Carbonyls

Question 1

How do you form an aldehyde?

Answer 1

oxidation of a primary alcohol

Question 2

How do you form a ketone?

Answer 2

oxidation of a secondary alcohol

Question 3

Are carbonyls reactive or unreactive to electrophiles?

Answer 3

unreactive

Question 4

How can nucleophiles react with carbonyls?

Answer 4

the carbon in the C=O bond is slightly positive as the O is electronegative

Question 5

What are the properties of carbonyls?

Answer 5

polar
high MP and BP
soluble in water

Question 6

Why is the MP and BP of carbonyls lower than alcohols and carboxylic acids?

Answer 6

there is no hydrogen bonding between molecules

Question 7

Why do carbonyls have a high MP and BP?

Answer 7

permanent dipole-dipole interactions between molecules

Question 8

What are the 4 reactions you need to know for carbonyls?

Answer 8

2,4-DNP
oxidation of aldehyde to carboxylic acid - Tollens test
nucleophilic addition x2:
1. reduction of carbonly compund to an alcohol
2. addition of HCN

Question 9

What happens when an aldehyde or ketone react with 2,4-DNP?

Answer 9

produces and orange to yellow precipitate

Question 10

How can you identify a sepcific carbonyl compound from reacting with 2,4-DNP?

Answer 10

finding the melting point of the recrystallised derivative and compare the MP to other known MP of carbonyl compunds

Question 11

What is the method of recrystallisation?

Answer 11

1. add minimum amount of hot solvent to dissolve crystals
2. cool solution
3. filter the crystals and rinse with cold solvent to leave them dry

Question 12

What is tollens reagent made of?

Answer 12

silver nitrate
ammonium hydroxide
sodium hydroxide

Question 13

How can an aldehyde be detected by Tollen’s test?

Answer 13

Tollen’s reagent oxidises aldehydes to carboxylic acids
warmed
silver ions are reduced to metallic silver producing a shiny silver mirror

Question 14

What reducing agent is used to reduce carbonyls into alcohols?

Answer 14

NaBH4

Question 15

What is a hydride ion?

Answer 15

H- = has 2 lone pairs
acts as a nucleophile to reduce carbonyls into alcohols

Question 16

What is the steps of nucleophilil addition of carbonyls with hydride ions?

Answer 16

1. H- acts as a nucleophile, attacking the slightly + carbonyl C
2. the intermediate acts as a base, taking H+ from H2O
3. leaves a Hydroxide ion and an alcohol

Question 17

What is a hydroxynitrile?

Answer 17

an alcohol with CN (cyanine) attached

Question 18

Why is a reagent needed for the reaction of a carbonyl with HCN?

Answer 18

HCN is a weak acid = only slightly dissociates into H+ and CN- in aq solutions
the undissociated molecure, HCN, is a weak nucleophile = reacts very slowly

Question 19

What is the reagent used in reacting a carbonyl with HCN?

Answer 19

NaCN / H+ = acidified, aq, sodium cyanide solution

Question 20

What happens to an aldehyde and ketone when added to Fehling’s solution?

Answer 20

aldehyde = forms dark red precipitate
ketone = stays blue

Question 21

What is Fehling’s solution made from?

Answer 21

copper sulfate solution

Question 22

what are enantiomers?

Answer 22

pair of molecules that are mirror images of each other

Question 23

What are optical isomers?

Answer 23

stereoisomers so they have the same structural formula but different arrangement of atoms in space
they are non-superimpossable mirror images of each other

Question 24

What is a chrial centre?

Answer 24

a central carbon atom with 4 different groups attached

Question 25

why would a different reagent of HCN be used instead?

Answer 25

HCN is highly toxic so a different CN compound would be safer to use

Question 26

How can nucleophilic addition of carbonyls lead to enantiomers being produced?

Answer 26

the carbonyl functional group is planar
so the nuceophile can attack from above or below the plane
aldehydes form chiral centres with 4 diff groups attached
so it can form opitcal isomers depending whether the nucleophile attacked from the top or bottom
ketone do not form chiral products = symmetrical