the reactions of aldehydes and ketones knockhardy publishing 2015 specifications
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THE REACTIONSTHE REACTIONSOF ALDEHYDES OF ALDEHYDES AND KETONESAND KETONES
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING20152015
SPECIFICATIONSSPECIFICATIONS
CARBONYL COMPOUNDS - FORMULAECARBONYL COMPOUNDS - FORMULAE
Molecular C3H6O
Structural C2H5CHO CH3COCH3
Displayed
Skeletal
C = OH
C2H5
C = OCH3
CH3
H C C C H
H O H
H H
H C C C O
H H H
H H
O O
CARBONYL COMPOUNDS - NOMENCLATURECARBONYL COMPOUNDS - NOMENCLATURE
Aldehydes C2H5CHO propanal
Ketones CH3COCH3 propanone
CH3CH2COCH3 butanone
CH3COCH2CH2CH3 pentan-2-one
CH3CH2COCH2CH3 pentan-3-one
C6H5COCH3 phenylethanone
CARBONYL COMPOUNDS - CHEMICAL PROPERTIESCARBONYL 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
ALDEHYDES easily oxidised to acids
RCHO(l) + [O] ——> RCOOH(l)CH3CHO(l) + [O] ——> CH3COOH(l)
KETONES oxidised under vigorous conditions to acids with fewer carbons
C2H5COCH2CH3(l) + 3 [O] ——> C2H5COOH(l) + CH3COOH(l)
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism occurs 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 thedifference in electronegativities
alkenes are non-polar and are attacked by electrophiles
undergoing electrophilic addition
C=C ELECTROPHILESALKENES
CARBONYLS
NON-POLAR
C=O POLAR NUCLEOPHILES
ADDITION
ADDITION
Bond Attacking speciesGroup Polarity Result
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Reagent potassium cyanide – followed by dilute acid
Conditions reflux
Nucleophile cyanide ion CN¯
Product(s) hydroxynitrile (cyanohydrin)
Equation CH3CHO + HCN ——> CH3CH(OH)CN
2-hydroxypropanenitrile
Notes HCN is a weak acid and has difficulty dissociating into ions
HCN H+ + CN¯
Using ionic KCN produces more of the nucleophilic CN¯
Alternative reagent: HCN catalysed by alkali which shifts the above equilibrium in favour of CN¯
HIGHLY TOXICTAKE GREAT CARE
HIGHLY TOXICTAKE GREAT CARE
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 CN¯ acts as a nucleophile and attacks the slightly positive COne of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2 A pair of electrons is used to form a bond with H+
Overall, there has been addition of HCN
STEP 2STEP 1
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic addition
Step 1 CN¯ acts as a nucleophile and attacks the slightly positive COne of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2 A pair of electrons is used to form a bond with H+
Overall, there has been addition of HCN
STEP 2STEP 1
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacks from above
CN¯ attacks from below
Both optical isomers are produced in a racemic mixture
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITIONCARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacksfrom above
CN¯ attacksfrom below
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4
Conditions aqueous or alcoholic solution
Mechanism Nucleophilic addition (also reduction as it is addition of H¯)
Nucleophile H¯ (hydride ion)
Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols.
Equation(s) CH3CHO + 2[H] ——> CH3CH2OH
CH3COCH3 + 2[H] ——> CH3CHOHCH3
Notes The water provides a proton
CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH NaBHNaBH44
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4
Conditions aqueous or alcoholic solution
Mechanism Nucleophilic addition (also reduction as it is addition of H¯)
Nucleophile H¯ (hydride ion)
CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH NaBHNaBH44
Water is added
Primary alcoholPrimary alcoholAldehydeAldehyde
Reagent sodium tetrahydridoborate(III) (sodium borohydride), NaBH4
Conditions aqueous or alcoholic solution
Mechanism Nucleophilic addition (also reduction as it is addition of H¯)
Nucleophile H¯ (hydride ion)
CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH NaBHNaBH44
ANIMATED MECHANISM
THE TETRAHYDRIDOBORATE(III) ION THE TETRAHYDRIDOBORATE(III) ION BHBH44
B H HH
H
H
H
H
each atom needs one electron to complete
its outer shell atom has three
electrons to share boron shares all 3 electrons to form 3 single covalent bonds
B
H
H
H
The hydride ion forms a dative covalent bond
B
H
H
H
B H
CARBONYL COMPOUNDS - REDUCTION WITH CARBONYL COMPOUNDS - REDUCTION WITH HYDROGENHYDROGEN
Reagent hydrogen
Conditions catalyst - nickel or platinum
Reaction type Hydrogenation, reduction
Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols.Ketones are REDUCED to secondary (2°) alcohols.
Equation(s) CH3CHO + H2 ——> CH3CH2OH
CH3COCH3 + H2 ——> CH3CHOHCH3
Note Hydrogen also reduces C=C bonds
CH2 = CHCHO + 2H2 ——> CH3CH2CH2OH
CARBONYL COMPOUNDS - REDUCTIONCARBONYL COMPOUNDS - REDUCTION
Introduction Functional groups containing multiple bonds can be reduced
C=C is reduced to CH-CHC=O is reduced to CH-OHCN is reduced to CH-NH2
Hydrogen H• H2
H+ (electrophile) H¯ (nucleophile)
Reactions Hydrogen reduces C=C and C=O bonds
CH2 = CHCHO + 4[H] ——> CH3CH2CH2OH
Hydride ion H¯ reduces C=O bondsCH2 = CHCHO + 2[H] ——> CH2=CHCH2OH
Explanation C=O is polar so is attacked by the nucleophilic H¯C=C is non-polar so is not attacked by the nucleophilic H¯
CARBONYL COMPOUNDS - REDUCTIONCARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is reduced?
NaBH4
H2COMPOUND X
CARBONYL COMPOUNDS - REDUCTIONCARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is reduced?
C=O is polar so is attacked by the nucleophilic H¯
C=C is non-polar so is not attacked by the nucleophilic H¯
NaBH4
H2COMPOUND X
2,4-DINITROPHENYLHYDRAZINE2,4-DINITROPHENYLHYDRAZINE
Structure
Use reacts with carbonyl compounds (aldehydes and ketones)used as a simple test for aldehydes and ketonesmakes orange crystalline derivatives - 2,4-dinitrophenylhydrazonesderivatives have sharp, well-defined melting pointsalso used to characterise (identify) carbonyl compounds.
Identification / characterisationA simple way of characterising a compound (finding out what it is) is to
measure the melting point of a solid or the boiling point of a liquid.
C6H3(NO2)2NHNH2
2,4-DINITROPHENYLHYDRAZINE C2,4-DINITROPHENYLHYDRAZINE C66HH33(NO(NO22))22NHNHNHNH22
The following structural isomers have similar boiling points because of similarvan der Waals forces and dipole-dipole interactions. They would be impossibleto identify with any precision using boiling point determination.
Boiling point 213°C 214°C 214°C
Melting point of2,4-dnph derivative 209°C 248°C 265°C
By forming the 2,4-dinitrophenylhydrazone derivative and taking its melting point,it will be easier to identify the unknown original carbonyl compound.
CHO CHO CHO
Cl
Cl
Cl
THE CHEMISTRYTHE CHEMISTRYOF ALDEHYDES OF ALDEHYDES AND KETONESAND KETONES
THE ENDTHE END
©©2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
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