Lecture 13 - Carbonyl group chemistry and biochemistry

Question 1

Draw an aldehyde

Answer 1

Question 2

Draw a ketone

Answer 2

Question 3

Draw a carboxylic acid

Answer 3

Question 4

Draw an acyl chloride

Answer 4

Question 5

Draw an ester

Answer 5

Question 6

Draw a thioether

Answer 6

Question 7

Draw a thioester

Answer 7

Question 8

Draw an acid anhydride

Answer 8

Question 9

How are acid anhydrides formed? Hint: links to their name

Answer 9

• from two carboxylic acids
• water molecule is removed (hence anhydride)

Question 10

Describe the electronegativity of the carbonyl C=O bond. How does this effect reactivity?

Answer 10

• the oxygen is an EN. It is partially negatively charged
• the carbon is electrophilic. It is partially positively charged. Therefore, it is susceptible to attack from nucleophiles- regions of high electron density like lone pairs and negative charges.

Question 11

Draw the pi and sigma bonds in a carbonyl. Which bond is broken first?

Answer 11

• the pi bond is broken first because it is weaker.

Question 12

Draw the two mechanisms (so two possible products) for the reaction between this carbonyl compound and this nucleophile.

Answer 12

Question 13

Describe the inductive effect in this compound in regards to the hydrogens at the alpha position.
Also, draw and name the product of this mechanism.

Answer 13

The H atoms at the alpha position are acidic. The partially positive carbon draws electron density away from H atoms and towards them .

the product is an enolate.

Question 14

List these compounds in order of reactivity: acid anhydride, ester, ketone, acyl chloride, aldehyde and amide.

Answer 14

Most

acyl chloride
acid anhydride
aldehyde
ketone
ester
amide

Least

Question 15

order of reactivity is due to three things

Answer 15

leaving group ability, inductive effect and mesomeric effect.

Question 16

Why is acyl chloride more reactive (and a better acid) than acid anhydride. It links to leaving group ability

Answer 16

The Cl- is a better leaving group than the carboxylate of the acid anhydride.
This because the Cl- is more stable than carboxylate.
Makes acyl chloride a better acid (pka= -7) than acid anhydride (pka = ~5)

Question 17

Why are aldehydes more reactive than ketones (inductive effect)

Answer 17

Ketones have two alkyl group and aldehydes have one.

Alkyl groups push electron density towards the partially positive carbon atom

More alkyl groups= more electron density is pushed towards the partially positive carbon atom= less partially positively charged

The C=O bond is less polar in ketone than aldehyde. Therefore, the aldehyde is more reactive.

Question 18

The mesomeric effect is seen in both esters and amides. Why?

Answer 18

Esters and amides have lone pairs of electrons in unhybridized p orbitals adjacent to pi systems.

Question 19

Why are esters and amides not very reactive?

Answer 19

The positive mesomeric effect pushes electron density into the pi system. The partially positively charged carbon atom is therefore less electrophilic. Less electrophilic= less reactive.

Question 20

Why are esters more reactive than amides?

Answer 20

oxygen is more electronegative than nitrogen and has stronger attraction for electron, making the partially positive carbon a bit more positive

Question 21

Why do rotamers arise in amides?

Answer 21

Due to the mesomeric effect we get some double bond character in the C-N bond. This prevents rotation and creates rotamers.

Question 22

Label this rotamer.

Answer 22

E, trans. The R groups are on opposite sides of the C-N bond

Question 23

Amides are not basic. Why?

Answer 23

Due to the mesomeric effect- their lone pairs are not available.

Question 24

Amines are basic. Why?

Answer 24

No mesomeric effect- electrons are available.