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allylic group

group on carbon bond adjacent to double bond

benzylic group

group on carbon adjacent to benzene ring

effect of the leaving group being on a beta carbon to a double bond or benzene

increased solvolysis rates

effect of adding substituents on sn1 allylic or benzylic solvolysis

adding substituents increases the rate

regioisomers in allylic compounds

after the leaving group leaves, the allylic carbo cation is stabilized by resonance. major product adds to the more substituted carbon

benzylic regioisomers

there are no regioisomers for benzylic compounds because nonbenzylic compounds lack stability

bond dissociation energy of an allylic H

less than the bond dissociation energy of a non allylic H

radical halogenation

benzylic hydrogen is abstracted and a halogen is added (in light)

radical halogenation initiation

homolytic cleavage of Br2

radical halogenation propagation

homolytic cleavage of benzylic H (creates H radical and radical on the alpha carbon). Br radical and H radical form HBr. (radical substrate + HBr)

radical halogenation termination

1. extra Br radical bonds with radical carbon 2. two radical carbons form a bond

allylic halogenation

a competing reaction with addition to the double bond

allylic substitution

a radical mechanism. Br on the beta carbon to the double bond with HBr as a side product

NBS

reagent for allylic and benzylic bromination

NBS reaction conditions

with light or peroxides in CCl4

how does NBS prevent addition reactions

NBS permits the slow release of Br2. it is necessary to use CCl4 as the solution in which NBS is sparingly soluble

NBS bromination initiation step

a bromine atom is generated by homolytic cleavage of the N-Br bond in NBS

NBS bromination propagation step

1. a bromine radical from solution abstracts an allylic hydrogen 2.the HBR that forms reacts with NBS to produce a molecule of Br2

NBS bromination termination

the allylic radical reacts with the Br2

allylic and benzylic anions

stabilized by resonance

benzyl anion stability

more stable than an allylic anion because of more resonance structures

allylic Grignard reagents

allylic Grignard reagents undergo a rapid equilibration in which the MgBr group moves between the two partially negative carbons (MgBr on the beta carbon to double bond)

allylic rearrangement

not resonance structures but two distinct structures in equilibrium

different (related) allylic alkyl halides can produce

the same Grignard reagent (and a mixture of products when grignard is reacted with water)

does an allylic/benzylic group favor SN2 or E2

E2

effect of allylic/benzylic positions on SN2

rate is increased due to a stabilized transition state

selective oxidation of allylic and benzylic alcohols

oxidation (to carbonyl) occurs on the surface of the solid MnO2

benzylic oxidation of alkyl benzenes

strong oxidizing agent (KMnO4, Jones' reagent)(Cr VI in conc H2SO4) will oxidize alkyl side chains of any length on benzene rings to CO2H

vigorous conditions needed for benzylic oxidation of alkylbenzenes

high concentrations, high temperature, long reaction times

torsional strain

strain caused by atoms in covalent bonds approaching each other

equatorial position

preferred by bulkier substituents for less interactions

boat cyclohexane

highest energy cyclohexane conformation due to eclipsed and transannular strain

bredt's rule

bridgehead carbon bust be sp3 hybridized. any double bonds must exist away from that carbon

when pH=pKa

the acid is 50% dissociated

more acidic atom

more stable conjugate base

hyperconjugation

when the empty p orbital is in the same plane as the sp3 carbon/H and the electron density is sucked it. the reason for regioselectivity

what to remember about addition reactions

can undergo an alkyl shift or hydride shift to make a more stable transition state

hydride shift

doesn't happen as readily as alkyl shift because hydride acts as the nucleophile(50% of the products is the result of the hydride shift)

more stable alkene

more substituted

halogenation rate law

rate=k[alkene][HBr]

how many transition states does halogenation have

2

catalytic hydrogenation of alkenes/alkynes conditions

H2, Pt/C

hydration of a double bond conditions

H2O in acid

alkene + Br2 + H20

halohydrin

bromination mechanism

goes through a bromunium ion. the bromine sits above the p orbital from the double bond

oxymercuration-reduction

markovnikov addition of water. 1. Hg(OAC)2 in H20/THF 2. NaBH4, NaOH

mercurinium ion

similar to the bromonium ion. intermediate in oxymercuration reduction

hydroboration-oxidation

anti markovnikov addition of water (due to borane avoiding steric interactions, nucleophile is hydride) 1.BH3/THF 2. H2O2, OH-

alkene to aldehyde/ketone

1. O3 2. (CH3)2S

alkene to carboxylic acid

1. O3 2. H2O2/H2O

anti markovnikov addition of HBr

using peroxides

hydration of alkynes

forms an enol that becomes an aldehyde or ketone

Hg2+ catalyzed hydration of alkyne

alkyne to enol to alcohol

alkyne to alkene

H2 and Lindlar's catalyst

hydroboration oxidation of alkynes

alkyne to aldehyde. bulky reagent makes the 2nd addition not occur 1. dismal borane 2. H2O2/OH-

H2 and Lindlar's catalyst

alkyne to alkene, gives cis alkene (less stable)

H2 and NaNH3

alkyne to alkene, gives trans isomer

Preparation of acetylide anion

alkyne + NH2Na

enantiomers

have the same physical properties. react differently in polarized light or with other chiral molecules

is mCPBA syn or anti

anti

is OsO4 syn or anti

syn

is KMnO4 syn or anti

syn

oxymercuration reduction except there is a carbon chain with an OH on it

will do intramolecular williamson ether synthesis

nucleophilic epoxide opening stereochemistry

inverted on least substituted side, retained on more sub side