Carbonyl Chemistry 1: Intro

Question 1

What causes polarisation in the carbonyl group?

Answer 1

Difference in electronegativity between the carbon and oxygen atoms
Oxygen is highly electronegative so withdraws electron density from carbon

Question 2

Name different homogenous groups that contain the carbonyl group.

Answer 2

Carboxylic acids
Aldehydes
Ketones
Esters
Acyl chlorides
Acid anhydrides
Amides
Imines

Question 3

What is the systemic name of formaldehyde?

Answer 3

Methanal

Question 4

What is the systemic name of acetone?

Answer 4

Propanone

Question 5

Describe the physical properties of aldehydes and ketones.

Answer 5

Both can form hydrogen bonds with water molecules
Cannot act as hydrogen bond donors
Exhibit dipole-dipole intermolecular forces

Question 6

Describe the physical properties of carboxylic acids.

Answer 6

Carboxyl groups possess two electronegative oxygen atoms
Both entities are able to participate in hydrogen bonds
The hydroxyl group is able to aact as a hydrogen bond donor
Carbonyl is able to act as hydrogen bond acceptor
Extensive network of hydrogen bonds exists between carboxylic acids as a result
Unusually high melting and boiling points

Question 7

Describe the physical characteristics of esters.

Answer 7

Characteristic, sweet smells
Formed when carboxylic acids and alcohols react
Condensation reaction
Lack the ability to form hydrogen bonds due to lack of hydrogen connected to an oxygen
Considerably lower melting and boiling points compared to equilvalent carboxylic acids

Question 8

What does a double headed arrow represent?

Answer 8

Resonance structures

Question 9

Describe the hybridisation of the C and O atoms in carbonyl compounds.

Answer 9

They are both sp2 hybridised
The oxygen atom forms 1 alpha bond and 1 pi bond to the carbon atom and has 2 lone pairs
The carbon atom forms 1 alpha bond and 1 pi bond to the oxygen atom and has 2 additional alpha bonds to other substituents

Question 10

Describe the shape/ bond angles of carbonyls.

Answer 10

Trigonal planar at the C and O atoms
Bond angle 120
2 lone pairs on the O atom (in sp2 orbital)

Question 11

What is a resonance structure?

Answer 11

Different depictions of the same molecule showing ‘alternative’ distribution of electrons
Shows movement of electrons to the electronegative atom, creating a (-) region
And a positive region on the more positive atom
The real structure is somewhere between the two

Question 12

What is the most common reaction with carbonyls?

Answer 12

Nucleophilic addition
Due to the polarisation of the carbonyl bond
Allowing the delta positive carbon to attract nucleophiles

Question 13

How can you amplify electrophilicity of the carbonyl group?

Answer 13

By protonation
Makes the substrate positively charged overall and therefore more electrophilic (it wants to neutralise the positive charge)

Question 14

What are the two stages of nucleophilic addition to aldehydes?

Answer 14

1) Addition of the nucleophile
2) Protonation of the alkoxide intermediate by ‘acidic workup’ (essentially adding a H+ to be attracted to the O-)

Question 15

Why are aldehydes more reactive than ketones?

Answer 15

1) More steric hindrance in ketones: alkyl groups are bulky so have more steric hindrance than hydrogen atoms
2) Aldehydes have greater electron density so they reduce the positive charge on the carbon atom to make it less electrophilic

Question 16

What are Grignard reagents?

Answer 16

Carbon nucleophiles
R-MgBr
Can be used to add extra carbons to a molecule

Question 17

What nucleophiles would you use to generate more C-H bonds on a carbonyl molecule?

Answer 17

Hydride nucleophiles
I.e. NaBH4, LiAlH4

Question 18

What are tautomers?

Answer 18

Isomers of each other where a proton is transferred intramolecularly
Known as keto-enol tautomerism
They are different molecules NOT resonance structures

Question 19

In keto-enol tautomerism, which is the more stable form?

Answer 19

The ketone form

Question 20

How can enolates be used as nucleophiles?

Answer 20

Ketone reacts with strong base to form enolate ions which are good nucleophiles
Can react with a carbonyl compound to form a new C-C bond

Question 21

How many resonance structures do 1,3-dicarbonyls have?

Answer 21

Three major resonance structures
Negative charge is delocalised into both carbonyl bonds

Question 22

How does pKa relate to acidity?

Answer 22

Higher pKa = weaker acidity

Question 23

What is an imine?

Answer 23

Compound formed when primary amines are added to a carbonyl group
Leads to formation of a C=N bond and elimination of water

Question 24

Give a general equation for imine formation.

Answer 24

ROH + H2N-R’ <-> HRNR’ + H2O
(equilibrium)
Acid catalysed, optimal pH is 5

Question 25

What are the steps involved in the mechanism of imine formation?

Answer 25

1) Addition of amine
2) Protonation and deprotonation
3) Elimination of water
4) Deprotonation

Question 26

Compare the characteristics of carbonyl containing functional groups.

Answer 26

Aldehydes and ketones have same oxidation level and similar reactivity

Carboxylic acids, acyl chlorides, acid anhydrides, esters, and amides have the same oxidation level, varying reactivity with some similarities

Question 27

How do carboxylic acids tend to react?

Answer 27

As acids with a base to form stable carboxylate ions
Nucleophilic addition to carboxylic acids is very rare
Most nucleophiles are basic so act as bases with carboxylic acids

Question 28

Why is nucleophilic addition to a carboxylate anion highly unfavourable?

Answer 28

Because carboxylate anions already possess a negative charge so are unwilling to accept more electron density