Week 7: Alkyl Halides

Question 1

is a nucleophile electron rich/dense or electron poor?

Answer 1

dense/rich

Question 2

is a electrophile electron rich/dense or electron poor?

Answer 2

poor

Question 3

what happens during nucleophilic substitution?

Answer 3

the halogen on a hydrocarbon gets replaced by another, more stable atom/pieces

Question 4

can nucleophilic substitution (SN2) reactions occur on carbons with pi bonds?

Answer 4

no. SN2 can only involve sigma bonds

Question 5

what does it mean to say that an SN2 reaction is concerted?

Answer 5

It occurs all in one step, rather than in a step-wise fashion.

Question 6

what is the leaving group in an SN2 reaction?

Answer 6

The halogen that got replaced on the hydrocarbon

Question 7

what does ‘inversion of configuration’ describe?

Answer 7

describes how in an SN2 reaction, the stereochemistry of the substrate/electrophile inverts as it changes from reactant to product.

Question 8

what are the two configuration options for a SN2 substrate that can participate in inversion of configuration

Answer 8

R (clockwise) and S (counterclockwise)

Question 9

which type of electrophile carbon is best/fastest for SN2 reactions?

Answer 9

primary, followed by secondary, followed by tertiary (due to lower steric hinderance)

Question 10

what are the 2 reasons an SN2 nucleophile must approach the electrophile from behind?

Answer 10

1. coming the front way exposes the nucleophile to electron repulsion/steric hinderance from the leaving group 2. molecular orbitals on the front approach are not aligned properly for bonding

Question 11

what are the 2 consequences/implications for SN2 reactions?

Answer 11

1. gives you the ability to produce a specific enantiomer or diastereomer (on purpose) 2. gives you the ability to produce specific stereochemistry via double inversion/series of SN2 rxns; this is useful when you’re short on starting material.

Question 12

what 4 components must be considered when determining if/how well an SN2 reaction will proceed?

Answer 12

1. the substrate 2. the nucleophile 3. the electrophile 4. the solvent

Question 13

what 3 factors make a substrate a ‘good’ substrate for an SN2 reaction?

Answer 13

1. it has little steric hinderance 2. it has no branches to create steric hinderance 3. it has no possible chair configurations (ring with all sigma bonds)

Question 14

what 3 factors make a nucleophile ‘strong’ for an SN2 reaction?

Answer 14

1. it’s big (but not bulky/hindered)
2. it’s negative (via neg charge or lone pairs; neg charge more neg than lone pairs)
3. it’s polarizable (bigger nuc/more shell e- = more polarizability)

Question 15

what 2 factors make a leaving group ‘good’ for an SN2 reaction?

Answer 15

1. it’s the conjugate of a strong acid/able to manage e- density 2. it’s SIMILAR TO the conjugate of a strong acid (exp sufonates)

Question 16

what factor makes a solvent ‘good’ for an SN2 reaction?

Answer 16

it’s polar aprotic (doesn’t engage in distracting hydrogen bonding)

Question 17

what makes the sulfonate ion acid derivative a mesylate (OMS)?

Answer 17

It has a methyl group for the R group

Question 18

what makes the sulfonate ion acid derivative a triflate (OTF)?

Answer 18

it has a CF3 in the R position

Question 19

what makes the sulfonate ion acid derivative a tosylate (THS?

Answer 19

It has a toluene molecule in the R position

Question 20

what does tosylate (OTS) look like

Answer 20

Question 21

what does mesylate (OMS) look like?

Answer 21

Question 22

what does triflate (OTF) look like?

Answer 22

Question 23

what 2 characteristics are dead give aways that a carbon might be an electrophile?

Answer 23

1. the carbon has a partial postive charge (bc something electronegative like Cl is attached to it)
2. the carbon has a postive charge/empty p orbital

Question 24

what 3 characteristics are dead give aways that a structure might be a nucleophile?

Answer 24

1. it has a partial negative or outright negative charge
2. it has a lone pair
3. it’s a ring with a pi bond in it?

Question 25

a nucleophiles stronger in ‘like’ solvents, or weaker?

Answer 25

weaker. as like dissolves like, nucleophiles in ‘like’ slns (i.e. polar, protic, etc) get distracted by interactions with the solvent