CHEMISTRY FORM 6 SEM 3 Chapter 6.pdf
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Transcript of CHEMISTRY FORM 6 SEM 3 Chapter 6.pdf
6.1 Aldehyde and Ketone – Both aldehyde and ketone are carbonyl
compound (organic with C=O in it). Both has the same molecular
formula as CnH2nO
6.1.1 Nomenclature of aldehyde
� functioning group of aldehyde are and end with –al
butanal 3-methylpentanal 4-chloro-3-
methylbutanal
benzaldehyde m- chlorobenzaldehyde 2-phenylethanal
6.1.2 Ketone : functioning group of ketone are and end with –
one
propanone butanone Pentan-2-one
hexan-3-one 2,4-dimethyl
pentan-3-onecyclohexanone
� Explanation : Butane is a non-polar molecule, where molecules were
held by temporary dipole-induced dipole. Propanal and propanone are
polar molecule, which has a permanent dipole-permanent dipole
attraction forces. However, the dipole moment of ketone is greater
than aldehyde, so ketone usually have higher b.p than aldehyde.
Propan-1-ol has high boiling point due to strong hydrogen bond
between them, however, ethanoic acid has greater hydrogen bond than
propan-1-ol, since they form dimer between them
Molecule Butane (C4H10)Propanal
(C2H5COH)
Propanone;
CH3COCH3
Propan-1-ol
C3H7OH
Ethanoic acid
CH3COOH
RMM 58 58 58 60 60
Boiling point
(oC)1 48 56 97 118
δ+ δ- δ+ δ-
� Solubility : Lower aliphatic aldehydes such as methanal &
ethanal are soluble in water because they are able to form
hydrogen bond with water (as shown in diagram). Higher
member of aliphatic carbonyl compounds are insoluble as there
are presence of hydrophobic alkyl group
δ+ δ- δ+ δ-
� Isomerism of aldehyde and ketones : Given the molecular formula of
organic compound are C5H10O, out line all possible isomers of the
organic compound
6.2 Chemical preparation of aldehyde and ketone
6.2.1 Aldehyde
1. Oxidation of primary alcohol
� Controlled oxidation of alcohol by using acidified potassium
dichromate (VI)
� Prevention : Having excess alcohol over oxidant & distilled off
aldehyde.
+ H2O
propan-1-ol propanal
2. Using catalytic oxidation (Cu at 400oC) on a 10 alcohol
propan-1-ol propanal
� This process is also known as dehydrogenation as hydrogen is
produced.
6.2.2 Ketone
1. Oxidation of secondary alcohol
� Oxidation of alcohol by using acidified potassium dichromate (VI)
� Unlike oxidation on 1o alcohol, it does not need prevention as ketone
formed will not further oxidise to other substances.
+ H2O
Propan-2-ol propanone
2. Using catalytic oxidation (Cu at 400oC) on a 20 alcohol
propan-2-ol propanone
� This process is also known as dehydrogenation as hydrogen is
produced.
6.3 Chemical reaction of aldehyde and ketone
6.3.1 Reduction of aldehyde and ketone.
� Reagent : LiAlH4 (lithium aluminium hydride) in dry ether
propanal propan-1-ol
� From reaction above, we can tell that 10 alcohol is formed back using
reduction. Hence, we can summarised the reaction as
Propan-1-ol propanal
6.3.2 Oxidation of aldehyde
� Aldehyde can be further oxidised to form carboxylic acid.
� Reagent : KMnO4 / H+ (acidified potassium manganate (VII)
Propanal propanoic acid
� However, no further reaction for oxidation of ketone.
6.3.3 Addition reaction of aldehyde and ketone
� Reagent : HCN (hydrogen cyanide) + little KCN
� The nitrile compound formed is then further hydrolysed under acidic
condition, to form 2-hydroxybutanoic acid according to the equation
� The nitrile compound formed can also be reduced to form amine, where
� The mechanism can be describe as below.
� Reagent : R-MgBr (Grignard reagent)
� When aldehyde react with Grignard reagent (Chap 4.4.2), a secondary
alcohol is formed
KETONE
� Reagent : HCN (hydrogen cyanide) + little KCN
� The nitrile compound formed is then further hydrolysed under acidic
condition, to form butanoic acid according to the equation
� Reduction
� Reagent : R-MgBr (Grignard reagent)
� When ketone react with Grignard reagent (Chap 4.4.2), a tertiary alcohol is
formed.
6.3.4 Condensation reaction of aldehyde / ketone
For aldehyde :
� Reagent : (NO2)2C6H4NHNH2 (2,4-dinitrophenylhydrazine)
� An orange precipitate surfaced when 2,4-dinitrophenylhydrazine is
added to aldehyde or ketone
For ketone
� Reagent : (NO2)2C6H4NHNH2 (2,4-dinitrophenylhydrazine)
� Similar to aldehyde, an orange precipitate will surface after the
reaction
Test Aldehyde Ketone
Triiodof
orm
• Reagent : (I2 + NaOH) Iodine in
sodium hydroxide Only work for
ethanal as it has methyl-carbonyl
group
• Observation : a yellow crystal of
triiodomethane is observed
Equation :
CH3CH=O + 3 I2 + NaOH �
CHI3 + HCOO–Na+ + 3 HI
• Reagent : (I2 + NaOH) Iodine in
sodium hydroxide Only work for
those which has methyl-carbonyl
group
• Observation : a yellow crystal of
triiodomethane is observed
Equation : CH3COCH3 + 3 I2 + NaOH
� CHI3 + CH3COO–Na+ + 3 HIHI
Test Aldehyde Ketone
Fehling’s
solution
• Reagent : Fehling solution [solution of complex copper
(II) ion]
• Positive Test : only works for aldehyde
• Observation : blue solution turns to red precipitate of
Cu2O
Equation :
red ppt.
No reaction
occur for
ketone
Tollen’s
reagent
• Reagent : Tollen solution [solution of complex Ag(NH3)2]+]
• Positive test : only work for aldehyde
• Observation : colourless solution turn grey solid (silver
mirror)
Equation :
silver mirror
No reaction
occur for
ketone
� propanone � 2-bromopropane
� propanal � 2-hydroxybutanoic acid
� propene � propanone
� 2-butenal � butanoic acid
� Ethanal � Ethanamine
2. Outline a chemical test to distinguish between
a) propanal and propanone
b) ethanal and propanal
c) pentan-2-one and pentan-3-one
Reagent : Fehling / Tollen reagent (for aldehyde) ; Iodine in NaOH (for ketone)
Observation : Red brick precipitate formed when added propanal while no changes for
propanone
Equation : CH3CH2CHO + 2 Cu2+ + 5 OH- � CH3CH2COO
- + Cu2O + 3 H2O
Reagent : Iodine in NaOH
Observation : Yellow precipitate formed when added to ethanal but no changes for
propanal
Equation : CH3CHO + 3 I2 + OH- � HCOO- + CHI3 + 3 HI
Reagent : Iodine in NaOH
Observation : Yellow precipitate formed when added to pentan-2-one but no changes
for pentan-3-one
Equation : CH3CH2CH2COCH3 + 3 I2 + OH- �
CH3CH2CH2COO- + CHI3 + 3 H2O + HI
6.5 Natural Compound with Carbonyl Group – Carbohydrates
6.5.1 Monosaccharide
� Simplest form of carbohydrates that cannot be hydrolysed to simple sugar
� Examples : glucose and fructose
Glucose Fructose
Also known as aldose (functioning group
of CO–H)
Also known as ketose (functioning group
CO–CH3)
Open ring close ring open ring close ring
Adding glucose to Fehling solution will turn
the blue solution into a red precipitate
(positive test : aldehyde)
Adding fructose to Fehling will show no
changes to Fehling solution
6.5.2 Disaccharide
� Disaccharides are 2 monosaccharide joined together by glycosidic link.
� The process of joining 2 monosaccharides are condensation process as water
molecule is given off as side product. Molecular formula of disaccharide is
C12H22O11.Example
� 3 most common disaccharides
� => sucrose (sugar cane) => maltose (barley) => lactose (milk)
� Disaccharide can be break-up and formed back 2 monosaccharides by
hydrolysis of water
� Most disaccharides are non-reducing sugar.
6.5.3 Polysaccharides
� Polysaccharide ~ polymer containing long chains of
monosaccharide units. Example : starch and cellulose. They have
the empirical formula C6H10O5.
� All saccharides are bond using glycosidic ring and they can be
hydrolysed by heating with diluted acid where
(C6H10O5)n + n H2O n C6H12O6.
� Cellulose are mainly found in cell wall of plants. Cotton is almost
pure cellulose. It can be use to manufacture synthetic fiber
known as rayon.
→+boil;H
A , B , C
C , E
C , D , E
C
C , D
E
Chiral carbon atom is formed since C1 can rotate freely in different position
Element Carbon, C Hydrogen, H Oxygen, O
Mass 66.7 11.1 22.2
Mol = mass / mol 5.51 mol 11.1 mol 1.39 mol
Ratio 5.51 / 1.39 = 4 11.1 / 1.39 = 8 1.39 / 1.39 = 1
Orange precipitate is observed
Carbonyl compound
Butanone / CH3CH2COCH3
LiAlH4 in dry ether
Alcohol / hydroxyl group
CH3CH2CH(OH)CH3
Cl2 under UV
Cl2 under AlCl3
Iodine in NaOH
C gives yellow precipitate / crystal while F will not
C6H5COCH3 + I2 + OH- � C6H5COO- + CHI3 + HI
mass of CN needed = 0.03 x 60 = 1.8 g [1]
Mr = 154.5, amount = 1.8/154.5 = 0.012 mol [1]
H < D < G
� chlorine on the aryl ring is very inert /strong C-Cl bond / overlap between
unhybridise Cl with C in benzene ring1]
� chlorine on C=O is reactive because of highly δ+ carbon atom bonded to
electronegative O and Cl/ due to inductive effect [1]
CH3CHO + HCN CH3CH(OH)CN →KCN
Nucleophilic Additional reaction
CH3CH(OH)CN + 2 H2O + H+ � CH3CH(OH)COOH + NH4
+
hydrolysis
CH3CHO → CH3CH(OH)COOH
44 90 or
4.40 g → 9.00 g
% yield = 5.40 ×100 / 9.00
= 60%
E will give yellow precipitate when react with alkaline iodine solution
Product formed are CH3CH2CH2COO– + CHI3
D reduced to form pentan-3-ol [1] while E is reduced to pentan-2-ol, which is optical
active / chiral carbon atom