Week 7: Alkyl Halides Flashcards
is a nucleophile electron rich/dense or electron poor?
dense/rich
is a electrophile electron rich/dense or electron poor?
poor
what happens during nucleophilic substitution?
the halogen on a hydrocarbon gets replaced by another, more stable atom/pieces
can nucleophilic substitution (SN2) reactions occur on carbons with pi bonds?
no. SN2 can only involve sigma bonds
what does it mean to say that an SN2 reaction is concerted?
It occurs all in one step, rather than in a step-wise fashion.
what is the leaving group in an SN2 reaction?
The halogen that got replaced on the hydrocarbon
what does ‘inversion of configuration’ describe?
describes how in an SN2 reaction, the stereochemistry of the substrate/electrophile inverts as it changes from reactant to product.
what are the two configuration options for a SN2 substrate that can participate in inversion of configuration
R (clockwise) and S (counterclockwise)
which type of electrophile carbon is best/fastest for SN2 reactions?
primary, followed by secondary, followed by tertiary (due to lower steric hinderance)
what are the 2 reasons an SN2 nucleophile must approach the electrophile from behind?
- 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
what are the 2 consequences/implications for SN2 reactions?
- 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.
what 4 components must be considered when determining if/how well an SN2 reaction will proceed?
- the substrate 2. the nucleophile 3. the electrophile 4. the solvent
what 3 factors make a substrate a ‘good’ substrate for an SN2 reaction?
- 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)
what 3 factors make a nucleophile ‘strong’ for an SN2 reaction?
- it’s big (but not bulky/hindered)
- it’s negative (via neg charge or lone pairs; neg charge more neg than lone pairs)
- it’s polarizable (bigger nuc/more shell e- = more polarizability)
what 2 factors make a leaving group ‘good’ for an SN2 reaction?
- 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)
what factor makes a solvent ‘good’ for an SN2 reaction?
it’s polar aprotic (doesn’t engage in distracting hydrogen bonding)
what makes the sulfonate ion acid derivative a mesylate (OMS)?
It has a methyl group for the R group
what makes the sulfonate ion acid derivative a triflate (OTF)?
it has a CF3 in the R position
what makes the sulfonate ion acid derivative a tosylate (THS?
It has a toluene molecule in the R position
what does tosylate (OTS) look like
what does mesylate (OMS) look like?
what does triflate (OTF) look like?
what 2 characteristics are dead give aways that a carbon might be an electrophile?
- the carbon has a partial postive charge (bc something electronegative like Cl is attached to it)
- the carbon has a postive charge/empty p orbital
what 3 characteristics are dead give aways that a structure might be a nucleophile?
- it has a partial negative or outright negative charge
- it has a lone pair
- it’s a ring with a pi bond in it?
a nucleophiles stronger in ‘like’ solvents, or weaker?
weaker. as like dissolves like, nucleophiles in ‘like’ slns (i.e. polar, protic, etc) get distracted by interactions with the solvent
