Download study notes on ALDEHYDES & KETONES. and more Study notes Organic Chemistry in PDF only on Docsity!
Introduction
Carbonyl compounds are of two types, aldehydes and ketones. Both have a carbon-oxygen double bond often called as carbonyl group.
Both aldehyde and ketones possess the same general formula (^) C (^) n H 2 nO.
Structure : Carbonyl carbon atom is joined to three atoms by sigma bonds. Since these bonds utilise sp^2 - orbitals, they lie in the same plane and are 120° apart. The carbon-oxygen double bond is different than carbon-carbon double bond. Since, oxygen is more electronegative, the electrons of the bond are attracted towards oxygen. Consequently, oxygen attains a partial negative charge and carbon a partial positive charge making the bond polar. The high values of dipole
moment,
C O (2.3 – 2.8D) cannot be explained only on the basis of inductive effect and thus, it is proposed that carbonyl group is a resonance hybrid of the following two structures. C O C O
Preparation of carbonyl compounds
(1) From alcohols (i) By oxidation.
Ketone
|| agents
Mildoxidising Secondaryalcohol
| ' '
O R C R
OH R CH R
Aldehy de
|| agents
Mildoxidising Primary^2 alcohol
O R CH OH R C H Mild oxidising agents are (a) X 2 (Halogen) (b) Fenton reagent ( FeSO (^) 4 H 2 O 2 ) (c)
K (^) 2 Cr 2 O 7 / H (d) Jones reagent (e) Sarret reagent (f) MnO 2 (g) Aluminium tertiary butoxide [ Al ( O C ( CH 3 ) 3 ) 3 ] When the secondary alcohols can be oxidised to ketones by aluminium tert-butoxide, [( CH (^) 3 ) 3 CO ] 3 Al the reaction is known as oppenauer oxidation. Unsaturated secondary alcohols can also be oxidised to unsaturated ketones (without affecting double bond) by this reagent. The yield of aldehydes is usually low by this methods. The allylic alcohols can be converted to aldehydes by treating with oxidising agent pyridinium chloro-chromate ( C 5 H 5 NH ^ CrO 3 Cl^ ). It is abbreviated as PCC and is called Collin's reagent. This reagent is used in non-aqueous solvents like (^) CH 2 Cl 2 (dichloro methane). It is prepared by mixing pyridine, CrO 3 and HCl in dichloromethane. This is a very good reagent because it checks the further oxidation of aldehydes to carboxylic
C
O
Carbonyl group
Aldehydes and Ketones
Chapter
120° C
120°
120°
- bond - bond
O
acids and is suitable method for preparing , - unsaturated aldehydes. (ii) Dehydrogenation of 1° and 2° alcohols by Cu /300° or Ag /300° C.
2 / 300 || 2 H
O
R CHOH Cu^ C R C H
2
| / 300 || ' ' H
O R C R
OH R CH R Cu^ C (2) From carboxylic acids (i) Distillation of Ca, Ba, Sr or Th salts of monobasic acids
3
|| ( ) 2 ( ' ) 2 2 ' 2 CaCO
O RCOO Ca RCOO Ca R C R Thus in the product, one alkyl group comes from one carboxylic acid and other alkyl group from other carboxylic acid. Calcium salts of dibasic acid (1, 4 and higher) on distillation give cyclic ketones.
Distillation (^2) ||
|| | 2 Ca O O
CH C
O
O
CH C
25 ^ Distillation
|| ( CH ) COOCa
O O C
(ii) Decarboxylation or Dehydration of acids by MnO/ 300 °C. (a) This reaction takes place between two molecules of carboxylic acids. Both may be the same or different.
(b) If one of the carboxylic acids is HCOOH then this acid undergoes decarboxylation because this acid is the only monobasic acid which undergoes decarboxylation even in the absence of catalyst. Case I : When both molecules are HCOOH
formaldehyde
|| formicacid^3002
|| H
O OH HCOOH CO HOH H C
O H C MnO (^) C^
Case II : When only one molecule is formic acid.
H CO HOH
O OH H COOH R C
O R C MnO ^ C Aldehy de 2
/ 300 || Carboxy licacid formicacid
||
Case III : When none of the molecule is formic acid.
R CO HOH
O OH RCOOH R C
O R C MnO ^ C Ketone 2 / 300 || Carboxy licacid
||
(3) From gem dihalides : Gem dihalides on hydrolysis give carbonyl compounds (i) / Aldehy de Gemdihalid e^2
R CHX HOH ^ O H R CHO
(ii) ' '
| / || | R
O R R C
X
X
R C HOH ^ O H
This method is not used much since aldehydes are affected by alkali and dihalides are usually prepared from the carbonyl compounds. (4) From alkenes (i) Ozonolysis : Alkenes on reductive ozonolysis give carbonyl compounds R CH Alkene CH R (ii) H (i) OO ^ / Zn R CHO RCHO 2
3
' ' '
' (^) (i) || || (ii) / Alkene
3 2^ R
O R R C
O R C R
R C C R
R (^) O H (^) O Zn
This method is used only for aliphatic carbonyl compounds. (ii) Oxo process R CH CH CO H R CH CH CHO COCO (^) C atm 2 2 ( ) (^22150) , 300
2 8
Oxo process is used only for the preparation of aldehydes. (iii) Wacker process (a) CH CH PdCl air (^) / Cu^ / HOHCl CH 3 CHO Ethene^2
2
(b) 3
|| air
/ Alky lethene 2 2 2
(^2) CH
O R CH CH PdCl (^) /Cu^ HOHCl R C
(5) From alkynes
(6) From Grignard reagents
O+CaCO 3 Cyclopropanone
Cyclohexanone
O
R – C C – H
H 2 O / HgSO 4 / H 2 SO 4
O
R – C – CH 3
(i) SiO 2 BH 3 (ii) H 2 O 2 / OH
R – CH 2 –
CHO
O R' – C – Cl
O
R' – C –
R
(Only ketone)
HCOOC 2 H 5
O
H – C – R (Aldehyde) O
R – C – R'
R' COOC 2 H (^5) (Ketone)
R – MgX O (Excess)
(4) Gattermann formylation : This reaction is mainly given by alkyl benzenes, phenols and phenolic ethers.
(5) Houben – Hoesch reaction : This reaction is given by di and polyhydric benzenes.
(6) Reimer – Tiemann reaction : Phenol gives o- and p- hydroxy benzaldehyde in this reaction.
Physical properties of carbonyl compounds
(1) Physical state : Methanal is a pungent smell gas. Ethanal is a volatile liquid, boiling points 294 K. Other aldehydes and ketones containing up to eleven carbon atoms are colourless liquids while higher members are solids. (2) Smell : With the exception of lower aldehydes which have unpleasant odours, aldehydes and ketones have generally pleasant smell. As the size of the molecule increases, the odour becomes less pungent and more fragrant. In fact, many naturally occurring aldehydes and ketones have been used in blending of perfumes and flavouring agents. (3) Solubility : Aldehydes and ketones upto four carbon atoms are miscible with water. This is due to the presence of hydrogen bonding between the polar carbonyl group and water molecules as shown below :
With the increase in the size of alkyl group, the solubility decreases and the compounds with more than four carbon atom are practically insoluble in water. All aldehydes and ketones are, however, soluble in organic solvents such as ether, alcohol, etc. The ketones are good solvents themselves. (4) Boiling points : The boiling points of aldehydes and ketones are higher than those of non polar compounds (hydrocarbons) or weakly polar compounds (such as ethers) of comparable molecular masses. However, their boiling points are lower than those of corresponding alcohols or carboxylic acids. This is because aldehydes and ketones are polar compounds having sufficient intermolecular dipole- dipole interactions between the opposite ends of C O dipoles.
^ C O C O C O However, these dipole-dipole interactions are weaker than the intermolecular hydrogen bonding in alcohols and carboxylic acids. Therefore, boiling points
– +^ –^ +^ O +^ – + C O H H O = C
CH 3 CH 3
CHO
CH 3
CHO
(i) Zn ( CN ) 2 / HCl gas (ii) H 2 O/ Toluene o- methyl benzaldehyd e
p- methyl benzaldehyd OH OH^ e CHO
OH
CHO
(i) gas(ii) Zn H ( 2 CNO/ ) 2 / HCl +
Phenol o - salicylaldehyd e p - salicylaldehyd OCH (^3) OCH (^3) e CHO
OCH 3
CHO
(i) Zn ( CN ) 2 / HCl gas(ii) H 2 O/
Anisol o - methoxy benzaldehyde p - methoxy benzaldehyde
OH
(i) RCN / HCl gas(ii) / H Anhy. 2 O ZnCl 2 OH
OH
OH
Resorcino COR l (^) 2,4-dihydroxy OH^ ketone
(i) RCN / HCl gas(ii) / H Anhy. 2 O ZnCl 2 OH
OH
OH
COR
HO HO
Phloroglucinol (^) 2,4,6-trihydroxy ketone
OH OH
CHO
OH
CHO
(i) CHCl 3 /Alc.KOH/ (ii) H 2 O/H + phenol (^) (major) (Minor)
of aldehydes and ketones are relatively lower than the alcohols and carboxylic acids of comparable molecular masses. Among the carbonyl compounds, ketones have slightly higher boiling points than the isomeric aldehydes. This is due to the presence of two electrons releasing groups around the carbonyl carbon, which makes them more polar.
b.pt.2.52 322 DK
Acetaldehyde
3 :
C O
H
CH
b.pt2.88 329 DK
(^3) Acetone
3 :
..
C O CH
CH
(5) Density : Density of aldehydes and ketones is less than that of water.
Chemical properties of carbonyl compounds
Carbonyl compounds give chemical reactions due to carbonyl group and -hydrogens. Chemical reactions of carbonyl compounds can be classified into following categories. (1) Nucleophilic addition reactions (2) Addition followed by elimination reactions (3) Oxidation (4) Reduction (5) Reactions due to -hydrogen (6) Condensation reactions and (7) Miscellaneous reactions (1) Nucleophilic addition reactions (i) Carbonyl compounds give nucleophilic addition reaction with those reagents which on dissociation give electrophile as well as nucleophile. (ii) If nucleophile is weak then addition reaction is carried out in the presence of acid as catalyst. (iii) Product of addition reactions can be written as follows,
Adduct
| |
|| ' Addition '
OH
Nu
R H Nu R C R
O
R C
In addition reactions nucleophile adds on carbonyl carbon and electrophile on carbonyl oxygen to give adduct. (iv) Relative reactivity of aldehydes and ketones : Aldehydes and ketones readily undergo nucleophilic addition reactions. However, ketones are less reactive than aldehydes. This is due to electronic and stearic effects as explained below: (a) Inductive effect : The relative reactivities of aldehydes and ketones in nucleophilic addition reactions may be attributed to the amount of positive
charge on the carbon. A greater positive charge means a higher reactivity. If the positive charge is dispersed throughout the molecule, the carbonyl compound becomes more stable and its reactivity decreases. Now, alkyl group is an electron releasing group (+I inductive effect). Therefore, electron releasing power of two alkyl groups in ketones is more than that of one alkyl group in aldehyde. As a result, the electron deficiency of carbon atom in the carbonyl group is satisfied more in ketones than in aldehydes. Therefore, the reduced positive charge on carbon in case of ketones discourages the attack of nucleophiles. Hence ketones are less reactive than aldehydes. Formaldehyde with no alkyl groups is the most reactive of the aldehydes and ketones. Thus, the order of reactivity is:
Formaldehyde
C O H
H > Aldehy de
C O H
R > Ketone
C O R
R
(b) Stearic effect : The size of the alkyl group is more than that of hydrogen. In aldehydes, there is one alkyl group but in ketones, there are two alkyl groups attached to the carbonyl group. The alkyl groups are larger than a hydrogen atom and these cause hindrance to the attacking group. This is called stearic hindrance. As the number and size of the alkyl groups increase, the hindrance to the attack of nucleophile also increases and the reactivity of a carbonyl decreases. The lack of hindrance in nucleophilic attack is another reason for the greater reactivity of formaldehyde. Thus, the reactivity follows the order:
Form aldehyde
C O H
H > Acetaldehyde
3 C O H
CH > (^3) Acetone
3 C O CH
CH >
Di^3 - isopropy l^2 ketone
32
( )
( ) C O CH CH
CH CH > Di^3 - tert.^3 buty lketone
33
( )
( ) C O CH C
CH C
In general, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic analogues. For example, benzaldehyde is less reactive than aliphatic aldehydes. This can be easily understood from the resonating structures of benzaldehyde as shown below:
C
H O. .:
C
H O. .:
C
H O. .:
I II III
CH CH O H
O
CH C 3 3
|| 3 Hemiketal^3
3
| (^3) |
OH
OCH
CH C CH
Hemiacetals and hemiketals are - alkoxy alcohols. Case II : Addition catalysed by acid : In the presence of an acid one equivalent of carbonyl compound reacts with two equivalents of alcohol. Product of the reaction is acetal (in case of aldehyde) or ketal (in case of ketone).
H CHOH
O
R C 3
|| 2 Acetal
2
3
3
| | HO
OCH
OCH
R C H
R CHOH
O
R C 3
|| 2
Ketal
2
3
3
| | HO
OCH
OCH
R C R
(i) Formation of acetals and ketals can be shown as follows:
3
3
H O CH
H O CH
C O
R
R
HO
OCH
OCH
C
R
R
2 3
3
(ii) Acetals and ketals are gem dialkoxy compounds. (iii) High yield of acetals or ketals are obtained if the water eliminated from the reaction is removed as it formed because the reaction is reversible. (iv) Acetals and ketals can be transformed back to corresponding aldehyde or ketone in the presence of excess of water.
CHOH
O
HO R C R
OCH
OCH
R C R H 3
|| (Excess)^2 Ketal
3
3
| | 2
This reaction is very useful reaction for the protection of carbonyl group which can be deprotected by hydrolysis. Glycol is used for this purpose. Suppose we want to carry out the given conversion by LiAlH 4.
2 2 5 ^4
|| 3 CH COOC^ H^ LiAlH
O
CH C
CH CHOH
O CH C 2 2
|| 3 This can be achieved by protection of C O group and then by deprotection
Addition of Grignard reagents : Grignard reagents react with carbonyl compounds to give alcohols. Nature of alcohol depends on the nature of carbonyl compound.
Addition of water : Carbonyl compounds react with water to give gem diols. This reaction is catalysed by acid. The reaction is reversible reaction.
Ketone
|| R ' HOH
O R C
Gemdiol
| | R '
OH
OH
R C
Gem diols are highly unstable compounds hence equilibrium favours the backward direction. The extent to which an aldehyde or ketone is hydrated depends on the stability of gem diol. Stability of gem diols depend on the following factors: (i) Steric hindrance by + I group around - carbon decreases the stability of gem diols. + I group decreases stability of gem diol and hence decreases extent of hydration. (ii) Stability of gem diols mainly depends on the presence of – I group on - carbon. More is the – I power of the group more will be stability of gem diols. (iii) Intramolecular hydrogen bonding increases stability of gem diols. – I groups present on carbon having gem diol group increases strength of hydrogen bond. More is the strength of hydrogen bond more will be the stability of gem diol. Addition of terminal alkynes : This reaction is known as ethinylation.
" '
'
| |
|| Sod.saltofalky ne
R
ONa
R
R R C C C
O R C CNa R C
HO
H^
H^
O (i) H – C – H (ii) HOH /H
R^ –^ CH^2 OH^ 1°-alcohol O (i) R' – C – H (ii) HOH /H
OH
R' – CH – R 2°-
alcohol OH | R' – C – R' | R
3°-alcohol
O (i) R' – C – R' (ii) HOH /H
RMgX Grignard reagent
R – C – R
O
alky nol
| |
/ (^) " '
R
OH
R
HOH ^ H R C C C
(2) Addition followed by elimination reactions : This reaction is given by ammonia derivatives ( NH (^) 2 Z ). (i) In nucleophilic addition reactions poor nucleophile such as ammonia and ammonia derivatives requires acid as catalyst. (ii) If the attacking atom of the nucleophile has a lone pair of electrons in the addition product, water will be eliminated from the addition product. This is called a nucleophilic addition elimination. Primary amines and derivatives of ammonia react with carbonyl compounds to give adduct. In adduct nucleophilic group has lone pair of electrons. It undergoes elimination to give product known as imine. An imine is a compound with a carbon-nitrogen double bond.
(^) H H NH Z
O R C R
|| .^.
OH
NHZ
R C R
..
| | C An imine N Z R
R
^ HOH ^
The overall reaction can be shown as follows
Animine
2 2
C N R
R
R
C O NH Z HO
R
R
^ H^ ^
Different Imine formation with (^) NH (^) 2 Z is given below
Beckmann rearrangement : Ketoxime when treated with acid at 0° C it undergoes rearrangement known as Beckmann rearrangement. Thus acid catalysed conversion of ketoximes to N - substituted amides is called Beckmann rearrangement. Acid catalyst used are proton acids( H 2 SO 4 , HCl , H 3 PO 4 ) and Lewis acids ( PCl 5 , SOCl 2 , PhSO 2 Cl , RCOCl , SO 3 , BF 3 etc.) O CH C NH CH NOH
C H C CH PClHO N- pheny lacet amide^65
|| (ii)^3
(i)
Acetophenoxime
6 5 || 3 ^2 ^5
O
CH C NH CH
N OH
CH C CH PClHO N- methy l acetamide^3
|| (^3) || 6 5 (ii)(i)^2 ^5 6 5
In short product of the rearrangement can be obtained as follows:
O
R C NH R
R N
R C O H
Tautomerisation || ' || '
(3) Oxidation of carbonyl compounds (i) Oxidation by mild oxidising agents : Mild oxidising agents oxidise only aldehydes into carboxylic acids. They do not oxidises ketones. Main oxidising agents are: (a) Fehling solution : It is a mixture of two Fehling solution: Fehling solution No. 1 : It contains CuSO 4 solution and^ NaOH. Fehling solution No. 2 : It contains sodium potassium tartrate. (Roschelle salt). (b) Benedict's solution : This solution contains CuSO (^) 4 , NaOH^ and sodium or potassium citrate. Reacting species of both solutions is^ Cu oxidation no. of Cu varies from 2 to 1. These two oxidising agents oxidise only aliphatic aldehydes and have no effect on any other functional groups Benedict's solution and Fehling solutions are used as a reagent for the test of sugar (glucose) in blood sample.
N OH
C
R R
||
(c) Oxidation by organic peracids : Organic peracids oxidise aldehydes into carboxylic acids and ketones into esters. This oxidation is known as Baeyer
- Villiger oxidation. O R C O R
O R C R C^ HCOOOH
|| 6 5 || In case of aldehyde there is insertion of atomic oxygen (obtained from peracid) between carbonyl carbon and hydrogen of carbonyl carbon. In case of ketone, insertion of oxygen takes place between carbonyl carbon and - carbon. Thus the product is ester. This is one of the most important reaction for the conversion of ketones into esters. Vic dicarbonyl compound also undergo oxidation and product is anhydride.
R O
O C
O
R R C
O
C
O
R C C^ H COOOH || ||
|| || 6 5
Popoff's rule : Oxidation of unsymmetrical ketones largely take place in such a way that the smaller alkyl group remains attached to the CO group during the formation of two molecules of acids. This is known as Popoff's rule Example : CH (^) 3 CO CH 2 CH 3 [ O ]^ CH 3 COOH HOOCCH 3 (d) Baeyer- villiger oxidation :
O
H H C OH
O
O C
H
O
O
H C H || ||
| ||
OH
O
H CH C
O
O C
H
H O
O
CH C || 3 ||
| 3 ||
Reaction will be held if the oxidising agent is performic acid. (4) Reduction of carbonyl compounds
(i) Reduction of group into – CH 2 – group : Following three reagents reduce carbonyl group into CH 2 ^ groups: (a)^ HI /^ P / (b)^ Zn / Hg / Conc. HCl and
(c)
NH (^) 2 NH 2 / OH.
(ii) Reduction of carbonyl compounds into hydroxy compounds : Carbonyl group converts into CHOH group by LiAlH (^) 4 , NaBH 4 , Na / C 2 H 5 OH and aluminium isopropoxide.
R CHO NaBH R CH 2 OH (iii)Aluminiumisopropoxide (ii)
(i)LiAlH 4
^4
OH R CH R
O
R C R ' LiAlHNaBH^ '
| (iii)Aluminiumisopropoxide (ii)
|| (i) 4
^4
NaBH 4 is regioselective reducing agent because it reduced only. CHO in the presence of other reducible group. Example : (^3) Crotonaldehyde (^3) Crotonyl alcohol 2 CH CH CH CHO NaBH ^^4 CH CH CH CHOH
Hydride ion of NaBH 4 attack on carbonyl carbon during reduction. Example :
2 3
| 2 - Butanone^33 |
|| 3 24 CH CH
OD
D
CH CH C
O
CH C NaBDD (^) O ^
2 Butanone^3
|| 3 CH
O CH C
(iii) Reductive amination : In this reduction CO group converts into CH NH 2 group
Primaryamine
/ 2
Imine
3 2 CH NH R
R C NH R
R C O NH R
R H^ Ni
(iv) Reduction of ketones by Mg or Mg/Hg : In this case ketones undergo reduction via coupling reaction and product is vic cis diol.
Vic diol(pinacol)
| |
| (ii) |
|| (i) g/ ||
|| || cis
HOH
M Hg
OH
R
C R
OH
R
R R C
O
R
C
O
R
R C
When this reaction is carried out in the presence of Mg / Hg / TiCl 4 , the product is vic trans diol.
(v) Reduction of benzaldehyde by Na/C 2 H 5 OH : Benzaldehyde undergoes reduction via coupling reaction and product is vic diol.
O
– C –
OH
CH 3 – C – CH 2 –
CH 3
OD^ D
CH 3 – C – CH 2 –
CH 3
H
NaBD 4 H 2 O
NaBH 4 D 2 O
HI/P/
Zn/Hg/Conc. HCl
NH 2 – NH 2 /OH
R – CH 2 – R'
R – CH 2 – R'
R – CH 2 – R'
O
R – C –
R'
(Clemmenso n reduction)
(Wolff-kishner reduction)
(i) Hg – Mg – TiCl (ii) 4 HOH OH
HO
Vic trans diol
2 O
Cyclohexanone
HOH C H Na/CHOH
O
H
C
O
H
C H C 6 5 (i)(ii)
|| |
|| (^6 5) |
^2 ^5
vicdiol
OH
CH CH
OH
C H CH 6 5
| | 6 5 ^ (Bouveault-blanc reaction)
Aldehydes are reduced to 1° alcohols whereas ketones to 2° alcohols. If carbon – carbon double bond is also present in the carbonyl compound, it is also reduced alongwith. However, the use of the reagent 9-BBN (9– borabicyclo (3, 3, 1) nonane) prevents this and thus only the carbonyl group is reduced Example : CH CH CHO ^9 ^ BBN HOCH ^2 CH ^2 NH ^2
CH CHCH 2 OH
If reducing agent is NaH, reaction is called Darzen's reaction, we can also use LiAlH 4 in this reaction. If reducing agent is aluminium iso propoxide
3
CH
CH CH O Al. Product will be alcohol. This
reaction is called Meerwein – pondorff verley reduction (MPV reduction). The percentage yield of alkanes can be increased by using diethylene glycol in Wolf Kishner reduction. Then reaction is called Huang – Millan conversion. (vi) Hydrazones when treated with base like alkoxide give hydrocarbon (Wolf – Kishner reduction).
R CH R
NNH
R C R
O
R C R NH^ NH Hydrazone RONa 2
|| ||^.^2
(vii) Schiff's base on reduction gives secondary amines.
Secondary^2 amine
/ Schiff'sbase
' Aldehyde R CH O R^ NH ^2 R CH NR ' H^2 Ni R CHNHR
(5) Reactions due to - hydrogen (i) Acidity of - hydrogens : (a) - hydrogen of carbonyl compounds are acidic in character due to the presence of the electron withdrawing CO group.
(b) Thus carbonyl compounds having - hydrogen convert into carbanions in the presence of base. This
carbanion is stabilised by delocalisation of negative charge. O CH C R
|| 3 (moreEnolate stable)ion
| 2 (lessCarbanionstable)
|| 2 R
O CH C
O CH C R
(c) The acidity of - hydrogen is more than ethyne. pKa value of aldehydes and ketones are generally 19 – 20 where as pKa value of ethyne is 25. (d) Compounds having active methylene or methyne group are even more acidic than simple aldehydes and ketones.
3
|| 6 5 2
CH
O
C H CH C pKa 15. 9
3
|| 2
|| 6 5
CH
O
CH C
O
C H C pKa 8. 5
(ii) Halogenation : Carbonyl compounds having - hydrogens undergo halogenation reactions. This reaction is catalysed by acid as well as base. (a) Acid catalysed halogenation : This gives only monohalo derivative.
bromo acetone^2
|| Acetone 3 /^3
|| 3 ^2 ^ ^3 ^ CH^ Br
O
CH C
O
CH C CH Br CHCOOH (b) Base catalysed halogenation : In the presence of base all - hydrogens of the same carbon is replaced by halogens.
Carbonyl compounds having three - hydrogens give haloform reaction.
3 3 / || 3
|| (^2) CX RCOO CHX
O
CH R C
O
R C X^ OH OH
(iii) Deuterium exchange reaction : Deuterium exchange reaction is catalysed by acid ( D )as well as base( )
(^) OD. In both the cases all the hydrogens on only one - carbon is replaced by D.
CD R
O
CH R R C
O
R C D^ O OD 2
/ || 2
|| 2 ;
CD R
O
CH R R C
O
R C D^ O D
2 / || 2
|| (^2)
- Hydrogen is acidic due to strong – I group; – CO H O^ –. | ||
Base
2 3
|| 3 2 CH CH
O
CH CH C
|^3
| | (^3) | CH X
CH
O C X
CH (^) CH 3
| |
||
3 2 CH
X
X
C
O
CH CH C
X 2 /^ – OH (^) Excess
Cinnamaldehy de Cinnamyl alcohol
Mechanism : 6 5 3 /
CH CHO CH CHO^ OH C (^) 6 H 5 CH CH CHO HOH
Step I : HO H CH 2 CHO
H
O H CH C
O HOH CH C
| 2
|| 2
Step II : CH CHO
O
H
C H C
2
|| (^65) |
CH CHO
O
H
C H C 2
| (^65) |
OH
OH
H
HOH ^ CH C CH 2 CHO
| (^65) |
Step III :
CH CH CH CHO HOH
OH
CHO
H
CH
OH
C H CH | 6 5
| 6 5
In aldol condensation, dehydration occurs readily because the double bond that forms is conjugated, both with the carbonyl group and with the benzene ring. The conjugation system is thereby extended.
Crossed aldol condensation : Aldol condensation between two different aldehydes or two different ketones or one aldehyde and another ketone provided at least one of the components have -hydrogen atom gives different possible product
(a) CH CHO CH CH CHO dil NaOH Ethanal^33 Propanal^2
CHO CH CH CHOH CH CHO
CH CH
OH CH CH 3 2 2
| |^3 3 However crossed aldol condensation is important when only it the components has -hydrogen atom.
(^2) (Acrolein) (3-hy droxypropanal)
(^2 3) | 2 CH 2 CHO 2 CH CH^ CHO OH
CH O CHCHO CH H O
Intra molecular aldol condensation : One molecule Intramolecular condensation give aldol compounds Example :
CHO
OH
O CH ( CH 2 ) 5 CHO di. NaOH
(ii) Claisen – Schmidt reaction : Crossed aldol condensation between aromatic aldehyde and aliphatic ketone or mixed ketone is known as Claisen – Schmidt reaction. Claisen – Schmidt reactions are useful when bases such as sodium hydroxide are used because under these conditions ketones do not undergo self condensation. Some examples of this reaction are :
4 Pheny l 3 buten- 2 - one^3
|| 3 100 6 5
|| 6 5 ^3 ^ CH
O CH CH CH CH C
O C HCHO CH C
OH C
Test of aldehydes and Ketones (Distinction)
Table : 27. Test Aldehydes Ketones With Schiff's reagent
Give pink colour. No colour.
With Fehling's solution
Give red precipitate.
No precipitate is formed. With Tollen's reagent
Black precipitate or silver mirror is formed.
No black precipitate or silver mirror is formed. With saturated sodium bisulphite solution in water
Crystalline compound (colourless) is formed.
Crystalline compound (colourless) is formed. With 2, 4- dinitrophenyl hydrazine
Orange-yellow or red well defined crystals with melting points characteristic of individual aldehydes.
Orange-yellow or red well defined crystals with melting points characteristic of individual ketones. With sodium hydroxide
Give brown resinous mass (formaldehyde does not give this test).
No reaction.
With sodium nitroprusside and few drops of sodium hydroxide
A deep red colour (formaldehyde does not respond to this test).
Red colour which changes to orange.
Some commercially important aliphatic
carbonyl compounds
Formaldehyde : Formaldehyde is the first member of the aldehyde series. It is present in green leaves of plants where its presence is supposed to be due to the reaction of CO 2 with water in presence of sunlight and chlorophyll. (1) Preparation (i) 2 CH (^) 3 OH O 2 Platinised 300 400 asbestos C Formaldehy HCHO de
CH (^) 3 OH O KHCrSOO HCHO H 2 O 2 4
[ ]^2 2 ^7
(ii) CH (^) 3 OH 300 Cu 400 or Ag C Formaldehy HCHO de (iii) Heat Formaldehyde Calcium formate^2 Ca ( HCOO ) HCHO
(iv) CH (^) 2 CH 2 O 3 PdH ^2 Formaldehy HCHO de
(v) 2 Mo-oxide Formaldehyde Methane^4
CH O Catalyst HCHO
(vi) CO H 2 Elec. dischargeFormaldehy HCHO de (2) Physical properties (i) It is a colourless, pungent smelling gas. (ii) It is extremely soluble in water. Its solubility in water may be due to hydrogen bonding between water molecules and its hydrate. (iii) It can easily be condensed into liquid. The liquid formaldehyde boils at – 21° C. (iv) It causes irritation to skin, eyes, nose and throat. (v) Its solution acts as antiseptic and disinfectant. (3) Uses (i) The 40% solution of formaldehyde (formalin) is used as disinfectant, germicide and antiseptic. It is used for the preservation of biological specimens.
(ii) It is used in the preparation of hexamethylene tetramine (urotropine) which is used as an antiseptic and germicide.
(iii) It is used in silvering of mirror. (iv) It is employed in manufacture of synthetic dyes such as para-rosaniline, indigo, etc.
(v) It is used in the manufacture of formamint (by mixing formaldehyde with lactose) – a throat lozenges.
(vi) It is used for making synthetic plastics like bakelite, urea-formaldehyde resin, etc.
(vii) Rongalite – a product obtained by reducing formaldehyde sodium bisulphite derivative with zinc dust and ammonia and is used as a reducing agent in vat dyeing.
Acetaldehyde Acetaldehyde is the second member of the aldehyde series. It occurs in certain fruits. It was first prepared by Scheele in 1774 by oxidation of ethyl alcohol. (1) Preparation : It may be prepared by any of the general methods. The summary of the methods is given below (i) By oxidation of ethyl alcohol with acidified potassium dichromate or with air in presence of a catalyst like silver at 300° C. (ii) By dehydrogenation of ethyl alcohol. The vapours of ethyl alcohol are passed over copper at 300° C. (iii) By heating the mixture of calcium acetate and calcium formate. (iv) By heating ethylidene chloride with caustic soda or caustic potash solution. (v) By the reduction of acetyl chloride with hydrogen in presence of a catalyst palladium suspended in barium sulphate (Rosenmund's reaction). (vi) By the reduction of (^) CH (^) 3 CN with stannous chloride and HCl in ether and hydrolysis (Stephen's method). (vii) By hydration of acetylene with dil. H 2 SO 4 and (^) HgSO 4 at 60° C.
(viii) By ozonolysis of butene-2 and subsequent breaking of ozonide. (ix) Laboratory preparation : Acetaldehyde is prepared in the laboratory by oxidation of ethyl alcohol with acidified potassium dichromate or acidified sodium dichromate. K 2 (^) Cr 2 O 7 4 H 2 SO 4 K 2 SO 4 Cr 2 ( SO 4 ) 3 4 H 2 O 3 [ O ] [ CH 3 CH 2 OH O CH 3 CHO H 2 O ] 3 dichromatePotassium^2 27 Ethyl^3 alcohol^2 Sulphuric^2 acid^4
KCrO 3 CHCHOH 4 H SO
sulphatePotassium^2 42 sulphateChromic^4 3 Acetaldehy^3 de Water^2
K SO Cr ( SO ) 3 CHCHO 7 HO
To recover acetaldehyde, the distillate is treated with dry ammonia when crystallised product, acetaldehyde ammonia, is formed. It is filtered and washed with dry ether. The dried crystals are then
3
(^3) | CH
CH CH OH
- With hydroxylamine NH (^) 2 OH Forms formaldoxime CH (^) 2 O H 2 NOH ^ H 2 O
CH (^) 2 NOH
Forms acetaldoxime CH (^) 3 CH O H 2 NOH ^ H 2 O
CH (^) 3 CH NOH
- With hydrazine ( NH 2 NH 2 ) Forms formaldehyde hydrazone CH (^) 2 O H 2 N NH 2 ^ H ^2 O
CH (^) 2 NNH 2
Forms acetaldehyde hydrazone CH (^) 3 CH O H 2 NNH 2 ^ H 2 O
CH (^) 3 CH NNH 2
- With phenyl hydrazine ( C 6 H 5 NHNH 2 )
Forms formaldehyde phenyl hydrazone CH (^) 2 O H 2 NNHC 6 H 5 ^ H ^2 O
CH (^) 2 NNHC 6 H 5
Forms acetaldehyde phenyl hydrazone CH (^) 3 CH O H 2 NNHC 6 H 5 H^ 2 ^ O CH 3 CH NNHC 6 H 5
- With semicarbazide ( H 2 NNHCONH 2 )
Forms formaldehyde semicarbazone CH (^) 2 O H 2 NNHCONH 2 ^ H 2 O
CH (^) 2 NNHCONH 2
Forms acetaldehyde semicarbazone CH (^) 3 CH O H 2 NNHCONH 2 (^2) CH 3 CH NNHCONH 2 H^ O
- With alcohol ( C 2 H 5 OH )in presence of acid
Forms ethylal H (^) 2 C O 2 C 2 H 5 OH HCl
2 5
2 5 2 OC H
OCH
CH
Forms acetaldehyde diethyl acetal CH (^) 3 CHO 2 C 2 H 5 OH HCl
2 5
2 5 3 OC H
OCH
CHCH
- With thioalcohols ( C 2 H 5 SH )in presence of acid
Forms thio ethylal H (^) 2 C O 2 C 2 H 5 SH
2 5
2 5 2 SC H
SCH
CH
Forms acetaldehyde diethyl thioacetal CH (^) 3 CH O 2 C 2 H 5 SH
2 5
2 5 3 SC H
SCH
CHCH
- Oxidation with acidified K 2 Cr 2 O 7 Forms formic acid HCHO O HCOOH
Forms acetic acid CH (^) 3 CHO O CH 3 COOH
- With Schiff's reagent Restores pink colour of Schiff's reagent
Restores pink colour of Schiff's reagent
- With Tollen's reagent Gives black precipitate of Ag or silver mirror Ag (^) 2 O HCHO 2 Ag HCOOH
Gives black precipitate of Ag or silver mirror Ag (^) 2 O CH 3 CHO
2 Ag CH 3 COOH
- With Fehling's solution or Benedict's solution
Gives red precipitate of cuprous oxide 2 CuO HCHO Cu 2 O HCOOH
Gives red precipitate of cuprous oxide 2 CuO CH 3 CHO
Cu (^) 2 O CH 3 COOH
- Polymerisation Undergoes polymerisation Undergoes polymerisation Evaporatio n H^2 SO^4 Conc. dil. H 2 SO 4. Room temp. distill heat (^) H 2 SO 4 Conc. dil. H 2 SO 4. distill
nHCHO Paraformaldehy de
( HCHO ) n
3 HCHO Metaformaldehy de 3
( HCHO )
3 CH 3 CHO
Paraldehy d^3 e 3
( CH CHO )
4 CH 3 CHO
Metaldehy d^3 e 4
( CH CHO )
Dissimilarities
- With PCl 5 No reaction Forms ethylidene chloride
Cl
Cl CH (^) 3 CHO PCl 5 CH 3 CH
POCl 3
- With chlorine No reaction Forms chloral CH (^) 3 CHO 3 Cl 2 CCl 3 CHO
3 HCl
- With SeO 2 No reaction Forms glyoxal CH (^) 3 CHO SeO 2 CHO. CHO
Se H 2 O
- Iodoform reaction ( I 2 + NaOH ) No reaction Forms iodoform CH (^) 3 CHO 3 I 2 4 NaOH CHl (^) 3 HCOONa 3 NaI 3 H 2 O
- With dil. alkali (Aldol condensation)
No reaction Forms aldol CH (^) 3 CHO HCH 2 CHO
CH (^) 3 CH ( OH ) CH 2 CHO
- With conc. NaOH (Cannizzaro's reaction)
Forms sodium formate and methyl alcohol 2 HCHO NaOH HCOONa
CH (^) 3 OH
Forms a brown resinous mass
- With ammonia Forms hexamethylene tetramine (urotropine) 6 HCHO 4 NH 3 ( CH 2 ) 6 N 4 6 H 2 O
Forms addition product, acetaldehyde ammonia CH 3 CHO NH 3
2
3 NH
OH
CHCH
- With phenol Forms bakelite plastic No reaction
- With urea Forms urea-formaldehyde plastic No reaction
- Condensation in presence of Ca ( OH ) 2
Form formose (a mixuture of sugars) No reaction
Inter conversion of formaldehyde and acetaldehyde (1) Ascent of series : Conversion of formaldehyde into acetaldehyde (i) HCHO H^ Ni CHOH PCl CHCl KCN Alc. chlorideMethy l
3 alcoholMethy l
3 / Formaldehyde
2 5
HCl CH CN Na^ CHCHNH NaNO ^2 Ethy l^3 amine 2 2
/Alcohol cy anideMethy l
3
Acetaldehy^3 de
(dil.) Ethyl^3 alcohol 2 2 27
CH CHOH^2 4 CHCHO
KCrO
H^ SO
If acetone would be in excess in ketal condensation or catalyst ( ZnCl 2 /dry HCl ) is used then three moles of acetone undergoes condensation polymerisation and form a compound called ‘ Phorone ’.
CH
H
H
CH
CH C O
|^3 3
|^3 3
CH
CH C CH
CH
H
H
CH
CH C O
|^3 3
Molecular mass of phorone = 3 mole of acetone – 2 mole of H (^) 2 O
Reformatsky reaction: This reaction involves the treatment of aldehyde and ketone with a bromo acid ester in presence of metallic zinc to form -hydroxy ester, which can be easily dehydrated into ,- unsaturated ester.
(a) Organo zinc^2 compound^25 BrCH (^) 2 COOC 2 H 5 Zn Benzene Br Zn CHCOOCH
(b) Addition to carbonyl group
2 2 5
3 2
| (^53) |
| 2 2 3
3 CHCOOC H
CH
OZnBr
CH C CH
ZnBr C O CHCOOCH CH
CH
| 2
|^3 |
/ 3
COOC H
CH
CH
OH
CH C OH ZnBr
HOH ^ H
2 5
|^3 3 CH COOCH
CH CH C (4) Uses (i) As a solvent for cellulose acetate, cellulose nitrate, celluloid, lacquers, resins, etc. (ii) For storing acetylene. (iii) In the manufacture of cordite – a smoke less powder explosive. (iv) In the preparation of chloroform, iodoform, sulphonal and chloretone. (v) As a nailpolish remover. (vi) In the preparation of an artificial scent (ionone), plexiglass (unbreakable glass) and synthetic rubber. (5) Tests (i) Legal's test : When a few drops of freshly prepared sodium nitroprusside and sodium hydroxide solution are added to an aqueous solution of acetone, a wine colour is obtained which changes to yellow on standing. (ii) Indigo test : A small amount of orthonitrobenzaldehyde is added to about 2 ml. of acetone and it is diluted with KOH solution and stirred. A blue colour of indigotin is produced. (iii) Iodoform test : Acetone gives iodoform test with iodine and sodium hydroxide or iodine and ammonium hydroxide.
Table : 27.3 Comparison between Acetaldehyde and Acetone Reaction Acetaldehyde Acetone Similarities
- Reduction with H 2 and Ni or LiAlH 4
Forms ethyl alcohol CH (^) 3 CHO H 2 Ni CH 3 CH 2 OH
Forms isopropyl alcohol CH (^) 3 COCH 3 H 2 CH 3 CHOHCH 3
- Clemmensen's reduction ( Zn / Hg and conc. HCl )
Forms ethane (an alkane) CH (^) 3 CHO 4 H CH 3 CH 3 H 2 O
Forms propane (an alkane) CH (^) 3 COCH 3 4 H CH 3 CH 2 CH 3 H 2 O
- Addition of HCN Forms acetaldehyde cyanohydrin Forms acetone cyanohydrin
CH
CH
CH C
3
(^3) |
C = O dry . ZnClHCl ^ ^2
C O
Acetone ( molecule)
phoron
CN
OH
CH 3 CHO HCN CH 3 CH
CN
OH
( CH 3 ) 2 CO HCN ( CH 3 ) 2 C
- Addition of NaHSO 3 White crystalline derivative
SO Na
OH
CHCHO NaHSO CHCH 3
White crystalline derivative
SONa
OH
CH CO NaHSO CH C 3
- Grignard reagent followed by hydrolysis
Forms isopropyl alcohol CH (^) 3 CHO CH 3 MgI ( CH 3 ) 2 CH OMgI H^2^ ^ O CH 3 CHOHCH 3
Forms tertiary butyl alcohol ( CH (^) 3 ) 2 CO CH 3 MgI ( CH 3 ) 3 COMgI H^2 O ( CH 3 ) 3 COH
- With hydroxylamine ( NH 2 OH )
Forms acetaldoxime (an oxime) CH (^) 3 CHO H 2 NOH CH 3 CH NOH
Forms acetoxime (an oxime) ( CH (^) 3 ) 2 CO H 2 NOH ( CH 3 ) 2 C NOH
- With hydrazine ( NH 2 NH 2 )
Forms acetaldehyde hydrazone CH (^) 3 CHO H 2 NNH 2 CH 3 CH NNH 2
Forms acetone hydrazone ( CH (^) 3 ) 2 CO H 2 NNH 2 ( CH 3 ) 2 C NNH 2
- With phenyl hydrazine( C 6 H 5 NHNH 2 )
Forms acetaldehyde phenylhydrazone CH 3 CHO H 2 NNHC 6 H 5 CH (^) 3 CH NNHC 6 H 5
Forms acetone phenyl hydrazone ( CH 3 ) 2 CO H 2 NNHC 6 H 5 ( CH (^) 3 ) 2 C NNHC 6 H 5
- With semicarbazide ( H 2 NNHCONH 2 )
Forms acetaldehyde semicarbazone CH 3 CHO H 2 NNHCONH 2 CH (^) 3 CH NNHCONH 2
Forms acetone semicarbazone ( CH 3 ) 2 CO H 2 NNHCONH 2 ( CH (^) 3 ) 2 C NNHCONH 2
- With PCl 5 Forms ethylidene chloride (Gem dihalide)
Cl
Cl CH (^) 3 CHO PCl 5 CH 3 CH
Forms isopropylidene chloride (Gem dihalide)
Cl
Cl ( CH (^) 3 ) 2 CO PCl 5 ( CH 3 ) 2 C
- With chlorine Forms chloral (Gem trihalide)
CH (^) 3 CHO Cl 2 CCl 3 CHO
Forms trichloro acetone (Gem trihalide) CH 3 (^) COCH 3 Cl 2 CCl 3 COCH 3
- With alcohols Forms acetal (a diether)
2 5
2 5 3 2 2 5 3 OC H
OCH
CH CHO CHOH CHCH
Forms ketal (a diether)
2 5
2 5 ( 3 ) 2 2 2 5 ( 3 ) 2 OC H
OCH
CH CO CHOH CH C
- With SeO 2 Forms glyoxal
CH (^) 3 CHO SeO 2 CHOCHO Se H 2 O
Forms methyl glyoxal ( CH (^) 3 ) 2 CO SeO 2 CH 3 COCHO Se H 2 O
- Iodoform reaction ( I 2 (^) NaOH )
Forms iodoform Forms iodoform
- Bleaching powder Forms chloroform Forms chloroform
- Aldol condensation with mild alkali
Forms aldol 2 CH (^) 3 CHO CH 3 CHOHCH 2 CHO
Forms diacetone alcohol 2 CH (^) 3 COCH 3 ( CH 3 ) 2 C ( OH ) CH 2 COCH 3
- Polymerisation Undergoes polymerisation Does not undergo polymerisation but gives condensation reaction
- With NH 3 Forms acetaldehyde ammonia Forms diacetone ammonia
