Nucleophilic substitution: SN1

Question 1

What are the basic steps of an SN1 reaction?

Answer 1

Leaving groups departs, leaving the carbenium ion which is attacked by Nu-

Question 2

Which step is the rate determining step?

Answer 2

The slowest step

Question 3

Is the rate determining step reversible or irreversible?

Answer 3

Reversible

Question 4

Why is the fastest step irreversible?

Answer 4

There is electrostatic attraction between the negative nucleophile and the positive carbenium ion

Question 5

What is hyperconjugation?

Answer 5

Electrons from the sp3 hydrogen are donated into the empty p orbital in the same plane

Question 6

What is more effective hyperconjugation or pi conjugation

Answer 6

Pi conjugation

Question 7

What does S stand for?

Answer 7

Substitution

Question 8

What does N stand for?

Answer 8

Nucleophilic

Question 9

What does the 1 stand for?

Answer 9

Unimolecular

Question 10

What does unimolecular mean?

Answer 10

Only one molecule involved in the rate determining step

Question 11

What do the graphs for rate kinetics show?

Answer 11

If you change one molecule and keep the other constant, the rate increases
If you change the other while you keep the other constant, the rate stays the same
Shows only one is involved in RDS

Question 12

How many transition states are there for SN1 kinetics diagram?

Answer 12

two

Question 13

What are the transition states?

Answer 13

R—X

Nuc —- R

Question 14

Which transition state is higher in energy?

Answer 14

R—-X

Question 15

Which activation energy is the highest?

Answer 15

The one which causes the loss of the leaving group meaning that one is determining

Question 16

What are the two axes for a reaction diagram?

Answer 16

Energy on Y (With a scale)

Reaction co-ordinate on X

Question 17

What are the two SN1 substrate effects?

Answer 17

Stability of the carbenium ion intermediate

Leaving group ability of X-

Question 18

How does the stability of the carbenium ion intermediate effect?

Answer 18

The stability of the first transition state determines the rate of reaction, partially positive charge on the carbon atom is stabilised

Question 19

What is able to stabilise a carbenium intermediate?

Answer 19

Any electron donating group attached to the electron deficient carbon

Question 20

What are the most to least stable carbenium ions?

Answer 20

Tertiary
Secondary
Primary
Methyl

Question 21

How do alkyl groups help to stabilise the carbenium ion?

Answer 21

They are electron donating

Question 22

How does hyperconjugation stabilise the carbenium ion?

Answer 22

The alkyl groups stabilise the carbenium ion by donating electrons from pi bonds when overlapped with the p orbital

Question 23

Which type of conjugation is more effective at stabilising carbenium ions; pi or hyper?

Answer 23

Pi conjugation

Question 24

What does pi conjugation result in?

Answer 24

A positive carbenium ion where the positive charge is spread across two or more atoms

Question 25

How does pi conjugation stabilise a carbenium ion?

Answer 25

electrons in the filled pi bond are donated to the empty p orbital

Question 26

Where can a nucleophile attack in pi conjugation?

Answer 26

Occur at either end of the delocalised carbenium ion and mostly attacks at the less hindered end of the allyl cation

Question 27

What are the rates of SN1 reactions controlled by?

Answer 27

Electronics

Question 28

What is the relative rate of a primary alkyl and why?

Answer 28

0.07 | all sn2 reactions are likely - unstable carbenium ion intermediate

Question 29

What is the relative rate of a secondary alkyl and why?

Answer 29

0.12 | Poor SN1 but can still take place, not very stable carbenium ion

Question 30

What is the relative rate of a tertiary alkyl and why?

Answer 30

2,100 | Very good SN1 due to stability of the tertiary carbenium ion

Question 31

What type of conjugation do primary, secondary and tertiary alkyls have?

Answer 31

Hyperconjugation

Question 32

What is the relative rate of an allylic compound?

Answer 32

1 | Good SN1, pi conjugation

Question 33

What is the relative rate of an allylic compound which is also tertiary?

Answer 33

130,000 | Extremely good SN1, has both a tertiary centre and pi conjugation

Question 34

What is the relative rate of an allylic compound with a phenyl ring?

Answer 34

7,700 | Good SN1, pi conjugation

Question 35

Why are pi conjugated systems good?

Answer 35

Conjugation means a primary carbenium ion could become a tertiary carbenium ion (resonance forms) which is more stable and causes a really fast reaction rate

Question 36

How can heteroatoms stabilise carbenium ions?

Answer 36

By delocalisation | They possess a lone pair pf electrons

Question 37

How much of the lone pair of the heteroatoms can be donated to stabilise the carbenium ion?

Answer 37

100%

Question 38

What is the order of effectiveness for pi, sigma or lone pair?

Answer 38

Lone pair >> Pi >>>> sigma

Question 39

Why is chloro methyl ether sometimes used?

Answer 39

Alkyl halide which reacts with a range of nucleophiles by SN1 reactions

Question 40

What conditions does acetal hydrolysis take place in?

Answer 40

Acid hydrolysis

Question 41

In acetal hydrolysis, how is the carbenium ion stabilised/

Answer 41

By the adjacent oxygen atoms

Question 42

What is the influence of the nucleophile in the SN1 reaction?

Answer 42

The nucleophile doesn't affect the rate of reaction as they aren't involved in the rate determining step

Question 43

Why do nucleophiles affect the nature of the SN1 product? eg alcohols

Answer 43

Nucleophile may attack C+ even though there is more water present but if the water attacks it will produce the starting product again

Question 44

What is the stereochemistry like for an SN1 reaction?

Answer 44

They cause racemisation (50:50 mixture of products)

Question 45

How is racemisation caused in an SN1?

Answer 45

There is no stereochemistry in the planar carbenium ion which means there is equal attack on either face by the nucleophile

Question 46

When will the product be racemic?

Answer 46

Always even if the starting material is enantiomerically pure

Question 47

What does reaction from starting material to product go via? (racemisation)

Answer 47

A planar intermediate