Search for notes by fellow students, in your own course and all over the country.
Browse our notes for titles which look like what you need, you can preview any of the notes via a sample of the contents. After you're happy these are the notes you're after simply pop them into your shopping cart.
Title: Introduction to Carbonyl Chemistry
Description: Well comprehensive notes on Introduction to Carbonyl Chemistry
Description: Well comprehensive notes on Introduction to Carbonyl Chemistry
Document Preview
Extracts from the notes are below, to see the PDF you'll receive please use the links above
SCH 206
SCH 206
Course outline
Aldehydes and Ketones;
Carboxylic Acids;
Carboxylic Acids derivatives;
R1
Amines
N
R2
and Phenols
R3
Dr
...
Solomon Derese; Chemistry Department
Room 118; sderese@uonbi
...
ke
Recommended text books
1
...
Organic Chemistry, Francis Carry
3
...
W
...
Dr
...
Solomon Derese
3
O
SCH 206
Carbonyl Compounds
R
Z
Two broad classes of compounds contain the carbonyl
group:
I
...
O
R
Aldehyde
R'
Ketone
An aldehyde has at least one H atom bonded to the
carbonyl group
...
Dr
...
Compounds that contain an electronegative atom
bonded to the carbonyl group (Carboxylic acids and
their derivatives)
...
The presence or absence of a leaving group on the
carbonyl carbon determines the type of reactions
these compounds undergo
...
Solomon Derese
5
SCH 206
Nature of the Carbonyl Group
The double bond between carbon and oxygen is
similar to an alkene C=C double bond
...
R1
R2
R3
R4
Alkene
Dr
...
Bond Bond distance Bond energy Dipole moment
C=C
134 pm
611 KJ/mol
0
...
5 D
Dr
...
Oxygen is
more electronegative than carbon (3
...
5); therefore, a carbon-oxygen double bond is polar, with
oxygen bearing a partial negative charge and carbon
bearing a partial positive charge
...
Conversely, oxygen atom carries a
partial negative charge, is a
nucleophilic site, and reacts with
electrophiles
...
Solomon Derese
8
SCH 206
Electronegativity
Nucleophilic (electron rich)
Reacts with electrophiles, e
...
H+
Electrophilic (electron deficient)
Reacts with nucleophiles
Uncrowded SP2 carbon
The electrophilicity of a carbonyl group derives
from resonance effects as well as inductive effects
...
Solomon Derese
9
SCH 206
The reactivity of carbonyl compounds is due to the
polarity of the carbonyl group that results from oxygen
being more electronegative than carbon
...
So
we can safely predict that it will be attacked by
nucleophiles
...
:Nu
Planar (sp2)
Dr
...
When a carbonyl group is attacked by a
nucleophile, the carbon atom undergoes a change
in hybridization (sp2 to sp3) and geometry (planar
to tetrahedral)
...
It depends on whether Z is a leaving
group or not
...
Solomon Derese
11
SCH 206
I
...
Dr
...
As the new bond to the nucleophile forms,
the p bond is broken, moving an electron pair out on
the oxygen atom
...
Dr
...
The net result is that the p bond is broken, two
new d bonds are formed, and the elements of H
and Nu are added across the p bond
...
Solomon Derese
14
SCH 206
Stereochemistry of the Nucleophilic Addition Reaction
H-Nu:
Racemic
mixture
H-Nu:
The carbonyl carbon is planar and nucleophiles can attack
it from either side (bottom as well as top face) equally
...
Dr
...
Reactions of carbonyl compounds when Z is a
leaving group – Nucleophilic Acyl substitution
Z= –OH (carboxylic acid), –OR (ester),
–Cl (acid chloride), –NH2 (amide), or
–OCOR (acid anhydride)
Nucleophilic Acyl Substitution Reaction
Carbonyl compounds that contain leaving groups
(electronegative elements) undergo Nucleophilic
Acyl Substitution Reaction
...
Solomon Derese
16
Mechanism of Nucleophilic Acyl Substitution
Reaction
Step I: Nucleophilic attack
SCH 206
Tetrahedral intermediate
The nucleophile (:Nu–H) attacks the electrophilic
carbonyl, forming an sp3 hybridized intermediate
...
Dr
...
To do so, an
electron pair on oxygen re-forms the p bond, and Z
leaves with the electron pair in the C – Z bond
...
This reaction is called
Nucleophilic Acyl Substitution
...
Solomon Derese
18
SCH 206
A compound that has an sp3 carbon bonded to an
oxygen atom generally will be unstable if the sp3
carbon is bonded to another electronegative
element
...
For an aldehyde or ketone to undergo nucleophilic
acyl substitution reaction, the tetrahedral
intermediate would need to eject a hydride ion
(H:-) or an alkanide ion (R:-)
...
Dr
...
Solomon Derese
20
SCH 206
Comparison of Carbonyl Reaction Types
I
...
II
...
III
...
Dr
...
The electrophilicity of the carbonyl carbon
The relative reactivities of carbonyl
O
compounds with nucleophiles is mainly
attributed to the amount of positive charge
R1
Z on the carbonyl carbon
...
If the
partial positive charge is dispersed throughout the
molecule, then the carbonyl compound is more stable
and less reactive
...
Solomon Derese
22
SCH 206
In general electron withdrawing groups increase
the electrophilicity of the carbonyl carbon while
electron
donating
groups
decrease
the
electrophilicity of the carbonyl carbon
...
The accessibility of the carbonyl carbon
Sterically hindered carbonyl compound react
slower with nucleophiles as a result aldehydes (one
alkyl group) are more reactive than ketones (two
alkyl groups)
...
Solomon Derese
23
SCH 206
In general, aldehydes are more reactive than
ketones toward nucleophilic attack
...
Aldehydes
Ketones
O
R1
Less steric hindrance
with only one R group
Less crowded
Only one R stabilizes
the positive charge
...
Solomon Derese
Two R’s increase
steric hindrance
More crowded
R2
Two R’s stabilizes the
positive charge
...
Electronic effect
Recall that alkyl groups are electron donating
...
The d+ charge of an aldehyde is less stabilized than a
ketone
...
Dr
...
Most reactive
Least reactive
Aromatic carbonyl compounds are less reactive
than aliphatic carbonyl compounds, for its
delocalized p orbitals can also act as electron
source
...
Solomon Derese
26
SCH 206
The bulkier the alkyl group the less reactive
...
Dr
...
a)
b)
Dr
...
They have a generalized
structure as follows:
This resonance contribution leads to:
a) Partial double bond character on the C-Y
bond, and
b) Reduced partial positive charge at the
carbonyl carbon
...
Solomon Derese
29
SCH 206
The particular extent of the effect depends on the type
of derivative
...
Electron release from the substituent Z not only
stabilizes the carbonyl group, it decreases the positive
character of the carbonyl carbon and makes the
carbonyl group less electrophilic
...
The greater the electron-donating
powers of Z, the slower the rate
...
Solomon Derese
30
SCH 206
a) Acid halide
The halogens have considerable electronegativity
...
The carbon porbital is 2p
...
All in all the resonance
contribution (overlap) from halogens is weakest
...
Very small contribution
Dr
...
Dr
...
Dr
...
The second resonance structure has such a
significant contribution that the C-N bond acquires
a considerable double bond character; reflected in
the restricted rotation of this bond
...
Solomon Derese
34
SCH 206
The amount of positive charge on the carbonyl
carbon is directly related to its susceptibility to
nucleophilic attack
...
>
>
>
>
≈
>
Most
reactive
Least
reactive
Least
stable
Most
stable
Dr
...
Otherwise catalysts must be used that will
increase either the electrophilicity of the
carbonyl carbon or the nucleophilicity of the
nucleophile
...
Dr
...
A contributing resonance structure puts the
positive charge on the carbonyl carbon
...
Dr
...
g
...
Step II: Nucleophilic attack
Dr
...
Step I: Nucleophilic attack
O
R1
R2
Aldehyde/Ketone
Dr
...
Dr
...
They are unusually acidic for
hydrogen atoms attached to
carbon
...
Solomon Derese
SCH 206
pKa = 44
H
H
H
H
H
H
pKa = 51
41
SCH 206
Why are a-hydrogens of carbonyl compounds
acidic?
An a-hydrogen is more acidic because the negative
charge formed on the a-carbon when a base
abstracts the proton is not localized on this carbon
but through resonance is also on the oxygen
...
Solomon Derese
42
SCH 206
The greater acidity of a-hydrogens arises because
the negative charge on the resulting enolate anion
is delocalized by resonance, thus stabilizing it
relative to an alkane, alkene, or alkyne anion
...
Solomon Derese
43
SCH 206
When an enolate anion reacts with a proton
donor, it may do so either on oxygen or on the acarbon
...
Protonation on oxygen gives an enol (alkene
alcohol) form
...
Dr
...
The keto and enol forms of carbonyl compounds
are constitutional isomers, but of a special type
...
Interconvertible keto and enol forms are called
tautomers, and their interconversion is called
tautomerization
...
Solomon Derese
45
SCH 206
Tautomers are constitutional isomers that differ in
the location of a double bond and a hydrogen
atom
...
A keto tautomer has a C=O and an additional C – H
bond
...
Dr
...
Tautomerization, the process of converting one
tautomer into another, is catalyzed by both acid
and base
...
Dr
...
It is
difficult to prevent tautomerization even if care is
taken to remove all acids and bases from the
solution
...
However, the equilibrium favors the keto form
largely because a C=O is much stronger than a C=C
...
Dr
...
0001%
The percentage of enol tautomer is even less for
carboxylic acids, esters, and amides
...
Dr
...
For a long time attempts to prepare compound A were
thwarted by its ready isomerization to compound B
...
Write a reasonable mechanism for this isomerization
...
Solomon Derese
OH
H2O
H
OH
O
A
B
50
SCH 206
II
...
Suggest reasonable explanations for each of the following
observations:
a) Optically active piperitone ([a]D=32) is converted to
racemic piperitone on standing in a solution of sodium
ethoxide in ethanol
...
Dr
Title: Introduction to Carbonyl Chemistry
Description: Well comprehensive notes on Introduction to Carbonyl Chemistry
Description: Well comprehensive notes on Introduction to Carbonyl Chemistry