Class 12 Chemistry Aldehydes and Ketones Notes

Class 12 Chemistry Aldehydes and Ketones Notes
Chapter 13 – Aldehydes and Ketones | Nepal eNotes
CHEMISTRY • CHAPTER 13

Aldehydes and Ketones

Exact typing reconstruction from the supplied handwritten notes.

Original Scanned PDF – View Notes

Open PDF Full Screen
Open PDF in Google Drive
PDF Page 1

Aldehyde and ketono

The organic compound having at least one carbonyl group is called aldehyde or ketone.

R−C(=O)−

Eg

Aldehyde

H−C(=O)−H

formaldehyde

methanal

CH3−C(=O)−H

Acetaldehyde

ethanal

CH3−CH2−C(=O)−H

propanal

H−C(=O)−CH2−C(=O)−H

propane 1,3 dial

C6H5−CHO

Benzaldehyde

Ketone

CH3−C(=O)−CH3

Acetone

(Nail polish remover)

CH3−C(=O)−CH2−CH3

Butanone

CH3−C(=O)−CH2−CH2−CH3

pentan−2−one

CH3−C(=O)−CH2−C(=O)−CH3

pentane−2,4−dione

C6H5−C(=O)−CH3

Acetophenone

PDF Page 2

Isomerism

Chain isomers

CH3−CH2−CH2−CHO    and    CH3−CH(CH3)−CHO

Butanal     2−methylpropanal

CH3−CH2−CH2−C(=O)−CH3    and    CH3−CH(CH3)−CH2−C(=O)−CH3

keton−2−one     4−methylpentan−2−one

Position isomers

CH3−C(=O)−CH2−CH2−CH3

pentan−2−one

CH3−CH2−C(=O)−CH2−CH3

pentan−3−one

alcohol does not show any positional isomers.

Functional isomers

CH3−CH2−CHO    and    CH3−C(=O)−CH3

propanal     Acetone (propanone)

NOTE: ethanal and methanal doesn’t show functional isomers.
PDF Page 3

Tautomerism

The isomers which are inter convertible and exist in equilibrium state are called tautomerism. aldehyde and the ketone gives enol form is a tautomerism.

CH3−C(=O)−CH3 ⇌ CH3−C(OH)=CH2

ketone     enol form / propan−2−ol

CH3−CH2−C(=O)−CH3 ⇌ CH3−CH=C(OH)−CH3

propanal     propan−1−ol

General preparation of aldehyde and ketone

1. By oxidation of alcohol

Primary alcohol on oxidation gives aldehyde which further oxidized into carboxylic acid.

CH3−CH2−OH + [O] KMnO4/H+→ CH3−CHO [O]→ CH3−COOH

ethanol → ethanal → ethanoic acid

Secondary alcohol on oxidation gives ketone which further oxidation in drastic condition gives carboxylic acid with one less no of carbon.

CH3−CH(OH)−CH3 + [O] KMnO4/H+→ CH3−C(=O)−CH3 [O], drastic condition→ CH3−COOH + CO2 + H2O

propanol → acetone

PDF Page 4

2. By dehydrogenation of alcohol

When vapour of alcohol passed over heated copper then dehydrogenation take place.

Primary alcohol undergoes dehydrogenation gives aldehyde.

CH3−CH2−OH Cu, 300°C→ CH3−CHO + H2

Ethanal (acetaldehyde)

Secondary alcohol undergoes dehydrogenation gives ketone.

CH3−CH(OH)−CH3 Cu, 300°C→ CH3−C(=O)−CH3 + H2

propan−2−ol / isopropyl alcohol → propanone (acetone)

3. By ozonolysis of alkene

alkene undergoes ozonolysis gives aldehyde or ketone.

CH3−CH=CH3 O3, Zn/H2O→ CH3−CHO + H−CHO + H2O2

propene → ethanal + methanal

CH3−CH=CH2 O3, Zn/H2O→ CH3−CHO + H−CHO + H2O2

propene

PDF Page 5
2−methylpropene O3, Zn/H2O→ CH3−C(=O)−CH3 + H−CHO + H2O2

From Acid halide

4. Rosenmund reduction rxn

Acid halide undergoes reduction in presence of rosenmund catalyst gives aldehyde. rosenmund (Pd/BaSO4) is used to control further reduction of aldehyde into alcohol.

CH3−COCl + 2[H] Pd/BaSO4, Δ→ CH3−CHO + HCl

ethanoyl chloride → Acetaldehyde

C6H5−COCl + 2[H] Pd/BaSO4, Δ→ C6H5−CHO + HCl

Benzoyl chloride → Benzaldehyde

ketone are obtain by the rxn of acid chloride with dialkyl cadmium.

2CH3−COCl + (CH3)2Cd → CH3−CO−CH3 + CdCl2

Acetone / propanone

5. From gem-dihalide

PDF Page 6
CH3−CHBr2 + 2KOH aq→ CH3−CH(OH)2 −H2O→ CH3−CHO

? bromoethane → unstable → Acetaldehyde

CH3−CBr2−CH3 + 2KOH aq→ CH3−C(OH)2−CH3 −H2O→ CH3−CO−CH3

2,2-dibromopropane → unstable

Properties

  • lower no of aldehyde are irritating and unpleasant order with volatile in nature.
  • Higher no of aldehyde and ketone has pleasant order.
  • Aldehyde and the ketone are soluble in H2O but solubility decreases on increasing molecular mass.
Fig: Intermolecular H-bond R−C O δ− H−O−H δ+ δ−

Boiling point of aldehyde and the ketone is higher then alkane having comparable molecular mass due to the presence of electronegative oxygen atom which possess dipole dipole interaction but lower than alcohol, carboxylic acid and water.

PDF Page 7

Chemical properties

1. Rxn with HCN

Aldehyde and the ketone react with HCN gives cyanohydrin which is followed by acidic hydrolysis gives carboxylic acid.

CH3−CHO + HCN → CH3−CH(OH)−CN H3O+→ CH3−CH(OH)−COOH

Acetaldehyde → Acetaldehyde cyanohydrin → α-hydroxy propanoic acid

CH3−CO−CH3 + HCN → CH3−C(OH)(CN)−CH3 H3O+→ CH3−C(OH)(COOH)−CH3

Acetone → Acetone cyanohydrin → 2-hydroxy-2-methyl propanoic acid

2. Addition with sod. bisulphite (NaHSO3)

CH3−CHO + HSO3Na → CH3−CH(OH)−SO3Na

Acetaldehyde sod. bisulphite

CH3−CO−CH3 + HSO3Na → CH3−C(OH)(SO3Na)−CH3

acetone sodium bisulphite

PDF Page 8

3. Rxn with ammonia

formaldehyde (methanal) react with ammonia gives hexamethylene tetramine called (urotropine) which is used as urinary antiseptic.

6HCHO + 4NH3 → (CH2)6N4 + 6H2O

methanal → hexamethylene tetramine (urotropine)

Fig: Urotropine N N N N CH₂ CH₂ CH₂ CH₂ CH₂
PDF Page 9

4. Rxn with derivative of ammonia

a) Action with hydroxylamine (H2N−OH)

Aldehyde and the ketone react with hydroxylamine gives aldoxime and ketoxime.

CH3−CHO + H2N−OH → CH3−CH=N−OH + H2O

Acetaldehyde → Acetaldoxime

CH3−CO−CH3 + H2N−OH → CH3−C(=N−OH)−CH3 + H2O

Acetone → Acetoxime

b) Action with hydrazine (H2N−NH2)

Aldehyde and the ketone react with hydrazine gives hydrazone.

CH3−CHO + H2N−NH2 → CH3−CH=N−NH2 + H2O

acetaldehyde hydrazone

CH3−CO−CH3 + H2N−NH2 → CH3−C(=N−NH2)−CH3 + H2O

Acetone hydrazone

PDF Page 10

c) Action with phenyl hydrazine (H2N−NH−C6H5)

CH3−CHO + H2N−NH−C6H5 → CH3−CH=N−NH−C6H5

acetaldehyde phenyl hydrazone

CH3−CO−CH3 + H2N−NH−C6H5 → CH3−C(=N−NH−C6H5)−CH3 + H2O

Acetone phenyl hydrazone

d) Action with Semicarbazide (H2N NHCONH2)

CH3−CHO + H2N−NH−CONH2 → CH3−CH=N−NH−CONH2 + H2O

acetaldehyde semicarbazone

CH3−CO−CH3 + H2N−NH−CONH2 → CH3−C(=N−NH−CONH2)−CH3 + H2O

Acetone semicarbazone

PDF Page 11

Rxn with 2,4-DNP (2,4-DNP test) (lab test of carbonyl compound)

2,4-dinitrophenyl hydrazine

Carbonyl compound (aldehyde or ketone) when react with 2,4-DNP gives orange yellow crystal. So carbonyl compound this rxn is used to detect carbonyl compound in lab called 2,4-dinitrophenyl test. DNP not useful for aliphatic or aromatic.

Acetaldehyde + 2,4-DNP → Acetaldehyde−2,4-DNP + H2O

orange yellow crystal

Acetone + 2,4-DNP → Acetone−2,4-DNP + H2O

orange yellow crystal

PDF Page 12

Tollen’s Test (silver mirror test)

Ammonical AgNO3 soln is called Tollen’s reagent. when aldehyde (aromatic as well as aliphatic) react with tollens reagent then Ag is deposited on the wall of test tube. This rxn is called silver mirror test. This rxn only suitable for aldehyde but not ketone.

CH3−CHO + 2[Ag(NH3)2OH] → CH3−COONH4 + 2Ag↓ + 3NH3 + H2O

silver mirror

Fehling test

Alkaline soln of copper sulphate containing potassium tartarate is called Fehling soln. When aliphatic aldehyde react in fehling soln gives brick red ppt of Cu2O (cuprous oxide). This rxn is used in test only aliphatic aldehyde but not aromatic aldehyde and the ketone.

CH3−CHO + 2Cu2+ + 4OH− → CH3−COOH + Cu2O↓ + 2H2O

brick red ppt

7) Benedict soln test

Alkaline soln of copper sulphate containing pot-citrate is called benedict soln. Benedict soln is used to diagnosis the diabetes. when benedict soln react with aliphatic aldehyde gives brick red ppt of cuprous oxide.

PDF Page 13
CH3−CHO + 2Cu2+ + 4OH− → CH3−COOH + 2Cu2O↓ + H2O

from benedict soln    Bricky red ppt.

8. Clemmensen’s redn rxn

aldehyde and the ketone when react with Zn/Hg and conc HCl undergo reduction gives alkane. In this rxn carbonyl group is converted in methylene − methylene group (−CH2).

CH3−CHO + 4[H] Zn−Hg / conc HCl→ CH3−CH3 + H2O

ethanal → ethane

CH3−CH2−CHO + 4[H] Zn−Hg / conc HCl→ CH3−CH2−CH3 + H2O

propanal → propane

C6H5−CHO + 4[H] Zn−Hg / conc HCl→ C6H5−CH3 + H2O

Benzaldehyde → Toluene

CH3−CO−CH3 + 4[H] Zn−Hg / conc HCl→ CH3−CH2−CH3 + H2O

propanone → propane

PDF Page 14

9. Wolf-Kishner redn rxn

Aldehyde and ketone undergoes reduction in presence of hydrazine and strong alkali or ethylene glycol gives alkane.

CH3−CHO + 4[H] H2N−NH2, KOH / 200°C→ CH3−CH3 + H2O

ethanal → ethane

CH3−CH2−CHO + 4[H] H2N−NH2, KOH / Δ→ CH3−CH2−CH3 + H2O
C6H5−CHO + 4[H] H2N−NH2, KOH / Δ→ C6H5−CH3 + H2O

Toluene

CH3−CO−CH3 + 4[H] H2N−NH2, KOH / Δ→ CH3−CH2−CH3 + H2O

propanone → propane

10. Aldol Condensation

When two molecule of aldehyde or ketone undergoes condensation in presence of dilute NaOH gives β−hydroxy aldehyde or ketone which is called aldol. This rxn is known as Aldol condensation. In case of aldol condensation carbonyl compound must be contain α−hydrogen.

PDF Page 15
CH3−CHO + CH3−CHO dil NaOH→ CH3−CH(OH)−CH2−CHO

Aldol / 3−hydroxybutanal

CH3−CO−CH3 + CH3−CO−CH3 dil NaOH→ 4−hydroxy−4−methyl pentan−2−ol

11. Cross aldol condensation

The aldol condensation which is carried out betn two different molecule of aldehyde or ketone is called cross-aldol condensation.

HCHO + CH3−CHO dil NaOH→ HO−CH2−CH2−CHO

β−hydroxybutanal (aldol)

CH3−CO−CH3 + HCHO dil NaOH→ CH3−C(OH)(CH3)−CH2−CHO

3−hydroxy−3−methyl butanal

12. Cannizzaro’s rxn

When two molecule of aldehyde without α−hydrogen react with conc NaOH undergoes self oxidation and self reduction produce sod. salt of carboxylic acid and alcohol.

PDF Page 16
HCHO + HCHO conc NaOH, Δ→ HCOONa + CH3OH

methanal → sod. formate + methyl alcohol

2C6H5CHO conc NaOH, Δ→ C6H5COONa + C6H5CH2OH

Sod. benzoate + Benzyl alcohol

13. Action with PCl5

Aldehyde and ketone react with PCl5 gives gemdihalide.

CH3−CHO + PCl5 → CH3−CHCl2 + POCl3

ethanal → 1,1−dichloroethane

CH3−CO−CH3 + PCl5 → CH3−CCl2−CH3 + POCl3
Benzaldehyde + PCl5 → Benzylchloride
PDF Page 17

14. Redn of aldehyde and ketone

Primary aldehyde undergoes reduction gives primary alcohol.

CH3−CHO + 2[H] LiAlH4→ CH3−CH2−OH
CH3−CH2−CHO + 2[H] Ni/Pt→ CH3−CH2−CH2−OH
C6H5−CHO + 2[H] LiAlH4→ C6H5−CH2OH

Benzyl alcohol

ketone undergoes reduction gives secondary alcohol.

CH3−CO−CH3 + 2[H] LiAlH4→ CH3−CH(OH)−CH3

propanone → propane−2−ol

15. Action with phenol

Formaldehyde react with phenol in acidic medium gives ortho and para hydroxylbenzyl alcohol which undergoes polymerization produce bakelite. bakelite is used to prepare thermo-setting plastic.

HCHO + Phenol H+→ o-hydroxybenzyl alcohol + p-hydroxybenzyl alcohol
PDF Page 18
Fig: Bakelite CH₂ CH₂ CH₂ CH₂ OH OH OH OH OH OH

Formalin and its uses

40% aq soln of formaldehyde is called formalin.

It is used as a preservative for biological specimens and milk.

  • Formalin is used as a general antiseptic.
  • It is used to manufacture ebonite, dyes, synthesis plastic, bakelite etc.

Uses of acetaldehyde

  • It is used to manufacture acetic acid, acetic anhydride, chloral, chloroform, etc.
PDF Page 19
  • It is used in the production of dyes, polymer, plastic etc.
  • It is used as a reducing agent.

Uses of acetone

  • It is used as solvent for acetylene, resin, cellulose etc.
  • It is used in the manufacture of iodoform, chloroform, chlorobenzene, thermosetting plastic etc.
  • It is used as a nailpolish remover and for removing stains.

Iodoform rxn

Carbonyl compound containing acyl group (CH3−CO−) when react with alkaline iodine gives yellow ppt of iodoform.

CH3−CHO + 3I2 + 4NaOH → CHI3↓ + HCOONa + 3NaI + 3H2O

Acetaldehyde (ethanal) → yellow ppt iodoform

CH3−CO−CH3 + 3I2 + 4NaOH → CHI3↓ + CH3COONa + 3NaI + 3H2O

Acetone / propanone → Iodoform yellow ppt.

PDF Page 20

Aromatic aldehyde and ketone

C6H5−CHO    or    Ph−CHO    Ar−CHO

Benzaldehyde

C6H5−CO−CH3

Aromatic ketone (Acetophenone)

C6H5−CO−C6H5

Benophenone

General preparation of benzaldehyde

From Toluene

Toluene + [O] CrO3→ Benzaldehyde
Toluene + [O] CrO2Cl2, chromyl chloride→ Benzaldehyde

Etard rxn

Toluene + [O] SeO2 / Δ, conc H2SO4→ Benzaldehyde

[laboratory preparation]

PDF Page 21

Example of carbonylation rxn

Gatterman Koch rxn

Benzene + CO HCl→ Benzaldehyde

Properties

  • Benzaldehyde is an oily colour less liquid with poisonous bitter almond odour.
  • It is insoluble in H2O but soluble in organic solvent like ether, alcohol, benzene etc.
  • Benzaldehyde gives all properties of aliphatic aldehyde like addition of HCN, NaHSO3, reaction with PCl5, DNP test, Tollen test, Fehling test etc.
  • It also gives similar properties with derivative of ammonia, Clemmenson reduction rxn, wolf kishner redn rxn.

Perkins condensation rxn

Benzaldehyde + CH3CO−O−COCH3 CH3COONa→ Cinnamic acid + CH3COOH

ethanoic anhydride (acetic anhydride) → cinnamic acid + acetic acid

Benzoin condensation

Two molecule of benzaldehyde undergoes condensation gives benzoin and the rxn is known as benzoin condensation.

PDF Page 22
2 Benzaldehyde alc. KCN, condensation, Δ→ C6H5−CO−CH(OH)−C6H5

Benzoin

Resonating structure of benzaldehyde

The supplied source shows a sequence of resonance structures of benzaldehyde and a resonance hybrid.

CHO ↔ C⁺H−O⁻ − ↔ C⁺H−O⁻ − ↔ CHO − Resonance sequence and hybrid as represented in the source
PDF Page 23

Electrophilic substitution rxn

Electrophile are the electron deficient centre which is always attack on electron reach centre. According to the resonating str of benzaldehyde that contain +ve charge at ortho and para position that means ortho and para positive position is electron pore centre. Hence incoming electrophile attack on meta position to give meta substituted product. thereby benzaldehyde is the meta director toward electrophilic.

Halogenation

Benzaldehyde Cl2 / AlCl3→ m-chlorobenzaldehyde

Nitration

Benzaldehyde conc HNO3 / H2SO4, Δ→ m-nitrobenzaldehyde

Sulphonation

Benzaldehyde conc H2SO4 / Δ→ m-benzaldehyde sulphonic acid

Uses of Benzaldehyde

  1. It is used to manufacture cinnamic acid, dyes, perfume as well as flavouring agent for food stuff, syrup and medicine etc.
PDF Page 24

Preparation of aromatic ketone

Benzene + CH3COCl Anhy. AlCl3→ Acetophenone + HCl

ethanoyl chloride (acetic chloride) → Acetophenone

Benzene + CH3CO−O−COCH3 CH3COONa, Δ→ Acetophenone + CH3COOH

Acetic acid / ethanoic acid

KEEP LEARNING View All Posts

Discussion

Share a helpful question, idea, or explanation with other students.

Leave a Comment

Write a clear question, answer, or helpful explanation.
Your email will not be published.

Download Our Offline App

Study class-wise notes even when internet is not available. Get the app from Play Store.

Nepal eNotes offline app preview
Get it on Google Play