6784664 carbonyl compounds
TRANSCRIPT
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Carbonyl Compounds
(Chapter 35)
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Carbonyl compounds
C O Carbonyl group
sp2
hybridized carbonCoplanar bonds, 120 o bond anglep-p overlap bond
Two types of compounds
Aldehyde Ketone C O
R
R
C O
H
R
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Polarity of carbonyl bondDipole moment
propan-1-ol 1.69Dpropanone 2.88ethanal 2.72
C+ O
-
-bond -ve inductive effect
-bond readily polarizable(mesomeric effect)
C+
O-
C+
O:-
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Chemical reaction
No electrophilic additions (why?)
C O
H-Br
C+ OH
:Br -
C OH
Br
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Chemical reaction
No electrophilic additions (why?)
C OH
BrH
Big +ve,unstableproduct
Energetic Factor
(Unstable)
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Chemical reaction
No electrophilic additions (why?)
Ea
Big +ve E a ,
unstableTransitionstate
Kinetic Factor
(Unstable)
C+ OH
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Chemical reaction
No nucleophilic substitution
C ONu -:
C O -
Nu
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Nucleophilic Addition
C O
Nu-
:
H/R
R
Reactivity:1. Electrophilicity of C atom
of the C=O group.2. Strength of Nu: - 3. Steric effect at the carbonyl
group
C O -
Nu
H/R
R
E+ C E
CN
H/R
R
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Addition of HCN
C O
H/R
R+ H-CN C OH
CN
H/R
R
Reactivity: HCHO > CH 3CHO > ArCHO > CH 3COCH 3 > CH 3COR > RCOR > ArCOAr
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Nucleophilic Addition
Addition of HCN (using KCN+H +)
C OCN -:
C O -
CN
H/R
R
H/R
HCN
R
C OH
CN
H/R R
Cyanohydrin(2-hydroxynitrile)
Note:HCN is NOTused.Because1. Toxic2. Weak acid
little CN -
slow
fast
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Usefulness in organic synthesis
C OHCN
H/R
R H2O,H+
refluxC OH
COOH
H/R
R
c.H 2SO 4,heat
-C=C-COOH( , -unsaturated acid)
+1 carbon (longer carbonchain)
2 functional group
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Addition of sodium
hydrogensulphate(IV)
C O -
SO 3H
R
RRoom temp. Na +
C OH
SO 3- Na +
R R
Sodiumhydrogensulphate(IV)adduct, isolated ascolourless crystals
C O
R
RNa+ HSO 3-
..
C-S bond is formed as S is more nucleophilic than O
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Addition of sodium
hydrogensulphate(IV)Limit to aliphatic aldehydes and sterically
unhindered ketones (steric effect)
% product from 1mol NaHSO 3 in 1 hour:
C=OCH 3
H89%
C=O
CH 3
CH 3 56%
C=OCH 3
C2H5
36%
=O
35%
C=O
(CH 3)2CH
CH 3 12%
C=O
(CH 3)3C
CH 3 6%
C=O
C2H5
C2H
5
2%
C=OPh
CH 3 1%
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Addition of sodium
hydrogensulphate(IV)Reversible (can be reversed by aq. Alkali oracid by shifting eqm. position to LHS byHSO 3- + H +=> SO 2 , HSO 3- + OH - => SO 32-)
Use to purify liquid or gaseous carbonyl
compounds which are difficult to purifyby direct recrystallization.
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Addition-elimination
(condensation)
C O
R
RN:HO
H
H(Hydroxylamine)
HO N+
C O-
H
H
R
R
HO N C OH
H R
R
-H 2OHO N C
R
R(Oxime)
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Addition-elimination
C O
R
RNH-NH
2
....
NO 2
NO 2
2,4-dinitrophenylhydrazine(Bradys reagent)
NH-N=C....
NO 2
NO 2
2,4-dinitrophenylhydrazone(yellow or light orange crystals)
R
R
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Phenylhydrazone
Products have sharp and characteristicmelting point.
Used as the identification of the originalaldehyde and ketone
Note:
1. NH 3 does not react2. Predict the product obtained byadding H 2N-NH 2 to propanal.
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Oxidation
1. KMnO 4 /H+
, K 2Cr 2O7 /H+
(Strong oxidizing agent)
RCHO => RCOOHRCH 2COCH 2R => RCOOH + R CH 2COOH +
RCH 2COOH + R COOHC6H5CHO => C 6H5COOH requiring reflux for hours
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Oxidation2. Tollens reagent (silver mirror test)
Reagent: 2Ag + + 2OH - => Ag 2O + H 2OAg 2O + 4NH 3 + H 2O => 2Ag(NH 3)2OH
2[Ag(NH 3)2]+ + RCHO + 3OH -
=> RCOO-
+2H 2O + 4NH 3 + 2Ag (mirror)
No reaction with ketone
(Tollens reagent is a mild O.A.)
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Oxidation
3. Fehlings reagent
Reagent: alkaline solution of copper(II) tartrate
RCHO + 2Cu 2+ + 5OH - => RCOO - + 3H 2O + Cu 2O(Fehling) (brick-red)
Note: No reaction with Ketones and Aromatic Aldehydes
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Reduction
Reducing agent: LiAlH 4 Lithium TetrahydridoaluminateNaBH 4 Sodium Tetrahydridoborate
Both equivalent to a source of hydride ion, H-
.
RC O
H/RH -
R O - C
H/R H
R OHC
H/R H
H+
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Reduction
LiAlH 4 must be kept dryi.e. in solution of dry ether
LiBH 4 is less powerful,can be used in aqueous solution.
Reducing agent: H2 /Ni, similar to alkene
RC O
H/R
H2 /NiRCH 2OH
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Triiodomethane reaction
(RCOCH 3 + I 2 /OH - => RCOO - + CHI 3 ,yellow ppt.)
CH 3 C O
H/R
OH -
-H 2O
-CH 2 C O
H/R
X2 CH 2XC O
H/R-X -
CX 3 C O
H/R
CX 3
HO C O -
H/RCX 3- + R/HCOOH CHX 3 + RCOO -
X2
-X -
X2
-X -
OH -