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Chemistry 101 COPY > Substitution Reactions SN1 and SN2 > Flashcards

Substitution Reactions SN1 and SN2 Flashcards

1
Q

If substitution occurs at a chiral centre, what two things can occur?

A

Inversion Racemisation

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2
Q

If you start with a drug precursor and react it via an SN2 mechanism, how much of the drug with the required chirality will be obtained?

A

100%

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3
Q

If you start with a drug precursor and react it via an SN1 mechanism, how much of the drug with the required chirality will be obtained?

A

50%

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4
Q

What three things do we need to look at to determine whether an SN1 or SN2 reaction will occur (in order)?

A

Substrate

Nucleophile

Solvent

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5
Q

What is the main requirement of the substrate for an SN2 reaction?

A

To minimise steric hindrance

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6
Q

By what mechanism(s) can primary substrates react?

A

Primary substrates always react via SN2

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7
Q

What is the main requirement of the substrate for an SN1 reaction?

A

SN1 reactions need to be able to form a stable carbocation

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8
Q

By what mechanism(s) can tertiary substrates react?

A

Tertiary substrates always react via SN1

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9
Q

By what mechanism(s) can secondary substrates react?

A

Either SN1 or SN2

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10
Q

Is the rate of an SN2 reaction dependent on the concentration of the nucleophile?

A

Yes

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11
Q

Is the rate of an SN1 reaction dependent on the concentration of the nucleophile?

A

No

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12
Q

What is a good way to determine what mechanism a secondary substrate will use to react?

A

The strength of the nucleophile

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13
Q

How does a strong nucleophile effect an SN2 reaction?

A

Strong nucleophiles speed up an SN2 reaction (reactions with strong nucleophile are always SN2)

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14
Q

How does a weak nucleophile effect an SN2 reaction?

A

Weak nucleophiles slow down an SN2 reaction, allowing the SN1 mechanism to occur (weak nucleophiles tend to SN1)

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15
Q

How do we force the reaction in the forward direction?

A

Use a nucleophile which is stronger than the leaving group.

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16
Q

Why do primary substrates always react via SN2?

A

As they have the least sterically hindered delta positive carbon atom

17
Q

Why can secondary substrates react via both SN1 and SN2?

A

There is some steric hindrance but it is not overwhelming, but also forms a partially stable carbocation

18
Q

Why do tertiary substrates always react via SN1?

A

As they form the most stable carbocation

19
Q

Why do we use solvents in SN1 and SN2 reactions?

A

To make solids react because atoms in solids don’t collide

20
Q

What are the two types of solvents that can affect which mechanism occurs?

A

Polar Protic

Polar Aprotic

21
Q

What are polar protic solvents?

A

Solvents with an OH group

22
Q

What is a good way to look for a polar protic solvent?

A

Look for the suffix “Ol”

23
Q

What are polar aprotic solvents?

A

Polar solvents without an OH group

24
Q

How will reaction done with a secondary substrate in a polar protic substrate?

A

It will preferentially react via SN1 because the strength of the nucleophile is reduced and the carbocation is made stable

25
Q

Via what mechanism will a primary substrate react?

A

SN2

26
Q

Via what mechanism will a secondary substrate react?

A

SN1/SN2

27
Q

Via what mechanism will a tertiary substrate react?

A

SN1

28
Q

Via what mechanism will a secondary substrate with a strong nucleophile react?

A

SN2

29
Q

Via what mechanism will a secondary substrate with a weak nucleophile react?

A

SN1

30
Q

Via what mechanism will a secondary substrate with a strong nucleophile in a polar aprotic solvent react?

A

SN2

31
Q

Via what mechanism will a secondary substrate with a strong nucleophile in a polar protic solvent react?

A

SN1

32
Q

Via what mechanism will a secondary substrate with a weak nucleophile in a polar aprotic solvent react?

A

SN2

33
Q

Via what mechanism will a secondary substrate with a weak nucleophile in a polar protic solvent react?

A

SN1

34
Q

How do substitution reactions with a neutral nucleophile act?

A

They occur in the same way, but they need the extra step of deprotonising the positive product (acid/base), so you add a base (to remove the proton)

35
Q

If you use a weaker nucleophile than the leaving group, what will occur?

A

For every molecule of the nucleophile that reacts one molecule of the leaving group will form, forcing the reaction back to the starting materials

36
Q

How can we make a reaction with a weaker nucleophile than leaving group occur?

A

You make the leaving group better, and a weaker base than the nucleophile (you need something that’s replacing the leaving group to make it stable e.g. convert OH to H2O)

Chemistry 101 COPY (32 decks)

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