Module 6: Addition Reactions

Question 1

Addition is the opposite of

Answer 1

Elimination

Question 2

The C=C is converted to…?

Answer 2

Two new sigma bonds!

Question 3

The pi bond is an _ _ _ _ _ donor and can act as a _____ or a ______

Answer 3

Electron pair donor.

Nucleophile or Base.

Question 4

Addition and Elimination are equilibrating reactions. Which side is favored depends on….?

Answer 4

Temperature.

Question 5

Entropy Favours Elimination so _____ temperatures favor elimination

Answer 5

higher!

Question 6

Addition reactions are _______ favored so they occur at ____ temperatures.

Answer 6

Enthalpically

Low

Question 7

HX Addition Reaction Mechanism

Answer 7

1) Double bond attacks the hydrogen
2) alkene is protonated forming a carbocation intermediate
3) bromide ion functions as a nucleophile and attacks the carbocation intermediate

Question 8

Stereochemistry of HX addition: do we form an enantiomer?

Answer 8

Yes! Our nucleophile can attack from either side because the carbocation is trigonal planar.

Question 9

Acid Catalyzed (H2SO4) Hydration (H2O) to Form an alcohol.

Answer 9

Addition of water with an acid catalyst to form an alcohol.

1) Water acts as a base to deprotonate the acid
2) Water is deprotonated by the pi bond
3) water attacks the carbocation to form an alcohol
4) water in solution functions as a base to deprotenate the alcohol.

Question 10

Stereochemistry of Hydration: Do we form enantiomers?

Answer 10

Yes! if we form a carbocation (sp2 hybridized/planar) the addition of H2O/HX generates two stereoisomers as the nucleophile can attack from either side

Question 11

Thermodynamics of Hydration

Answer 11

If we are synthesizing an alcohol from an alkene, we would use excess water

If we are synthesizing an alkene from an alcohol, we would only use acid, and not add water to the reaction

Question 12

What are some problems with hydration reactions

Answer 12

1) Alcohols React Similarly

2) Intramolecular

Question 12

What are some problems with hydration reactions

Answer 12

1) Alcohols React Similarly

2) Intramolecular

Question 13

Hydroboration Oxidation [(1)BH3 *THF, (2)H2O2, NAOH)] places the OH on the….?

Answer 13

Least substituted carbon.

1) The less substituted carbon attacks the boron, and the more substituted carbon develops a delta+ which triggers a hydride shift
2) the delta negative hydrogen attacks the carbocation as its being formed.
3) The boron ends up on the least substituted carbon and the hydrogen ends up on the most subsititued carbon.
4) The H2O2, the sodium hydroxide replaces the BH2 and converts it into an OH, so the position of the OH on the least substituted carbon using these two reactions is a direct result of the first step of the reaction which we can predict

Can occur up to three different times

Question 14

Hydroboration Oxidation- Stereoselective:

H and OH are added in a ____ fashion across the double bond

Answer 14

the electron rich alkene attacks the boron, then the delta negative hydrogen attacks the carbocation that forms, all this occurs at the same time, the boron and hydrogen are added and delivered into the same face.
(SYN)

Question 15

Hydroboration Oxidation- Stereoselective:

If ___ or ___ chirality center is formed, a pair of enantiomers is formed by addition to either side of the alkene

Answer 15

one, two.

both H and OH can be added to one face or the other face.

Question 16

Hydrogenation (H2, Pt, Pd, Ni)

Answer 16

the addition of H2 across a C=C double bond.

Requires a metal catalyst

Alkene is converted to the corresponding alkane

SYN addition

Question 17

Hydrogenation: is stereospecific only ____ addition is observed.

Answer 17

Syn.

Two chirality centers are formed only the stereoisomers resulting from syn addition are obtained

Question 18

Catalytic Hydrogenation

Answer 18

1) The hydrogen molecule absorbs onto the metal surface and when it does it breaks the H-H bond.
2) The alkene binds to the metal surface as well through the pi bond, this facilitates the transfer of the hydrogens to the alkene, and because the alkene is absorbed on the surface its basically coming face to face with the hydrogens that are also bound on the surface.
3) Those to hydrogens are transferred to the same face of the alkene.

The metal surface is required to weaken the H-H bond of H2 and because the reaction is happening on the metal surface the hydrogens are transferred to the same face of the alkene.

Question 19

Halogenation: Is only praticle with…?

Answer 19

Br2 and Cl2

Question 20

Halogentation Stereoselective: We only get products from the _____ addition.

Answer 20

Anti.

Question 21

Halogenation (Br2)

Answer 21

1) Br2 (or Cl2) is nonpolar, but polarizable. Approach of a nucleophile will induce a dipole

Think of Br2 as a bromine atom bonded to a good leaving group

2) An electron rich alkene will attack the delta + bromine.
3) The attacked bromine forms bonds with both carbons preventing the formation of the carbocation forming a halonium ion.
4) the bromonium ion is electron withdrawing leaving both carbons delta +. The bromine ion will attack either on of the carbons and attacks from the opposite side of the first bromine.

Question 22

Halogenation is stereospecific

Answer 22

the stereochemistry of the starting alkene determines the stereochemistry of the product(s)

Question 23

Halohydrin Formation: Br2 and H2O

Answer 23

• Halohydrins – formed when halogenation is conducted in water
• Water acts as the nucleophile that attacks the bromonium ion
• There are many more H2O molecules compared to Br - ions, so H2O outcompetes Br - for the bromonium ion

Question 24

Halohydrin Formation: Reaction is Regioselective

Answer 24

The preference is an attack at the carbon who is best able support the positive charge (most substituted carbon), attacks from the opposite side

Question 25

Syn-Dihydroxylation (addition of OH and OH across the double bond).

The key reagent is _ _ _ _

Answer 25

Key reagent is OsO4 the other reagents are co-oxidants

Use of the co-oxidants generate only syn addition

Question 26

Syn dihydroxylation can also be achieved with _ __ _ _ but only under mild conditions (cold temperatures)

Answer 26

KMnO4

-Delivers both oxygen atoms concertedly added to the same face.

Intermediate is not isolated and NaOH is added to lead the syn addition of both groups.

Very strong oxidizing agent, more messy there is a byproduct that needs to be isolated.

Used a visual indicators for the presence of alkenes.

Question 27

Anti-Dihydrogenation is a two step process with 1) R_ _ _ _ and 2) _ _ _

Answer 27

RCO3H

H2O

Question 28

Anti dihydroxylation of an alkene is a two-reaction process

Answer 28

1)In the first step the alkene is reacted with a peroxyacid, The electron rich alkene attacks the electron deficient oxygen, as that carbon oxygen bond forms the delta negative oxygen takes the proton and those two electrons can form the other bond.
we form an epoxide in the first step

2)the epoxide can be isolated and in the second step reacted with H3O and then water acts as a nucleophile to open up this epoixde to form a trans diol

Question 29

Oxidative Cleavage of Alkenes (O3 with DMS or Zn/H2O)

Answer 29

If you take an alkene and react with ozone followed by a reducing agent DMS, the outcome of this reaction is to split that alkene into to two carbonyl compounds.

Fragment the pi bond and replace it with two carbonyl bonds.