Lecture 13 - Carbonyl group chemistry and biochemistry Flashcards
Draw an aldehyde
Draw a ketone
Draw a carboxylic acid
Draw an acyl chloride
Draw an ester
Draw a thioether
Draw a thioester
Draw an acid anhydride
How are acid anhydrides formed? Hint: links to their name
- from two carboxylic acids
- water molecule is removed (hence anhydride)
Describe the electronegativity of the carbonyl C=O bond. How does this effect reactivity?
- 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.
Draw the pi and sigma bonds in a carbonyl. Which bond is broken first?
- the pi bond is broken first because it is weaker.
Draw the two mechanisms (so two possible products) for the reaction between this carbonyl compound and this nucleophile.
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.
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.
List these compounds in order of reactivity: acid anhydride, ester, ketone, acyl chloride, aldehyde and amide.
Most
acyl chloride
acid anhydride
aldehyde
ketone
ester
amide
Least
order of reactivity is due to three things
leaving group ability, inductive effect and mesomeric effect.
Why is acyl chloride more reactive (and a better acid) than acid anhydride. It links to leaving group ability
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)
Why are aldehydes more reactive than ketones (inductive effect)
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.
The mesomeric effect is seen in both esters and amides. Why?
Esters and amides have lone pairs of electrons in unhybridized p orbitals adjacent to pi systems.
Why are esters and amides not very reactive?
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.
Why are esters more reactive than amides?
oxygen is more electronegative than nitrogen and has stronger attraction for electron, making the partially positive carbon a bit more positive
Why do rotamers arise in amides?
Due to the mesomeric effect we get some double bond character in the C-N bond. This prevents rotation and creates rotamers.
Label this rotamer.
E, trans. The R groups are on opposite sides of the C-N bond
Amides are not basic. Why?
Due to the mesomeric effect- their lone pairs are not available.
Amines are basic. Why?
No mesomeric effect- electrons are available.
