Carbonyl chemistry- Carbonyl overview

Question 1

Describe the bonding in a carbonyl bond

Answer 1

1. One strong sigma
2. One weaker pi-bond
3. Double bond is formed by overlap of the orbitals of an sp2 hybridised carbon atom with an sp2 hybridised oxygen atom
4. Sigma is formed by end on overlap of an sp2 orbital of carbon with an sp2 orbital of oxygen
5. Pi is formed by side on overlap of an unhybridized p-orbital on carbon with an unhybridized p orbital on oxygen.
6. The 2 remaining sp2 orbitals of oxygen atoms each contain a lone pair of electrons .
7. The two remaining sp2 orbitals of the carbon form the two other sigma bonds
8. All three sigma bonds to carbon are in the same plane and are separated by angles of 120 degrees.

Question 2

Describe the length and stability of c-o, c=c and c=o bond

Answer 2

1. C-O - longer and weaker bond, 153 pm +347
2. C=C- 134 pm, +612
3. C=O - shorter, stronger bond 121 pm, +742

Question 3

Where are the sites of reactivity on a carbonyl

Answer 3

1. Delta positive carbon- electrophilic and suscpetible to attack from nucleophiles
2. Delta negative oxygen- lewis basic oxygen so reacts with lewis acids/bronsted acids
3. Weakly acidic alpha-proton- can be deprotonated to form carbon nucleophile

Question 4

What is the approximate pKa of a carboxylic acid

Answer 4

1. 5

Question 5

What are the two basic mechanistic steps of a nucleophilic attack on a carbonyl

Answer 5

1. Nucleophilic attack on the carbonyl

2. Protonation of resulting anion or loss of leaving group

Question 6

What type of reaction do you get if there is no leaving group on a carbonyl

Answer 6

1. Nucleophilic addition

2. Forms alcohol

Question 7

What type of reaction do you get if there is a leaving group on a carbonyl

Answer 7

1. Nucleophilic substitution

2. Still have carbonyl

Question 8

What is the angle of nucleophilic attack and why

Answer 8

1. 107 degrees- Burgi-Dunitz angle
2. Nucleophiles donate pair of electrons into the pi*-antibonding orbital of the c=o
3. But oxygen with electron density repels it so there is not maximum overlap
4. So it pushes itself outside the plane and attacks at 107 degrees- no repulsion and bond forming can occur

Question 9

State the order of reactivity of carbonyls from most to least

Answer 9

1. Acyl chloride
2. Acid anhydride
3. Aldehyde
4. Ketone
5. Ester
6. Carboxylic acid
7. Amide

Question 10

Describe the order of reactivity difference between acyl chloride and amide

Answer 10

1. Acyl chloride have large difference in electronegativity and so have the most electrophilic carbon and pull electron density towards the Cl- inductive effect
2. They can’t donate a lot of electron density back into the pi system- mesomeric effect
3. Amide has the least electrophilic carbon as not a large difference in electronegativities
4. Amide has a lone pair on the nitrogen so p-orbital can overlap with 2p orbital in pi system. Gives lone pair away so easily so mesomeric effects are much stronger than inductive
5. Net result is carbon in amide is less electrophilic so less reactive towards nucleophiles

Question 11

Describe the reactivity position of ketones and aldehydes

Answer 11

1. They add electron density into the system
2. But do this by pushing through the bond which is not as effective as pushing through orbitals and lone pairs
3. They add electron density to carbonyl compound but not as much as if they had lone pairs on oxygen or nitrogen

Question 12

Why is the inductive effect for acyl chlorides stronger than the mesomeric effect

Answer 12

1. Poor orbital overlap

2. Chlorine 3p orbital with lone pair of electrons is larger

Question 13

Compare how electrophilic a carboxylate and carboxylic acid are

Answer 13

1. Carboxylate is less electrophilic than carboxylic acid
2. It has a full negative charge as well as the lone pair to donate back into c delta + so the carbon is a lot less electrophilic

Question 14

What is the pKa of the conjugate acid of the leaving group for acyl chloride

Answer 14

1. HCl= -5

Question 15

What is the pKa of the conjugate acid of the leaving group for acid anhydride

Answer 15

1. Carboxylic acid= 5

Question 16

What is the pKa of the conjugate acid of the leaving group for ester

Answer 16

1. Alcohol= 15

Question 17

What is the pKa of the conjugate acid of the leaving group for an amide

Answer 17

1. NH3= 30

Question 18

What is the pKa of the conjugate acid of the leaving group for an aldehyde and ketone

Answer 18

1. R-H= 50

Question 19

What does the pKa of the conjugate acid of the leaving group for aldehyde and ketones mean

Answer 19

1. Worst leaving group as high pKa so weak acid
2. Therefore aldehydes and ketones do not undergo substitution at the carbonyl centre- leaving group rubbish
3. Only undergo nucleophilic addition

Question 20

Why is an aldehyde more reactive than a ketone

Answer 20

1. Ketones have more alkyl groups surrounding the carbonyl
2. Inductive effects of alkyl groups surrounding the carbonyl reduce reactivity
3. Also pi* antibonding orbital is less sterically crowded in aldehydes- better overlap by nucleophile

Question 21

What are two things that affect the strength of an acid

Answer 21

1. Strength of H-X bond

2. Stability of conjugate base ( most important for organic acids)

Question 22

Why is the alpha proton acidic

Answer 22

1. pKa of alpha-hydrogen in aldehyde or ketone is 17-20

2. The enolate ion produced is stabilised by resonance so that is why they are relatively acidic

Question 23

Which resonance form of enolate ions is more stable

Answer 23

1. The resonance form with the negative charge on the electronegative oxygen and c=c bond is more stable

Question 24

What does the acidity of the alpha hydrogen mean

Answer 24

1. Enolate ions are strong nucleophiles

2. Can react with various electrophiles to introduce a new substituent at the alpha-position