ALDEHYDES & KETONES CONTENTS Prior knowledge
Bonding in carbonyl compounds Structural differences Nomenclature Preparation Identification Oxidation Nucleophilic addition Reduction Before you start it would be helpful to
ALDEHYDES & KETONES Before you start it would be helpful to know the functional groups found in organic chemistry know the arrangement of bonds around carbon atoms recall and explain the polarity of covalent bonds CARBONYL COMPOUNDS - STRUCTURE
Structurecarbonyl groups consists of a carbon-oxygen double bond the bond is polar due to the difference in electronegativity Difference ALDEHYDES -at least one H attached to the carbonyl group C = O H C = O H CH3 CARBONYL COMPOUNDS - STRUCTURE
Structurecarbonyl groups consists of a carbon-oxygen double bond the bond is polar due to the difference in electronegativity Difference ALDEHYDES -at least one H attached to the carbonyl group KETONES -two carbons attached to the carbonyl group C = O H C = O H CH3 C = O CH3 C = O C2H5 CH3 CARBONYL COMPOUNDS - FORMULAE
MolecularC3H6O StructuralC2H5CHOCH3COCH3 Displayed Skeletal C = O H C2H5 C = O CH3 HCCCH HOH H H HCCCO HHH H H O O CARBONYL COMPOUNDS - NOMENCLATURE
Aldehydes C2H5CHOpropanal KetonesCH3COCH3propanone CH3CH2COCH3butanone CH3COCH2CH2CH3pentan-2-one CH3CH2COCH2CH3pentan-3-one C6H5COCH3phenylethanone CARBONYL COMPOUNDS - FORMATION
ALDEHYDES Oxidation of primary (1) alcoholsRCH2OH [O] > RCHO H2O beware of further oxidationRCHO [O] > RCOOH Reduction of carboxylic acids RCOOH [H] > RCHO + H2O CARBONYL COMPOUNDS - FORMATION
ALDEHYDES Oxidation of primary (1) alcoholsRCH2OH [O] > RCHO H2O beware of further oxidationRCHO [O] > RCOOH Reduction of carboxylic acids RCOOH [H] > RCHO + H2O KETONES secondary (2) alcoholsRCHOHR [O] > RCOR H2O [O] means oxidizing agent like Sodium/potassium dichromate(VI)/potassium manganate (VII) OXIDATION OF PRIMARY ALCOHOLS
PRIMARY ALCOHOLS OXIDATION TO ALDEHYDES DISTILLATION Aldehyde has a lower boiling point so distils off before being oxidisedfurther to get higher yield of Aldehyde OXIDATION OF PRIMARY ALCOHOLS
If you want to produce carboxylic acid from primary alcohol, heat underrefluxand use excess/sufficient oxidizing agent to get higer yield ofCarboxylic acid Reactants = Primaryalcohol and excessoxidizing agent OXIDATION OF SECONDARY ALCOHOLS
SECONDARY ALCOHOLS OXIDATION TO CARBOXYLIC ACIDS REFLUX Note : volatile compounds Chemical compounds that transition to gas at low temperatures Reflux condenser prevents volatilecompounds like ethanal, ethanol andethanoic acid vapours from leavingthe flask Aldehyde condenses back into the mixture and gets oxidised to the acid Why 1 and 2 alcohols are easily oxidised and 3 alcohols are not
OXIDATION OF ALCOHOLS Why 1 and 2 alcohols are easily oxidised and 3 alcohols are not For oxidation to take place easily you must have two hydrogen atoms onadjacent C and O atoms. 1 2 3 OXIDATION OF ALCOHOLS H H R C O + [O] R C O + H2O H H H H
Why 1 and 2 alcohols are easily oxidised and 3 alcohols are not For oxidation to take place easily you must have two hydrogen atoms onadjacent C and O atoms. H H RC O [O] RC O H2O HH 1 2 3 H H RC O [O] RC O H2O RR This is possible in 1 and 2 alcohols but not in 3 alcohols. R H RC O [O] R PHYSICAL PROPERTIES OF CARBONYL COMPOUNDS
Boiling points Carbonyl compounds are polarso induced dipole-induced dipole as well as London forces However, hydrogen bonding is possible between oxygen atoms and carbonyl groups and OH group in water Simpler aldehydes such as methanal is freely soluable in water. The simplest keytone, propanone mixes freely with water CARBONYL COMPOUNDS - IDENTIFICATION
Method strong peak around cm-1 in the infra red spectrum Method formation of an orange precipitate with 2,4-dinitrophenylhydrazine Although these methods identify a carbonyl group, they cannot tell the difference between an aldehyde or a ketone. To narrow it down you must do a second test. 2,4-DINITROPHENYLHYDRAZINE
Structure Use reacts with carbonyl compounds (aldehydes and ketones) used as a simple test for aldehydes and ketones makes orange crystalline derivatives - 2,4- dinitrophenylhydrazones C6H3(NO2)2NHNH2 How would you use the purified product to identify the compound ??
Measure melting temperature / point Compare with literature/database / known value CARBONYL COMPOUNDS - IDENTIFICATION
Differentiationto distinguish aldehydes from ketones, use a mild oxidising agent Tollens Reagent ammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes to form carboxylic acid butnotoxidise ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+ the silver(I) ion is reduced to silver Ag+(aq)+ e > Ag(s) the test is known as THE SILVER MIRROR TEST Warming To liens' reagent with an aldehyde produces a precipitate of silver which coats clean glass with a shiny layer of silver so that it acts like a mirror (left). There is no reaction with a ketone (right) CARBONYL COMPOUNDS - IDENTIFICATION
Differentiationto distinguish aldehydes from ketones, use a mild oxidising agent Fehlings Solution / Benedicts solution contains a copper(II) complex ion giving a blue solution on warming, it will oxidise aldehydes to form carboxylic acid butnot oxidise ketones the copper(II) is reduced to copper(I) a red precipitate of copper(I) oxide, Cu2O, is formed The silver mirror test is the better alternative as it works with all aldehydes Ketones do not react with Tollens Reagent or Fehlings Solution Fehling's reagent is used to test for aldehydes
Fehling's reagent is used to test for aldehydes. The reagent has a blue colour as it contains copper(II) ions. The test tube in the middle contains Fehling's reagent that has been reduced by an aldehyde, to form an orange-brown precipitate of copper(I) oxide. The test tubes on the left and right contain Fehling's reagent and ketones. Ketones are very similar to aldehydes but do not react with Fehling's reagent, hence the colour is unchanged. TRIIODOMETHANE (IODOFORM) REACTION WITH ALDEHYDES AND KETONES
Triiodomethane (iodoform) reaction can be used to identify the presence of a mythylgroup next to carbonyl group Mythyl group next to carbonyl group Note: An alternative reagent for idoform reaction is a mixture ofpotassium iodide and sodium chlorate(I) solutions CARBONYL COMPOUNDS - CHEMICAL PROPERTIES
OXIDATION provides a way of differentiating between aldehydes and ketones mild oxidising agents are best aldehydes are easier to oxidise powerful oxidising agents oxidise ketones to a mixture of carboxylic acids ALDEHYDESeasily oxidised to acids RCHO(l) [O] > RCOOH(l) CH3CHO(l) [O] > CH3COOH(l) KETONESoxidised under vigorous conditions to acids with fewer carbons C2H5COCH2CH3(l) [O] > C2H5COOH(l) CH3COOH(l) Oxidation by potassium dichromate
Acidified potassium dichromate CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanismoccurs with both aldehydes and ketones involves addition to the C=O double bond unlike alkenes, they are attacked by nucleophiles attack is at the positive carbon centre due to the difference in electronegativities alkenes are non-polar and are attacked by electrophiles undergoing electrophilic addition Group Bond Polarity Attacking species Result ALKENES C=C NON-POLAR ELECTROPHILES ADDITION CARBONYLS C=O POLAR NUCLEOPHILES ADDITION CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Reagenthydrogen cyanide - HCN (in the presence of KCN) Conditionsreflux in alkaline solution Nucleophilecyanide ion CN (Mostly KCN also added with HCN) Product(s)hydroxynitrile (cyanohydrin) Equation CH3CHO HCN> CH3CH(OH)CN 2-hydroxypropanenitrile NotesHCN is a weak acid and has difficulty dissociating into ions HCN H CN the reaction is catalysed by alkali which helps produce more of the nucleophilic CN CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O STEP 1 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
MechanismNucleophilic addition Step 1CN acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM Plane polarization of light ??
Light oscillates in many planes (directions) , when light is passed through a Polaroid . It oscillates in only one plane Light osccilating in many directions Optical isomerism Optical isomerism is an example of stereo-isomerism.
It occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. Put simply, they are mirror images of each other (see the diagram below). Molecules like this are said to be chiral (pronounced ky-ral), and the different forms are called enantiomers. Optical isomers can occur when there is an asymmetric carbon atom An asymmetric carbon atom is one which is bonded to four different groups 2 enantiomers / Optical isomers do not superimpose
Does not superimpose CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles CN attacks from one side CN attacks from other side CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles CN attacks from one side (above) CN attacks from other side(below) ANIMATED MECHANISM TO SHOW HOW DIFFERENT ISOMERS ARE FORMED
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM TO SHOW HOW DIFFERENT ISOMERS ARE FORMED 2 enantiomers form reacimic mixture
CN attacks from both sides so forms 2 optical isomers /enantiomers which forms a recimic mixture 2 enantiomers form reacimic mixture How reacimic mixture forms ??
Rotate plane polarized light to anit clockwise (to the left) and is called the () enantiomer. Rotate plane polarized light to clockwise (to the right) and is the (+) enantiomer A mixture containing equal concentrations of the (+) and () enantiomers is not optically active (it will not rotate the plane of polarisation). It is called a racemic mixture or racemate. Optical isomerism and reaction mechanisms
The optical activity of the reactants and products of organic reactions can help chemists to determine the mechanisms of reactions. This is illustrated by the outcomes of the SN1 and SN2 mechanisms in nucleophilic substitution reactions. During the one,step SN2 mechanism, the three groups that remain attached to the central carbon atom are turned inside out. The molecule is inverted like an umbrella in a high wind (Figure 6.12). This means that an optically active halogenoalkane gives rise to an optically active alcohol if substitution takes place by the SN2 mechanism. In the two-step SN2 mechanism, a planar intermediate is formed after the first step. However, attack by the nucleophile during the second step can happen from either side of the planar intermediate. The result is that starting with one optical isomer of a halogenoalkane leads to a product which is a racemic mixture of the two forms of the chiral alcohol. This means that the product is optically inactive (Figure 6.13). The product of this reaction above has a chiral centre
Theproduct of this reaction above has a chiral centre. Would you expect the reaction to produce a solution that rotates the plane of plane-polarized light? Explain your answer.?? No As: Reaction site/carbonyl/aldehyde/moecule is planar Attack (equally likely) from both sides (gives) racemic mixture CARBONYL COMPOUNDS - REDUCTION WITH NaBH4
Reagentsodium tetrahydridoborate(III) (sodium borohydride), NaBH4 Conditionsaqueous or alcoholic solution MechanismNucleophilic addition(also reduction as it is addition of H) NucleophileH (hydride ion) Product(s)AlcoholsAldehydes are REDUCED to primary (1) alcohols. Ketones are REDUCED to secondary (2) alcohols. Equation(s)CH3CHO [H]>CH3CH2OH CH3COCH [H]>CH3CHOHCH3 NotesThe water provides a proton CARBONYL COMPOUNDS - REDUCTION WITH LiAlH4
LiAlH4 is a powerful reducing agent whichconverts aldehydes to primary alcohols and keytones to secondary alcohols. LiAlH4 is easily hydrolized so, reagent is dissolved in dry ether dry ether CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN
Reagenthydrogen Conditionscatalyst-nickel or platinum Reaction typeHydrogenation, reduction Product(s)AlcoholsAldehydes are REDUCED to primary (1) alcohols. Ketones are REDUCED to secondary (2) alcohols. Equation(s)CH3CHO H >CH3CH2OH CH3COCH H2>CH3CHOHCH3 NoteHydrogen also reduces C=C bonds CH2 = CHCHO H >CH3CH2CH2OH