Module 6: Addition Reactions of Alkenes

Question 1

What drives the addition reaction?

Answer 1

alkene pi bond electron richness

base to grab a proton or a nucleophile to attack an electrophile (electron-loving, wants neg charge)

Question 2

What is the result of addition reactions?

Answer 2

adding reagents to the alkene to remove pi bond and form sigma bonds, C=C pi bond converts to 2 new sigma bonds

Question 3

What does the electron rich pi bond act as in an addition reaction?

Answer 3

1. base to grab proton and form carbocation
2. nucleophile to grab + charge from electrophile and form carbocation

Question 4

What is the relationship between temperature, entropy, and enthalpy in addition reactions?

Answer 4

Addition reactions are favoured by enthalpy because sigma bonds are stronger and more stable than pi bonds and in an addition reaction, 1 pi (alkene) and 1 sigma (elec or h+) is broken and 2 sigma are formed which is an exothermic reaction

Addition reactions are not favoured by entropy because 2 molecules are combined to form one product so entropy decreases

Question 5

What is the relationship between addition and elimination reactions?

How does temperature determine addition vs elimination reactions?

Answer 5

remain in equilibrium with each other as opposite reactions and the reaction that is favoured depends on temperature

low temp = enthalpy dominates = addition reactions favoured

high temp = entropy dominations (∆G = ∆H - T∆S) = elimination favoured

Question 6

When a carbocation is formed from an alkene, what is the hybridization? What does this indicate for an electron rich species attacking the carbocation?

Answer 6

sp2 hybridized = trigonal planar = nuc attack from either side = product in R and S form

Question 7

Why is 3º carbon more favourable?

Answer 7

lower activation energy = favoured path

Question 8

What is a hydration reaction?

Answer 8

Alkene + water = add an H+ to one carbon of the alkene and OH to the other carbon (this product is an alcohol)

Question 9

What is the difference between an alkene hydration reaction and an alkene HX reaction?

Answer 9

Requires a little bit of acid catalyst

Question 10

Is a hydration reaction regiospecific or regioselective?

Answer 10

Regiospecific = major product is OH on more highly substituted carbon

Question 11

What factors determine the stability of the carbocation?

Answer 11

Carbocation with resonance = most stable
More substituted = more stable
Electronwithdrawing group = inductive effect = polar, destabilizes carbocation = less stable (ex. Br)

Question 12

Why is there a racemic mixture formed during a hydration reaction of alkene?

Answer 12

If there is a new chiral centre formed, the carbocation intermediate is trigonal planar = sp2 = nuc attack of water from either side = R or S enantiomers can form

Question 13

How are hydration reactions related to elimination reactions?

Answer 13

Hydration reaction = type of addition reaction = equilibrium can be controlled with Le Chatelier’s principle:

Dilute acid catalyst = hydration
Concentrated acid catalyst = elimination

Excess water = synthesize alcohol (addition/hydration)

Only acid (no water) = synthesize alkene from alcohol (elimination)

Question 14

What reaction is similar to hydration?

Answer 14

Reaction of alkene with alcohol (also requires dilute acid catalyst)

Question 15

When are intramolecular reactions favoured?

Answer 15

When forming stable ring sizes

Question 16

What is a hydroboration-oxidation reaction? How is this different from a hydration reaction?

Answer 16

Example of addition reaction

Reactants: alkene + (1) BH3 (2) H2O2; NaOH (base)
Product: OH on least substituted carbon

Compared to hydration reaction:
1. OH on least instead of most substituted carbon from alkene
2. Two step reaction instead on one
3. Products are opposite regioisomers

Question 17

What is the mechanism for the hydroboration oxidation reaction?

Answer 17

1. Least substituted carbon attacks boron because H is similar EN to C and H is more EN than B = polar bond
2. More substituted carbon develops a delta + which triggers a hydride shift

Question 18

How is the hydroboration oxidation reaction stereoselective?

Answer 18

Electron rich alkene attach electron poor boron, and carbocation starts to form: B adding to least subbed and H attacked most subbed carbon occurs at the same time

So

H and BH2 (and therefore OH) are added to the same face, same face of alkene, in a syn fashion so the H and OH are cis (makes two products which are enantiomers)
- both H and OH are up or both are down

ANTI (TRANS) not formed*

Question 19

What occurs during a hydroboration oxidation reaction if a chiral centre is formed?

Answer 19

A pair of enantiomers is formed by addition to either side of the alkene, even when multiple chiral centres

Question 20

What is the catalytic hydrogenation addition of H2 reaction?

Answer 20

1. alkene reacts with H2 and metal catalyst (M)
2. product: H added to both carbon (alkene to alkane)

Question 21

What are the potential metal catalysts for a catalytic hydrogenation of H2 reaction?

Answer 21

Pt, Pd, Ni

Question 22

Is the catalytic hydrogenation of H2 reaction stereoselective or stereospecific?

Answer 22

stereospecific: two chiral centres formed only syn addition is observed with hydrogenation (Hs add on the same face of the alkene), so only sin stereoisomers result

Question 23

Why is a metal catalyst required for catalytic hydrogenation of H2 reaction?

Answer 23

metal surface binds (absorbs) the H2 breaking the H-H bond

metal surface also binds to alkene and this facilitates the transfer of the H’s onto the alkene

alkene’s one face is exposed to the H atoms, so added on the same face, syn, across the pi bond

Question 24

What would happen if metal catalyst was not present for a catalytic hydrogenation of H2 reaction?

Answer 24

addition of H2 is too slow bc of high activation energy

Question 25

What is a halogenation reaction?

Answer 25

alkene reacts with Br2 or Cl2 and the halogens add one to each carbon on the C=C

Question 26

Is halogenation stereoselective or stereospecific?

Answer 26

stereoselective - halogenation occurs with anti/trans addition (halogens on opposite C’s are are opposite sides)

stereospecific - stereochemistry to starting alkene determines the stereochemistry of the products

Question 27

Why is halogenation only practical with Cl2 and Br2?

Answer 27

I2 is poor and F2 is too violent

Question 28

How can the addition of Br2 visually show the presence of alkene?

Answer 28

Br2 = reddish brown
alkene = clear, colourless

Br2 + alkene product = clear, colourless

Br2 + non-alkene product = reddish brown

–> use to determine presence of alkene

Question 29

What are the properties of Br2 and Cl2?

Answer 29

non polar but inducing polar bond possible with the presence of a nucleophile (alkene acts as nucleophile)

• think of Br2 as a bromine atom bonded to a good leaving group

Question 30

What is the halogenation mechanism?

Answer 30

1. nucleophilic pi bond induces X2 polarity
2. pi bond attacks delta positive X and e- from bond goes to delta negative X
3. C-Br bond forms on one C but other C is NOT carbocation otherwise we would get products from both sun and anti additions
4. lone pair on Br in the C-Br forms bond with positively charged other C; this traps the forming carbocation to form a halonium ion (3 membered ring)
5. the halonium ion has a positive charge (very e- deficient) so Br- that formed from pi bond attack, attacks one of the delta positive Cs on the side OPPOSITE to the first Br + because it blocks that side

Question 31

What is a halohydrin reaction? Why does this occur?

Answer 31

halogenation reaction conducted in water, which acts as nucleophile and attacks bromonium ion (tri-ring species) in Sn2 rxn so that Br and OH are anti

water is more abundant that Br- ions, so H2O outcompetes Br- for bromonium ion

Question 32

How is halohydrin reaction regioselective?

Answer 32

nuc (H2O) attacks the C in the C=C that is best able to support positive charge – the most substituted

nuc (H2O) also attacks from side opposite to halogen

Question 33

What is a dihydroxylation reaction?

Answer 33

addition reaction where alkene converts to sigma with a OH on each C

Question 34

What is syn dihyroxylation?

Answer 34

both OH on the same side of the double bond (cis) in concerted mechanism

Question 35

What are the reagents that induce an alkene to convert to a sigma bond with 2 OHs in dihydroxylation in cis/syn conformation? Why are these reagents causing syn conformation?

Answer 35

OsO4 (catalyst, toxic) and NMO (alkyl peroxide) as a co-oxidant or KMnO4 (mild conditions)

this occurs bc OHs are added in a concerted process

Question 36

What is the syn dihydroxlyation reaction mechanism?

Answer 36

1. pi bond attacks O on KmnO4 (or OsO4), electron from bond transferred to Mn (or Os)
2. pi bond on other O attacks other C on the C=C

(1+2 at the same time)

3. forms cyclic osmate ester which reacts with Na2SO3/H2O or NaHSO3/H2O
4. product is OH of each C in syn manner

Question 37

What is anti-dihydroxlyation?

Answer 37

two step process of converting alkene C=C to adding OH trans to each C

Question 38

What is the anti dihydroxylation reaction mechanism?

Answer 38

1. react alkene with “per” acid - RCO3H (peroxyacid) to form epoxide (3 membered ring containing O)
2. epoxide of (1) react with H3O+

Question 39

Is dihydroxylation stereoselective or stereospecific?

Answer 39

stereospecific: stereochemistry of reagent determines reaction outcome

Question 40

What is the oxidative cleavage of alkenes reaction?

Answer 40

alkene reacted with

1) O3 (ozone) - addition rxn - ozonolysis
2) reducing agent like Me2S (DMS) or Zn + H2O

converts alkene to carbonyl compound

Question 41

What does the oxidative cleavage of alkenes product look like?

Answer 41

the pi bond split perpendicularly and O is attached to double bond

Question 42

What is the ozonolysis reaction mechanism?

Answer 42

1. alkene attacks O in O3, e- transfers to middle O in O3
2. other side O in O3 attacks other C
3. forms initial ozonide (molozonide) - unstable
4. stable ozonide –> mind reducing agent –> 2 carbonyl groups

Question 43

Does benzene or phenyl/aryl rings react in the same way as an alkene in a ring?

Answer 43

no not under the same conditions bc aromatic rings = too stable bc electrons in pi bonds are highly stabilized