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CHEM 203 > SN2 > Flashcards

SN2 Flashcards

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1
Q

What makes a good nucleophile?

A

(1) anionic (always in this class!)
(2) usually participates in Sn2 (so attack right away)
(3) may separate into metal ion
(4) strong base
(5) conjugate base of weak acid

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2
Q

What makes a poor nucleophile?

A

(1) neutral (always in this class!)
(2) usually participates in Sn1 (so wait for carbocation usually but there are exceptions)
(3) No metal ion (bc neutral)

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3
Q

When a poor nucleophile is used in a mechanism, what is the last step?

A

Deprotonation

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4
Q

What is the rate-determining step in a mechanism with a good nucleophile?

A

LG leaves

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5
Q

Is SN2 bimolecular or unimolecular? What does this mean?

A

bi bc k = [alkyl halide] [Nu]

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6
Q

What do you have to do in an SN2 with a weak nucleophile?

A

After LG leaves, you’ll have a charged atom, so deprotonate it

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7
Q

Rank stability: tertiary carbocation, secondary carbocation, primary carbonation, and methyl

A

most to least: tertiary carbocation, secondary carbocation, primary carbonation, and methyl

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8
Q

Steps in an SN1? Why are they like this?

A

Bc SN1 is tertiary, the molecule is oo crowded for a nucleophile to attack right away, so
(1) LG leaves to make a tertiary carbonation
(2) nucleophile attack

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9
Q

Rate law in SN1

A

rate = k [alkyl halide]

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10
Q

When can SN2 bimolecular or unimolecular? What does this mean?

A

Usually bi but can be uni if LG leaves right away bc slow (rate-determing) step only depends on 1 molecule

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11
Q

What arrows should you use in mechanisms?

A

equilibrium arrows

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12
Q

What do you have to do in an SN1 with a weak nucleophile?

A

Well, after the (1) LG leaves, you’re gonna have a carbocation, so u need to (2) do a nucleophilic/general attack to change the + charge to a + charge on the nucleophile & then (3) use the LG (charged neg) to deprotonate the nucleophile to get rid of charges

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13
Q

Do secondary alkyl halides use SN1 or SN2?

A

SN2 if strong, anionic nucleophile
SN1 if weak, neutral nucleophile

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14
Q

Do methyl halides use SN1 or SN2?

A

SN2 if strong, anionic nucleophile
SN2 if weak, neutral nucleophile

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15
Q

Do tertiary alkyl halides use SN1 or SN2?

A

SN1 if strong, anionic nucleophile
SN1 if weak, neutral nucleophile

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16
Q

Do primary alkyl halides use SN1 or SN2?

A

SN2 if strong, anionic nucleophile
SN2 if weak, neutral nucleophile

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17
Q

What mechanism (SN1 or SN2) is used if a primary carbon has resonance that forms a carbocation?

A

SN1

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18
Q

How should you label bonds/antibonds?

A

sigma/pi (and star if antibonding) and then the 2 atoms forming the bond

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19
Q

Where does an atom tend to attack?

A

The bigger orbital lobe & the side with fewer e- (bc fewer e- means less e- density to create e- repulsion, repelling the atom)

20
Q

When do you have maximum energy stabilization?

A

better energy match between HOMO and LUMO

21
Q

How do you know which side is favored at equilibrium?

A

Side with better energy match between HOMO and LUMO (and so maximal energy stabilization)
Also can use SERHI

22
Q

What’s lower in energy sigma bond between CH or sigma bond between CCl? Why?

A

CH bc H less EN

23
Q

How do basicity and nucleophilicity compare?

A

Opposite relationship

24
Q

How can you tell if a molecule is a better or worse nucleophile?

A

More basic molecules are better nucleophiles. More basic molecules are less stable (less EN for example & so cannot hold the - charge as well) and so more likely to react.
important: basicity is opposite to stability

25
Q

How do you name a HOMO?

A

LP S- sp3 (add charge too if able)

26
Q

How do you name a LUMO?

A

sigma star CCl (add charge too if able)

27
Q

How does I compare to Cl and Br in terms of basicity and nucleophilicity?

A

I is a weaker base than Cl & Br but a stronger nucleophile than both because it has a farther reach and is more polarizable

28
Q

Good nucleophile exceptions?

A

Neutral: CH3SH

29
Q

How do you write in a molecule that was added into the mechanism externally?

A

Write it in above the arrow

30
Q

When can primary and secondary alkyl halides use SN1?

A

If they give a stabilized carbocation

31
Q

How do primary and secondary benzylic halides usually proceed?

A

SN1

32
Q

Benzylic Halide

A

Benzene ring attached to an alkyl halide

33
Q

Can primary/methyl carbocations exist?

A

No! But, you can draw them in resonance structures, since resonance structures don’t exist

34
Q

How do allylic halides proceed (SN1 vs SN2)?

A

SN1

35
Q

Allylic Halide

A

Carbon attached to Carbon that is double-bonded to another C

36
Q

Are SN1 or SN2 products scrambled? What does this mean?

A

SN1 products are scrambled, which means they have a racemic (50/50 mixture of enantiomers) mixture

37
Q

When do you use HOMO and LUMO?

A

When making a new bond

38
Q

Hyperconjugation

A

Stabilizing effect where neighboring sigma bonds stabilize a carbocation by overlapping with the empty 2p orbital (NOT making new bond)

39
Q

For an allyl cation, how do the HOMOs look in an MO diagram dealing only with p orbitals?

A

In phase (shading all on same side)

40
Q

For an allyl cation, how do the LUMOS lookin an MO diagram dealing only with p orbitals?

A

Out of phase (shading on opposite sides)

41
Q

How is water as a leaving group?

A

Very good!

42
Q

How is OH as a leaving group?

A

Bad, so usually protonate first to become waterr

43
Q

How should we draw resonance structures within a mechanism?

A

Draw one resonance arrow (just 1, even if >1 resonance structures] & draw the resonance structures in [ ]

44
Q

What does CH3Br do (SN1 or SN2)?

A

SN2 only

45
Q

When is a Hofmann product the major product? Why?

A

When there is a strong, bulky base because that pathway is easier to access

CHEM 203 (5 decks)

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