Chirality in Reactions

Question 1

LiAlH4 / H3O+

Answer 1

Carbonyl to OH and H

Question 2

KOH

Answer 2

Deprotonates O (eg on alcohol)

Question 3

What does a base do to an alkane with H and a leaving group (X) anti-periplanar to each other do?

Answer 3

E2 elimination

Question 4

m-CPBA

Answer 4

Forms stable 3 member ring with two C atoms that previously had a pi bond (syn addition)

Question 5

Br2

Answer 5

Forms a brominium ion which then is attacked by the other bromine to get anti addition of two bromines across a double bond

Question 6

H-X

Answer 6

Breaks double bond, forms carbocation, which is then attacked by the X ion

Most stable carbocation indicates where X- will attack

Question 7

Acid catalyzed hydration

Answer 7

Add a molecule of H2O across a double bond (mix of stereoisomers)

Question 8

NBS

Answer 8

Forms a brominium ion

Question 9

NBS / H2O, THF

Answer 9

Adds OH to a brominium ion

Question 10

Bulky base
(reagent has leaving group)

Answer 10

Elimination
(axial requirement for six member rings)

Question 11

H2O / heat
(on reagent with leaving group)

Answer 11

Elimination
(pi bond formation)

Question 12

NaBH4

Answer 12

Carbonyl to OH and H

Question 13

OsO4
NMNO

Answer 13

Endo or exo addition of two OH molecules across a pi bond

Question 14

LiB(Et)3H

Answer 14

(same as any metal with BX4)
Carbonyl to OH and H

Question 15

StereSELECTIVE reaction

Answer 15

Reaction where only one of a set of stereoisomers is formed exclusively

Question 16

StereoSPECIFIC reaction

Answer 16

Reaction where one stereoisomer of starting material produces one stereoisomer of product

Question 17

How does Sn2 reaction work?

Answer 17

Nucleophile donates electrons into sigma* orbital of reagent at an angle of 180 degrees, forms a trigonal bipyramidal transition state, then leaving group leaves forming product with inverted symmetry

Nucleophile and electrophile both involved in RDS

Question 18

What is the inversion of Sn2 reaction called?
What kind of reaction is Sn2?

Answer 18

Walden Interconversion
Stereospecific reaction

Question 19

How doe Sn1 reaction work?

Answer 19

Leaving group leaves (RDS), forming a trigonal planar transition state, then nucleophile attacks carbocation

Two faces of carbocation are enantiotopic, resulting in enantiomeric products of equal probability

Question 20

When will an Sn1 reaction never happen?

Answer 20

If the leaving group is attached to a carbon that cannot form a planar carbocation

Question 21

Elimination reactions

Answer 21

Two sigma bonds —> single pi bond
(Two groups expelled)

Question 22

Bredt’s Rule

Answer 22

An elimination reaction which forms a bridgehead alkene will never happen (but sometimes does)

Too much strain on sp2 carbon

Question 23

E2 Reactions

Answer 23

Concerted, single step mechanism elimination of two groups forming a pi bond

Question 24

What configuration is required for E2?
What kind of reaction is E2?

Answer 24

Two groups being eliminated must be anti-periplanar
Stereospecific reaction

Question 25

E1 Reaction

Answer 25

Leaving group must leave first (RDS), molecule can then rotate and elimination can occur from any conformation (multiple products)

Product distribution determined by alkene stability

Question 26

What may happen with E1/Sn1 reactions?

Answer 26

They have the same first step so they may compete

Question 27

E1cb

Answer 27

Nucleophile attacks H, and forms a carbanion which has free rotation, then leaving group leaves forming pi bond

Question 28

Addition Reactions (types)

Answer 28

HX Reactions
Acid catalyzed hydration
Addition of halogens (X2)
Epoxidation
Hydroboration

Question 29

How do addition reactions work?

Answer 29

One pi bond —> two sigma bonds
Can either be syn (same face) or anti (opposite face)
Both syn and anti additions are stereospecific

Question 30

HX Reactions

Answer 30

Addition of H-X to alkene, which is regioselective
Unsymmetrical alkene gives predominance to one regioisomer over another

X attacks more substituted carbon (Markovnikov)

Question 31

Acid-catalyzed hydration (and related reactions)

Answer 31

Addition of H2O across an alkene
Strong acid used with water (poorer nucleophile)
Forms discrete carbocations, so there is very little stereoselectivity (mix of products)

Question 32

Addition of halogens (X2)

Answer 32

Brominations - anti addition is preferred for alkenes that do not have substituents that strongly stabilize a carbocation

Chlorination - similar effect to Br2, but smaller so a worse bridging atom (less pronounced effect)

Question 33

Epoxidation

Answer 33

Same as halogenation, but the three membered ring is formed with oxygen and is stable
Generally occur via a peroxy acid (-OOH group) which provide an electrophilic O atom

Question 34

What increases rate in epoxidation reactions?

Answer 34

Electron withdrawing groups on peroxy acid
Electron donating group on alkene
Strain on double bond

Question 35

What kind of approach is preferred for epoxidation of exocyclic alkenes?

Answer 35

Axial approach

Question 36

Hydroboration

Answer 36

Alkene reacts with BH3 or BR3 (any combination) to form alkylboranes (boron = electrophile, accepts electrons)

Question 37

Hydroboration mechanism

Answer 37

Pi bond donates electrons (attacks) empty p-orbital of B atom, the hydride is transferred to the other C atom (concerted syn process), then B atom bonds to less substituted carbon atom