Aldehydes and Ketones

Question 1

Functional Group

Answer 1

Carbonyl

-C=O

Question 2

Formation of Aldehydes

Answer 2

Alcohol dehyrogenation

Oxidation of a 1° R-X

Question 3

Formula of Aldehydes

Answer 3

Cn H2n+1 CHO

Written in this format to distinguish from Alcohols

R – C = O
|
H

Question 4

Formula of Ketones

Answer 4

O
||
R – C – R’

Rs mustn’t be an H

R’ means that the 2 Rs don’t have to be the same

1st member is propanone

Question 5

Formation of Ketones

Answer 5

Oxidation of a 2° R-X

Can use relux as Ketones can’t be [O] further

Acidified CrO7 2-

Question 6

Aldehyde - Suffix for naming

Answer 6

-al

Question 7

Ketone - Suffix for naming

Answer 7

-one

Question 8

B.P trends for Aldehydes / Ketones

Answer 8

b.p of (corresponding) alkane polar molecule)

As aldehyde gets larger, the effect of the polar C=O becomes less significant

Question 9

H bonding conditions

Answer 9

H is bonded to F/O/N (E.N element)

Question 10

Solubility of Aldehydes/Ketones

Answer 10

Small A/Ks are soluble in H2O

Larger A/Ks become less and less soluble in H2O and more soluble in non polar substances

(effect of polar C=O group)

Question 11

Uses of aldehydes

Answer 11

Methanal in H2O = Formalin

Used to preserve biological specimens/organs/bodies

Question 12

Natural products

Answer 12

Smell in almonds

Question 13

Uses of ketones

Answer 13

Hormones

• testosterone
• progesterone

Propanone (Acetone)

• organic solvent
• nail polish remover

Question 14

Reactions of Aldehydes to form Carboxylic Acids

Answer 14

Oxidation

a) with H+/Cr2O7 2-
(orange to green)
Cr3+

b) with H+/MnO4-
(purple to colourless)

if not acidified, brown ppt. of MnO2 will form instead of Mn2+

Mn7+ –> Mn2+

Question 15

Distinguishing between Aldehydes and Ketones

Answer 15

1. Tollen’s Reagent (Silver Mirror Test)

2. Fehling’s Solution

Question 16

Fehling’s solution

Answer 16

alkaline solution (NaOH)

Cu2+ ions

               (OX. Agent) Aldehyde + Cu2+ --> Carboxylic Acid + Cu2O
            blue solution                           red ppt

Question 17

Tollen’s reagent

Answer 17

Ammoniacal Silver Nitrate
(excess NH3)

Clean test tube - otherwise black Ag due to [O]

Aldehyde + Ag+ –> Carboxylic Acid + Ag
silver mirror on test tube

Question 18

Reducing reagents for the reduction of Aldehydes and Ketones

Answer 18

Sodium tetrahydridoborate - NaBH4 (aq)

Lithium tetrahydrioaluminate - LiAlH4 (aq)
more powerful

Question 19

Reaction of Aldehydes and Ketones with HCN - reactants

Answer 19

HCN is generated in situ
(poisonous gas)

NaCN/KCN + dil. H2SO4 generates HCN

==> NaCN/KCN act as a catalyst

Question 20

Overall Reaction Mechanism for addition of HCN to Aldehydes

Answer 20

H H O:-
\ NaCN + H2SO4 \ /
C = O ——————-> C (oxoanion)
/ / \
R R CN

                                                      |
                                                      | 

                                              H       OH
                                                  \    /
                                                    C       (hydroxy nitrile) 
                                                  /     \
                                           H3C         CN

Extra electrons from the O- of the oxoanion get a curly arrow pointing to the imaginary line between the H and the O

H is bonded to CN and the bond between these 2 goes to the CN

e.g. 2-hydroxypropanenitrile

Question 21

pH of HCN

Answer 21

pH 4 - 5

Question 22

Reduction Reaction of Aldehyde with H2 and Metal Catalyst

Answer 22

H H OH
\ Ni/Pt \ /
C = O + H2 ——————-> C
/ / \
R R H

Question 23

Reduction Reaction of Aldehyde with H2 an Metal Catalyst

Answer 23

R R OH
\ Ni/Pt \ /
C = O + H2 ——————-> C
/ / \
R R H

Question 24

Reduction reaction of Aldehydes and Ketones with NaBH4 and LiAlH4 conditions

Answer 24

NaBH4

• aqueous alkaline solution e.g NaOH
• warm with aldehyde/ketone

LiAlH4

• in dry ether
• otherwise LiAlH4 will react with H2O
• r.t

Question 25

Type of reaction between carbonly compounds and HCN?

Answer 25

Nucleophilic Addition C=O is being attacked by an nucleophile ( :CN - )

Question 26

Nitrile group to Carboxylic Acid

Answer 26

Hydrolysis --CN + H+ + H2O --> --COOH + NH4+

Question 27

Nucleophilic Addition Reaction Mechanism - Cyanide ion and Carbonyl Stage 1

Answer 27

- C = O is polarised ==> C is open to attack by a nucleophile e.g. - CN:

Question 28

Nucleophilic Addition Reaction Mechanism - Cyanide ion and Carbonyl Stage 2

Answer 28

Intermediary compound formed is highly reactive ( -O: ) and reacts with an H+ ion (from H2O/HCN/dilute acid)

Question 29

Test to detect presence of carbonyl compounds?

Answer 29

Condensation Reaction with 2,4-DNPH (2,4-Dinitrophenylhydrazine)

Question 30

Carbonyl + 2,4-DNPH: what happens?

Answer 30

2 molecules join together and eliminate a small molecule (here, H2O) orange ppt.

Question 31

Why doesn't 2,4-DNPH react with amines, esters & carboxylic acids?

Answer 31

extra resonance structure delocalises some of the positive charge away from the carbonyl atom onto the adjacent hetero-atom ==> the C atom is less prone to attack by the nucleophilic 2,4-DNPH

Question 32

Observation for Tollen's reagent and Aldehyde

Answer 32

Silver mirror formed in the inside of test tube

Question 33

Equation for Tollen's reagent and Aldehyde

Answer 33

Oxidation RCHO + H2O ----> RCOOH + 2e- + 2H+ Reduction e- + Ag+ ---> Ag(s) RCHO + 2Ag+ + H2O ----> RCOOH + 2Ag + 2H+

Question 34

Observation for Fehling's solution and Aldehyde

Answer 34

Blue solution to red ppt.

Question 35

Equation for Fehling's solution and Aldehyde

Answer 35

Oxidation RCHO + H2O ----> RCOOH + 2e- + 2H+ Reduction 2e- + Cu2+ + 2OH- ---> Cu2O(s) + H2O