12th Class Chemistry Chapter 12 Aldehydes and Ketones Short Questions Answer

1.How aldehdes occur in nature?

a) Aldehydic group is present in most of the sugars. b) Aldehdes are the principal constituent of a number of essential oils. c) The essential oils are used as fragrant and flavours.

2.Whar are those factors which make aldehdes more reactive than ketones?

The presence of hydrogen atom with the carbonyl group in aldehydes makes less steric hinderance as compared to ketone.In ketones the alkyl groups are electron-donating and they decrease the electrophilic caracter of carbon atom.

3.How formalin is prepared on the commercial scale from methyl alcohol?

4.How acetaldehyde is prepared from ethl alcohol in the laboratory?

5.Which product is obtained by geating calcium acetate?

6.What type of polarity is present in carbonyl group?

Carbonyl group is polar due to the difference fo electronegativity between carbon and oxygen.SO carbon becomes electrophilic center adn the oxygen as nucleophilic center.The reactivity of carbonyl group is due to this polarity.

7.How acids and bases acting as catalysts increase the reactivity of aldegydes and ketones?

8.Addition of HCN on aldehdes andketones is a base catalyzed reaction.What happens when products are hydrolysed?

9.HCN is not directly used in aldehydes and ketones.Give reasons?

HCN is a poisonous gas and it is difficult to handle it.NaCN and HCI are directly added to aldehdes and ketones which generate HCN in the vessel and is immediately consumed for the addition purposes.

10.How a-hydroxy acids are produced from aldehydes and ketones?

11.The addition product of an aldehyde and a ketone with NaHSO₃ is very useful reaction.Justify it?

Bisulphite addition compounds of aldehydes and ketones are white ppts.So these ppts.are separated treated with acids and aldehydes and ketones are regenerated.In this way separation of aldehdes and ketones from other comounds can be done.Moreover we can distinguish between methyl ketones and other ketones by this reaction.

12.Why NaHSO₃ does not give the reaction with 2-pentanone?

2-Pentanon is such a ketone in which both the alkyl groups are big sized.Due to steric hinderance NaHSO₃ feels difficulty for adding at such a ketone.

13.How acetaldehdyde in the presence of a strong base gives 2-butenal?

14.For aldol condensation the presence of a-hydrogen is must in aldehydes and ketones.Give reasons?

Aldol condensation proceeds through the formation of carbanion.This ion is produced when the acidic hydrogen at the a-carbon is removed by the base which acts as catalyst.In the absence of a-hydrogen carbanion cannot be prduced so aldol condensation cannot takes place.

15.Why the aldehdes with no a-hydrogen give Cannizzaro’s reaction?

If a-hydrogen is not available in aldehydes and ketones then carbanion is not produced and aldol condensation cannot take place.But OH^– ion attacks the carbonyl carbon atom and then hydride shift reaction takes place to give the self – oxidation reduction reaction.
CH₃CHO does not give this reaction due to a-H in it.
(CH₃)₃– C-CHO does not give a-H and so gives self oxidation reduction reaction.

16.Justify that Cannizzaro’s reaction is self-oxidation reduction reaction?

17.What products are obtained when benzaldehyde reacts with 50% KOH?

18.How iodoform is prepared from ethanol and acetaldhyd?

Acetaldehyde has one methyl group attached with the carbnyl group which is converted into iodoform.
CH₃CHO+3l₂+4NaOH…………………. CHI₃+HCOONa+3NaI+3H₂O
CH₃CH₂OH+4I₂+6NaOH……………….CHI₃+HCOONa+5Nal+5H₂O
Actually ethanol is first oxidized to CH₃CHO with I₂ and NaOH.

19.Only methyl ketones are able to give haloform reaction but other ketones do not.Give reasons?

The – CH₃ group attached with the carbonyl group is converted to CHX₃.Other methyl groups cannot be converted into such compounds.
For example CH₃COCH₃ give CHI₃ with I₂ and a base.2.-pentanone is a methyl ketone.It give yellow ppt.But 3-pentanone cannot give this ppt.

20.With the haloform reaction we can decrease the number of carbon atoms in aldehydes and ketones.How?

One of the carbon atoms of aldehydes and ketones which is present in the form of CH₃-group is detached from the molecule and CHX₃ is produced.The atom less than the original aldehyde or ketone.

21.How acetophenone can be converted into sodium salts of benzoic acid?

22.Polymerization of formaldehyde gives six membered cyclic comounds.justify it?

HCHO undergoes self addition reaction and meter formaldehyde is produced.

23.justify that the carbonyl group of acetaldehyde is no more there is its polymer i.e.paraldehyde?

24.NH₃ reacts with aldehydes and ketones to give cyanohydrins and then hydrazones.Justify it?

25.The reactions of hydroxylamine hydrazine and phenyl hydrazine with aldehydes and ketones are just like the reactions of NH₃ with aldehyde and ketone.Justify it?

26.NH₃ reacts with formaldehyde in different manner from other aldehydes and ketones.justify it?

27.2,4-Dinitrophenylhydrazine can be used to have a distinction between aldehydes and ketones from other organic comounds.How?

2,4-Dinitrophenyl hydrazine reacts with aldehydes and ketones to give deep yellow colour ppt.but other compounds can not do so.The reaction is just like NH₃ and its derivatives.

28.How the reduction rocess of aldehydes and ketones give alcohols?

29.How aldehydes react with alcohols to give hemiacetals and acetals?

30.Which are those weak oxidizing agents which can oxidize aldehydes but not the ketones?

Tollen’s reagent Fehling’s solution and Benedict’s solution are weak oxidizing agents.They can oxidize aldehydes into corresponding carboxylic acids and ppts.Of Ag metal and CH₂O are produced.Ketones fo not give such reactions.

31.Tollen’s test is also called silver mirror test justify it?

32.Fehling’s solution reacts with aldehydes to give red ppt.Justify it?

Fehling’s solution is a mixture is a mixture of CuCI₂ NaOH and tartaric acid.It is reduced by aldehydes and red ppt.to Cu₂O is produced.Aldehydes are themselves oxidized to correspcnding acids.

33. Benedict’s solution reacts with aldehydes to give red ppt.Justify it?

34.Give the uses of formaldehyde?

Formaldehyde is used for the manufacture of Bakelite and dyes.It is used to prepare formalin which is used as the antiseptic disinfectant germicide fungicide and preservation of biological specimens.It is also used in the silvering of mirrors and many medicines.

35.Give the uses of acetaldehyde?

Acetaldehyde is used for the production of acetic acid acetic anhydride n-butanol ethanol vinyl acetate paraldehyde and ethyl acetate.Moreover we can prepare choloralhydrate and phenolic resins.

36.How do you compare formaldehyde and acetaldehyde?

Formaldehyde gives Cannizzare’s reaction but acetaldehyde gives aldol condensation.Formaldehyde gives bakelite but acetaldehyde can not do so.Formaldehyde does not give haloform reaction but acetaldehyde does.

37.How do you compare aldehydes and ketones?

Aldehydes give silver mirror test give red ppt.with Fehling’s solution and red ppt.with Benedict’s solution but ketones can not give these reactions.Moreover aldehydes can polymerize but ketones cannot.

38.What is Formalin?

Formalin is 40% solution of formaldehyde with 8% CH3OH and 52% water.It is antibacterial and is used for the preservation of biological specimens.

39.How different carboxylic acids are prepared by oxidation of ketones?