Aldehydes & Ketones Aldehydes & Ketones:
Aldehydes
and
ketones
contains
the
same
functional
group,
the
carbonyl
group
(>
C
=
O).
Aldehydes behave as reducing agents due to presence of reducing hydrogen atom where as ketones have no such property so aldehydes easily reacts with oxidising agent. Even HCOOH shows some properties of carbonyl compounds due to presence of – CHO group. HCOOH behaves as reducing agent while in other acids no reducing hydrogen atom, so no reaction with oxidising agent.
Preparation
(i.)
Methodology:
Oxidation
of
Alcohols:
(a.) From PCC or pyredeniumchlorochromate. Which is pyridine, CrO 3 and HCl, in equal ratio
By Jones reagent (CrO 3 and aq. CH3COCH3) reaction goes till acids as it is a strong oxidising agent where as by PCC and Collins reagent (ii.)
reaction
stops Rosenmund’s
at
aldehyde. Reduction:
Reduction of acid halide into aldehydes by Pd and BaSO 4 is known as rosenmund reduction, only aldehydes can be prepared by this reaction.
HCOCl break down into CO + HCl so the first aldehyde from rosenmund reduction is CH 3CHO. Here Pd act as catalyst and BaSO 4 as
catalytic
poison
to
prevent
(iii.)
conversion
of
Stephan’s
aldehyde
into
alcohol
by
further
reduction.
Reduction:
When Cyanides are partially reduced by means of stannous chloride and hydrochloric acid followed by hydrolysis yield aldehyde. Ketones can not be prepared by this process.
Here [H2SnCl 4] is formed which works as powerful reducing agent.
(iv) Oxidation of Alkyl halide by dimethyl sulphoxide:
HCOCl break down into CO + HCl so the first aldehyde from rosenmund reduction is CH 3CHO. Here Pd act as catalyst and BaSO 4 as
catalytic
poison
to
prevent
(iii.)
conversion
of
Stephan’s
aldehyde
into
alcohol
by
further
reduction.
Reduction:
When Cyanides are partially reduced by means of stannous chloride and hydrochloric acid followed by hydrolysis yield aldehyde. Ketones can not be prepared by this process.
Here [H2SnCl 4] is formed which works as powerful reducing agent.
(iv) Oxidation of Alkyl halide by dimethyl sulphoxide:
(v) From Grignard Reagent
(vi) Dry distillation of calcium salts of acids
(vii) Oxidation of vicinal glycols by periodic acid
This reaction can also be done by lead tetrs tetr s acetate. We have already studied stud ied the mechanism of this reaction in last chapter.
o
(viii) Action of MnO on acids at 300 C temprature
(ix) By Hydrolysis of Acetoacetic ester
o
(x) Catalytic Dehydrogenetion by Cu at 300 C
(xi) Oppenauer oxidation
This is specific method for oxidation of secondary alcohol to ketone. Secondary alcohol is heated with aluminium iso-propoxide in a ketone (usually acetone)- secondary alcohol is oxidised to ketone whereas ketone is reduced to secondary alcohol.
Actually this is a reversible reaction and equilibrium can be shifted in either direct ion by appropriate manipulation.
For example, by taking an excess of secondary alcohol, ketone can be reduced; this reduction is called Meerwein Ponndorf - Verley reduction which has been discussed later.
The uniqueness of Oppenauer oxidation lies in the fact that it selectively oxidises hydroxy group which means that if t he compound besides hydroxy group has some other oxidisable functionality, the latter will remain unaffected under these conditions.
For example :
(xii) Hydrolysis of gemdihalide
Properties
(i) Nucleophilic addition reaction
Six electron system of carbocation is formed if primary attack of electrophile takes place, otherwise it is a eight electron system which is more stable when primary attack of nucleophile takes place.
So Primary attack here is of the nucleophile due to more stable intermediate oxygen anion of eight electron system. The reactivity of different carbonyl compounds towards formation of nucleophilic addition decreases in the following order :
This order arises due to the following two factors :
We have seen above that carbon of the carbonyl group is susceptible to (i) El ectronic factor : the attack of nucleophile due to the presence o f positive charge on it (i.e. on carbon). As the intensity of positive charge on carbon decreases, t he nucleophilic attack would occur less readily. On the other hand, if intensity of positive charge increases nucleophilic at tack would occur more readily. Alkyl group exerts + I effect , due to which it reduces the intensity of positive charge on carbonyl carbon, which decreases in the following order :
Intensity of positive charge on carbonyl carbon decreases. Hence t he reactivity of these compounds towards nucleophilic addition also decreases in this order.
The change in C - C - O angle as a result of nucleophilic addition may be (ii ) Steri c factor : noted.
It is obvious that nucleophilic addition causes decrease in C – C – O bond angle and hence groups are pushed closer. The bigger groups oppose coming closer, hence t he reactivity of carbonyl group decreases. Size of methyl group is much larger t han hydrogen, hence acetaldehyde is less react ive than formaldehyde. Similarly, acetone will be less reactive than acetaldehyde.
(a) Reaction with HCN
In the presence of alkali the dissociation of HCN increases because HO- of alkali trap H + of HCN so that ionisation of HCN increases.
But in presence of acid the dissociation is suppresed due to common ion effect. Thus the reaction shifts to backward direction.
(b) Reaction with NaHSO 3 (sodiumbisulphite)
This reaction is most versatile test for separating carbonyl compounds from noncarbonyl compounds, it first form white crystalline solid which when passed with dry HCl gas re -form carbonyl compound.
Here the carbonyl compounds are reobtained by reaction with dry HCl, so these react ion are used for separation test.
But always remember that for separation of aldehydes & ketones is done by Tollen’s reagent + (ammonical AgNO3 ) i.e. [Ag(NH 3)2 ], where aldehydes reacts to form white metallic silver.
(c) Reaction with sodio - derivatives of 1 - alkynes
(d) Reaction with C 2 H 5SH (Ethanethiol)
Aldol Condensation (ii)
Aldol
Aldehydes or ketones with by
active
Condensation
when treated with dilute alkali like NaOH,K 2CO3 etc. undergo nucleophilic addition
intermediate
carbanion
to
form
known
as
aldols.
Various basic reagents such as potassium hydroxide, aqueous alkali carbonate, alkali metal alkoxides, etc., may be used. The reaction
is
not
favourable
for
all
type
of
ketones
but
applicable
in
some
special
cases
like
CH 3COCH3.
Aldol condensation are of different type and it can occur between
(i) two identical or different aldehydes, (ii) two identical or different ketones and (iii) an aldehyde and a ketone.
When the condensation is between two different carbonyl compounds, it is called crossed aldol
condensation.
(a) Simple Aldol
Above reaction is also called as Claisen Schimdt Condensation where one is aromatic carbonyl compound and other is aliphatic carbonyl
compound.
In cross aldol condensation possibility of more than one product is always there so these reactions are not very much useful for synthatic chemistry. For example :
Acetone can also undergo aldol type reaction in the presence of dilute alkali (base catalyst) or dry HCl gas.
Intraaldol condensation: Dicabonyl compounds, having two carbonyl groups within the same molecule, undergo intramolecular aldol condensation reactions. Even bases as weak as sodium carbonate are adequate in these reactions.
Intramolecular aldol condensations proceed best when five- or six- membered rings result because these are of minimum strain
ring.
(iii)
Ald ehyd es which do not ha ve any
Cannizaro’s
Reaction
, when treated with con centrated solution of NaOH or KOH, undergo
simultaneous oxidation and reduction (disproportionation) forming a salt of carboxylic acid and alcohol.
Here 'R' should be – H or phenyl but not alkyl group with presence of
.
The reaction follows third-order law (second order in aldehyde and first order in base),/ i.e., rate
. This suggests the reaction between the first-formed anion (from base and aldehyde) and another molecule of aldehyde in the ratedetermining step.
In the presence of a high concentration of base, the reaction follows fourth-order law (second order in both two molecules of base), i.e., rate
The hydride ion is directly transferred from one molecule of the aldehyde to the other, and does not become free in solution has been proved by the observation that the recovered alcohol does not contain deuterium when the reaction is performed in the presence of D
2
O.
Cannizaro Reaction of HCHO Transfer of hydride ion due to back donation of oxygen charged electron give out hydride ion which is used for reduction of other carbonyl molecule so oxidation of one molecule gives acid salt while other by reduction forms alcohol.
(a) Simple cannizaro's reaction
Mechanism of Cannizaro's of benzaldehydeMechanism of Cannizaro's of benzaldehyde M echanism
of
Cannizaro' s
of
benzaldehyde
(b)
Cross
Reaction
of with
Cannizaro's
carbonyl
alkali
compounds
forms
each
Reaction
(does
molecules
of
not acid
having and
alcolol.
Whenever HCHO is present, there are only two main products otherwise there will be four products in all other cases. Because HCHO
One
contains
maximum
of
the
reducing
hydrogen
most
atom
so
easily
important
undergo
oxidation
applications
crossed
is
process.
the Cannizzaro
reaction
between
(iv)
Bimolecular
formaldehyde
Reduction
and
other
or
aldehydes
Pinacol
containing
Reaction
Two molecules of acetone undergo reduction in the presence of Mg/Hg to form Pinacol. Upon treatment with mineral acids, 2,3dimethyl 2,3- butane diol (pinacol) is converted into methyl ter-butyl ketone (pinacolone). The 1,2-diol undergo dehydration in such a way that rearrangement of the carbon skeleton occurs. Other 1,2 diols undergo analogous reactions, which are known as inacol
type
pinacolone
rearrangement.
Mechanism
Pinacol – Pinacolone
type
Rearrangement
As the migrating group migrates with its electron pair, the more nucleophilic group might be expected to migrate. Thus, the order of
migratory
attitude
amongst
the
aryl
groups
-anisyl
is >
-tolyl
>
phenyl
>
-chlorophenyl,
etc.
Remember, electron-attracting groups will retard the migration. The migratory aptitude amongst the alkyl groups is Me 3
C
>
Me
2
CH > Me. However, the stability of the initially formed carbocation may offset the migratory attitude order. Thus, in the compound 1, 1-dimethyl-2, 2-diphenyl glycol, the resonance-stabilized carbocation (I) is formed instead of (II) and so it is the methyl group and
not
the
phenyl
group
which
migrates,
contrary
to
the
above
sequence.
Steric
hinderance
-anisyl
may
group
affect
migrates
the 1000
rate
of
times
migration –
faster
than
o -anisyl
group.
Migrating group attacks from the trans side or back side of to the leaving group. This has significant aspect in cyclic systems. Thus, the two isomers of 1, 2-dimethyl-cyclohexane-1, 2-diol give different products due to different orientations of the methyl and hydroxyl
groups.
The
one
(III)
in
which
the
Me
and
OH
groups
are
trans to each other gives 2, 2-dimethylcyclohexanone by methyl shift. The other (IV) in which the Me and OH groups are cis to each other undergoes ring methylene group shift instead of Me-shift with consequent ring contraction to give 1-acetyl-1methylcyclopentane
(V).
Applications
Ketones
from
cyclic
diols
Pinacol rearrangement has been also applied to prepare ketones which are very difficult to prepare by other method.
(v)
Reaction
with
NH3
When carbonyl compounds reacts with ammonia, nucleophilic addition reaction occurs but products are formed according to praportions
between
ammonia
and
carbonyl
compound.
For
example
(vi)
Reaction
with
Ammonia
Derivatives
NH 2
– Z is an example of nucleophilic addition elemination reaction where – Z groups are following in nature. First attack from nucleophilic site of nitrogen atom to electrophilic carbon atom then elimination of water occurs simultaneously.
These reactions are usually carried out in weakly acidic medium because weak acid catalyses the reaction by protonating carbonyl oxygen but in presence of excess of acid nucleophile is also protonated which reduces its reactivity, so optimum pH is necessary. When
zine
compounds
reacts
with
carbonyl
compound
they
form
crystalline
solid
as
zone
derivatives.
The
reaction
with
Always
2,4 – DNPH
is
used
for
remember,
sepration
of
carbonyl
presence
compound
with
of
noncarbonyl
compound.
traces
of
creates better polarity in the carbonyl compound and more electrophilic nature of carbon atom but on the other hand presence of excess
H
+
in
the
ammonia
derivative
makes
it
a
bad
nucleophile.
For
example
when
NH
2
NH 2
(hydrazine)
in
presence
of
excess
of
acid
converted
into
NH
2
N +
H 3
cation (hydrazinium) which is a bad nucleophile. Hence optimum pH is a very important condition for this reaction,on ly tr acer amount +
are required.
of
H
Distinction between aldehydes & Ketones
(vii)
Distinction
between
aldehydes
&
Ketones
Aldehydes having reducing hydrogen atom whereas ketones don’t, thus only aldehydes reacts with oxidising agent and forms respective
(a)
product.
Ammonical
AgNO 3
(Tollen’s
reagent)
Aldehydes reacts with silveroxide to form white precipitate of metallic silver while ketones cannot, due to absence of reducing hydrogen atom.
(b)
Reaction
with
H gCl 2
(Mercuric
chloride)
Aldehydes reacts with mercuric chloride to form white precipitate of mercurous chloride which changes into black precipitate of metallic
(c)
mercury.
Fehling's
solution
There are two solutions which contains cupric oxide as oxidising agent, when reacts with aldehydes it forms red precipitate o f cuprous
oxide.
C 6
H 5
CHO and their aromatic derivatives do not give test with Fehling's solution because aromatic aldehydes are not good reducing agents.
It
gives
reaction
with
tollen's
reagent
due
to
more
oxidising
nature
of
Ag
2
O
than
CuO.
(Ag
contains
higher
reduction
(d)
potential
than
copper
in
electrochemical
Benedict's
series)
Solution
Similar chemical reaction and cupric oxide are present in benedict's solution but presence of citrate gives differant complex. It reacts
with
aldehydes
to
form
red
precipitate
of
Cu
2
O.
It
cannot
reacts
with
benzaldehyde
and
its
aromatic
derivatives.
(e)
Schiff’s
Reagent
When dilute solution of p-rosaniline hydrochloride, pink in colour, passed through sulphurdioxide gas forms colourless solution known as schiff's reagent. This restore it colour by reducing nature of aldehyde while ketones gives no response.
(viii)
Reformatsky
Reaction
When
an
usually
an
reacts
with
carbonyl
.
compound
This
in
the
presence
of
is
zinc
metal known
Reformatsky
When,
to
produce
a as
reaction.
a
mixture
of
the
carbonyl
compound,
and zinc in dry solvent benzene is carefully heated under reflux when zinc undergo dissolution. Zinc may be activated by adding traces of iodine, or copper powder. The mixture is then treated with ice-cold dilute sulphuric acid and benzene layer separated. Benzene
is
distilled
off
when
is
obtained.
Aldol
condensation,
Knovengel’s
reaction,
Perkin
reaction
and
Reformatsky
reaction
ase
are
catalysed
reaction so these reactions are carbanian active process. The advantage of using zinc in place of magnesium is that the organo-zinc compounds
are
less
so
they
do
that
reactive not
than
normally
(ix)
the react
organo-magnesium with
their
derivatives
own
ester
of groups.
Perkin
synthesis
In Perkin reaction, synthesis has been effected between aromatic aldehydes and aliphatic acid anhydrides in the presence of sodiu m or
In
potassium
this
reaction
salt
active
of
species
the
also
acid
comes
corresponding
in
the
presence
to
of
the
base
as
anhydride,
carbanion
to
yield
–
(C H2COOCOCH3).
Besides simple aromatic aldehydes, their vinyl derivatives, heterocyclic aldehydes and even phathalic anhydride (as the carbonyl component)
give
this
reaction.
When carbonyl compounds reacts with acetic anhydride in the presence of base to form active carbanian species, which give nucleophilic addition with carbonyl compounds. But remember, absence of base gives simple fission of anhydride (no carbanian) which
on
reaction
with
carbonyl
forms
stable
alkyledene
acetates.
Toluene oxidation by chromic acid forms benzoic acid while in the presence of acetic anhydride reaction stops at benzaldehyde due to formation of stable intermediate benzeledene di-acetate which on hydrolysis again form – CHO group. So protection of – CH 3
group oxidation at – CHO group, we use chromic acid and anhydride mixture. Following reactions are given below
Witting reaction (x)
Witting
reaction
Witting reaction gives an important and useful method for the preparation of alkenes by the reaction of aldehydes or ketones with
alkylidenetriphenylphosphorane
(Ph 3P
=
CR 2)
or
simply
called
as
phosphorane.
The Witting
reagent
, alkylidenetriphenylphosphorane, is prepared by reaction of trialkyl or triarylposphine usually the latter with an alkyl halide in ether
solution.
Finally
resulting
phosphonium
salt
is
reacted
with
a
strong
base
(such
as
C
6
H 5
Li,
BuLi,
NaNH
2
,
NaH,
C
2
H 5
ONa,
etc.)
which
removes
a
haloacid
to
give
the
reagent,
methylenetriphenyl
phosphorane
(II).
In end, carbonyl compound is directly treated with the etherna l solution of the above reagent to form many compounds.
Mechanism
The reaction go through by the nucleophilic attack of the ylide on the carbonyl carbon. The dipolar complex (betain) so formed undergo electronic exchange decomposes to olefin and triphenylphosphine oxide through a four-centred transition state.
The mechanism is strongly supported by an example that an optically active phosphonium salt reacts to produce a phosphine oxide with
retention
of
(xi)
configuration
in
Lederer
the
final
product.
Manasse’s
Reaction
When phenol is treated with 40% aqueous solution of formaldehyde (formalin) in the presence of a dilute acid or alkali at low temperature,
a
mixture
This
of
o-and
p-hydroxy
reaction
benzyl
alcohol
is
is
formed.
called
Lederer-Manasse
reaction.
On heating for a short time, these compounds unergo condensation reaction with themselves and unchanged phenol and give linear polymers
by
elimination
of
water.
These reactions are the basis of the preparation of phenol formaldehyde resins. These materials were developed by Backland and are
hence
called
akelite.
They are thermoplastic solids soluble in many organic solvents. When warmed with hexa methylene tetramine. (CH 2
6
N 4
, which splits up to formaldehyde and ammonia, further methylene bridges are formed and a three-dimensional polymer results.
Baeyer – villiger
(xii)
rearrangement
Baeyer--Villiger
rearrangement
is an example of the migration of a group from carbon to electron-deficient oxygen.The reaction first involves the oxidation of ketones to esters by the treatment with peracids such as peracetic acid, performic acid, meta chloroperbenzoic acid (MCPBA), perbenzoic
acid,
pertrifluoroacetic
acid,
permonosulphuric
acid,
etc.
This
reaction
can
also
be
done
by
H
2
O 2
and
Cyclic
base.
ketones
are
converted
to
lactones
with
expansion
of
ring.
(xiii)
Beckmann
The
acid-catalyzed
rearrangement
conversion
of
ketoximes
to
N -substituted
amides
is
known
as
Beckmann
.
The
rearrangement
reaction
is
catalysed
by
acidic
reagents
such
as,
H
2
SO 4
,
SOCl
2
,
P
2
O 5
,
PCl
5
,
Al
2
O 3
C 6
H 5
SO 2
Cl,
H
3
PO 4
and
The
many
reaction
proceeds
by
the
migration
of
a
others.
group
from
carbon
to
electron-deficient
nitrogen.
Some aldoximes undergo the rearrangement process in the presence of polyphosphoric acid (PPA) but the reaction is not a general one. The migration of the group not depends on the migrational activity but upon the orientation of the group in relation to the OH group.
It
is
found
that
the
migrating
group
is
always
anti
(i.e.,
trans to
the
hydroxyl
group.
So
we
can
Mechanism
(xiv)
say
that,
the
reaction
is
stereospecific.
of
Benzilic
acid
rearrangement
reaction
(modified
intra
molecular
cannizaro's
reaction)
When we mix a strong base to a carbonyl group first the formation of an anion takes place and the reversal of the anionic charge may cause removal of the attached group, but in case of 1,2-diketone the attched group may migrate to the adjacent electrondeficient
Thus,
carbonyl
benzil
on
reaction
with
a
strong
enzilic
.
Basically
carbon
base
forms
benzilic
acid
forming
(salt),
reaction
acid
this
reaction
is
intracannizaro
reaction
is
known
as
rearrangement
where
formation
of
benzillic
acid
takes
place.
Barium hydroxide (barayta water) is more effective than sodium or potassium hydroxides due to strong basic nature. Alkoxide ions (methoxide,
ethoxide,
t -butoxide,
etc.)
in
place
of
hydroxide
ion
give
the
corresponding
esters.
Phenoxide ions are too weak for nucleophile to attack. Besides aromatic 1, 2-diketones, aliphatic and heterocyclic diketones, for example o -quinones can also undergo this type of reaction.
Polymerization Reactions (xv)
Polymerization
Reactions
When two or more molecules combine to form bigger molecule accompanied by the loss of simple molecule like water, alcohol or ammonia,
etc.
the
process
is
called
condensation
polymerisation.
But if the number of molecules (of the same substance or of different substances) combine to form larger molecule, the process is called
polymerisation.
Condensation
and
Condensation
polymerisation
and
products
of
some
carbonyl
polymerisation
compounds
product
are
of
as
follows
-
formaldehyde
(a) Condensation with ammonia : Formaldehyde condenses with ammonia to form hexa methylene tetramine (urotropine)
The
(b)
reaction
Condensation
occurs
with
in
phenol
the
-
Lederer
following
-
Manasse’s
manner
reaction
Formaldehyde condenses with phenol in alkaline medium to form o-and p-hydroxy benzyl alcohol which further condenses with
:
:
phenol
to
give
polymer,
This
is
called
Bakelite
. Lederer-Manasse
(c)
reaction.
Formation
Formaldehyde
and
of
acetaldehyde
penta
combine
to
erythritol
form
trihydroxy
(Claisen-Schmidt
which
(d)
reacts
:
aldehyde reaction)
with
alkali
Formation
to
give
pentaerythritol
of
(Cannizaro
formose
reaction)
:
:
On treating formaldehyde with dilute barium hydroxide (baryta water), a mixture of several sugars (monosaccharides) is formed
which
is
called
formose.
Above
reaction
resembles
aldol
Polymers
condensation
of
Formaldehyde
three
Trioxan
distilling
formaldehyde
formaldehyde
not
have
a
:
polymers
or
with
does
formaldehyde
forms
(a)
On
although
:
trioxymethylene
small
quantity
of
dilute
:
sulphuric
acid,
trioxan
is
formed.
(b)
On
allowing
formaldehyde
to
stand,
Paraformaldehyde
it
slowly
changes
to
trioxan.
:
When a concentrated aqueous solution of formaldehyde is evaporated off to dryness, a long linear polymer is obtained which is called araformaldehyde.
On heating paraformaldehyde, formaldehyde is regenerated. Hence formaldehyde is transported in the form of paraformaldehyde. when
paraformaldehyde
is
heated
in
a
sealed
tube
at
115
o
C,
trioxan
(c
is
)
formed.
Polyoxymethylene
:
When dilute sulphuric acid is added into cold aqueous solution of formaldehyde, a white insoluble solid is formed which is called olyoxymethylene
.
This
is
also
a
linear
polymer.
Condensation
and
(a)
Reaction
In
this
case
chain
polymerisation
length
is
longer
than
products
in
paraformaldeh yde.
of
with
acetaldehyde
ammonia
:
Ammonia reacts with acetaldehyde to form simple nucleophilic addition product- acetaldehyde ammonia which subsequently losses
water
to
give
acetaldemine.
(b)
Polymerisation:
Acetaldehyde
when
treated
with
hydrogen
chloride
gas
at
0°C
forms
a
solid
tetramer
called
metaldehyde
.
However,
if
temperature
is
not
controlled,
a
trimer,
called
araldehyde
is
formed
which
is
liquid.
