Ketones and aldehydes
Aldehydes (
) and ketones (
) are both carbonyl compounds.
They are organic compounds in which the carbonyl carbon is connected to C
or H atoms on either side. An aldehyde has one or both vacancies of the
carbonyl carbon satisfied by a H atom, while a ketone has both its vacancies
satisfied by carbon.
Naming Aldehydes and Ketones
Ketones are named by replacing the -e in the alkane name with -one. The
carbon chain is numbered so that the ketone carbon, called the carbonyl
group, gets the lowest number. For example,
would be named 2butanone because the root structure is butane and the ketone group is on the
number two carbon.
Alternatively, functional class nomenclature of ketones is also recognized by
IUPAC, which is done by naming the substituents attached to the carbonyl
group in alphabetical order, ending with the word ketone. The above example
of 2-butanone can also be named ethyl methyl ketone using this method.
If two ketone groups are on the same structure, the ending -dione would be
added to the alkane name, such as 2,5-heptanedione.
Aldehydes replace the -e ending of an alkane with -al for an aldehyde. Since
an aldehyde is always at the carbon that is numbered one, a number
designation is not needed. For example, the aldehyde of pentane would
simply be pentanal.
The -CH=O group of aldehydes is known as a formyl group. When a formyl
group is attached to a ring, the ring name is followed by the suffix
"carbaldehyde". For example, a hexane ring with a formyl group is named
cyclohexanecarbaldehyde.
Boiling Points and Bond Angles
Aldehyde and ketone polarity is characterized by the high dipole moments
of their carbonyl group, which makes them rather polar molecules. They are
more polar than alkenes and ethers, though because they lack hydrogen, they
cannot participate in hydrogen bonding like alcohols, thus making their
relative boiling points higher than alkenes and ethers, yet lower than alcohols.
Typical bond angles between the carbonyl group and its substituents show
minor deviations from the trigonal planar angles of 120 degrees, with a
slightly higher bond angle between the O=C-R bond than the R-C-R bond on
the carbonyl carbon (with R being any substituent).
Preparing Aldehydes and Ketones
Preparing Aldehydes
Partial oxidation of primary alcohols to aldehydes
This reaction uses pyridinium chlorochromate (PCC) in the absence of water
(if water is present the alcohol will be oxidized further to a carboxylic acid).
From fatty acids
(HCOO)2Ca + HEAT ----> HCHO + CaCO3 (CH3COO)2Ca + HEAT ----> acetone
+ CaCO3 (CH3COO)2Ca + (HCOO)2Ca ---->ethanaldehyde
Stephen reduction
RCN + nCl2 ----> RCH=NH2+Cl− ----> on hydrolysis gives RCHO
Here sulfur is used as a poisoner so that aldehyde formed doesn't get
oxidised to the carboxylic acid. See the Wikipedia article for more detail.
Rosenmund reaction
RCOCl + Pd + BaSO4 + S ---->RCHO for solvent xylene is used
Preparing Ketones
From Grignard reagents
RCOOR' + R'MgX ---->RCOR + R'OH
R'
|
R'
|
OH
|
RC=O + R'-MgX ---->R-C-OMgX----->R-C-OH + Mg-X
|
O-R'
|
OR'
|
OR'
From nitriles
RCN + R'MgX ----> RCOR'(after hydrolysis) HCN does not react with RMgX as
HCN has acidic hydrogen which results in RH being formed.
From gem dihalides
RCCl2R + strong base ----> RCOR
Oppenaur oxidation
Reagent is Aluminium tert. butoxide solvent is acetone
ROH + ACETONE ----> Ketone + isopropyl alcohol this oxidation does not
affect double bonds in this oxidation ketone act as a oxidizing agent this is
exact opposite to merrwine pondroff reduction
Friedel-Crafts acylation of aromatic compounds
An aromatic ring reacts with a carboxylic acid chlorine (RCOCl) in the
presence of AlCl3 to form an aryl ketone of the form ArCOR.
Oxidation of secondary alcohols to ketones
A secondary alcohol can be oxidised into a ketone using acidified potassium
dichromate(VI) and heating under reflux.
The orange dichromate(VI) ion, Cr2O72-, is reduced to the green Cr3+(aq)
ion.
Other reactions which produce either aldehyde or ketone
Ozonolysis of alkenes
The cleavage of an alkene with ozone and then subjected to water and zinc.
Hydration of alkynes
Water is added to an alkyne in a strong acid. The strong acid used is sulfuric
acid and mercuric acid.
Keto-enol tautomerism
In the presence of an acid (H+) or a base (OH-), the aldehyde or ketone will
form an equilibrium with enols, in which the double bond of the carbonyl
group migrates to form double bond between the carbonyl and the alpha (α)
carbon.
In the presence of an acid, protonation of the oxygen group will occur, and
water will abstract an alpha (α) hydrogen.
In the presence of a base, deprotonation of the alpha hydrogen will occur,
and a hydrogen from water will be abstracted by the carbonyl oxygen.
This is an important feature of ketone and aldehydes, and is known as the
keto-enolic tautomery or keto-enol tautomerism, i.e. the equilibrium of
carbonyl compounds between two forms.
It must be stressed that the keto and the enol forms are two distinct
compounds, not isomers. They are known as tautomers of each other. The
presence of α-hydrogen is necessary for this equilibrium: those compounds
not possessing it are called non-enolizable ketones.
Mechanism of enol-keto tautomerism
Reactions of Aldehydes and Ketones
Reactions with the carbonyl carbon
Since aldehydes and ketones contain a polar carbonyl group, the partially
positive carbon atom can act as an electrophile. Strong and weak nucleophiles
are able to attack this carbonyl carbon, resulting in a net addition to the
molecule.
Nucleophilic addition
With cyanide, nucleophilic addition occur to give a hydroxynitrile:
RR'C=O + CN- + H+ → RR'COHCN
e.g. propanone → 2-hydroxymethylpropanonitrile
Reactions with the carbonyl oxygen
The partially negative oxygen can act as a nucleophile, or be attacked by
electrophiles.
Oxidation
Using a strong oxidizing agent such as the Tollens' Reagent (Ag2O in aqueous
ammonia) acidified dichromate, Benedict's/Fehling's reagent (essentially
alkaline Cu+2); aldehydes but not ketones may be oxidized into carboxylic
acids. This is one way to test for the presence of an aldehyde in a sample
compound: an aldehyde will become a carboxylic acid when reacted with
Tollens' reagent, but a ketone will not react. when aldehydes react with
fehling solution a red precipitate is obtained (due to formation of Cu2O) .
Reactions of carbonyl groups
The main reactions of the carbonyl group are nucleophilic additions to the
carbon-oxygen double bond. As shown below, this addition consists of adding
a nucleophile and a hydrogen across the carbon-oxygen double bond.
Due to differences in electronegativities, the carbonyl group is polarized. The
carbon atom has a partial positive charge, and the oxygen atom has a
partially negative charge.
Aldehydes are usually more reactive toward nucleophilic substitutions than
ketones because of both steric and electronic effects. In aldehydes, the
relatively small hydrogen atom is attached to one side of the carbonyl group,
while a larger R group is affixed to the other side. In ketones, however, R
groups are attached to both sides of the carbonyl group. Thus, steric
hindrance is less in aldehydes than in ketones.
Electronically, aldehydes have only one R group to supply electrons toward
the partially positive carbonyl carbon, while ketones have two electronsupplying groups attached to the carbonyl carbon. The greater amount of
electrons being supplied to the carbonyl carbon, the less the partial positive
charge on this atom and the weaker it will become as a nucleus.
Addition of water
The addition of water to an aldehyde results in the formation of a hydrate.
The formation of a hydrate proceeds via a nucleophilic addition mechanism.
1. Water, acting as a nucleophile, is attracted to the partially positive
carbon of the carbonyl group, generating an oxonium ion.
2. The oxonium ion liberates a hydrogen ion that is picked up by the
oxygen anion in an acid-base reaction.
Small amounts of acids and bases catalyze this reaction. This occurs because
the addition of acid causes a protonation of the oxygen of the carbonyl group,
leading to the formation of a full positive charge on the carbonyl carbon,
making the carbon a good nucleus. Adding hydroxyl ions changes the
nucleophile from water (a weak nucleophile) to a hydroxide ion (a strong
nucleophile). Ketones usually do not form stable hydrates.
Addition of alcohol
Reactions of aldehydes with alcohols produce either hemiacetals (a functional
group consisting of one —OH group and one —OR group bonded to the same
carbon) or acetals (a functional group consisting of two —OR groups bonded
to the same carbon), depending upon conditions. Mixing the two reactants
together produces the hemiacetal. Mixing the two reactants with hydrochloric
acid produces an acetal. For example, the reaction of methanol with ethanal
produces the following results:
A nucleophilic substitution of an OH group for the double bond of the
carbonyl group forms the hemiacetal through the following mechanism:
1. An unshared electron pair on the alcohol's oxygen atom attacks the
carbonyl group.
2. The loss of a hydrogen ion to the oxygen anion stabilizes the oxonium
ion formed in Step 1.
The addition of acid to the hemiacetal creates an acetal through the following
mechanism:
1. The proton produced by the dissociation of hydrochloric acid
protonates the alcohol molecule in an acid-base reaction.
2. An unshared electron pair from the hydroxyl oxygen of the hemiacetal
removes a proton from the protonated alcohol.
3. The oxonium ion is lost from the hemiacetal as a molecule of water.
4. A second molecule of alcohol attacks the carbonyl carbon that is
forming the protonated acetal.
5. The oxonium ion loses a proton to an alcohol molecule, liberating the
acetal.
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