Org 7 - Aldehydes & Ketones

Question 1

What is the general structure of an aldehyde ?

Answer 1

suffix: -al

Question 2

What is the general structure of a ketone ?

Answer 2

Suffix: -one

Question 3

What is the functional group of aldehydes and ketones ?

Answer 3

The carbonyl group C=O

Question 4

Describe the Carbonyl functional group, including bond angles

Answer 4

Planar, with bond angles of 120º

The Carbonyl Carbon atom has 3 σ bonds.

The C=O bond is shorter and stronger than the C-O.

Question 5

What is an Aryl group ?

Answer 5

Benzene minus 1 hydrogen e.g. C₆H₅ Phenyl

Question 6

Name five important features of the Carbonyl group

Answer 6

1. Resonance
2. Polarity
3. Acidity of the alpha hydrogen
4. Keto-enol tautomerisation
5. Hydrogen bonding

Question 7

Sketch the two resonance structures of a ketone

Answer 7

Question 8

Why is polarity important in the Carbonyl group ?

Answer 8

It may participate in nucelophilic or electrophilic reactions. Nucleophiles attack the partially positive carbon, and electrophiles attack the partially negative oxygen.

In both of these reactions, the character of the double bond is altered.

Question 9

Where is the alpha carbon located in a Carbonyl compound ?

Answer 9

The alpha carbon is always adjacent to the functional group.

Question 10

What is the relationship between resonance structures and stability ?

Answer 10

If you can draw resonance structures for a compound, then it displays increased stability.

Question 11

Explain why acidity of the alpha hydrgen occurs in a carbonyl compound ?

Answer 11

The acidity is a result of the resonance stabilisation of the alpha carbanion formed.

Question 12

Are the beta and gamma hydrogens also acidic in a carbonyl compound ?

Answer 12

No, only the alpha hydrogen is acidic

Question 13

What happens to acidity if the alpha hydrogen is situated between 2 Carbonyl groups ?

Answer 13

It will display increased acidity

Question 14

What are tautomers ?

Answer 14

Constitutional isomers that readily interconvert (= tautomerization).

This happens so quickly that they are usually considered to be the same compound.

Question 15

What is an enol ?

Answer 15

Alkene & Alcohol

Question 16

What are conjugated double bonds ?

Answer 16

Double bonds separated by a single bond

Question 17

Sketch the equilibrium present in tautomerization

Answer 17

Question 18

Describe tautomerization ?

Answer 18

Fast interconnection between constitutional isomers

e.g. Carbonyl and Enol

Question 19

How do Carbonyl groups participate in hydrogen bonding ?

Answer 19

Question 20

Why don’t aldehydes and ketones form hydrogen bonds with themselves ?

Answer 20

There is no hydrogen on the carbonly oxygen

Question 21

What influences reactivity of the Carbonyl compounds ?

Answer 21

The carbonyl group is the functional group, but adjacent groups can influence reactivity, e.g. an e- withdrawing group adjacent to the carbonyl carbon will increase the partially positive charge on the carbon making the carbonyl group more attractive to a nucleophile.

An e- donating ligand would decrease the reactivity of the carbonyl group.

Question 22

How does steric hindrance influence the reactivity of the carbonyl compounds ?

Answer 22

Generally, aldehydes oxidise and more easily undergo nucleophilic addition than ketones.

Question 23

Explain oxidation of Carbonyl compounds ?

Answer 23

Aldehydes oxidise to Carboxylic acids (using agents such as KMnO₄, CrO₃ (Jones reagent), HNO₃, Ag₂O (Tollens)

Question 24

Describe a diagnostic tool for the presence of an aldehyde group ?

Answer 24

Using Tollen’s reagent (Ag₂O) metallic silver Ag is produced if an aldehyde is present.

Ketones cannot be oxidised to Carboxylic acids.

Question 25

Describe 7 reactions that result in the formation of aldehydes or ketones ?

Answer 25

* Ozonlysis * REDOX * Hydroformylation * 1º alcohols → Aldehydes (Oxidation) * 2º alcohols → Ketones (Oxidation) * Friedel-Crafts Acylation * Hydration of terminal alkynes

Question 26

Explain the difference in boiling points between alcohols and ketones ?

Answer 26

Alcohols exhibit hydrogen bonding, ketones exhibit dipole-dipole bonding (weaker intermolecular forces) therefore ketones have a lower boiling point.

Question 27

Why do alkanes have a lower boiling point than ketones or alcohols ?

Answer 27

* Alkanes - Intermolecular (london) forces * Ketones - Dipole dipole forces * Alcohols - hydrogen bonding London forces \< dipole dipole \< hydrogen bonding (In terms of relative bond strength)

Question 28

What is the impact of increasing alkyl groups on reactivity of the carbonyl group ?

Answer 28

The more alkyl groups present (and therefore, the longer the carbon chain), the more stable the carbonyl group. There is more polarisation in an aldehyde than a ketone, therefore a ketone is more stable.

Question 29

What are the two classes of reactions that carbonyl compounds undergo ?

Answer 29

* Nucleophilic addition reactions at the C=O bond * Reactions at adjacent positions

Question 30

Describe nucleophilic addition reactions in Carbonyl compounds ?

Answer 30

Question 31

Describe reduction of aldehydes and ketones ?

Answer 31

Reduction with Grignards reagent yields alcohols: * Formaldehyde → 1º Alcohol * Other aldehydes → 2º Alcohol * Ketones → 3º Alcohol N.B. Other reducing agents include NaBH₄, and LiAlH₄

Question 32

What is the general formula of a Primary amine ?

Answer 32

R-NH₂

Question 33

What is the product when a Carbonyl compound reacts with a primary amine ?

Answer 33

An Imine (Schiff base) is formed. The Carbonyl group is replaced with a C=N-R group.

Question 34

What is the product when a Carbonyl compound reacts with a secondary amine ?

Answer 34

An enamine is formed

Question 35

What is a secondary amine ?

Answer 35

Question 36

What is the product when a tertiary amine reacts with a Carbonyl compound ?

Answer 36

There is no reaction

Question 37

What is an Aldol ?

Answer 37

An aldol is both an aldehyde and an alcohol, e.g. the intermediate product in aldol condensation reactions

Question 38

What is Aldol condensation ?

Answer 38

The Aldol undergoes a dehydration reaction, producing a C-C bond in the condensation product (an enal - alkene and aldehyde)