Post on 24-Dec-2021
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3.8 Aldehydes and Ketones
Carbonyls: Aldehydes and Ketones
.
Carbonyls are compounds with a C=O bond.They can be either aldehydes or ketones
If the C=O is on the end of thechain with an H attached it is analdehyde.The name will end in –al
CH3CHO ethanalCH3COCH3
If the C=O is in the middle ofthe chain it is a ketoneThe name will end in -one
propanone
Solubility in water
The smaller carbonyls are solublein water because they can formhydrogen bonds with water.
C
CH3
CH3
OO H
H
Intermolecular forces in carbonylsPure carbonyls cannot hydrogen bond, butbond instead by permanent dipole forces.
Reactions of carbonylsIn comparison to the C=C bond inalkenes, the C=O is stronger and doesnot undergo addition reactions easily.
The C=O bond is polarised becauseO is more electronegative thancarbon. The positive carbon atomattracts nucleophiles. This is in contrast to the electrophiles
that are attracted to the C=C .
Oxidation Reactions
Primary alcohol
Tertiary alcoholsSecondary alcohol
aldehydes carboxylic acidketones
do not oxidise
Potassium dichromate K2Cr2O7 isan oxidising agent that causesalcohols and aldehydes tooxidise.
Key point: Aldehydescan be oxidised tocarboxylic acids, butketones cannot beoxidised.
Reaction: aldehyde carboxylic acidReagent: potassium dichromate (VI) solution anddilute sulfuric acid.Conditions: heat under reflux
Oxidation of Aldehydes
RCHO + [O] RCO2H
+ [O]CO
C
H
H
C
HH
H
H
C C
O
O HH
C
H
H
H
H
Full equation for oxidation3CH3CHO + Cr2O7
2- + 8H+ 3 CH3CO2H + 4H2O + 2Cr3+
Observation: the orange dichromate ion(Cr2O7
2-) reduces to the green Cr 3+ ion
Aldehydes can also be oxidised using Fehling’ssolution or Tollen’s reagent. These are used astests for the presence of aldehyde groups
Tollen’s Reagent
CH3CHO + 2Ag+ + H2O CH3COOH + 2Ag + 2H+
Reagent: Fehling’s solution containing blue Cu 2+ ions.Conditions: heat gentlyReaction: aldehydes only are oxidised by Fehling’s
Solution into a carboxylic acid. The copper (II) ionsare reduced to copper(I) oxide . .
Observation: Aldehydes :Blue Cu 2+ ions in solutionchange to a red precipitate of Cu2O. Ketones donot react.
Fehling’s solution
CH3CHO + 2Cu2+ + 2H2O CH3COOH + Cu2O + 4H+
Reagent: Tollen’s reagent formed by mixingaqueous ammonia and silver nitrate. Theactive substance is the complex ion of[Ag(NH3)2]+ .
Conditions: heat gentlyReaction: aldehydes only are oxidised by
Tollen’s reagent into a carboxylic acid. Thesilver(I) ions are reduced to silver atoms
Observation: with aldehydes, a silver mirror formscoating the inside of the test tube. Ketonesresult in no change.
1
CH3
C
CH3
O
δ+
δ-
nucleophile
C
O
C
H
H
H
H
C C CO
HH
H
HH
H
δ+ δ-δ-
Dr.Ashar Rana www.chemistryonlinetuition.com asherrana@chemistryonlinetuition.com
C
H
CH3 CH3
O
2
Reduction of carbonylsReducing agents such as NaBH4 (sodium tetrahydridoborate)or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonylsto alcohols.
Aldehydes will be reduced to primary alcohols Ketones will be reduced to secondary alcohols.
propanone
C C CO
HH
H
HH
HC
H
H
C
H
C
H
H
H OH
H+ 2[H] CO
C
H
H
C
HH
H
H
+ 2[H] C O H
H
H
C
H
H
C
H
H
Hpropanal Propan-1-ol
Propan-2-ol
Reagents: NaBH4 In aqueous ethanolConditions: Room temperature and pressure
NaBH4 contain a source ofnucleophilic hydride ions (:H-)which are attracted to the positivecarbon in the C=O bond.
Nucleophilic Addition Mechanism H+ from water or weak acid
Catalytic HydrogenationCarbonyls can also be reduced using catalytic hydrogenation
Reagent: hydrogen and nickel catalyst
Conditions: high pressure
CH3CHO + H2 CH3CH2OHExample Equations
CH3COCH3 + H2 CH3CH(OH)CH3
Addition of hydrogen cyanide to carbonyls to form hydroxynitriles
Reaction: carbonyl hydroxynitrileReagent: sodium cyanide (NaCN) and dilute sulfuricacid.Conditions: Room temperature and pressureMechanism: nucleophilic addition
NC C
R
H
OH
hydroxynitrile
The NaCN supplies thenucleophilic CN- ions. TheH2SO4 acid supplies H+ ionsneeded in second step of themechanism
CH3COCH3+ HCN CH3C(OH)(CN)CH3
CH3CHO + HCN CH3CH(OH)CN
When naming hydroxy nitrilesthe CN becomes part of themain chain and carbon no 12-hydroxy-2-methylpropanenitrile
2-hydroxypropanenitrile
Nucleophilic Addition MechanismH+ from sulfuric acid
NC C
CH3
CH3
OH
NC C
CH3
H
OH
We could use HCN for this reaction but itis a toxic gas that is difficult to contain.KCN/NaCN are still, however, toxic,because of the cyanide ion.
CH3
CCH3
O
:H-
δ+
δ-
: H+
C
H
CH3 CH3
O H-
CH3
CCH3
O
:CN-
δ+
δ-
: H+-
C
CN
CH3 CH3
O
C
CN
CH3 CH3
O H
Dr.Ashar Rana www.chemistryonlinetuition.com asherrana@chemistryonlinetuition.com
CH3C
H
O
3
:CN--NC: C
CH3
O
H
C
NC CH3
OH
H
C
CH3 CN
OH
HThere is an equalchance of eitherenantiomer formingso a racemate forms.No optical activity isseen.
Nucleophilic addition of HCN to aldehydesand ketones (unsymmetrical) when thetrigonal planar carbonyl is approached fromboth sides by the HCN attacking species:results in the formation of a racemate.
Mechanism for the reaction (drawn the same for both enantiomers)
:CN-
δ+
δ-
: H+-
C
CN
CH3 H
O
C
CN
CH3 H
O H
Dr.Ashar Rana www.chemistryonlinetuition.com asherrana@chemistryonlinetuition.com