Transcript Carbonyl Compounds

ORGANIC CHEMISTRY
CHM 207
CHAPTER 7:
CARBONYL COMPOUNDS
(ALDEHYDES AND KETONES)
NOR AKMALAZURA JANI
SUBTOPICS
Nomenclature
Physical properties:
- boiling points
- water solubility
• Reactions:
- Oxidation
- Reduction
- Condensation with 2,4-dinitrophenylhydrazine
- Nucleophilic addition
- Haloform reaction
• Uses of carbonyl compounds.
•
•
ALDEHYDES AND KETONES
• Functional group: carbonyl group
C O
 Aldehyde: one hydrogen atom is bonded to the
carbon in the carbonyl group.
 Ketone: the carbon atom in the carbonyl group is
bonded to two hydrocarbon groups.
R
O
O
C
C
R'
ketone
R
H
aldehyde
R, R' = substituents
Naming Aldehydes
The IUPAC names of aldehydes are
obtained by dropping the –e and adding
-al to the name of the parent hydrocarbon.
butane
butanal
al
• The parent hydrocarbon is the longest chain
that carries the –CHO group.
• This chain has 4 carbon atoms.
3
2
1
4
• The parent hydrocarbon is the longest
chain that carries the –CHO group.
• This chain has 5 carbon atoms.
5
4
3
2
1
• The –CHO group is always at the beginning
of the carbon chain. The carbonyl carbon is
numbered as carbon 1.
5
4
3
2
1
3-methylpentanal
• The common names of aldehydes are derived
from the common names of the carboxylic acids.
• The –ic acid or –oic acid ending of the acid name is
dropped and is replaced with the suffix –aldehyde.
butyric acid
butyraldehyde
NOMENCLATURE OF CYCLIC ALDEHYDES
• Aliphatic aldehydes containing a ring as well as aromatic aldehydes
in which the aldehyde (-CHO) group is attached directly to the
benzene ring are named by adding suffix carbaldehyde to the
name of the corresponding hydrocarbon.
CHO
56 1
4 2
3
cyclohexanecarbaldehyde
CHO
CH3
2-methylcyclohexanecarbaldehyde
• Naming aromatic compounds:
CHO
CHO
NO2
benzaldehyde
4-nitrobenzaldehyde
 If the aldehyde group is not attached directly to the
benzene ring, the aldehyde is named as an aryl
derivatives of the corresponding aldehyde.
3
CH2CHO
phenylethanal
2
1
CH=CHCHO
3-phenylpropenal
Naming Ketones
• The IUPAC name of a ketone is derived from the name of
the alkane corresponding to the longest carbon chain
that contains the ketone-carbonyl group.
• The parent name is formed by changing the –e ending of
the alkane to -one.
propane
propanone
one
• If the carbon chain is longer than 4 carbons,
it’s numbered so that the carbonyl carbon has
the smallest number possible, and this number
is prefixed to the name of the ketone.
This end of the chain is closest to the C=O.
Begin numbering here.
1
2
3
4
5
6
IUPAC name: 3-hexanone
New IUPAC name: hexan-3-one
• The common names of ketones are derived by
naming the alkyl or aryl groups attached to the
carbonyl carbon followed by the word ketone.
ethyl
propyl
ethyl propyl ketone
NOMENCLATURE OF CYCLIC KETONES AND
AROMATIC COMPOUNDS
•
•
The parent name is formed by changing the
cycloalkane to -one.
Carbonyl carbon is designated C1.
–e ending of the
O
O
612
5 3
4
CH3
cyclohexanone
4-methylcyclohexanone
 Aromatic compound:
- phenyl is used as part of the name.
O
O
C CH3
C
phenylethanone
diphenylmethanone
• A ketone or aldehyde group can also be named as a substituent
on a molecule with another functional group as its root.
• The ketone carbonyl is designated by the prefix oxo• The –CHO group is named as a formyl group.
• Carboxylic acids frequently contain ketone or aldehyde groups
named as substituents.
O
O
O
CH3CH2 C CH2 C H
5
4
3
2
1
3-oxopentanal
3
4
2
5
1
COOH
2-formylbenzoic acid
6
O
O
C H
CH3 C CH2 C OH
4
3
2
1
3-oxobutanoic acid
PHYSICAL PROPERTIES OF ALDEHYDES AND
KETONES
 BOILING POINTS
- Polarization of the carbonyl group creates dipole-dipole
attractions between the molecules of ketones and
aldehydes.
- this attractions resulting in higher boiling points for
ketones and aldehydes than for hydrocarbons and ethers of
similar molecular weights.
- did not have O-H and N-H bonds → can not form hydrogen
bonds with each other.
- boiling points of ketones and aldehydes are lower than
alcohols of similar molecular weight.
Boiling points of alkane, ether, aldehyde, ketone and
alcohol of similar molecular weight.
O
CH3CH2CH2CH3
butane
bp 0oC
CH3 O CH2CH3 CH3CH2 C H
O
CH3 C CH3
CH3CH2CH2-OH
1-propanol
methoxyethane
propanal
acetone
bp 8oC
bp 49oC
bp 56 C
o
bp 97oC
bp alkane < bp ether < bp ketone, bp aldehyde < bp alcohol
bp ketone, bp aldehyde > bp alkyl halides
 WATER SOLUBILITIES
- ketones and aldehydes have lone pairs of electrons
and can act as hydogen bond acceptors with other
compounds having O-H or N-H bonds.
- for example, the –OH hydrogen of water or an alcohol
can form a hydrogen bond with unshared electrons on
a carbonyl oxygen atom.
R
R
C O
R'
C O
δHδ+ H
H
R
H
δ+ δ-
O
O
δ+
hydrogen bonding
δ+
δ-
δ+
δ+
hydrogen bonding
δ-
• Because of the hydrogen bonding, ketones and aldehydes are
good solvents for polar hydroxylic substances such as alcohols.
• Ketones and aldehydes are soluble in water.
- ketones and aldehydes with up to 4 carbon atoms are fairly
soluble in water.
- the solubility of ketones and aldehydes in water decreases
with the increasing length of the carbon chain.
REACTIONS OF ALDEHYDES AND
KETONES
•
•
•
•
•
Oxidation
Reduction
Condensation with 2,4-dinitrophenylhydrazine
Nucleophilic addition
Haloform reaction
OXIDATION
 OXIDATION OF ALDEHYDES WITH KMnO4 AND K2Cr2O7
[O]
aldehydes
carboxylic acids
O
+
CH3CHO
ethanal
KMnO4/H
CH3-C-OH
heat
ethanoic acid
+
CHO
benzaldehyde
KMnO4/H
heat
O
C-OH
benzoic acid
• When an aldehyde is heated with potassium dichromate (VI) solution
acidified with dilute H2SO4, the solution changes colour from orange to
green.
O
R
C
O
+
H
aldehyde
Cr2O2-7
(orange)
K2Cr2O7/H
R-C-OH
heat
carboxylic acid
3+
Cr
(green)
 Ketones are resistant to oxidation.
 Oxidation only occurs if the ketone is boiled with a strong
oxidising agent under reflux for a prolonged period of time.
The oxidation of ketones involves breaking C-C bonds.
 REACTIONS OF ALDEHYDES WITH TOLLENS’ REAGENT:
SILVER MIRROR TEST
- Tollens’ reagent is called ‘ammoniacal silver nitrate’
solution.
- contains the silver amine complex ion, [Ag(NH3)2]+
- Tollens’ reagent is a mild oxidising agent.
- when aldehyde is warmed with Tollens’ reagent, the
colourless complex ion, [Ag(NH3)2]+ is reduced by aldehyde
to grey metallic silver.
- the precipitate forms a silver mirror on the walls of test
tube.
• Equation:
CH3CHO + 2 [Ag(NH3)2]+ + OH- → CH3COO- + 2Ag(s) + 2NH4+ + 2NH3
aldehyde Tollen’s reagent
grey metallic silver
• A simplified equation:
CH3CHO + 2 Ag+ + H2O → CH3COOH + 2Ag(s) + 2H+
• General equation:
RCHO + 2Ag+ + H2O → RCOOH + 2Ag(s) + 2H+
* Tollens’ test is used to distinguish aldehydes from
ketones. Ketones DO NOT react with Tollens’s reagent.
 REACTIONS OF ALDEHYDES WITH FEHLING’S
SOLUTION
- Fehling’s solution contains a copper (II) complex ion.
- Fehling’s solution: mixing copper (II) sulphate solution
with a solution of sodium potassium tartrate in NaOH
(aq).
- When Fehling’s solution is warmed with aldehydes, the
deep blue colour of the Fehling’s solution dissapears
and a brick-red (reddish-brown) precipitate of copper (I)
oxide (Cu2O) is obtained.
O
R
C
O
H
2Cu2+ + 5OH-
aldehyde
Fehling's solution
(blue colour)
R
C O-
Cu2O + 3H2O
copper (I) oxide
(brick-red precipitate)
• Fehling’s solution can be used to distinguish
between:
a) Aldehydes and ketones (ketones do not
react with Fehling’s solution).
b) Aliphatic aldehydes and benzaldehyde
(benzaldehyde does not react with Fehling’s
solution).
REDUCTION
• Aldehydes and ketones can be reduced to alcohols using:
a) lithium aluminium hydride (LiAlH4)
b) sodium borohydride (NaBH4)
c) catalytic hydrogenation
O-
O
OH
+
R
C
H
LiAlH4 or NaBH4 or H2, Ni
R
aldehyde
C
H
H
R
C
H
H
H
o
1 alcohol
O
R
C R'
O
LiAlH4 or NaBH4 or H2, Ni
ketone
+
H = diluted acid such as H2SO4
-
OH
+
R
C R'
H
R
C R'
H
H
o
2 alcohol
Examples:
O-
O
CH3
C
H
LiAlH4
CH3
ethanal
C
OH
H
H+
CH3
C
H
H
H
ethanol
O
CH3
C
O
CH3
propanone
H2/Ni
CH3
-
C CH3
H
OH
H+
CH3
C
CH3
H
2-propanol
CONDENSATION WITH 2,4DINITROPHENYLHYDRAZINE
• Abbreviation for 2.4-dinitrophenylhydrazine is 2,4-DNP.
• A solution of 2,4-DNP in methanol and H2SO4: Brady’s reagent.
• Aldehydes reacts with 2,4-DNP at room temperature to give a
yellow-orange precipitate of 2,4-dinitrophenylhydrazone.
REAGENT
POSITIVE TEST
NO2
H
C
O
H2N N
H
NO2
H
benzaldehyde
room
C
temperature
R'
C
O
NO2
H2O
benzaldehyde 2,4-dinitrophenylhydrazone
(yellow-orange precipitate)
NO2
H2N N
N N
H
2,4-dinitrophenylhydrazine
R
NO2
R
NO2
H
room
temperature
R' C
NO2
N N
NO2
H2O
H
2,4-dinitrophenylhydrazine
•
2,4-Dinitrohydrazones have characteristic sharp melting points.
•
The formation of a yellow or orange precipitate when 2,4-DNP
reacts with an organic compound at room temperature is used
a) As chemical test for aldehydes or ketones,
b) To identify an aldehyde or a ketone by measuring the melting point
of the 2,4-dinitrophenylhydrazone formed.
NUCLEOPHILIC ADDITION
• The carbonyl groups in aldehydes and ketones are polarised because
of the difference in the electronegativity of carbon and oxygen.
• The carbon atom carries a partial positive charge while oxygen atom
carries a partial negative charge.
• Aldehydes and ketones are susceptible to attack both by nucleophiles
at the carbonyl carbon atom and by electrophiles at the oxygen atom.
δ+
δ-
C
O
nucleophilic attack
electrophilic attack
Nucleophilic addition of hydrogen cyanide
O
R C R'
OH
HCN
ketone
R C R'
CN
cyanohydrin
example
O
CH3
C CH3
propanone
OH
HCN
CH3
C CH3
CN
2-hydroxy-2-methylpropanenitrile
O
OH
R C
OH
+
H
HCN
R C CN
aldehyde
H2O/H
R C COOH
H
NH4+
H
cyanohydrin
carboxylic acid
example
O
CH3
C
OH
H
OH
+
HCN
ethanal
CH3
C CN
H2O/H
CH3
H
C COOH
H
2-hydroxypanenitrile
2-hydroxypropanoic acid
(lactic acid)
MECHANISM
O
O
C
CN
C
+
H
CN
OH
C
CN
+
NH4
HALOFORM REACTION
• IODOFORM TEST
- a solution of I2 in an alkaline medium such as NaOH or KOH is a
oxidising agent.
- when ethanal warmed with this solution, triiodoethanal will be
formed as the intermediate product.
- triiodoethanal then reacts with the base to form a yellow
precipitate of triiodomethane (iodoform).
CH3CHO + 3I2
CI3CHO
+ 3HI
triidoethanal
Cl3CHO + -OH
CHI3 + HCOOiodoform
•
Iodoform test is useful for the methyl ketone group (CH3C=O) in
ethanal and methyl ketones.
 If an alkaline solution of iodine is warmed with an organic
compound and a yellow precipitate of triiodomethane is
produced, the organic compound is likely to be one of
the following:
OH
ethanol CH3
C
O
H
ethanal CH3
H
OH
a secondary alcohol with the CH
3
O
a ketone with the CH3
C H
C
group
CH
group
• Iodoform test can be used to distinguish:
i) ethanal from other aldehydes, because ethanal is the only
aldehydes that gives a positive iodoform test.
ii) ethanol and secondary alcohols that contains the CH3CH(OH)group give a positive iodoform test.
iii) methyl ketones (ketones that contain CH3CO- group) give
positive iodoform test.
For example, propanone and phenylethanone give a yellow
precipitate, but 3-pentanone and diphenylmethanone give
negative iodoform tests.
O
O
C CH3
3I2
phenylethanone
O
C C I
warm
O
I
3HI
I
-
NaOH
C C I
I
+
C O Na
CHI3
I
The overall reaction is
O
C CH3
phenylethanone
O
3I2
NaOH
heat
C O- Na+
sodium benzoate
CHI3
3HI
iodoform
(yellow precipitate)
USES OF CARBONYL COMPOUNDS
• Formalin (40% aqueous solution of methanal):
- as disinfectant
- as a preservative for biological specimens
• Vanillin (C8H8O3):
- a strong vanilla odour and used for food flavouring.
• Camphor (C10H16O):
- used medically as an inhalant for colds.
• Cyclohexanone:
- starting material for production of nylon.
• Used as solvents, starting materials and reagents. For
example, propanone.