3.2 Synthesis b - substitution reactions

Question 1

What is a substitution reaction?

Answer 1

an atom is replaced by another atom

Question 2

What are the 3 different types of substitution reaction?

Answer 2

• nucleophilic
• electrophilic
• radical

Question 3

What is a nucleophilic substitution reaction?

Answer 3

when a nucleophile provides an electron pair for bonding with the compound being transformed

Question 4

What are the two different types of nucleophilic substitution reactions?

Answer 4

• Sn1
• Sn2

Question 5

What is an Sn1 mechanism?

Answer 5

there is one species in the rate determining step and occurs in a minimum of 2 steps via a trigonal planar carbon intermediate

Question 6

What is the process of an Sn1 mechanism?

Answer 6

• begins with the loss of the leaving group (RDS)
• to give a carbocation
• this can then undergo attack by a WEAK nucleophile at either face

Question 7

What type of nucleophile takes part in an Sn1 reaction?

Answer 7

Weak nucleophiles

Question 8

What is a cation?

Answer 8

positive ion

Question 9

What is an anion?

Answer 9

negative ion

Question 10

Explain the role of inductive stabilisation of the carbocation intermediate in Sn1 reactions

Answer 10

• the rate of the first step (leaving group) depends on how stable the carbocation is
• the more stable, the faster the first step will occur
• an electron donating group will stabilise a carbocation
• carbocations with the most R groups are more stable

Question 11

What is an Sn2 reaction?

Answer 11

a nucleophilic substitution reaction with two species in the rate determining step. It occurs in a single step via a five centred, trigonal bipyramidal transition state

Question 12

What order is an Sn1 mechanism?

Answer 12

1st order

Question 13

What order is an Sn2 mechanism?

Answer 13

2nd order

Question 14

What is the process of an Sn2 reaction?

Answer 14

one bond is broken and one bond is formed in a concerted way

Question 15

Where does the nucleophile attack?

Answer 15

180 degrees from the leaving group

Question 16

What is steric hinderance?

Answer 16

this describes hoe a molecules physical structure can affect its ability to react

Question 17

Explain increases and decreases in steric hinderance?

Answer 17

• the least number of R groups in the transition state mean that the least steric hinderance between the nucleophile and the electrophile centre
• as the number of R groups increases, these R groups shield the backside of the electrophilic carbon, slowing the rate of reaction

Question 18

What is an electrophilic substitution reaction?

Answer 18

when a functional group from a compound is substituted with an electrophilic species. it involves 3 steps:
1) generation of an electrophile
2) formation of a carbocation
3) removal of a proton

Question 19

What is a radical substitution?

Answer 19

• using free radicals for substitution
• swapping one atom for another using free radicals