Chapter 18 : Carbonyl Compounds

Question 1

Definition of nucleophilic addition

Answer 1

The mechanism of the reaction in which a nucleophile attacks the carbon atom in a carbonyl group and adds across the C==O bond, EG, aldehydes or ketones reacting with hydrogen cyanide.

Question 2

Definition of hydroxynitrile

Answer 2

An organic compound containing both an -OH and a -CN group.

Question 3

Definition of condensation reaction

Answer 3

A reaction in which two organic molecules join together and in the process eliminate a small molecule, such as water or hydrogen chloride

Question 4

Definition of tollen’s reagent

Answer 4

An agueousn solution of Silver nitrate in excess ammonia solution. It is used to distinguish between aldehydes and ketones. It gives a positive ‘silver mirror’ test when warmed with aldehydes, but no change is observed with ketones

Question 5

Definition of Fehlings solution

Answer 5

An alkaline solution containing copper ([]) ions used to distinguish between aldehydes and ketones. A positive test is one in which is clear blue solution gives a red/orange precipitate when war,ed with aldehydes, but no change is observed with ketones

Question 6

Definition of infra-red spectroscopy

Answer 6

A technique for Identifying compounds bases on the change in vibrations of particular atoms which infra-red radiation of specific frequencies is absorbed

Question 7

Aldehydes are formed from the oxidation of…..

Answer 7

Primary alcohols

Question 8

Ketones can be formed from the oxidation of. ….

Answer 8

Secondary alcohols

Question 9

Both aldehydes and ketones contain the carbonyl group , C=O but what is the difference ? (2)

Answer 9

● in aldehydes the carbonyl group is joined to just one other carbon atom

●in ketones , the carbonyl group is joined to 2 other carbon atoms

Question 10

The names of aldehydes are derived from ? Plus the simplest examlle

Answer 10

Derived from the name of the alkane with the “e” at the end replaced by -“al”

E.g methanal

Question 11

How are ketones named ?

Answer 11

The name of the alkane with the “e” at the end replaced by “-one” e,g. Propanone

Question 12

The preparation of an aldehyde :

1. What is the general formula
2. What are the 2 conditions
3. An example using propan-1-ol

Answer 12

1. Primary alcohol + Oxygen atom from oxidising agent ➡ aldehyde + water
2. Conditions :● potassium dichromate (VI) , K2Cr2O7 acidified in sulfuric acid●(gently heated ) the aldehyde needs to be distilled off as it forms
3. Example

CH3CH2CH2OH + [O] ➡CH3CH2CHO + H2O
propan-1-ol propanal

Question 13

In the preparation of an aldehyde ,

1. why can it be distilled and
2. why is it distilled off from the reaction vessel?

Answer 13

1. it can be distilled off because the aldehyde product will always have a lower BP than its corresponding alcohol
2. If the aldehyde is not distilled as soon as it forms, further heating with acidified potassium dichromate will oxidise the aldehyde formed further into a carboxylic acid

Question 14

Preparation of a ketone :

1. What is the general equation
2. What 2 conditions are there
3. Example using propan-2-ol

☆☆☆Fact: ketones can not be oxidised further (therefore,no need for distilling )

Answer 14

1. Secondary alcohol + Oxygen atom from oxidising agent ➡ ketone + water
2. Conditions :● potassium dichromate, acidified with sulfuric acid , K2Cr2O7● heat

Question 15

Chemical reduction of aldehydes and ketones form what in general. …?

Answer 15

Alcohols

Question 16

Reduction of aldehydes and ketones :

1. What is the general equation for each
2. 2 options for conditions

☆☆☆Fact : the equation is the reverse of the oxidation one just with the aldehyde or ketones with [ H ] ( number in front of H if you need to balance out the Hydrogens on the equation )

Answer 16

1. Aldehyde + reducing agent ➡primary alcoholKetone + reducing agent ➡ secondary alcohol
2. Conditions options:

● warming the ketone / aldehyde with aqueous alkaline solution of NaBH4

                         😜  Or

●adding LiAlH4, dissolved in dry ether at room temperature

Question 17

Nucleophilic addition with HCN :

1. Instead of the C=C breaking like in normal addition , what bond breaks because aldehydes and ketones don’t have a C=C bond ?
2. Example of propanal + HCN
3. What happens to the Carbon chain in this reaction

Answer 17

1. The C=O bond

CH3CH2 OH
\ I
C =O + HCN ➡ CH3CH2 -C-C☰N
/ I
H H
propanal 2-hydroxybutanenitrile

3. It increases the length of the carbon chain in the original aldehyde molecule by one carbon atom

Question 18

The nitrile group produced from nucleophilic addition with HCN of an aldehyde or ketone can then easily have 2 main reactions.

The nitrile group then can be easily : 2 reactions

Answer 18

1. Hydrolysed to a carboxylic acid

2. Reduced to an amine

Question 19

What are the 2 conditions to hydrolysis a nitrile group (-C☰N) to a carboxylic acid and an example of the ionic equation :

Answer 19

Conditions :

● refluxing with

● dilute hydrochloric acid

Example :

-CN + H^+ + H2O ➡ -COOH + NH4^+

Question 20

The reduction of a nitrile group (-C☰N) gives an amine :

What is the condition and an example of the ionic equation ?

Answer 20

Condition :

● sodium and ethanol

Example of the ionic equation :

-CN + 4 [H] ➡-CN2NH2

Question 21

What is the carbonyl group?

Answer 21

The C=O

Question 22

The mechanism of nucleophilic addition with HCN to form a nitrile :

1. What is the nucleophile?
2. What makes the C atom open to attack by the nucleophile?
3. Describe the first step + an example only using the groups affected
4. Described the second step + a general example of the groups affected

Answer 22

1. The cyanide Ion , CN^- is the nucleophile
2. The carbon atom is ope to attack from the CN^- Ion be the carbonyl group (C=O) is polarised due to the high electronegativity of the Oxygen atom , leaving the Carbon with a s+ partial charge making it vulnerable to attack.

🤓Description of first step :

A negatively charged intermediate forms which is highly reactive and quickly reacts with an H^+ Ion ( from HCN, from the dilute acid or the H2O present in the reaction mixture ).

s+  ⤵                         I
   C=O            ➡       -C-O: ^- 
  ⤴ s-                         I
\:CN^-                          CN
                              Intermediate 

🤔 first arrow is from the :CN^- to the C atom

🤔 the 2nd arrow is from the C=O bond to the oxygen atom

4. Description : in this step a ☆☆☆ 2-hydroxynitrile ☆☆☆ product always forms . (* 1 exception is if you start with methanal then you get hydroxyethanenitrile with no 2 in front )

I
-C- O:^- ⤵⤵ I
I H - CN ➡ -C-OH + :CN^-
I
CN
2-hydroxynitrile

🤔 the first arrow is from the oxygen to the H atom

🤔 the 2nd arrow is from the H-CN bond to the C

Question 23

What are the 3 tests for aldehydes and ketones

Answer 23

1. Testing with 2,4 - DNPH
2. Testing with Tollen’s reagent
3. Testing with Fehling’s solution

Question 24

Testing with 2,4 -dinitrophenylhydrazine (2,4- DNPH) :

1. A positive test, if an aldehyde or ketone is present, will give a …..?
2. The reaction with 2 4 -DNPH and an aldehyde /ketone is an example of a (……….) Reaction
3. To identify the compound that precipitated what can be done ?

☆☆☆ Fact : other classes of organic compound that also contain the carbonyl group (e.g. Carboxylic acids and esters) do NOT form precipitate

Answer 24

1. • test = deep orange precipitate
2. A condensation reaction
3. By refrigerator to melting point data , from this a specific aldehyde /ketone can be identified ( not a favourable method though to identify, rather use other solutions )

Question 25

What are the 2 tests we can use to distinguish between aldehydes and ketones?

Answer 25

1. Testing with Tollens' reagent | 2. Testing with Fehling's solution

Question 26

Testing with Tollens' reagent;: 1. what is Tollens' reagent consist of? 2. What are the results for a positive test for an aldehyde and of a ketone ? ☆☆☆ Fact : when warmed the Ag^+ are reduced to silver atoms in the redox reaction with an aldehyde

Answer 26

1. Silver nitrate in excess ammonia 2. ■Result with an aldehyde : silver " mirror" on inside of tube ■Result with a ketone : no change, it remains a colourless mixture

Question 27

Testing with Fehling's solution : 1. What does Fehling's solution consist of? 2. What are the results with an aldehyde and ketone Condition. ..must be warmed with

Answer 27

1. It is an alkaline solution containing copper (II) ions , Cu^2+ 2. Results : ■ Aldehyde = Cu^2+ ions are reduced to Cu^+ . The clear blue Fehling's solution turns an opaque red/orange colour as a precipitate of copper (I) oxide forms {redox reaction} ■ ketone is not oxidised, so Fehling's solution remains blue

Question 28

Reactions to form tri- iodomethane : 1. It reacts with methyl ketones (CH3CO -group ) to form what colour precipitate? ☆note that ethanal a aldehyde also has a CH3CO group and so also has the same precipitate * exception , it is the only aldehyde 2. To hives a positive test the CH3 and C=O have to be .... 3. What is the reagent being used and an example of one ( it's a condition )? 4. The reaction involves 2 steps , describe each using an general equation (just the parts affected ) ☆☆☆ FACT : Yellow crystal of tri-iodomethane can be identified by their MP of 119°C

Answer 28

1. Yellow 2. +test= CH3 and C =O have to be next to each other 3. An alkaline solution of iodine , e.g. I2, NaOH 4. Step 1: The carbonyl compound is halogenated-the 3 H-atoms in the CH3 group are replaced by iodine atoms Step 2: the intermediate is hydrolysed to form the yellow precipitate of tri-iodomethane, CHI3 Step 1 step 2 I2, NaOH (aq) NaOH (aq) Halogenation hydrolysis RCOCH3 ➡ RCOCI3 ➡ RCOO^-Na^+ + CHI3 a methyl ketone tri-iodomethane Key : R - alkyl group in a methyl ketone

Question 29

Tri-iodomethane can be used to test for the presence of a secondary alcohol : 1. What is the group of atoms being tested 2. What are the 2 products of this reaction 3. What is formed before the reaction can follow the general equation of reactions to form tri-iodomethane

Answer 29

1. CH3CH (OH) Group 2. Tri-iodomethane and the sodium salt of a carboxylic acid 3. First the CH3CH (OH) is oxidised by the alkaline iodine solution. This forms the methyl ketone , RCOCH3, which reacts via the 2 steps of tri-iodomethane reactions .

Question 30

Infa-red spectroscopy helps to identify?

Answer 30

Organic compounds by their absorption of energy in the Infa-red range of wavelengths, matching their spectrum to a database of known Infa-red spectra.

Question 31

You are ....

Answer 31

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