ALDEHYDES AND KETONES
FILL IN THE BLANKS
Q.01: Fill in the blanks:
(i) Aldehydes are the first oxidation product of _________. (primary alcohols)
(ii) Ketones are the first oxidation product of __________. (secondary alcohols)
(iii) Aldehydes and ketones undergo ________ addition reactions. (nucleophilic)
(iv) Formaldehyde reacts with _________ to give primary alcohol. (Grignard reagent)
(v) Acetaldehyde reacts with ________ to give 2-butanol. (ethyl magnesium bromide)
(vi) Aldehydes are strong _________ agents. (reducing)
(vii) The oxidation of an __________ always gives a carboxylic acid. (aldehyde)
(viii) The reduction of a __________ always gives a secondary alcohol. (ketone)
(ix) Formaldehyde gives _________ test with Tollen’s reagent. (silver mirror)
(x) Acetaldehyde gives a ________ precipitate with Fehling’s solution. (brick red)
TRUE/FALSE
Q.02: Indicate True or False:
(i) Formaldehyde is used in the silvering of mirrors. (TRUE)
(ii) Ketones combine with alcohols in the presence of HCl gas to form acetals. (FALSE)
CORRECT: Aldehydes combine with alcohols in the presence of HCl gas to form acetals.
(iii) Acetaldehyde undergoes Cannizzaro’s reaction. (FALSE)
CORRECT: Acetaldehyde doesn’t undergo Cannizzaro’s reaction.
(iv) Aldol condensation reaction is given by only those aldehydes and ketones which contain an a-hydrogen atom. (TRUE)
(v) Cannizzaro’s reaction is given by only those aldehydes containing no a-hydrogen atom. (TRUE)
(vi) Propanal and propanone behave differently with Tollen’s reagent. (TRUE)
(vii) Acetone reacts with sodium bisulphite to give a yellow crystalline product. (FALSE)
CORRECT: Acetone reacts with sodium bisulphite to give a white crystalline product.
(viii) Acetone on reduction gives a primary alcohol. (FALSE)
CORRECT: Acetone on reduction gives a secondary alcohol.
(ix) 40% aqueous solution of formaldehyde is called formalin. (TRUE)
MCQs
Q.03: Multiple choice questions.
(i) The carbon atom of a carbonyl group is:
(a) sp hybridized
(b) sp2 hybridized
(c) sp3 hybridized
(d) None of these
ANSWER: (b) sp2 hybridized
EXPLANATION: The carbon of carbonyl group has two single bonds on one side and a double bond with oxygen on other. This gives triangular planar arrangement of the hybrid orbitals. This arrangement allows the parallel overlapping of perpendicular unhybrid p-orbitals, making pi bond. Thus, carbon atom of carbonyl group is sp2 hybridized to allow the formation of pi bond between carbon and oxygen atom.
(ii) Formalin is:
(a) 10% solution of formaldehyde in water
(b) 20% solution of formaldehyde in water
(c) 40% solution of formaldehyde in water
(d) 60% solution of formaldehyde in water
ANSWER: (c) 40% solution of formaldehyde in water
EXPLANATION: Formalin is a mixture of 40 % formaldehyde, 8% methyl alcohol and 52 % water.
(iii) Which of the followings will have the highest boiling point?
(a) Methanal
(b) Ethanal
(c) Propanal
(d) 2-Hexanone
ANSWER: (d) 2-Hexanone
EXPLANATION: 2-Hexanone is a much bigger molecule than others, having a polar carbonyl functional group. So, it has both dipole-dipole forces and strong London dispersion forces due to which its boiling point is comparatively higher than others.
(iv) Ketones are prepared by the oxidation of:
(a) Primary alcohol
(b) Secondary alcohol
(c) Tertiary alcohol
(d) None of these
ANSWER: (b) Secondary alcohol
EXPLANATION: Secondary alcohol has hydroxyl group (OH) on a carbon which is further attached to two carbon atoms. So, when this OH group is oxidized, the resulting carbonyl group has two carbon atoms on either side of the carbonyl carbon. This makes what is called a ketone.
(v) Acetone reacts with HCN to form a cyanohydrin. It is an example of:
(a) Electrophilic addition
(b) Electrophilic substitution
(c) Nucleophilic addition
(d) Nucleophilic substitution
ANSWER: (c) Nucleophilic addition
EXPLANATION: HCN generates CN– in acidic medium which is a nucleophile. It attacks the electrophilic carbon of acetone carbonyl group. Pi bond breaks and the pi electrons are transferred to O-atom which shares them with proton (H+). The resulting product is acetone cyanohydrin which is a nucleophilic addition product.
(vi) Which of the following compounds will not give iodoform test on treatment with I2/NaOH?
(a) Acetaldehyde
(b) Acetone
(c) Butanone
(d) 3-Pentanone
ANSWER: (d) 3-Pentanone
EXPLANATION: Only that aldehyde or ketone gives iodoform test which has a methyl group attached to carbonyl carbon. 3-Pentanone doesn’t have any methyl group attached to carbonyl carbon, so it doesn’t give iodoform test.
(vii) Which of the following compounds will react with Tollen’s reagent?
(a) CH3—COH
(b) CH3—CO—CH3
(c) CH3—CO—OH
(d) CH3—CO—CH2—CH3
ANSWER: (a) CH3—COH
EXPLANATION: Tollen’s reagent is a mild oxidizing agent. It can oxidize only aldehydes, not ketones or carboxylic acids. So, only acetaldehyde (CH3—COH) will give Tollen’s test.
(viii) Cannizzaro’s reaction is not given by:
(a) Formaldehyde
(b) Acetaldehyde
(c) Benzaldehyde
(d) Trimethylacetaldehyde
ANSWER: (b) Acetaldehyde
EXPLANATION: Cannizzaro’s reaction is given by those carbonyl compounds which have no a-hydrogen. Acetaldehyde has three a-hydrogens, so it does not give Cannizzaro’s reaction.
(xi) Which of the following compounds will react with both aldehydes and ketones?
(a) Grignard reagent
(b) Tollen’s reagent
(c) Fehling’s reagent
(d) Benedict’s reagent
ANSWER: (a) Grignard reagent
EXPLANATION: Tollen’s reagent, Fehling’s reagent & Benedict’s reagent all are mild oxidizing agent which can oxidize only aldehydes, not ketones. So, only Grignard reagent can react with both aldehydes and ketones.