Exam 3 Reactions Flashcards by Samantha Boyer

Dehydrohalogenation SM

alkyl halide
usually 3°, or 2° with stronger conditions

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E1 DEHYDROHALOGENATION MECHANISM

halogen leaves
C+ rearrangement
weak base deprotonates beta H; H e- make alkene

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E1 dehydrohalogenation solvent

good ionizers - water, alcohol

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E1 dehydrohalogenation rate law

rate = k[RX]

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base used in E1 dehydrohalogenation
small or bulky?

weak
small yields Zaitsev product
bulky yields Hofmann product

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E1 dehydrohalogenation byproducts

Sn1

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how to favor E1 over Sn1?

heat

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E2 dehydrohalogenation SM configuration requirement

anticoplanar
if cyclohexane, X and H must both be axial

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E2 dehydrohalogenation SM

alkyl halide
usually 2° or 3°

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E2 dehydrohalogenation rate law

rate = k[RX][B]

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best base characteristics for E2 dehydrohalogenation

strong
nonpolarizable
matched with solvent
small (for Zaitsev product)
large (for Hofmann product)

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how to favor E2 dehydrohalogenation over Sn2

polar protic solvent
heat

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E2 dehydrohalogenation stereospecificity

2 “wedges” end up one one side of alkene (longways)

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Dehydration of alcohol SM

an alcohol

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E1 DEHYDRATION MECHANISM

hydroxy attacks H of acid to make a good LG
C-O bond breaks and water leaves
C+ rearrangement
water attacks & removes beta H - H e- form alkene

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E1 dehydration SM and acid

2° and 3° alcohol required
phosphoric/sulfuric acid

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E2 DEHYDRATION MECHANISM

acid attacks hydroxy to make good LG
water leaves, and water attacks beta H - H e- make alkene

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E2 dehydration SM & acid

1° alcohol
concentrated H2SO4

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Hydrohalogenation SM

alkene

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HYDROHALOGENATION MECHANISM

alkene attacks H (or other electrophile), splitting it from X
C+ rearrangement
X attacks C+

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hydrohalogenation product

alkyl halide
pair of enantiomers

hydrohalogenation variation

organic peroxide rxn
HBr
RO-OR solvent
radical mechanism

hydrohalogenation of alkynes SM and their products

terminal alkyne yields vinyl halides or geminal dihalides (depends on molar ratio)

internal alkyne yields 4 products, no Markovnikov application

HX addition to alkynes
1HX:1SM =

vinyl halide alkene

HX addition to alkynes 2HX:1SM =

geminal dihalide alkane

ACIDIC WATER HYDRATION MECHANISM

1. alkene attacks H of acid 2. C+ rearrangement 3. water attacks C+, giving protonated alcohol 4. another water deprotonates alcohol & regenerates acid

OXYMERCURATION-DEMERCURATION HYDRATION MECHANISM

1. oxymercuration—alkene attacks Hg, Hg attacks C, forming mercurinium ion ring 2. water attacks more stable partial + C, and Hg moves to the other C 3. another water deprotonates alcohol, forming organomercurial alcohol 4. demercuration—addition of NaBH4 drives SN2-type reaction, where Hg is replaced with H

why is mercuration used?

to get a non-rearranged hydrated product

species involved in oxymercuration-demercuration

- SM: alkene - Hg(OAc)2 - THF-H2O solvent - NaBH4 hydride - product: non-rearranged alcohol

How is alkyoxymercuration different from oxymercuration? (2)

1. Hg(O2CCF3) is used 2. OR is attached to product, not OH; results in ether, not alcohol

Mercuric ion-catalyzed hydration of alkyne product

ketone

MERCURIC HYDRATION OF ALKYNE MECHANISM

1. alkyne attacks Hg; mercury attaches to less sub. C 2. water attacks C+, forming orthomercurial alcohol 3. another water deprotonates water, giving alcohol 4. alkene attacks proton from acid; Hg is replaced by H 5. keto-enol tautomerization occurs

type of Hg salts used in mercuric hydration of alkynes

Hg (II) - HgO, HgSO4, Hg(OAc)2

hydroboration TS

4-membered ring, with Boron aligned with less sub. C

stereoselectivity of hydroboration

syn - H and Boron add to same face of alkene Boron adds to less hindered face if both faces are equally hindered, 2 products result

species involved in hydroboration of alkenes

- BH3 (or B2H6 dimer) - THF - H2O2, OH-

boron reagent used in hydroboration of alkynes

Sia2BH

rearrangement involved with hydroboration of alkynes

enol-aldehyde tautomerization

HALOGENATION OF ALKENE MECHANISM

1. halonium ion formation—alkene attacks X, X-X bond polarizes, X- breaks away 2. X- opens 3-membered ring with backside attack

type of X-X addition to alkenes

anti

produces PVC

halogenation of alkenes

halogenation of symmetrical trans substrate gives... of asymmetrical substrate gives...

meso product enantiomers

halogenation of cis substrate gives...

pair of enantiomers

halogenation solvent

CCl4

halogenation of alkynes occurs in ________ conditions

anhydrous

halohydrin SM and product

SM: alkene product: OH and X on alkane

halohydrin solvent

H2O

Nu and El for halohydrin formation

OH- is Nu X+ is El

type of halohydrin addition

trans (anti)

HALOHYDRIN MECHANISM

1. alkene attacks X-X, polarizing bond; X+ attacks one C; X- detaches 2. water opens 3-membered ring from backside 3. another water deprotonates alcohol

Halohydrin -- is OH or X on more sub. C?

Markovnikov -- OH is on more sub. C

2 variations of halohydrin formation

1. ROH solvent gives ether, not OH 2. salt in H2O gives mixed product, ex) Cl and Br on different products