Basic Organic Chemistry III

1
Organic Chemistry Review

• Part II

2
Organic Reactions

1. Addition
2. Elimination
3. Substitution
4. Rearrangement
5. Condensation
6. Esterification
7. Hydrolysis
8. Oxidation Reductions
9. Combustion

3
Addition Reactions

• The components of an organic molecule AB are
  added to the carbon atoms in a CC bonds.
• Involve the conversion of a p bond into 2 s
  bonds.
• General form A B ? C

4
Addition Reactions

• Symmetrical alkenes produce one product.
• Unsymmetrical alkenes produce racemic mixtures.

5
Addition Reactions

• Alcohols are often produced by addition
  reactions.
• Initial attack by the p bond of an alkene on a
  Hd of H3O produces a carbocation.
• The carbocation then undergoes nucleophilic
  attack by a lone pair of electrons from H2O.
• This is followed by elimination of H to form the
  alcohol.

6
Addition Reactions
7
Addition Reactions

• With symmetrical alkenes, addition of hydroxyl
  group produces one type of alcohol.

8
Addition Reactions

• With unsymmetrical alkenes, addition of hydroxyl
  group produces different types of alcohols
  depending on the location of the double bond

9
Addition Reactions

• Formation of hemiketals hemiacetals
• Reactions between an acohol and either a ketone
  or aldehyde.

10
Elimination Reactions

• The removal or elimination of adjacent atoms
  from a molecule.
• Two s bonds are lost, replaced by a new p bond.
• The dehydration reaction of alcohols to generate
  alkene proceeds by heating the alcohols in the
  presence of a strong acid, such as sulfuric or
  phosphoric acid, at high temperatures. 

11
Elimination Reactions

• The required range of reaction temperature
  decreases with increasing substitution of the
  hydroxyl carbon
• 1 alcohols 170 - 180C
• 2 alcohols 100 140 C
• 3 alcohols 25 80C

12
Elimination Reactions

• If the reaction is not sufficiently heated, the
  alcohols do not produce alkenes, but they react
  with one another to form ethers (Williamson Ether
  Synthesis).

13
Elimination Reactions

• General form A ? B C

14
Elimination Reactions

• 1 Alcohols

15
Elimination Reactions

• 2 Alcohols

16
Elimination Reactions

• In dehydration reactions of alcohols, hydride or
  alkyl shifts relocate the carbocation to a more
  stable position.
• The dehydrated products are a mixture of alkenes,
  with and without carbocation rearrangement.

17
Elimination Reactions

• Hydride or alkyl shifts are the result of
  hyperconjugation. The interaction between the
  filled orbitals of neighboring carbons and the
  singly occupied p orbital in the carbocation
  stabilizes the positive charge in carbocation.
• The tertiary cation is more stable than a
  secondary cation, which is more stable than a
  primary cation.

18
Elimination Reactions

• Hydride shift

19
Elimination Reactions

• Alkyl shift

20
Substitution Reactions

• Nucleophilic substitution reactions.
• An electronegative atom is replaced by another
  more electronegative atom, called a stronger
  nucleophile.
• The stronger nucleophile must possess a pair of
  electrons and have a greater affinity for the
  electropositive carbon atom than the original
  electronegative atom.
• A s bond is replaced by another s bond .

21
Substitution Reactions

• General form A B ? C D
• Non-polar reactions

22
Substitution Reactions

• Polar reactions

23
Rearrangement Reactions

• Are isomerisation reactions.
• An organic molecule changes structure.
• Constitutional change in carbon skeleton.
• Reaction may involve changes in bond type.
• General form A ? B

24
Rearrangement Reactions
25
Condensation Reactions

• Two molecules combine to form one single molecule
  with the loss of a small molecule.
• When this small molecule is water, it is known as
  a dehydration reaction.
• Other possible small molecules lost include
  hydrogen chloride, methanol, or acetic acid.

26
Condensation Reactions

• When two separate molecules react, their
  condensation is termed intermolecular.
• The condensation of two amino acids to form a
  peptide bond (red) with expulsion of water (blue).

27
Condensation Reactions

• When a condensation is performed between
  different parts of the same molecule, the
  reaction is termed intramolecular condensation.
• In some cases this leads to ring formation.

28
Condensation Reactions
29
Esterification Reactions

• Esters are obtained by refluxing a carboxylic
  acid with an alcohol in the presence of an acid
  catalyst.
• The reaction is driven to completion by using an
  excess of either the alcohol or the carboxylic
  acid, or by removing the water as it forms.
• Alcohol reactivity order   CH3OH gt 1o gt 2o gt 3o
  (steric effects).

30
Esterification Reactions

• A carboxylic acid and an alcohol react together
  under acidic conditions to form an ester and lose
  water.

31
Esterification Reactions

• Esters can also be made from other carboxylic
  acid derivatives, especially acyl halides and
  anhydrides, by reacting them with the appropriate
  alcohol in the presence of a weak base.
• If a compound contains both hydroxy- and
  carboxylic acid groups, then cyclic esters or
  lactones can form via an intramolecular reaction.
  Reactions that form 5- or 6-membered rings are
  particularly favorable.

32
Esterification Reactions
Pericyclic esters
33
Hydrolysis

• A reaction in which water is a reactant, and
  becomes part of the reaction product.
• A number of organic compounds undergo hydrolysis
  with water, such as amides, esters,
  halogenoalkanes and acyl halides.

34
Hydrolysis

• Reactions require a catalyst.
• The catalyst is either an acid (H ions) or
  alkali (OH- ions).
• Hydrolysis might involve refluxing in the
  presence of dilute hydrochloric acid or sodium
  hydroxide solution.

35
Hydrolysis

• In the overall reaction, a bond in an organic
  molecule is broken.
• A water molecule also breaks into ions.
• The -OH group from water is added to one end of
  the organic molecule and the remaining H atom is
  added to the other.

36
Hydrolysis of an Ester

• The addition of a strong acid, such as dilute
  hydrochloric acid, is required to free the
  carboxylic acid molecule.
• In the base-catalyzed, the carboxylic acid
  molecule loses a proton to a hydroxide ion.

37
Hydrolysis of Amides Nitriles

• Amide acid catalyzed - HCl
• Nitrile acid catalyzed HCl or H2SO4

38
Hydrolysis of Halogenalkanes
39
Hydrolysis of Aromatics
40
Summary of Hydrolysis Reactions

• The hydrolysis of a primary amide
• RCONH2 H2O    ?    RCOOH NH3
• The hydrolysis of a secondary amide
• RCONHR' H2O    ?    RCOOH R'NH2

41
Summary of Hydrolysis Reactions

• The hydrolysis of an ester
• RCOOR' H2O    ?    RCOOH R'OH
• The hydrolysis of a halogenoalkane
• RBr H2O    ?    ROH H Br-

42
Reduction Oxidation (REDOX) Reactions

1. Oxidation States
2. Oxidations
3. Reductions

43
Definitions

• Oxidation-Reduction reactions
• Involve changes in oxidation state at one or more
  atoms.
• Can often be identified by changes in the number
  of oxygen atoms at a particular position in the
  hydrocarbon skeleton or in the number of bonds
  between carbon and oxygen at that position.
• It is not consider an oxidation or reduction
  reaction
• Addition or loss of H, H2O, HX.

44
Definitions

• Oxidation
• The oxidation state increases
• Loss of H2
• Loss of a C-H bond
• Addition of O or O2
• Formation of a C-O bond or equivalent (C-Cl, C?N,
  C-S)
• Addition of X2 (halogens)

45
Definitions

• Reduction
• The oxidation state decreases
• Addition of H2 or H-
• Formation of a C-H bond
• Loss of O or O2
• Loss of a C-O bond or equivalent
• Loss of X2.
• An increase in the number of hydrogen atoms in a
  hydrocarbon is often an indication of a reduction.

46
Oxidation States

• Carbon oxidation states are assigned on the basis
  of the electronegativity of attached atoms.
• For each bond to a more electronegative atom give
  1.
• For each bond to a less electronegative atom
  (even H) give 1.
• For each bond to carbon give 0.

47
Oxidation States
48
Oxidation States

• In nitrogen-containing compounds, the number of
  carbonnitrogen bonds changes with the oxidation
  state of carbon.

49
Oxidation States
50
Assign oxidation states to all atoms in the
following structure
H C
O
H
H
C HO
C
H
C
H
H
H
51
Assign oxidation states to all atoms in the
following structure
H1 C-2
-2 O
1 H
-2 C 1 HO 3
H1 H1
-3 C
-2
C
H
1H
H
1
1
52
Problem

1. Identify if the following reactions are
   oxidation-reduction reactions.
2. For any that are, identify the atoms that are
   oxidized and reduced.

Br I
NaI NaBr
H2
O
OH
MnO2
H2O
KMnO4
K-O
53
Problem
No, both Br and I are more electronegative than
C
-2
H2 Yes, the carbon atoms are reduced, the H2
molecule is oxidized
54
Problem
55
Summary of Oxidation States
56
REDOX Reactions of Alcohols

• Alcohols can undergo either oxidation or
  reduction type reactions.
• Oxidation is a loss of electrons.
• Reduction is a gain of electrons.

57
Oxidation of Alcohols

• 1 and 2 alcohols are easily oxidized by a
  variety of reagents.
• The most common reagents used
• Pyridinium chlorochromate (PCC)
• Potassium permanganate
• Thermal dehydrogenation

58
Oxidation of Alcohols

• The most common reagent used for oxidation of 2
  alcohols to ketones is chromic acid, H2CrO4.
• 3 alcohols are resistant to oxidation because
  they have no hydrogen atoms attached to the
  oxygen bearing carbon (carbinol carbon).

59
Oxidation of 1 Alcohols

• 1 alcohols are easily oxidized just like 2
  alcohols.
• The product of oxidation is an aldehyde.
• The aldehyde is easily oxidized to an acid as a
  result of over-oxidation.
• A reagent that selectively oxidizes a 1 alcohol
  to an aldehyde is pyridinium chlorochromate, PCC.

60
Oxidation of 2 Alcohols

• The alcohol and chromic acid produce a chromate
  ester, which then reductively eliminates the Cr
  species.
• The Cr is reduced (VI ? IV), the alcohol is
  oxidized to a ketone.

61
Summary of Oxidation of Alcohols
62
Reduction of Alcohols

• Normally an alcohol cannot be directly reduced to
  an alkane in one step.
• The OH group is a poor leaving group and hydride
  displacement cannot happen.
• Instead, the hydroxyl group is easily converted
  into other groups that are better leaving groups,
  and allow reaction to move forward.

63
Reduction of Alcohols

• Commons reagents are tosyl chloride and lithium
  aluminum hydride (LiAlH4).
• The reaction involves the formation of a
  tosylate.
• The tosylates can undergo either substitution or
  elimination reactions.

64
Reduction of Alcohols

• The tosylate reduces to cyclohexane very easily
  with lithium aluminum hydride.

65
Reduction of Carboxylic Acids

• Carboxylic acids are reduced to 1 alcohols.

66
Reduction of Esters

• Esters are reduced to 1 alcohols.

67
Reduction of Amides

• Amides are reduced to 1, 2, or 3 amines.

68
Reduction of Aldehydes

• Aldehydes and ketones are reduced to 1 and 2
  alcohols respectively.

69
Summary REDOX Reactions
70
Combustion Reactions

• The reaction of an organic molecule with oxygen
  to form carbon dioxide, heat/energy and water.

71
Combustion Reactions

• Alkanes
• Alkenes
• Alcohols