CHEM 232- Classifying Nucleophiles

What are characteristics of a good nucleophile and strong base

-C -, N -, O -

-Trapped charge (no resonance, no conductive effect, etc.)

-Small R group

What are characteristics of a poor nucleophile and strong base

-N -, O -

-Bulky R group

What are characteristics of a good nucleophile and weak base

- Charge is delocalized

- More polarizable and less electronegative than neutral oxygen

- conjugate base that are more stable than good Nu/strong base

What are characteristics of a poor nucleophile and weak base

Neutral oxygen species

For neutral nucleophiles, as we go ___________ a column, our atom gets ____________ and becomes more polarizable. As polarizability increases, it becomes a ______________ nucleophile.

down; bigger; better

What is solvolysis?

when the solvent acts as the nuceophile

During SN2 as substitution increases, relative rate becomes.....

slower

What is hydrolysis?

When H2O is both solvent and nucleophile

Define hyper conjugation

The stabilizing interaction that results from an interaction between the electrons in a sigma bond (usually C-C or C-H) and an adjacent empty or partially filled p orbital or pi orbital.

In SN1, only __________ ____________ _________ is active in the RDS, which makes it a _______ step reaction coordinate.

electron leaving group; two

SN1 Mechanisms:

Better leaving groups ______________ rate.

Changing the nucleophile has ______ __________ on rate.

More substitution at reactive (alpha) carbon ____________ rate.

If the reactive (alpha) carbon is a stereo center, the ________ stereoisomers form.

increases

no effect

increases

two

What are characteristics of radical halogenation?

sp2 hybridized

hyper conjugation stabilize

trigonal planar

flat

Carbocations are always:

- ________ hybridized

- The p orbital is ______

- _________ _________, flat

- __________ deficient, ___________ charged

- ___________ energy species

sp2

empty

trigonal planar

electron; positively

high

What is the first step of SN1?

What happens?

Dissociation

Leaving group leaves, bond breaks, and rate determining step happens

What is the second step of SN1?

What happens

Does the nucleophile attack from one or both sides?

Coordination

Nucleophile attacks, bonds made

Both

What is the third step of SN1?

What happens?

Acid/Base (deprotonation)

Proton transfers

Is water is a weak or strong nucleophile?

WEAK !!!

In SN1:

Nucleophile is the attacking ___________ _________

The electrophile is the _______________

electron pair

carbocation

You can only make stereoisomers if _______________ a stereo center

attacking

SN1 reaction rate:

A better _________ (more _____ conj. base) leaves faster.

leaving group; stable

SN1 reaction rate:

What is the Hammond Postulate?

More __________ at the alpha carbon _____________ the carbocation intermediate through ______________.

Share electron density to stabilize the carbocation/ late TS ++

substitution; stabilizes; hyper conjugation

Which two cannot SN1:

Methyl and primary or secondary and tertiary?

Why?

Methyl and primary because they are too high in energy and less stable

Hyperconjugation:

involves an interaction between.....

it stabilizes both ___________ and ______________.

is a ___________ effect than conjugation.

the interacting orbitals must be aligned on ___________ atoms.

p and sigma orbitals

radicals; carbocations

weaker

aligned

What is the most effective way to share electrons?

Both resonance and hyperconjugation do what?

Resonance

Share electron density

What do polar aprotic solvents do to the reaction rate? How do they stabilize the carbocation?

increase the reaction rate

through H-bonding

The nucleophile in SN1 is _________, which makes it a two step reaction.

The nucleophile in SN2 is ________, which makes it a one step reaction.

weak

strong

Which mechanism is this rate for:

rate= k [Nu][e-LG]

Sn2

What are the 5 key characteristics to SN2 mechanisms?

- 1 step reaction, concerted

- inversion, backside attack, stereospecific

- needs a good leaving group and a strong nucleophile

- less hindered (less crowded) electrophile (methyl, primary) is best

- max rate with polar parotid solvent

What mechanism is this rate for:

rate= k [e-LG]

SN1

What are the four key characteristics of SN1 mechanisms?

- 2 step mechanism, carbocation intermediate

- may form 2 stereoisomers

- needs to have a good leaving group and a stable carbocation (secondary or tertiary alpha carbons are best)

- maximize rate with polar protic solvent

If you have a secondary carbon leaving group, what determines what mechanism it will be?

if the nucleophile is strong then it is.......

if it is weak then it is.....

the nucleophile

SN2

SN1