Lecture (9)
VI-Aldehydes and Ketones
1-Introduction:
Aldehydes are compounds of the general formula R─CHO; ketones are
compounds with the general formula R─CO─R' the groups R and R'
may be aliphatic or aromatic. Aliphatic aldehydes and ketones have the
general formula CnH2nO. Both aldehydes and ketones contain the
carbonyl group C=O, and are often referred to as carbonyl compounds. It
is the carbonyl group that largely determines the chemistry of aldehydes
and ketones. The carbonyl group consists of a sigma bond (σ) and a (П)
bond. The carbonyl group is polar; the oxygen has two pairs of unshared
electrons.
2-Nomenclature of aldehydes and ketones:
(A)-Common name:
I-Aldehydes (names derived from acids)
Simple aldehydes are usually named after the acids produced by them on
oxidation. The ending(ic) of the name of the acid is replaced by the word
aldehyde.
Examples:
HCOOH (formic acid)
HCHO (formaldehyde)
CH3COOH (acetic acid)
CH3CHO (acetaldehyde)
C2H5COOH (Propionic acid)
C2H5CHO (propionaldehyde
C3H7COOH (Butyric acid)
C3H7CHO (Butyraldehyde)
II-Ketones:
The simple Ketones are named by a functional group name. The alkyl
groups attached to the carbonyl group are named, and then the word
ketone is added.
Examples:
CH3COC2H5
Ethyl methyl ketone (MEK)
CH3COCH3
Dimethyl ketone (Acetone)
(B)-IUPAC name:
In the IUPAC system, the name of an Aldehyde is drived from the name
of the parent alkane by changing the final (-e) to (-al).
No numbering is needed as the CHO group always on carbon (1).
The name of ketone named by changing the (-e) of the alkane name to (one).
Numbering is used when necessary.
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Examples:
● CH3CHO
●CH3CH=CHCHO
● CH3COCH2CH2CH3
●CH3COCH=CHCH3
Ethanal
2-butenal
2-Pentanone
3-Penten-2-one
3-Preparation of aldehydes and ketones:
(A)-Oxidation of alcohols:
K2Cr2O7/H2SO4
RCH2OH
RCHO
(1°alcohol)
aldehyde
K2Cr2O7/H2SO4
R1R2CHOH
R1COR2
(2°alcochol)
ketone
(B)-Hydration of alkynes:
H2SO4/H2O
(1)-HC≡CH
Acetylene
HgSO4
H─ C = C ─H
H OH
Vinyl Alcohol
CH3CHO
acetaldehyde
H2SO4/H2O
(2)-H3C─C≡CH
Propyne
HgSO4
(C)-Ozonolysis of alkenes:
O3
(1)-H2C=CH2
Ethylene
Zn/H3O+
CH3─C=CH2
OH
Propenyl Alcohol
CH3COCH3
acetone
2HCHO
formaldehyde
O3
(2)- (CH3)2C=C(CH3)2
2,3-dimethyl-2-butene
+
Zn/H3O
(CH3)2CO
acetone
4-Physical Properties:
(A)-The polar carbonyl group makes aldehydes and ketones polar
compounds, and hence they have higher boiling point than non polar
compounds of compared molecular weight. By themselves, they are not
capable of intermolecular hydrogen bonding since they contain hydrogen
bonded only to carbon, as a result they have lower boiling points
compared to alcohols or carboxylic acids.
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Example:
Propionic acid (141°C B.P)>n-butyl alcohol (118°C B.P)> ethyl methyl
ketone (80°C B.P)>n- butyraldehyde (76°C B.P)>n-Pentane (36°C B.P)
(B)-The lower aldehydes and ketones are appreciably soluble in water,
because of hydrogen bonding between solute and solvent molecules.
Border line Solubility is reached at about five carbons after that the
solubility in water decreases with increasing their molecular weights.
(C)-Aldehydes and ketones are soluble in the usual organic solvents like
ether and alcohols.
5-Chemical reactions:
Addition reactions of aldehydes and ketones:
●The carbon-oxygen double bond is polar and may be attacked either by
nucleophile or electrophile.
●The carbonyl group is stabilized by adjacent alkyl groups. Which are
electron releasing. Ketone with two R groups more stable than an
aldehyde with only one R.
Formaldehyde with no alkyl groups is the most reactive of the aldehydes
and ketones.
RCOR
RCHO
HCHO
Ketone
aldehyde
formaldehyde
Increasing reactivity
(A)-Reaction with alcohols:
●The product of addition of one molecule of an alcohol to an aldehyde
called Hemiacetal, while the product of addition of two molecules of
alcohol is called Acetal (with loss of H2O).
●HemiKetal and Ketal are the responding terms used for ketone
products.
●all these reactions are catalyzed by a trace of strong acid.
R'OH, H+
RCHO
OR'
R─ C ─H
R'OH, H+
OH
Aldehyde
OR'
R─C─H + H2O
OR'
hemiacetal (OH+OR on C)
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acetal (two OR on C)
e.g.:
CH3COH
2CH3CH2OH, H+
OCH2CH3
CH3 C H
+H2O
OCH2CH3
acetal
Acetaldehyde
(B)-Reaction with Grignard reagent:
The reaction of formaldehyde with grignard reagent yields a primary
alcohol. Other aldehydes yield secondary alcohols. Ketones yield tertiary
alcohols.
H2O, H+
RMgX + HCHO
RCH2OH + MgXOH
Formaldehyde
(1°alcohol)
+
H2O, H
RMgX + R'CHO
R'RCHOH + MgXOH
(2°alcohol)
+
H2O, H
RMgX+R'COR''
RR'R"COH + MgXOH
(3°alcohol)
(C)-Oxidation of aldehydes and ketones:
K2Cr2O7/H2SO4
RCHO
Aldehyde
RCOOH
carboxylic acid
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