Addition reactions

Question 1

Addition reactions

Answer 1

An overall replacement of a (weak) pi-bond with a (strong) sigma-bond.

Question 2

What are the characteristics of an addition reaction?

Answer 2

• Reaction requires an acid
• Rate increases with acidity
• Rate is independent of nucleophile concentration
• involves 2 steps (first step - slow)

Question 3

What is Markovnikov’s rule?

Answer 3

In an addition reaction with an alkene/alkyne and HX:
• The hydrogen from HX will bond to the carbon atom with the greater number of Hydrogens in the starting alkene/alkyne.
• X will end on the side with more substituents

Question 4

What are the steps in an addition reaction?

Answer 4

1. Pair of electrons from the pi bond is donated to the electrophile
2. then nucleophile becomes a carbocation (electrophile)
3. accepts electrons from a nucleophile, e.g. Br-

Question 5

What is the order of carbocation stability in an addition reaction?

Answer 5

• 3° > 2° > 1°
• When markovnikov’s rule is followed (when H is added to side with most Hs), product is the major product and is most stable

Question 6

What happens if no nucleophile is added ?

Answer 6

• when an electrophile is added, e.g. H+
• pi bond electrons donated to H+
• nucleophile then becomes electrophile (carbocation)
• then this gets repeated and a polymer is formed

• ref. lecture 20

Question 7

Oxidation reaction

Answer 7

Refers to the loss of hydrogen (-H) or the addition of oxygen (+o) to an organic molecule

Question 8

Reduction reaction

Answer 8

Refers to the loss of oxygen (-O) or the addition of hydrogen (H+) to an organic molecule

Question 9

Oxidation of alcohols

Answer 9

1° > aldehydes&raquo_space; carboxylic acids

2° > ketones

3° > cannot be oxidised
- there are no C-H bonds to break (so nowhere for C-O bond to form)

Question 10

Rate of an addition reaction

Answer 10

Rate is independent of nucleophile concentration

Question 11

What steps occur when a nucleophile attacks a carboxyl group?

Answer 11

1. Nucleophile approaches, and pi bond electrons are transferred to O atom (this is intermediate where negative charge is on oxygen atom)
2. The electrons on the O then returns to become a double bond (C=O) - in the presence of acid, an alcohol will be produced
3. X leaves (leaving group)

This is more simply known as a nucleophilic substitution where X is the leaving group

Question 12

How are acid chlorides formed?

Answer 12

Carboxylic acids and thionyl chloride

Question 13

What are acid chlorides useful for?

Answer 13

Acid chlorides are useful intermediates for the preparation of esters and amides.

Question 14

How are esters formed?

Answer 14

Acid chlorides + alcohols = esters

Question 15

How are amides formed?

Answer 15

• acid chlorides + primary or secondary amines = primary or secondary amide
• tertiary amines are unreactive

Question 16

Reaction of carboxylic anhydrides?

Answer 16

• carboxylic anhydrides have similar reactivity to acid chlorides
• reacts with alcohols to produce esters
• react with amines to produce amides

• one equivalent of carboxylic acid is released (by-product) lec. 20

Question 17

Nucleophilic addition of hydride (H-)

Answer 17

Leads to reduction of carbonyl groups