Reactions of Alkenes Alkenes generally react in an addition mechanism

35 Pages·2016·1.12 MB·English

by  Michael Biewer

Reactions of Alkenes

Alkenes generally react in an addition mechanism

(addition – two new species add to a molecule and none leave)

R X Y
X Y
H R
R R H
Have already observed using a H+ electrophile (HBr or H+/H O) that a carbocation
2
intermediate is generated which directs the regiochemistry

Whenever a free carbocation intermediate is generated there will not be a stereopreference
due to the nucleophile being able to react on either lobe of the carbocation

(already observed this with S 1 and E1 reactions)

N
Br
Br
H+ H
H C
Br
3
CH CH
2 3
Obtain racemic mixture
of this regioisomer
Reactions of Alkenes

There are three questions to ask for any addition reaction

R X Y
X Y
H R
R R H
1)  What is being added?

(what is the electrophile?)

2)  What is the regiochemistry?

(do the reagents add with the X group to the left or right?)

3)  What is the stereochemistry?

(do both the X and Y groups add to the same side of the double bond or opposite sides?)

All of these questions can be answered if the intermediate structure is known
Reactions of Alkenes

Dihalogen compounds can also react as electrophiles in reactions with alkenes

Possible partial bond structures

!+ !+
Br Br
Br Br Br !+ or

Br
!+
More stable partial
positive charge

Experimentally it is known, however, that rearrangements do nor occur with Br addition

2
-therefore free carbocations must not be present

The large size and polarizability of the halogen can stabilize the unstable carbocation

With an unsymmetrical alkene, however, both bonds to the bromine need not be equivalent

Called a “Bromonium” ion

-this structure will direct further reactions

Br Br Br Does not rearrange, therefore this
carbocation must not be present
Dihalogen Addition

The bromonium ion thus forms a partial bond to the carbon that can best stabilize

a positive charge which will then react with the bromide nucleophile

!+ Br
Br Br Br
Br !+
Br
Due to the 3-centered intermediate, dihalogen additions occur with an anti addition

H Br H Br
H C H C
3 3
Br Br
CH
3 CH H
H3C Br H 3 Br CH
3
Obtained product

Not obtained
Formation of Halohydrins

When water is present when a dihalogen is added to a double bond,

then water can react as the nucleophile with the halonium (e.g. bromonium) ion

!+ OH
Br Br Br !+ Br
Br
H O
2
Favored product

While water is a weaker nucleophile than bromide,

because it is the solvent there is a much greater concentration present

The halonium ion thus directs both the regiochemistry (oxygen adds to the carbon that can
best stabilize the partial positive charge) and the stereochemistry (due to the three membered
ring the oxygen must add anti to the the bromine already present)

!+ CH
3
Br !+
H Br
H O
Br Br
CH CH 2 OH
3 H 3
D D H D
The halohydrin is named according to which halogen is present

(chlorohydrin, bromohydrin, iodohydrin)
Halogenation of Alkynes

Dihalogen can be added to alkynes in addition to alkenes

The reaction is similar to alkenes with the main difference being the presence of two π bonds
thus allowing reaction to occur twice for a total of 4 halogens adding to the compound

Br Br
H C Br
Br Br 3 Br Br
H C CH CH
3 3 H C 3
3
Br CH
3 Br Br
With one addition, obtain Second addition is favored,
trans vicinal dihalogen

hard to stop at alkene stage
as alkene is more reactive
than alkyne

Due to difference in reactivity,

it is possible to selectively add to an alkene in the presence of an alkyne

Br
Br Br
1 equiv.
Br
Oxymercuration

An alkene can also be hydrated using mercury salts (called oxymercuration)

Mercury diacetate [Hg(OAc) ] is a common reagent

2
which loses one acetate to generate an electrophilic source of mercury

H
CH !+
O O O 3 AcO
Hg !+
H
CH
O O O H 3
Hg Hg
H
The electrophilic mercury reacts with an alkene to form a mercurinium ion which is similar
to bromonium ions in that a three membered ring is formed with a partial bond to the carbon
that can best handle the partial positive charge

Water can then react (which is typically the solvent for these reactions) in an anti addition

!+
CH
AcO 3
Hg !+ H O NaBH OH
2 AcOHg 4
CH OH
H 3
H H H
The mercury can subsequently be removed with sodium borohydride to form the alcohol
Routes to Hydrate an Alkene

Different routes have been seen to hydrate an alkene,

each route though offers different advantages and often an entirely different product

CH
3
Markovnikov product

CH H+/H O CH
3 2 H C 3 Generate free carbocation that
3
H C CH HO CH rearranges to more stable 3˚ cation

3 3 3
1) BH •THF
3
CH HO CH
3 2) H O , NaOH 3
2 2 Anti-Markovnikov

H C CH H C CH
3 3 3 3
OH
1) Hg(OAc) , H O
CH 2 2 CH Markovnikov product

3 2) NaBH 3
4 Do not generate free carbocation
H C CH H C CH
3 3 3 3 therefore no rearrangements occur
Epoxidation

To form an epoxide from an alkene, need to generate an electrophilic source of oxygen

Previously we have observed oxygen acting as a nucleophile

and reacting with carbocation sites

A peroxy acid (or peracid) is a source of electrophilic oxygen

!- !-
O O
!+
!- O
H C !+OH H C !+O H
3 3
!-
Acetic acid

Peracetic acid

(called peracid or peroxy acid)

Due to the high electronegativity for oxygen, typically the oxygen atoms

in an organic compound have a partial negative charge (therefore nucleophilic)

In a peracid, however, the terminal oxygen is already

adjacent to an oxygen with a partial negative charge

The terminal oxygen thus has a partial positive charge and thus is electrophilic
Epoxidation

When an alkene reacts with a peracid, an electrophilic reaction occurs

where the π bond reacts with the electrophilic oxygen

O CH O CH
3 3
H
H O O
O
O
CH CH
3 3
CH CH
3 3
The reaction forms an epoxide (oxirane) with a carboxylic acid leaving group

Due to the cyclic transition state for this reaction, the two new bonds to oxygen form SYN

O
RCO H
CH 3 CH
3 3
CH CH
3 3
O
CH
RCO H 3
H C 3
3
CH CH
3 3