ch 7 continued

trans alkenes

are more stable than cis alkenes

one

stereospecific reactions occur when there is only ____ avalible beta hyrdogen

antiperiplanar

leaving group and hydrogen group have to be in new window (must be opposite of each other

coplanar

so that p orbitals can overlap

red flag

single beta hydrogen is a ___ _____ for E2

anticoplanar (antiperiplanar)

staggered and proceeds by a much lower energy transition state compared to syn coplanar conformtaion

E2

mechanisms depend on the configuration of the starting alkyl halide

syn coplanar

higher in energy so we usually only see antiplanar

antiperiplanar

in most cases elimination occurs only via this conformtaion leading to a specific stereoisomeric product

stereospecifc

only makes one product

proton

the stereospecificity of the substrate is only relevant when the beta position has one _______.

two protons

when beta position has ____ ________ either can be arranged (via rotation of single bond) so that it is antiperiplanar to leaving group

E

most stable stereoisomeric product from two protons. this makes the reaction stereoselective

stereoselective

favors alkene with two biggest groups opposite

regiospecific

only one beta hydrogen avalible.

regioselective

two or more beta hydrogens avalible

E2 mechanism steps

1. assess regiochemical outcome (how many unique (no symmetrey) B carbons are there (w H atoms)

2. assess stereochemical outcome and make leaving group and beta hydrogen antiperiplanar if they arent already by rotating alpha and beta bond

exception 3

leaving group must be axial on a ring. and beta hydrogen sterochemistry must be opposite aka anti to leaving group

trans

in cyclohexanes only hydrogens that are beta carbons and that are _____ to the leaving group can undergo elimination

both dashed/wedged

if leaving group and beta hydrogen on a ring are this then E2 cannot occur

regioselective

lots of beta carbons

minor

there can be many ______ products

one beta hydrogen

red flag. we can only make E or Z but not both

simultaneously

unimolecular substitution and elimination reactions (SN1 and E1) happen almost always ___________.

mix

unimolecular substitution and elimination reactions occur at the same time and you will often see a ____ of SN1 and e1 reaction products

stable

SN1 and E1 always make the most _______ product (zaitsev and or trans/ E)

reagent

is the key to telling the difference between the unimolecular and bimolecular

stepwise

SN1 and E1 proceed ___________ instead of concerted like SN2 and E2

weak solvent (neutral)

favors unimolecular reaction

first order kinetics

only depends on one (nucleophile or base) rate= k(substrate)

ionization

the first step of SN1 and E1 is ___________, which is taking a neutral molecule and making it charged ‘ionized’. leaving group leaves to form a carbocation intermediate

nucleophile

reagent acts as a __________ so this is SN1

base

reagent acts like a ________ so rxn is E1

solvolysis

E1 and SN1 are often termed _________ because the reagent is both nucleophile/vase and the solvent.

Ea

first step of leaving group leaving to form a carbocation has a high __ (activation barrier) due to becoming more unstable

SN1 steps

1. loss of a leaving group

2. nucleophilic attack

E1 steps

1. loss of a leaving group

2. proton transfer

rate determining

the highest energy transiiton state in SN1 and E1 is for the formation of the carbocation intermediate. so formation of the cabrocation is the _____ __________ ____ (slowest step w highest Ea).

tertiary

unimolecular reactions are fastest for _______ substrates due to the fact that we must form a carbocation intermediate

SN2

likes unhindered substrate like methyl and primary

E2 SN1 E1

all like tertiary substartes

primary

only under resonance stabilized conditions can ________ carbocations undergo SN1/E1 reactions

vinyl & aryl

halides will not undergo substitution/ elimination reactions

SN1

we want tertiary and resonance

best to worst leaving groups

I-, Br-, Cl-, F-

protic

to promote unimolecular reaction, use polar ______ solvent (can hydrogen bond)

less solvated

less stable

more solvated

more stable

hydrogen

the protic solvent will _______ bond with the nucleophile, stabilizing it, while the leavin group leaves first

SN1/E1

a polar protic solvent will also stabilize the full and partial charges that form during _____ mechanism thus lowering the activation barrier

aprotic

SN2 is favored by this

protic

Sn1 and E1 favored

SN2 and SN1 and E1

can both occur with aprotic or protic, is just slower when it is not what is favored

charged

reactions rarley end with _______ product. proton transfer occurs at last step after nucleophilic attack to neutralize molecule

stable

if a more ______ carbocation can be formed, a rearrangement will occur

nucleophile

one way to favor SN1 over E1 is to use reagent that is a ________ only

mixture

in reactions where a carbocation rearrangemnet is possible, a _______ of products is usually obtained. in general, rearranged product predominates because carbocation rearrangement is an intramolecular process (faster than intermolecular processes)

carbocation geometry (SN1)

sp2 trigonal planar and flat. can be attacked from either side. there is slight preference for inversion of configuration of product due to leaving groups steric prescence but both products are formed. stereoselective WE OBSERVE BOTH inversion and retention of configuration

regiochemistry E1

no exceptions, always makes zaitsev and tras/ E if possible.

heat

favors E1 over SN1

last step

a proton is removed from a beta carbon adjacent to the sp2 hybridized C+. the less sterically hindered/ lower energy / more stable product will be the major product. this is typically the E1 product with the two largest groups opposite of each other