Lecture 20 - Alkenes as electrophiles

Question 1

Are aromatics USUALLY nucleophiles or electrophiles? What type of reaction to they undergo?

Answer 1

usually weak nucleophiles- regions of high electron density due to pi system - and so undergo electrophilic substitution reactions

Question 2

Aromatics can however sometimes be ……………. and undergo what type of reaction?

Answer 2

• electrophiles
• SnAR

Question 3

alkenes with isolated double bonds are nucleophilic or electrophilic? What type of reaction do they undergo?

Answer 3

nucleophilic - undergo electrophilic addition reactions

Question 4

Are carbonyls electrophilic or nucleophilic?

Answer 4

electrophilic - the carbonyl carbon is partially positively charged.

Question 5

Draw an alpha beta unsaturated carbonyl. These are not isolated alkenes. What do they contain instead?

Answer 5

conjugated double bond

Question 6

Draw an alpha, beta unsaturated carboxylic acid

Answer 6

Question 7

Draw an alpha, beta unsaturated ketone. What do we call these molecules?

Answer 7

enone

Question 8

draw the resonance forms for an enone. Explain how it has two electrophilic sites.

Answer 8

resonance forms show that electrophiles can attack two sites- two electrophilic sites. The partially positive carbon and the position at which there is a carbocation in one of the resonance forms

Question 9

Because there are two regions of electron density in alpha,beta unsaturated carbonyls, there are two routes of reactions with nucleophiles. Name them

Answer 9

1,2 / direct - the nucleophile attacks the carbonyl group directly

1,4 addition / conjugate addition / MICHAEL addition

Question 10

Draw the mechanism for a 1,2 / direct reaction between an alpha,beta unsaturated carbonyl with a nucleophile.

Answer 10

Question 11

Draw the mechanism 1,4 addition / conjugate addition / MICHAEL addition between an alpha,beta unsaturated carbonyl with a nucleophile.

Answer 11

Question 12

list carbonyls in order of reactivity. How does this effect the routes of reaction that their alpha, beta unsaturated versions react.

Answer 12

most :
- acyl chloride
- aldehyde
- ketone
- ester
- amide
Least

The nucleophile readily reacts at the 1,2 position for alpha, beta unsaturated acyl chloride because it is so reactive.

Amides are unreactive so the nucleophile attacks at the 1,4 position

Question 13

Electrophiles are toxic to the human body. Why?

Answer 13

we are nucleophilic

Question 14

Give three common nucleophiles we usually find in the body

What compounds are these typically found in?

Answer 14

R-OH
R-NH
R-SH

amino acids, enzymes and peptides

Question 15

Show how an alpha, beta unsaturated amide can change the shape of an enzyme, which contains the nucleophilic group SH. Why is this so bad?

Answer 15

If the shape of the enzyme is changed, it is denatured and so cannot perform it’s function.

Question 16

Draw the structure for glutathione. It is a tripeptide. What three amino acids does it contain? What is the important nucleophilic group it contains?

Answer 16

• glycerine
• cysteine
• glutamate

The SH group

Question 17

Glutathione sacrifices itself. What does this mean?

Give an example.

Answer 17

it reacts with toxic electrophiles - the nucleophilic SH group reacts with toxic electrophiles.

For example, reacts with paracetamol- converts into a toxic electrophile

Question 18

If you overdose on paracetamol it is toxic. Show the compound that paracetamol oxidises into- show the reaction and name the compound.

Answer 18

NAPQI

Question 19

If you overdose on paracetamol, all of the glutathione is used up to mop it up. So, you have to treat the person who has overdosed with WHAT? Draw and name the compound.

Answer 19

NAC - N-acetyl cysteine