Nucleophilic Substitution At Sp3 Centers Flashcards

1
Q

How can we control which SN mechanism will occur?

A

Choice of solvent

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2
Q

what solvent is used for SN1 reactions and why

A

Polar protic
Polar: stabilises the separation of charge that increases as the reaction reaches the transition state (greater dipole moment) -> specifically for the anion/leaving group (favours ion formation)
Protic: solvation of the leaving group helps separate and stabilise it

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3
Q

Examples of polar protic solvents

A

Water, ethanol, acetic acid

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4
Q

How does greater polarity effect of a solvent affect the rate of SN1 reaction?

A

Great polarity means better charge separation meaning that it is easier to separate and stabilise the charges in the transition state thus lowering its energy -> faster SN1 rate

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5
Q

What are examples of polar aprotic solvents?

A

Acetone, DMF, DMSO

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6
Q

What are the 2 factors that determine the leaving group ability?

A
  1. Strength of C-X bond
  2. Stability of X-
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7
Q

What is the relationship between pKa and leaving group ability?

A

Higher pKa means that it is less acidic thus less likely to form H+ and the anion thus is not a good leaving group (vice versa)

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8
Q

What are the 3 ways to make OH a leaving group? what is the disadvantage of the first way?

A
  1. Protonating the OH group using an acid
    -> disadvantage is that the rest of the molecule might be unstable in acid
  2. Phosphorus halides
  3. Tosylation
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9
Q

What is the name of the reaction that allows for alcohols to directly react with nucleophiles? what is the reagent used called?

A

Mitsunobu reaction

di-ethyl azo dicarboxylate (DEAD)

-> forms SN2 reaction product (inversion)

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10
Q

Under what condition can a comparison of basicity be used to determine nucleophilicity?

A

When the attacking atom is the same

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11
Q

How does acidity of the conjugate base affect nucleophilicity?

A

If pKa is higher then species prefers being in protonated form rather than deprotonated. Since conjugate base is deprotonated form the higher then pKa the more reactive (more nucleophilic) the species is

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12
Q

Why are soft nucleophiles more nucleophilic than hard nucleophiles?

A

Soft nucleophiles have electrons that are less tightly held by nucleus (more polarisable) thus their HOMO sits at a higher energy level compared to the tightly bound HOMO in hard atoms. Thus the HOMO of the nucleophile would be closer in energy to the LUMO of the electrophile which produces better overlap (more favourable)

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13
Q

What is the relationship between energy difference of HOMO/LUMO and rate of reaction

A

Lower energy difference higher rate of reaction (inversely proportional)

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14
Q

how to make primary amines from halogenoalkane?

A

Azide (N3) ion then reduction with LiAlH4

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15
Q

If 2 reactants result in the same product after SN reactions how to determine which is the major one and why

A

More substituted is more stable thus becomes major product

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16
Q

what is more significant, sigma conjugation or resonance/delocalisation?

A

resonance/delocalisation

17
Q

what are the 3 things that can stabilize the transition state of an SN2 reaction?

A

benzylic, allylic and carbonyl compounds (no carbonyl for SN1)

18
Q

why do carbonyls not stabilise carbocation intermediates?

A

the carbonyl is electron withdrawing which destabilises the carbocation

19
Q

how does the dielectric constant determine the stability of the leaving group?

A

higher dielectric constant means that the ionic form is more stable, thus would make it easier to separate the charges for the leaving group (step 1) in an SN1 reaction

20
Q

what are the reagents required for tosylatio n? why is this reaction used?

A

TsCl + pyridine to deprotonate the species after being combined with the alcohol

it generates a good leaving group in TsO- which means good for nucleophile attack

21
Q

do SN2 reactions prefer softer or harder nucleophiles and why?

A

softer nucleophiles because their HOMO is closer in energy to the LUMO of the electrophile therefore better overlap which increases the rate of reaction

THEY DO NOT LIKE HARD NUCLEOPHILES VERY IMPORTANT

22
Q

what reagents are required to reduce azides into primary amines

A

LiAlH4 in ether or pd with H2