Module 6: Addition Reactions of Alkenes Flashcards
What drives the addition reaction?
alkene pi bond electron richness
base to grab a proton or a nucleophile to attack an electrophile (electron-loving, wants neg charge)
What is the result of addition reactions?
adding reagents to the alkene to remove pi bond and form sigma bonds, C=C pi bond converts to 2 new sigma bonds
What does the electron rich pi bond act as in an addition reaction?
- base to grab proton and form carbocation
- nucleophile to grab + charge from electrophile and form carbocation
What is the relationship between temperature, entropy, and enthalpy in addition reactions?
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
What is the relationship between addition and elimination reactions?
How does temperature determine addition vs elimination reactions?
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
When a carbocation is formed from an alkene, what is the hybridization? What does this indicate for an electron rich species attacking the carbocation?
sp2 hybridized = trigonal planar = nuc attack from either side = product in R and S form
Why is 3º carbon more favourable?
lower activation energy = favoured path
What is a hydration reaction?
Alkene + water = add an H+ to one carbon of the alkene and OH to the other carbon (this product is an alcohol)
What is the difference between an alkene hydration reaction and an alkene HX reaction?
Requires a little bit of acid catalyst
Is a hydration reaction regiospecific or regioselective?
Regiospecific = major product is OH on more highly substituted carbon
What factors determine the stability of the carbocation?
Carbocation with resonance = most stable
More substituted = more stable
Electronwithdrawing group = inductive effect = polar, destabilizes carbocation = less stable (ex. Br)
Why is there a racemic mixture formed during a hydration reaction of alkene?
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
How are hydration reactions related to elimination reactions?
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)
What reaction is similar to hydration?
Reaction of alkene with alcohol (also requires dilute acid catalyst)
When are intramolecular reactions favoured?
When forming stable ring sizes
What is a hydroboration-oxidation reaction? How is this different from a hydration reaction?
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
What is the mechanism for the hydroboration oxidation reaction?
- Least substituted carbon attacks boron because H is similar EN to C and H is more EN than B = polar bond
- More substituted carbon develops a delta + which triggers a hydride shift
How is the hydroboration oxidation reaction stereoselective?
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*
What occurs during a hydroboration oxidation reaction if a chiral centre is formed?
A pair of enantiomers is formed by addition to either side of the alkene, even when multiple chiral centres
What is the catalytic hydrogenation addition of H2 reaction?
- alkene reacts with H2 and metal catalyst (M)
- product: H added to both carbon (alkene to alkane)
What are the potential metal catalysts for a catalytic hydrogenation of H2 reaction?
Pt, Pd, Ni
Is the catalytic hydrogenation of H2 reaction stereoselective or stereospecific?
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
Why is a metal catalyst required for catalytic hydrogenation of H2 reaction?
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
What would happen if metal catalyst was not present for a catalytic hydrogenation of H2 reaction?
addition of H2 is too slow bc of high activation energy
What is a halogenation reaction?
alkene reacts with Br2 or Cl2 and the halogens add one to each carbon on the C=C
Is halogenation stereoselective or stereospecific?
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
Why is halogenation only practical with Cl2 and Br2?
I2 is poor and F2 is too violent
How can the addition of Br2 visually show the presence of alkene?
Br2 = reddish brown
alkene = clear, colourless
Br2 + alkene product = clear, colourless
Br2 + non-alkene product = reddish brown
–> use to determine presence of alkene
What are the properties of Br2 and Cl2?
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
What is the halogenation mechanism?
- nucleophilic pi bond induces X2 polarity
- pi bond attacks delta positive X and e- from bond goes to delta negative X
- C-Br bond forms on one C but other C is NOT carbocation otherwise we would get products from both sun and anti additions
- 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)
- 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
What is a halohydrin reaction? Why does this occur?
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
How is halohydrin reaction regioselective?
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
What is a dihydroxylation reaction?
addition reaction where alkene converts to sigma with a OH on each C
What is syn dihyroxylation?
both OH on the same side of the double bond (cis) in concerted mechanism
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?
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
What is the syn dihydroxlyation reaction mechanism?
- pi bond attacks O on KmnO4 (or OsO4), electron from bond transferred to Mn (or Os)
- pi bond on other O attacks other C on the C=C
(1+2 at the same time)
- forms cyclic osmate ester which reacts with Na2SO3/H2O or NaHSO3/H2O
- product is OH of each C in syn manner
What is anti-dihydroxlyation?
two step process of converting alkene C=C to adding OH trans to each C
What is the anti dihydroxylation reaction mechanism?
- react alkene with “per” acid - RCO3H (peroxyacid) to form epoxide (3 membered ring containing O)
- epoxide of (1) react with H3O+
Is dihydroxylation stereoselective or stereospecific?
stereospecific: stereochemistry of reagent determines reaction outcome
What is the oxidative cleavage of alkenes reaction?
alkene reacted with
1) O3 (ozone) - addition rxn - ozonolysis
2) reducing agent like Me2S (DMS) or Zn + H2O
converts alkene to carbonyl compound
What does the oxidative cleavage of alkenes product look like?
the pi bond split perpendicularly and O is attached to double bond
What is the ozonolysis reaction mechanism?
- alkene attacks O in O3, e- transfers to middle O in O3
- other side O in O3 attacks other C
- forms initial ozonide (molozonide) - unstable
- stable ozonide –> mind reducing agent –> 2 carbonyl groups
Does benzene or phenyl/aryl rings react in the same way as an alkene in a ring?
no not under the same conditions bc aromatic rings = too stable bc electrons in pi bonds are highly stabilized
