Orgo Prelim 3

What are factors that Affect SN and E reactions?
Solvent, Nucleophilicity/Basicity, Leaving Group, and the Reactant
Which solvents are the best polar solvents?
SN and E reactions
What are some characteristics of SN2 reactions as polar solvents?

  • polar aprotic
  • stabilze (+) which allows the nucleophile to attack
  • need good nucleophile
  • ex: DMSO, DMF, HMPA, acetonic

What are some characteristics of SN 1 reactions as polar solvents?

  • polar protic
  • stabilizes (+) and (-)
  • can separate charges (?)
  • ex: H2O and alcohol

What are some characteristics of E2 reactions as polar solvents?

  • strong branched base and conjugate acid
  • polar protic and aprotic (removes B-H)
  • ex: CH3O in CH3O/DMSO

What are some characteristics of E1 reactions as polar solvents?

  • polar protic
  • stabilize (+) and (-)
  • solvolysis
  • competes with SN1
  • ex: H2O and alcohol

Is a charged atom (I) higher or lower in nucleophilicity than an alcohol?

higher.

 

because:

charged species > lone pair e > H2O or alcohol

 

What are some factors about polar protic solvents in terms of nucleophilicity?

  • polar protic: SN1 and E1 reactions
  • H-bonding donors and acceptors
  • stabilize carbocation and anion
  • acids R-X ionization or solvolysis
  • ex: H2O and alcohol

What are some factors of polar aprotic solvents in terms of nucleophilicity?

  • polar aprotic: SN2, E2
  • little or no h-bonding
  • stabilizes (+) so nucleophile can attack
  • ex: acetone, DMSO, DMF, HMPA

in terms of basicity, do weak bases have higher or lower pKa values?

lower.

 

because

CH3 > H2N > RO > F > Cl > Br > I

pKa values decrease –>

How do SN2 reactions differ in polar protic and polar aprotic reactions in terms of nucleophilicity and basicity?

In polar protic: nucleophilicity & basicity both decrease

In polar aprotic: nuclephilicity increases and basicity decreases

For polar protic compounds (eg alkoxide anion), how do basicity and nucleophilicity differ in terms of carbon orders (teritiary, etc)?

nucleophilicity: 1o > 2o > 3o

basicity:  3o > 2o > 1o

What is the leaving group related to and which leaving group is the best?

related to the stability of an atom; the best leaving group is a weak base=conjugate base of a strong acid

 

I > Br > Cl > > F

Which is better – a bigger leaving group or a smaller leaving group? Why?
The bigger leaving group is better because it has a delocalized charge.
In terms of an SN2 reactant, which carbon order is the strongest? What are two important factors?

1o > 2o > > > (3o)

 

2 important factors: steric factor and branching

Both slow the reaction to allow for a backside attack!

In terms of the reactant, what is the carbon order from highest to lowest? What must you also have?

3o > 2o > 1o

 

must have beta-hydrogens and anti-configuration is preferred

What is the carbon order from highest to lowest in SN1 and E1 reactions? What factor is important?

3o > 2o > > > (1o)

 

Electronic factor is important to stabilize carbozation

What you need to remember about SN2 reactions

  • bimolecular
  • Rate = k[Rx][nu]
  • back side attack! Me>1>2>>3
  • TS (no intermediate)one step
  • need a good nucleophile (SH-, -OCH3, -OH, -I)
  • need a good LG
  • iversion –>stereochemistry
  • polar aprotic solvent
  • E2 product possible

What you need to remember about E2 reactions

  • bimolecular
  • Rate = k[Rx][base]
  • TS concerted
  • B-H’s best if anti (or at least coplanar)
  • strong branched base helps
  • Zaitsen’s Rule = most branched alkenes preferred
  • sometimes competes with SN2

What you need to know about SN1 reactions

  • unimolecular
  • rate = k[R-X]
  • carbocation intermediate ( 3 > 2 >>>1)
  • branching desirable
  • 2-step reaction
  • racemic products/rearrangements possible (look at 2o)
  • ok nucleophile (H2O, H2S, CH2OH, Cl-)
  • but need good LG
  • polar protic solvent; solvolysis stabilizes (+) and (-)
  • E1 products are probable

What to know about E1 reactions

  • unimolecular
  • carbocation intermediate (3>2>>>1)
  • branching helps
  • rearrangements pssible
  • Zaitsen’s rule (most substituted alkene)
  • polar protic solvent
  • solvolysis (no strong base needed)
  • competes with SN1

What happens when a methyl halide or a primary alkyl halide reacts with a Lewis base?

;

Na+CH3CH2O + Br-CH2CH3–?

The Lewis base replaces the halogen, which gets expelled as halide ion

 

—> CH3CH2O-CH2CH3 + Na+ Br

What happens in a substitution reaction

A Lewis base acting as a nucleophile donates an electron pair to an eletrophile to displace a leaving group.

 

Or in the words of Ganem: something goes in, something gets kicked out.

Show an example of substitution reaction (SN2 reaction)

CH3O + CH3-Cl —–> CH3-O-CH3 + Cl

 

 

elimination reaction
reaction in which two or more groups are lost from within the same molecule
β-elimination
an elimination that involves the loss of two or more groups from adjacent carbons (conceptually the reverse of an addition to an alkene)
what are alpha-carbons and beta-carbons

alpha-carbons: the carbon bearing the halogen (in alkyl halides)

 

beta-carbon: the carbons adjacent to the alpha-carbon

What promotes the beta-elimination reaction of an alkyl halide? What are usually used as solvents?
strong bases promote; the conjugate acid alcohols of these bases are usually used as solvents.
In the presence of strong Lewis bases, what are the typical reactions for primary and tertiary alkyl halides

primary: substitution

tertiary: β-elimination

what is the rate of a reaction a function of
the rate of a reaction is a function of the concentrations of the reactants (because molecules are more likely to collide at a high concetration than a low)
What is the rate law for A + B —> C when both A and B double in concentration? What if only the concentration of A is double and B has no effect?

both double: rate = k[A][B]

only A doubles=k[A]

What is the overall kinetic order for a reaction

the overall kinetic order for a reaction is the sum of the powers of all the concentrations in the rate law.

 

the kinetic order in each reactant is the power to which its concentration is raised in the rate law.

why is the rate law of a reaction so important
tells about the reaction mechanism, the concentration terms of the rate law indicate which atoms are present in the transtition state of the rate limiting step.
What does SN2 stand for

S=substitution

N=nucleophilic

2=bimolecular

what does bimolecular mean
the rate limiting step involves two species
what is the relationship between the rate law and the machanism

  1. the concentration terms of the rate law show what atoms are involved in the rate-limiting step
  2. mechanisms that are not constitent with the rate law are ruled out
  3. the simplest mechanisms consistent with the rate law is used

what three different ways can a substitution reaction occur at the stereocenter

  • with rentention of configuration at the stereocenter
  • with the inversion of configuration at the stereocenter
  • with a combination of the 2 (inversion and retention)

What would retention of the configuration at the asymmetric  carbon result from?
if the nucleophile comes in to the same side as the leaving group leaves; known as frontside attack
what would cause inversion of configuration at the symmetric carbon
if the nucleophile comes in to one side and the leaving group leaves from the opposite side (3 other groups must invert); known as backside attack
which stereochemical configuration is generally observed in all SN2 reactions

inversion of configuration is usually observed in SN2 reactions at carbon stereocenters.

 

 

why is backside substitution preferred in the SN2 reaction?
the orbital containing the donated the electron pair must initially interact with an inoccupied molecular orbital of the alkyl halide?
steric factor

any effect on a chemical phenomenom, such as a reaction, caused by van der Waals repulsions

 

SN2 reactions of branched alkyl halides are retarded by a steric effect

why do elimination ractions compete with SN2 reactions of secondary and teriary alkyl halides

secondary and teriary alkyl halides react so slowly in SN2 reactions that the rates of elimination raectionscompete with the rates of substitution.

 

the rates of the SN2 of teritary alkyl halides are so slow that elimination is the only reaction observed.

when are the less basic nucleophiles more nucleophilic
when the nucleophiles in the nucleophilic atom are from the same group but different periods of the periodic table
solvent proticity

whether the solvent is protic, has the most effect on rate.

 

 the change of solvent will have a great effect on the rate of its SN2 reactions depending on the strength of the ion

what strongly accelerates the SN2 reactions of nucleophiles
eliminating the possibility of hydrogen bonding
In E2 reactions, what do syn– and anti-elimination mean

syn- the dihedral angle between the C-H and C-X bond is 0°  (the H and V groups leave from the same side of the reference plane)

 

anti– the dihedral angle between the C-H and C-X bonds is 180° (H and X groups leave from opposite sides of the reference plane

What are three reasons anti-elimination is preferred

  1. its transition state has a staggered conformation, which is more stable than syn-, therefore anti- is faster
  2. the base and the leaving group are on opposite sides of the molecule
  3. more favorable because all electron displacements are backside

what is usually the predominant product an an E2 reaction
the most stable alkene isomer (has the most alkyl substituents); these isomers are formed in the greatest amount
when does the greatest use of the E2 reaction for the preparation of alkenes occur
when the alkyl halide has only one type of beta-hydrogen and only 1 product is possible.
in a reaction with an alkyl halide with beta-hydrogens + a Lewis base, what reaction do you get if a Lewis base acts like a nuclephile? what if a Bronsted base?

nucleophile: SN2 reaction

Bronsted base: E2 reaction

what are the two variables that determine which reaction will be the major process

  1. the structure of the alkyl halide (at both the alpha and beta carbons)
  2. the structure of the base

when asked to predict how a given alkyl halide will react, what 3 major questions must you ask

  1. is the alkyl halide primary, secondary, or tertiary
  2. is a lewis base present (if yes, is it a good nucleophile, strong Bronsted base, or both?)
  3. what is the solvent? (polar protic, polar aprotic, or mixtures of both?)

carbene
a species with a divalent carbon atom
alpha-elimination
elimination of 2 groups from the same atom
x

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