Amine
3° Amine
•Amines are compounds derived from ammonia
•Amines tend to be associated with strong, often unpleasant
odors
Putrescine NH2(CH2)4NH2
Cadaverine NH2(CH2)5NH2
Amino Acids
• The building blocks
of proteins are
amino acids
• Amino Acids have
an amino group
and a carboxylic
acid on them
• When the
ribosome forms a
protein from amino
acids, it does so in
a condensation
reaction that forms
an amide
Organic Chemistry 2:
Important Reactions
In the biological world, organisms are capable
of synthesizing or degrading nearly any
molecule
For the remainder of this class, we are going to
look at the chemistry of basic biological
molecules and the reaction mechanisms
these molecules are involved in
Reaction Types
We are going to focus on 3 basic reaction types:
1. Nucleophilic Substitution Reactions: An electron
rich atom (nucleophile) attacks a electron
deficient atom
2. Acid-Base Catalysis: Certain amino acid side
chains of enzymes can accept or donate protons,
making them act like acids (donate protons) or
bases (accept protons)
3. Condensation Reactions: The involve the
combining of two molecules to form a larger
molecule and a smaller one
•
The reverse reaction is called a Hydrolytic Reaction.
We’ll look at those as well.
Nucleophilic Substitution Reactions
Terminology:
• Nucleophile: An electron rich atom. May be
negatively charged or have an available lone pair
• Electrophile: An electron poor atom. May or may
not have a positive charge
Nucleophilic Substitution Reactions
• Two types of Sn Reactions Exist
• They are classified and named based upon the slowest (rate
limiting) step: Sn1 and Sn2
• The General form of these reactions is:
R:X + :Z --> R:Z + X
:Z is the nucleophile
X is the leaving group
• In a condensation reaction for the formation of a lipid from a
glycerol and a fatty acid, a glycerol hydroxyl is the nucleophile
attacking the carbonyl carbon of the carboxylic acid
• Remember: The nucleophile attacks atoms of partial positive
charge
Types of Nucleophilic Substitution Reactions
• Sn1: The rate is dependent on the
leaving group leaving
– Stands for: Substitution Nucleophilic
Unimolecular
Note: The first step is the dissociation of the chlorine
Types of Nucleophilic Substitution Reactions
• Sn2: The rate is dependent on the nucleophile
and the substrate forming a bond at the SAME
TIME the leaving group dissociates
– Stands for: Substitution Nucleophilic
Bimolecular
•Steric hindrances may
prevent Sn2 reactions
•The concentration of
both reactants affects
the rate
Nucleophilic Substitution Reactions
• Actually, many reactions are mixtures of Sn1
and Sn2 mechanisms
• Many factors affect whether a reaction
proceeds via the Sn1 or Sn2 route, including:
–
–
–
–
Nucleophilicity
Bond polarizability
Leaving group stability
Solvent composition
• Think about these factors, you will see them
again in Organic Chemistry
General Acid-Base Catalysis
• In these reactions, groups accept or donate
protons, thereby acting as acids or bases
• In proteins, acid-base catalysis is mediated
by side chains containing:
– Imidazole, hydroxyl, carboxyl, sulfhydryl, amino
and phenol groups
• For an enzyme catalyzed reaction in which
the enzyme abstracts a proton from a
substrate, the protein is acting like a base
+
-
General Acid-Base Catalysis
• For an enzyme catalyzed reaction in which the
enzyme donates a proton to a substrate, the protein
is acting like an acid
R-H+ + R-O- --> R + R-OH
We would call this General Acid Catalysis
• For an enzyme catalyzed reaction in which the
enzyme abstracts a proton from a substrate, the
protein is acting like a base
R-H+ + R-OH --> R + R-OWe would call this General Base Catalysis
General Acid-Base Catalysis: An Example
• Keto-Enol Tautomerization
Uncatalyzed
General acid catalysis: Partial
proton transfer from an acid
lowers the free energy of the
high-free energy carbanionlike
transition state of the keto-enol
tautomerization
General base catalysis: The
rate can be increased by partial
proton abstraction by a base.
Concerted acid-base
catalyzed reactions involve
both processes occurring
simultaneously.
Adapted from Voet, Voet and Pratt. Fundamentals of Biochemistry, 3rd Ed. 2008.
General Acid-Base Catalysis: An Example
Enzymatic Degradation of 4-Nitrophenylacetate
proceeds via a General Acid-Base mechanism
•Imidazole nitrogen extracts proton from water
initiating the reaction
Condensation Reactions
• Two molecules
combine with the
generation of a
smaller molecule
Condensation Reactions
• Reaction of Acetic Acid and Ethanol
Looking at the Reaction Mechanism
1. The carbonyl carbon is:
•
•
Electron deficient
In a trigonal planar geometry
•
120º between substituents
2. The carbonyl oxygen is pulling electrons
towards it
•
Resonance stabilization
3. The Lone Pair of the alcohol oxygen can
react with the carbonyl carbon to set the
whole thing in motion
4. Remember your VSEPR Geometry
Condensation Reactions: Making Lipids from Sugars
and Fatty Acids
• Your cells can
synthesize
lipids from
glycerol and
fatty acids in a
condensation
reaction

Condensation Reactions: Polymerizing Carbohydrate
Monomers
Condensation Reactions: Forming a Peptide Bond
1.
2.
What are the
amino acids in
the figure?
What function
group is
formed?
Its not really this
simple, but it
illustrates a
point!
Hydrolysis: The Opposite of Condensation
•In a hydrolytic reaction, we add the elements of water (H+ and OH-) across a
bond
•Many enzymes use this kind of reaction to degrade polymers
•Lipases: Hydrolyze lipid esters
•Glycosidases: Hydrolyze carbohydrate polymers
•Peptidases: Hydrolyze peptide bonds
•Compound Name + ase : Usually indicates a hydrolase (but not always!)
•If it isn’t a compound name and ase, then it usually does something else:
•Lyase
•Reductase
•Kinase
•Transferase
Hydrolysis of Sugar Polymers
• We add water
across the
Glycosidic Bond of
Maltose to break it
and generate 2
monomers
• Catalyzed by a
glycosidase
(Maltase perhaps?)
Hydrolysis of Peptides
•
•
Dipeptide (What are
the amino acids) is
hydrolyzed to ???
Catalyzed by a
peptidase or a
protease