Substitution + Elimination - Orgo Exam 2

What are the reactants of a substitution reaction?

R (carbon containing group) - LG (leading group)

What are the products of a substitution reaction?

R-Nucleophile + Leaving Group

Substitution reactions occur with

nucleophile and solvent

halogen attached to carbon is

leaving group

the nucleophile has what to make a new bond?

lone pair

what is SN1

substitution nucleophiles 1st order rate

what is SN2

substitution nucleophiles 2nd order rate

the SN1 mechanism has how many steps?

two

the SN2 mechanism has how many steps?

1

what is the first part of an SN1 reaction

R-X, breaks the bond

what is the second part of SN1

nucleophile binds to R (carbon intermediate), X is leaving group

what happens in SN2

Nucleophile binds to R and leaving group breaks bond

what is the difference between SN1 and SN2

SN2 has no carbon intermediate and occurs in one fluid step

mechanisms must follow specific sequences of

bond making and breaking

each bond made or broken must have

a curved arrow

curved arrows represent

the movement of electrons

the arrow points towards

where the electrons are going, in either making or breaking a bond

each step in a mechanism is balanced with respect to

charges and atoms

this is an example of which reaction

SN1

this is an example of which reaction

SN2

what is the rate equation for SN1

rate=K[R-X]

what is the rate equation for SN2

rate=K[R-X][Nuc]

Why doesn’t the rate reaction include a nucleophile for SN1

it is not involved in the rate determining (slow) step

if you double the rx of SN1

the rate doubles

if you double the nucleophile in SN1

nothing changes

if you double the rxs and the nuc in SN1 (whole reaction)

the rate doubles

if you double the rxn in SN2

the rate doubles

if you double the nuc in SN2

the rate doubles

if you double the rxn and the nuc in SN2 (whole rxn)

the rate quadruples

which does this energy diagram represent?

SN1

which does this energy diagram represent?

SN2

the carbocation intermediate is_______ in energy than R-X or R-Nuc in SN1

higher

why is the first hump in SN1 higher in energy than the second

the 1st step was slow and required more energy

** review stereo chemical outcomes

what is the first rule in deciding if SN1 or SN2

the structure of the alkyl halide

what is the second rule in deciding if SN1 or SN2

the strength of the nucleophile (weak or strong)

what is the third rule in deciding if SN1 or SN2

leaving group ability

what is the 4th rule in deciding if SN1 or SN2

solvent

what variables are MOST important in deciding SN1 or SN2?

structure of the alkyl halide and strength of the nucelophile

SN1 has what reaction with a Methyl

no reaction

SN1 has what reaction with a 1° carbon

no reaction

SN1 has what reaction with a 2° carbon

slow rxn

SN1 has what reaction with a 3° carbon

fast rxn

SN2 has what reaction with a methyl carbon

fastest rxn

SN2 has what reaction with a 1° carbon

fast rxn

SN2 has what reaction with a 2° carbon

slow rxn

SN2 has what reaction with a 3° carbon

no rxn

if the rxn is with a 1° or methyl it is likely

SN2

if the rxn is with a 3° it is likely

SN1

if the rxn is with a 2° it is likely

SN1 or SN2

SN1 follows what rules for the structure of alkyl halides?

carbocation stability rules (3° = most stable, methyl = least stable)

more adjacent carbons =

mosre stable carbocations

hyper-conjugation is when

bonds on carbon adjacent to carbocation can donate electrons to carbocation and stabilize it

SN2 prefers 1° or methyl because

less crowding and unobstructed paths with less carbons

in SN1 the strengh of a nucleophile

doesn’t matter much and is usually a weak nuc and solvent

strong nucleophiles usually have

negative charges or neutral strong nuc

example of strong nucleophiles (negative charges)

OH^-, OR^-, R₂N^-, N₃^-, CN^-, carboxylic ether^-, R-S^-, X^-

examples of neutral strong nuc (less EN atoms with lone pairs)

R-NH₂ (amines), NH₃, R-SH, R₃P (phosphines), PH₃

neutral strong nucs are

less EN atoms that have lone pairs

weak nucleophiles are

neutral EN atoms with lone pair and proton

examples of weak nucleophiles

ROH (alcohol), H₂O, Carboxylic acids

if the decisions for SN1 vs SN2 do not line up that means

it is not a substitution reaction

steps 3 and 4 for deciding SN1 vs SN2

influence rates and trends but are not deciding factor in SN1 vs SN2

When the leaving group leaves it gains a _____ charge

negative

the bigger the size of the LG,

the faster the reaction

I is able to take on a larger negative charge than CL because

I is spread out as a larger atom

solvents can stabilize

intermediates and LGs

the more polar the solvent

the faster the SN1 and SN2

all SN1 reactions will have

polar protic solvents

when the solvent is the same as the nucleophile its called

solvoysis

examples of non-polar solvents

haxanes, diethyl ether, ethyl acetate, chloroform, dichloromethane, bentane, tolvene

examples of polar solvents

SMSO, DMF, acetonitrile

in between polar and non polar solvent

THF

examples of polar protic solvents

alcohols, H₂O, carboxylic acids

polar protic solvents are

weak nucleophiles (and often solvolysis)

polar protic solvents

stabilize LG in slow step of SN1 (preventing reverse reaction)

this is an example of an

elimination reaction

the reactions of an elimination reaction are

alkyl halides

the products of elimination reactions are

alkene + H-Basem + X^- (lg)

the elimination reaction occurs in a

base

base and nucleophiles are often

interchangeable (for elimination rxn)

substitution and elimination reactions are both

reactions of alkyl halides

elimination reactions are also often called

β-hydride elimination

this is an example of

E1

this is an example of

E2

E1 rate law is

rate=K[Rx]

E2 rate law is

K[Rx][Base]

this reaction coordination diagram is an example of

E2

this reaction coordination diagram is an example of

E1

E1 and SN! happen…

together

why do E1 and SN1 happen together

they have the same slow first step

elimination can often form multiple products

true

the more substituted alkene will be the

major elimination product

a more substituted alkene is the alkene with

more carbons attatched to the α and β carbons of alkene

the more substituted alkene has the most carbons attached and is therefore

the most stable

substitution and elimination can have

the same reactants but different products (compete with each other)

to draw elimination products make

C=C bonds between α- carbon (where x is attatched) and β-carbon (adjacent carbon)

which carbon is the α-carbon in elimination

carbon where the X is attached

where is the β-carbons in elimination

carbons adjacent to the α-carbon