Unit V (5) Lecture Notes: Structure & Prep of Alkenes

lecture notes information

polar protic solvents favor

SN1 reactions

polar aprotic solvents favor

sn2 reactions

solvent interactions with nucleophiles can

drive reactions toward SN1

When attempting substitution with strong nucleophiles in aprotic solvents on sterically hindered substrates

elimination reactions occur instead

neopentyl effect causes

steric hindrance next to electrophilic carbon

neipentyl effect results in 

alkene formation instead of substitution 

In Alkene nomenclature…

ane becomes ene

Numbering rules in alkene nomenclature are

double bond gets lowest possible number in parent chain

vinyl group

directly attached to sp2 carbon

allyl

one carbon away from double bond 

structural properties of SP2 hybridication

both carbons in double bond are trigional planar

cis and trans isomers are

physically distinct isomers

stereoisomers in alkenes

can not interconvert at room temp and rotation requires breaking of a pi bond

alkene cis/trans require different

subsitueints on both carbons of double bond 

C/Z nomenclature critical rule

compare subsitueints on same carbon only

Z (zusammen)

high priority groups on same side

E (entegen)

high priority groups on opposite sides

a common mistake in EZ nomenclature is to

never compare across a double bond 

more subsituated alkens are

more stable

cis isomers are less

stable due to steric interactions

sp2 carbons are

more electronegative than sp3

hyperconjugation occurs when alkyl groups

release electrons into alkene

cyclohexane conformation:

half chair

cyclooctene

first ring size allowing trans double bond 

in industrial alkene formation dehydrogenation heats

alkenes to 750 degrees celc to produce ethylene and hdyrogen gas industrially

What are the key lab sysnthesis methods of aleknes 

beta eliminaition, dehydrohalogenation, dehydrtion of alc, important reagent recogntiion

secondary/tertiary alcs in Hbr/HCL undergo

SN1 reactions

non-nucleophilic conjugate bases

(sulfuric acid, phosphoric acid) lead to eliminiation instead of subsitution

dehydration reactions require elevated

temperature

in eliminiation reactions you must consider 

beta positions 

most subsituted alknes forms

preferetially

E-alknes form preferetially over

z alkenes

E1 mechanism charactertics

carbocation forms, water acts as base, rate depends only on substrate concentration

tertiary carbons perfer

E1 mechanisms 

carbocation rearrgangements in E1

same as in SN1 like methyl and hydride shifts

E2 mechanism

concerted, rate depends on both, requires strong base, most eliminiations of alkykl halids goes by e2

most eliminiation of alkyl halides goes by

E2

Tertiary carbons are also prefered by

E2 ( opposite of sn2)