Carbonyls, acids and esters Flashcards Preview

A2 Chemistry (Unit 1: F234) > Carbonyls, acids and esters > Flashcards

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1

What is the carbonyl group?

C=O

2

How is the double bond in the carbonyl group formed?

- The double bond consists of 2 types of bonds between the C and O, the σ bond and the π bond.

- The σ bond is formed by the front-on overlap of orbitals in the C and O atoms, each contributing 1 electron to the bonding pair of electrons.

- The π bond is formed by the side-on overlap of p orbitals in the C and O atoms, each contributing 1 electron the the bonding pair of electrons, forming 2 bonding regions; one above and one below the σ bond.

- The front-on overlap is a better overlap so the σ bond is stronger than the π bond.

3

What is the difference between the C=C bond and the C=O bond?

O is much more electronegative than C and attract the bonding pairs of electrons more, resulting in the C=O bond being polar whereas the C=C bond is not.

4

How are carbonyls named?

- Aldehydes are named with -al as the suffix and the numbering of the carbon the carbonyl is attached to ensures the smallest number possible.

- Ketones are named with -one as the suffix and the numbering of the carbon the carbonyl is attached to also ensures the smallest number possible.

5

How are aldehydes made?

- Oxidation of primary alcohols with a suitable oxidising agent; for example, acidified dichromate ions (K2Cr2O+ H2SO4) will convert the primary alcohol into an aldehyde, then into a carboxylic acid.

- The aldehyde needs to be distilled as it's being made in order to prevent it from becoming a carboxylic acid.

- As the reaction occurs, the colour of the dichromate solution changes from orange to green.

6

How are ketones made?

- Oxidation of secondary alcohols with a suitable oxidising agent, for example, acidified dichromate ions (K2Cr2O7 + H2SO4) will convert the secondary alcohol into a ketone.

- Because the ketone is the only and final product in this reaction, the reaction may be carried out under reflux to ensure that all the alcohol has been converted into ketones.

- As the reaction occurs, the colour of the dichromate solution changes from orange to green.

7

How are carbonyls reduced?

Carbonyls can be reduced by heating them with a suitable reducing agent (e.g. NaBH4, Sodium borohydryde). Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols. NaBH4 is a relatively weak reducing agent so will only attack the carbonyl groups on a multi-functional group compound.

8

What is the mechanism for the reduction of carbonyls?

Nucleophilic addition

9

What are the steps involved in nucleophilic addition?

1. NaBH4 is a source for H- ions, which acts as the nucleophiles.

2. The δ+ C atom in the C=O bond attracts the H- ion, which attacks the C atom to form a new C-H bond with its lone pair of electrons.

3. The π bond breaks and a negatively charged intermediate is formed on the O atom in the C=O bond.

4. The O- intermediate donates a pair of electrons to an H atom in H2O, forming a dative O-H bond. An O-H bond breaks in H2O, with the electron pair going to the oxygen atom to form an OH- ion.

5. An alcohol and a hydroxide ion is formed.

 

10

What is the general formula for the reduction of an aldehyde?

R-CHO + 2 [H] → R-CH2OH

11

What is the general formula for the reduction of a ketone?

R-CO-R' + 2 [H] → R-CH(OH)-R'

12

What is the test for a carbonyl?

1. 2,4-DNPH (2,4-Dinitrophenylhydrazine) is added to a mixture of methanol and sulfuric acid to make Brady's reagent.

2. If an orange precipitate is formed after adding Brady's reagent to the compound in question, then the compound is a carbonyl.

13

How is an aldehyde distinguished from a ketone?

1. Determine that the compound is a carbonyl via the 2,4-DNPH test.

2. Tollen's reagent is made by making Ag2O from NaOH and AgNO3 and then redissolving in aqueous ammonia.

3. Heat the compound in question with a few drops of Tollen's reagent in a test tube. If a 'silver mirror' is formed on the sides of the test-tube, the compound is an aldehyde. If not, the compound is a ketone.

4. Ag2O is a weak oxidising agent and will oxidise an aldehyde to a carboxylic acid to form silver solid, but will not oxidise a ketone.

14

How is the exact identity of a carbonyl identified after performing the 2,4-DNPH and silver mirror test?

1. The orange 2,4-DNPH derivative of the carbonyl from the first test is purified with recrystallisation and then filtered under reduced pressure.

2. The purified orange/yellow crystal then undergoes melting point analysis and the results are compared to a data table in order to determine the carbonyl compound derives from.

15

What is the functional group present in carboxylic acids?

Carboxyl group (-COOH).

16

What is the solubility of carboxylic acids in water?

Carboxylic acids are very soluble in water due to the fact that they contain a polar hydroxyl (-OH) group which is capable of making hydrogen bonds with water molecules. However, the solubility decreases as the size of the molecule increases becasue the size of the non-polar part of the molecule incapable of hydrogen bonding with water also increases, making it harder for the molecule to dissolve.

17

Why are hydroxyl groups on carboxylic acids more acidic than those on alcohols?

A lone pair of electrons on the O in the carboxyl -OH groups can partially delocalise across the 2 Os (C-O- and C=O) in the carboxyl group when -OH dissociates to -O-. This means that -O- only partially exists in carboxylic acids, whereas -Oexists fully in dissociated alcohols. The high -ve charge on the alcohol -O- quickly reattracts any dissociated H+ ions, so alcohol -OH do not dissociate easily, whereas the partial -ve charge on the -O- has a weaker attraction towards dissociated H+ ions so carboxyl -OH dissociate more easily than alcohol -OH.

18

What is the name given to salts of carboxylic acids formed from typical acid reactions?

Carboxylate salts.

19

What is the test for a carboxylic acid?

React with carbonate ions to produce effervescence or use indicator to test pH.

20

How is an ester made?

Heating an alcohol with a carboxylic acid in the presence of a concentrated acid catalyst (e.g. sulfuric acid). The reaction is:

R-OH + R'-COOH ⇔ R-COO-R' + H2O

The equilibrium lies to the right since the concentrated acid also acts as a dehydrating agent and removes water formed from this reaction, shifting p.o.e to the right.

21

What is a more efficient method for forming an ester?

Gently heating an acid anhydride with an alcohol to form an ester and a carboxylic acid. The reaction is non-reversible therefore the yield is higher than that of a carboxylic acid-alcohol reaction. The equation is:

R-COOCO-R + R'-OH → R-COO-R' + R-COOH

22

What is hydrolysis?

A reaction with water or hydroxide ions that that breaks a compound into two compounds.

23

How can an ester be hydrolysed by an acid?

When making an ester with a carboxylic acid and an alcohol with acid catalyst, the reaction is reversible, with the reverse reaction being the hydrolysis reaction. The catalyst increases rate of forward and reverse reaction equally, but p.o.e shifted to the right as concentrated acid is a dehydrating agent and removes water. During acid hydrolysis reaction, an ester is refluxed with a dilute acid catalyst to eliminate the dehydrating properties of the catalyst and move p.o.e bach toward the left. The equation is:

R-COO-R' + H2O ⇔ R-COOH + R'-OH

24

How can an ester be hydrolysed by an alkali?

Esters can also be hydrolysed by alkali. This reaction is non-reversible therefore produces a higher yield than the acid hydrolysis. The equation is:

R-COO-R' + OH- → R-COO- + R'-OH

Instead of the acid, the carboxylate salt is produced since any acid produced by the reaction immediately reacts with the alkali to form a salt.

25

What are some of the uses for esters?

Esters are responsible for many natural flavours and aromas found in plants.

- Oil of wintergreen is the ester that can be obtained by steam distilling organic plant matter. It can be used as an ointment for mild pain relief.

- Benzyl ethanoate is an important ester in the jasmine flower aroma and is used in many perfums and fragrences, as wells as to give the apple/pear taste in some drinks. 

26

What is the difference between a fat and an oil?

Fats have melting points above room temperature and oils have melting points above room temperature. This means that fats are solid and oils are liquids at room temperature.

27

What are fatty acids?

Carboxylic acids made from long lengths of hydrocarbon varying in structure containing a number of single and double bonds. Most natural fatty acids have an even number of carbon atoms.

28

What is the systematic name for glycerol?

Propane-1,2,3-triol

29

What is a triglyceride?

Most commonly consumed plant and animal fats are triglycerides. They consist of a glycerol molecule bonded to 3 fatty acid molecules via ester bonds as a result of 3 separate condensation reactions.

30

What is the difference between an unsarturated and a saturated fats?

A saturated fat contains fatty acids with hydrocarbon chains containing no double bonds.

A monounsaturated fat contains one double bond and a polyunsaturalted fat contains more than 1 double bond.