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-3methylbutanal

benzaldehyde

m- chlorobenzaldehyde

2-phenylethanal

6.1.2 Ketone : functioning group of ketone are one

and end with –

propanone

butanone

Pentan-2-one

hexan-3-one

2,4-dimethyl pentan-3-one

cyclohexanone

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




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

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 • Reagent : (I2 + NaOH) Iodine in sodium hydroxide Only work for ethanal as it has methyl-carbonyl group

Triiodof • Observation : a yellow crystal of triiodomethane is observed orm Equation : CH3CH=O + 3 I2 + NaOH  CHI3 + HCOO–Na+ + 3 HI

Ketone • 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 • Reagent : Fehling solution [solution of complex copper (II) ion] • Positive Test : only works for aldehyde • Observation : blue solution turns to red precipitate of Fehling’s Cu2O solution Equation :

red ppt. • Reagent : Tollen solution [solution of complex Ag(NH3)2]+] • Positive test : only work for aldehyde • Observation : colourless solution turn grey solid (silver mirror) Tollen’s reagent Equation :

silver mirror

Ketone

No reaction occur for ketone

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 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 b) ethanal and propanal 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

c) pentan-2-one and pentan-3-one 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 6.5.1



Natural Compound with Carbonyl Group – Carbohydrates Monosaccharide

Simplest form of carbohydrates that cannot be hydrolysed to simple sugar Examples : glucose and fructose

Glucose Also known as aldose (functioning group of CO–H) Open ring close ring

Fructose Also known as ketose (functioning group CO–CH3) open ring close ring

Adding glucose to Fehling solution will turn Adding fructose to Fehling will show no changes to Fehling solution the blue solution into a red precipitate (positive test : aldehyde)

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 H + ; boil (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.

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

 KCN  →

CH3CH(OH)CN

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