SN1, SN2, E1, E2 Reaction Mechanisms

CH 6-8

SN

Nucleophile Substitution

E

Elimination

What order is an SN1 reaction?

First-order kinetics

How many steps are in an SN1 reaction?

two steps

What is the order of an SN2 reaction?

second-order kinetics

Is an SN2 reaction unimolecular or bimolecular?

Bimolecular

What happens in a bimolecular (SN2) reaction?

Both the alkyl halide and nucleophile appear in the rate equation

What is the reaction called in second-order kinetics? (SN2)

concerted reaction

What happens in a concerted reaction? (SN2)

bonds break and form at the same time

From what direction does the nucleophile approach the substrate in an

SN2 reaction?

only backside

What is a frontside attack?

The nucleophile approaches from the same side as the leaving group

What happens in a backside attack?

The nucleophile approaches from the side opposite the leaving group

Which attack from a nucleophile in an SN1 reaction results in retention of configuration around the stereogenic center?

frontside attack

What does retention of configuration mean?

The nucleophile and leaving group are in the same position relative to the other three groups on carbon. The nucleophile attacks from the right, and all other groups remain in the same position.

Which attack from a nucleophile in an SN2 reaction results in inversion of configuration around the stereogenic center?

backside attack

What does inversion of configuration mean?

The nucleophile approaches from the opposite side to the leaving group. The leaving group was drawn on the right, so the nucleophile attacks from the left. The nucleophile and leaving group are in the opposite position relative to the other three groups on carbon.

What is the name of the inversion of configuration in an SN2 reaction?

Walden inversion

How are the products of frontside and backside attacks related?

They are enantiomers

Why are the products of frontside and backside attacks enantiomers?

They are different compounds that are nonsuperimposable

Which attack happens in all SN2 reactions?

backside attacks

What is the relationship between SN2 reactions and R groups?

As the number of R groups on the central (alpha) carbon increases, the rate of the reaction decreases

What is the order from slowest to fastest an SN2 reaction will undergo with R groups?

tertiary alkyl halides, secondary alkyl halides, primary alkyl halides

What happens in an SN2 reaction when there is a tertiary alkyl halide?

They do not undergo the reaction

Why do tertiary alkyl halides not undergo SN2 reactions?

The number of R groups makes it not possible for the nucleophile to attack the central carbon atom

Why do SN2 reactions prefer less R groups?

There's fewer bulky R groups, so less steric hindrance, and the central carbon atom is easier to attack.

What is the general form of a second-order kinetics reaction rate equation?

rate = k[RX][:Nu-]

Is an SN1 reaction unimolecular or bimolecular?

Unimolecular

What happens in a unimolecular (SN1) reaction?

Only the alkyl halide is considered in the rate equation. The identity and concentration of the nucleophile have no effect on the reaction rate

Which step in an SN1 reaction is unimolecular?

the first step, when the leaving group departs from the compound and the carbocation is formed.

the slow step

What happens in a two-step (SN1) reaction?

bond breaking occurs before bond making

what happens in step one of an SN1 reaction?

heterolysis of the C-X bond makes a carbonation in the rate-determining step

what happens in step two of an SN1 reaction?

The nucleophile attacks the carbocation and forms a new bond

What is the carbocation in SN1 reactions?

They are formed as reactive intermediates

Which mechanism favors higher temperatures?

Elimination

Which mechanism favors lower temperatures?

SN

when bonds are broken, what happens regarding energy?

it is exothermic and is a positive value on an energy diagram

when bonds are formed, what happens regarding energy?

it is endothermic and is a negative value on an energy diagram

where are energy barriers?

Each step has its own, with a transition state at each energy maximum

if an SN1 mechanism has two steps, how many energy barriers are there?

2

What hybridization is a carbocation with 3 R groups in an SN1 reaction?

sp2

what geometry is an sp2 carbocation in an SN1 reaction? (3 R groups)

trigonal planar with a vacant p orbital above and below the plane

where the does the nucleophile attack a planar carbocation in SN1 reactions?

from both sides

What products are formed from SN1 reactions with a planar carbocation?

enantiomers

Because there is no preference for nucleophilic attack from either direction, what is made?

an equal amount of two enantiomers is formed (racemic mixture)

what is a racemic mixture?

where two enantiomers are formed in equal amounts as products

What is racemization?

The formation of equal amounts of two enantiomers from an enantiomerically pure starting material

What is the relationship between R groups and SN1 reactions?

As the number of R groups increases, the reaction rate increases

What is the order from slowest to fastest an SN1 reaction will undergo with R groups?

primary, secondary, tertiary alkyl halides

What alkyl halides will not undergo SN1?

primary alkyl halides and methyl

What is the general form of a first-order kinetics reaction rate equation?

rate = k[RX]

What intermediate does an SN1 reaction form?

Trigonal planar carbocation intermediate

Where does racemization occur in an SN1 reaction?

at a single stereogenic center

What is the relationship between the number of R groups there are and carbocation stability?

As the number of R groups increases, the stability also increases

What is the order of carbocation stability from weakest to strongest?

Methyl, primary, secondary, tertiary

What is the inductive effect?

electronic effects that occur through sigma bonds, electron donor groups (Z) stabilize a + charge (Z->Y+)

Electronic withdrawing groups (W) stabilize a - charge (W<-Y-)

What is the relationship between electron-donating R groups and carbocation stability?

The more electron-donating R groups, the greater the stability

What is the order of carbocation stability from weakest to strongest with electron-donating groups?

Methyl, primary, secondary, tertiary

What is hyperconjunctation?

Where the more C-H bonds are connected to the central/alpha carbon, the more stability the compound has

What is the Hammond Postulate?

The transition state of a reaction resembles the structure of the species (reactant or product) to which it is closer in energy.

Where is the transition state in endothermic reactions?

closer in energy to products

Where is the transition state in exothermic reactions?

closer in energy to reactants

What does an energy diagram look like for endothermic reactions?

The peak of the transition state is closer to the products side

What does an energy diagram look like for exothermic reactions?

The peak of the transition state is closer to the reactants side

What happens when the energy of the transition state is lowered in endothermic reactions?

The energy of activation (Ea) decreases and the reaction rate increases

The peak of the transition state is closer to the products side, therefore...

anything that stabilizes the product stabilizes the transition state too

According to the Hammond Postulate, a more stable product of an endothermic reaction should...

occur faster

What happens to the reaction rate of an exothermic reaction if the energy of the product is lowered?

little or no effect on the energy of the transition state

In an exothermic reaction, if Ea is unaffected, then...

the reaction rate is unaffected too

In an exothermic reaction, Ea is

similar for both products

What is the rate-determining step in an SN1 reaction?

the formation of the carbocation

is the rate-determining step of an SN1 reaction endothermic or exothermic?

endothermic

The stability of a carbocation in an SN1 reaction determines...

the rate of its formation

What is the relationship between transition states and carbocations?

A more stable transition state makes a more stable carbocation

In the SN1 reaction, the rate-determining step is endothermic so...

the more stable carbocation is formed faster

How is it determined if a reaction occurs by SN1 or SN2?

The alkyl halide, if the nucleophile is weak or strong, if the leaving group is good or poor, and if the solvent is protic or aprotic

What is the order of determining the reaction mechanism in a reaction?

the alkyl halide, strength of the base/nucleophile, temperature

More R groups favor

SN1 reactions

Less R groups favor

SN2 reactions

Methyl and primary halides undergo

only SN2 reactions

tertiary alkyl halides undergo

only SN1 reactions

secondary alkyl halides undergo

both SN1 and SN2 reactions

How does the strength of the nucleophile affect an

SN1 mechanism?

the rate is unaffected by the nucleophile's identity

Why does the strength of the nucleophile not affect an

SN1 mechanism?

because the nucleophile isn't included in the rate equation

Why does the strength of the nucleophile affect an

SN2 mechanism?

because the nucleophile is in the rate equation

strong nucleophiles in high concentrations favor

SN2 reactions

weak nucleophiles favor______ by _________

SN1 reactions by decreasing the rate of any competing SN2 reaction

The most common nucleophiles in

SN2 reactions have

a net negative charge

What are some strong bases/nucleophiles?

-OH, -OCH3, DBU, DBN, I-, Br-, NaOH-, KOCH3-

What are some weak bases/nucleophiles?

H2O, CH3OH

What does SN do to the products formed?

Determines their stereochemistry

How does the identity of the leaving group affect an

SN1 or SN2 reaction?

A better leaving group increases the rate of both

SN1 and SN2 reactions.

What does SN1 need to work as a reaction?

secondary or tertiary alkyl halide

weak nucleophile

low temperatures

What does SN2 need to work as a reaction?

primary or secondary alkyl halides

strong base or nucleophile

low temperatures

What does E1 need to work as a reaction?

weak base or nucleophile

high temperatures

What does E2 need to work as a reaction?

strong base and

high temperatures

What reaction works with a DBU reagent?

Only E2

Why does only E2 work for DBU reagents

Because there's too much steric hindrance for E1, SN1 and SN2, but it’s a strong base. It can remove a hydrogen from a beta carbon and make and alkene. It doesn’t need to attack the carbon center.

What kind of form/stereochemistry does a tetrahedron always have?

2 groups that are planar (in the same plane) and one wedged and one dashed

Why do the Elimination (E1 & E2) prefer higher temperatures in reactions?

Bond breaking requires more energy/heat to be done

What is the reaction rate for a unimolecular reaction?

only the alkyl halide is in the rate equation
rate = k[alkyl halide]