Download CHEM 202_ Part 2

Organic Chemistry II
CHEM 202
2nd Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
[email protected]
Chapter IV : Aldehydes & Ketones
In this chapter, we shall study the following topics:
• Nomenclature of aldehydes & ketones.
• Preparation of aldehydes & ketones.
• Main chemical reactions.
• Uses of aldehydes & ketones.
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Overview
* The general formula of aldehyde is RCHO while that of ketone is RCOR.
* Aldehydes and ketones are common in living systems such as ribose sugar
(aldehyde) and progesterone (ketone, female hormone).
*Many aldehydes and ketones have distinctive odors . Aldehydes are generally
pungent- smelling and ketones are sweet- smelling.
•The carbonyl group of aldehyde and ketones are characterized by:
•SP2 hybridization
• flatness
•Polar
•Two pairs of unshared valence electrons exist on oxygen atom.
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Nomenclature
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Preparation of aldehydes & Ketones
The most common way to prepare aldehyde or ketone in laboratory is by the oxidation
of alcohol. Aryl ketones can be prepared by Friedel - Crafts acylation.
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Reactions of aldehydes & Ketones:
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4.
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Addition reactions: (H2O, ROH, HCN, RMgX)
Addition – elimination reactions: ( RNH2, NH2NH2, phosphonium ylide)
Reduction
Oxidation
Halogenation
Addition reactions
Carbonyl group is polar, so the compound may be attacked by a nucleophile or electrophile
Carbonyl group is stabilized by adjacent alkyl groups (e-donor), so ketone is more stable
than aldehyde. Steric effect also play a role in the relative reactivities of aldehydes and
ketones.
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Reaction with water
Carbonyl compounds react with water to form 1,1 diol (gem-diol or hydrate)
Both formaldehyde and chloral are more reactive than most other aldehydes or ketones
because carbonyl carbon in each has a large amount of positive charge.
Reaction with alcohol
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Reaction with hydrogen cyanide
HCN can be obtained from reaction of KCN or NaCN with strong acid. HCN is toxic . It
forms cyanohydrin when reacts with carbonyl group.
The reaction requires alkaline condition such as buffer solution of NaCN -HCN
Cyanohydrins are usuful synthetic intermediate, CN group can be hydrolyized to carboxylic
or ester group. Hydroxyl group also can be replaced by amonia to form amino acid.
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Reaction with Grignard reagent
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Addition – Elimination reactions of carbonyl group
Reaction with ammonia and primary amine to form imine
Reaction with secondary amine to form enamine (vinyl amine)
CH3CHO + (CH3)2NH
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Reaction with nitrogen compounds to form hydrazone
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Reaction with phosphonium ylide (Witting reaction) to form alkenes
+ (C6H5)3 P= O
Triphenylphosphoine oxide
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Reduction of aldehydes & ketones
Aldehydes and ketones can be reduced to
alcohols (using H2 or metal hydrides), or to
hydrocarbons or amines (using ammino
compounds).
Hydrogenation reactions
If C=C exist in the carbonyl compounds, the reduction take place first at C=C and after that C=O can be reduced.
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Reduction with metal hydride
Aldehydes and ketones can be reduced to corresponding alcohols using strong
reducing agents such as LiAlH4 or mild reducing agents such as NaBH4
Metal hydride reduced only carbonyl group and can not reduce C=C or ester group if exist in
carbonyl compound
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Reduction to hydrocarbons
C=O group in aryl ketones can be reduced
to CH2 group using Wolff-Kishner
reduction (hydrazine reagent in alkaline
medium) or Clemmensen reduction (Zn
amalgam in acidic medium)
Reduction to amine compounds
The carbonyl group can be converted to amino group using ammonia or primary amine in
the presence of H2 and catalyst
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Oxidation of aldehydes & ketones
Aldehydes can be easily oxidized to corresponding carboxylic acids while ketones does not.
Oxidizing agents such as Pot. Permanganate, Pot. dichromate or Tollens reagent (Silver mirror).
Tautomerism and alpha hydrogen
The presence of hydrogen atom between two carbonyl groups make it acidic due to resonance
structures. Such kind of carbonyl group form tautomerism (special type of structural isomers), keto
form and enol form.
More stable by H-bond
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A ketone that form tautomerism can be oxidized by strong reducing agent such as
conc. HNO3
2 CH3CH2CO2H
CH3CH2CH2CO2H
+
CH3CO2H
Propanoic acid
butanoic acid
acetic acid
Alpha halogenation & haloform reaction
Ketones can be halogenated in acidic or basic medium at alpha carbon. The reaction is the basis of
haloform test (iodoform, chloroform, bromoform). The reaction can be used to get carboxylic acids.
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Use of aldehydes & ketones
Aldehydes and ketones can be used to synthesis of many organic compounds. In all these reactions,
carbonyl group can be retained (halogenation), or extended to more carbon skeleton (Grignard and
Witting reaction), or converted to another functional group (reduction)
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Homework (Ch.13)
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13.10
13.11
13.15
13.18
13.19
13.23
13.27
13.31
13.32
13.35
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Organic Chemistry II
CHEM 202
2nd Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
[email protected]
Chapter V : Carboxylic acids
In this chapter, we shall study the following topics:
• Nomenclature of carboxylic acid.
• Preparation of carboxylic acid.
• Acid strength.
• Reactions of carboxylic acids.
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Overview
Carboxylic group contains hydroxyl group and carbonyl group, its planar, polar and has
unshared electrons. Carboxylic compounds are important biologically and commercially.
Aspirin is a common carboxylic acid. They are weak acids (pKa around 5) compared with
mineral acids (pKa around -1), but they are more acidic than alcohols or phenols (because
of resonance structure).
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Nomenclature of carboxylic acids
IUPAC
Common names
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Preparation of carboxylic acids
(i) Hydrolysis of carboxylic acid derivatives
(ii) Oxidation of alcohols
(iii) Grignard reaction
Hydrolysis reactions
Oxidation reactions
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Grignard reaction
The reaction between Grignard reagent and CO2 in the presence of aqueous acid gives
carboxylic compounds
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Acid strength
It is the extent of ionization of acid in water. The greater the amount of ionization, the
stronger is the acid. The acidity of carboxylic compounds depends on its structure.
There are many factors affecting the stability of A- and consequently the strength of HA
such as: Electronegativity of atom in A- , Size of A- , hybridization of atom in A- ,
inductive effect of atoms attached to A- , resonance stabilization of A- , solvation of A-
Effect of EN
Effect of Size
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Effect of hybridization
Inductive effect
Effect of Substituted groups
Effect of resonance
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Reactions of carboxylic acids
(i) Neutralization
(ii) Esterification
(iii) Reduction
(i) Reaction with base (neutralization):
Acetate salt such as
(ii) Reaction with alcohols (Esterification)
Esterification is acid catalyzed and reversible . The rate depends on steric hindrance in the
alcohol and acid. Water is from by loss of H from alcohol and OH of acid.
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(iii) Reduction of carboxylic acid
Polyfunctional carboxylic acids
Dicarboxylic acids and carboxylic acid containing another functional group such as keto
group show unique chemical properties
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Homework (Ch.14)
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14.4
14.6
14.7
14.10
14.14
14.20
14.22
14.26
14.29
Write a report on :
“ Comparative study between acidity of
mineral acids and that of carboxylic acid
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