Rx Adisi (b).ppt

SULFURIC ACID Concentrated sulfuric acid Dilute sulfuric acid

two very different reagents.

CONCENTRATED SULFURIC ACID NO WATER PRESENT

O

18 M

H O S O H O

expanded octet representation

There is no species other than H2SO4 present.

6M AQUEOUS SULFURIC ACID ~0M

55.6 M

12 M

2 H3O+ + SO4-2

H2SO 4 + 2 H2O strong acid

6M

excess

ionizes almost completely

Three predominant species are present :

H H O

H O

H

H

water

+

hydronium ion

-

O O S O

O sulfate ion

-

ADDITION OF SULFURIC ACID AND WATER

ADDITION OF WATER AND SULFURIC ACID TO ALKENES THREE DIFFERENT REACTIONS 1) Alkyl hydrogen sulfates

C C

conc. + H2SO4

cold, concentrated, large amount (stoichiometric) 0o

C

H 2-6M

NOTE DIFFERENT CONDITIONS FOR EACH REACTION HERE AND ON THE NEXT SLIDE

+

OSO3H C C

2) Hydration to alcohol C C

18M

H2O

H2 SO4

OH C C H

dilute, aqueous

6M continued

3) Polymerization C C

conc H2SO4

warm, 18 M concentrated, small amt. smaller amount, (not stoichiometric)

H C C C C

excess monomer A Sections 8.9 and 8.14 ( discussed later … )

n

polymer

C C OSO3H

Alkene molecules add together to make long chains or polymers

A-A-A-A-A-A-A-A-A REACTION CONDITIONS ARE IMPORTANT !

CONCENTRATED SULFURIC Formation of Alkyl Hydrogen Sulfates

ALKYL HYDROGEN SULFATES Concentrated H2SO4

C C

cold

O

bisulfate ion

-O

( or hydrogen sulfate)

O

S O H OSO3H

O C C + H carbocation

H O S O H O FOLLOWS THE MARKOVNIKOFF RULE SAME MECHANISM AS HCl ADDITION

C C H

an alkyl hydrogen sulfate discussed later add water room temp

OH C C H

alcohol HYDROLYSIS

DILUTE SULFURIC ACID Formation of Alcohols

MECHANISM

dilute H2SO4

(HYDRATION)

H O H H2SO4 H2O

+ H O H used

higher conc. than SO42-

ADDITION OF WATER

alkene C C

sulfate is a poor nucleophile

H

FOLLOWS THE MARKOVNIKOFF RULE SAME MECHANISM AS HCl ADDITION

C C + H the catalyst is not used up

H + O H C C H

O H

H oxocation (oxonium ion)

H O C C H

alcohol

+ + H O H H regenerated

MARKOVNIKOFF ADDITION ( HYDRATION REACTIONS, H2SO4 + H2O )

CH3 + H2O CH2 + H2O

CH CH2 + H2O

H2SO4

CH3 OH

CH3 H2SO4

H2SO4

OH

CH CH3 OH

DIFFERENT WAYS OF WRITING REACTIONS Reactants specifically shown with a “+” sign, catalyst written over arrow. H2SO4

+ H2O

OH

Reactant(s) and catalyst both written over the arrow. H2SO4

OH

H2O

Solvent written over the arrow.

+ HCl

H2O

Cl

Chemists use all of these!

STEREOCHEMISTRY HX and H2O These additions are usually not stereospecific. Some compounds*, under some conditions*, may give stereoselective results. * Stereoselective examples will not be discussed in any detail here, you may assume these reactions are not stereospecific most of the time.

STEREOCHEMISTRY OF HX AND H2O ADDITIONS from H3O+

H2SO4 catalyst

H+

C CH2 CH2CH3 The carbocation is planar. In the second step, X or H2O can add equally 50/50 to either side (top or bottom).

Ph

HX or H2O

+

X

C CH3

CH3CH2 b a

X

50/50

b

Ph CH3CH2

Ph C + CH 3 CH3CH2 AQUEOUS SOLUTIONS

a

sp2

C

CH3

X

The resulting halides or alcohols are ENANTIOMERS (racemic mixture)

ADDITION OCCURS TO BOTH SIDES OF AN OPEN CARBOCATION H

H+ could add to either side left or right

CH3 H3C

H+

O H

CH3

H2O H2SO4

add to left side bottom

CH3

CH3

O H

OH +

H syn addition

CH3 H

H

CH3

OH

+ enantiomer

+

H DIASTEREOMERS

+ enantiomer

CH3 anti addition

EFFECT OF POLARITY OF THE SOLVENT CAUTION The addition to both sides of the C+ ion is common in H2O solutions, but if you change to a non-polar solvent, this may not happen.

100%

CH3

HBr (g) pentane

+ CH3

H

H

conc. HBr (in H2O)

CH3 Br

+

CH3 Br CH3

anti + syn

H

A “bridged” rather than an “open” cation is involved.

The typical result in a polar solvent like H2O.

CH3

H

Br

addition to top side only

The typical result in a non-polar solvent.

CH3 CH3

This will be explained later

anti

addition to both sides 50/50

ADDITION POLYMERS

ADDITION POLYMERS A + A + A + A

A

monomers +

+

+

A

A

polymer

+

ethylene (ethene)

+

A

linear

polyethylene

+

propylene (propene) many (n) monomers

polypropylene

nA

(A)n

branched

polymer

SOME NOMENCLATURE A

Monomer (The alkene that is polymerized)

A-A

A Dimer (Two units)

A-A-A

A Trimer (Three units)

A-A-A-A

A Tetramer (Four units)

A-(A)n-A

A Polymer (Many units)

Depending on the reaction conditions and stoichiometry the chains formed can have lengths that vary from two units (dimer) to thousands of units (polymer).

PREPARATION OF ADDITION POLYMERS CATIONIC MECHANISM

POLYMERIZATION ( lots of alkene, smaller amount of H2SO4 )

concentrated, smaller amount, not cold

Polymerization C C

conc H2SO4

H C C C C

excess

n repeating unit

A cationic polymerization catalyst (Ziegler Catalyst) can be used instead of H2SO4.

C C OSO3H

polymer

CATIONIC MECHANISM styrene

H

+

small amount

H +

H2SO4

(+)

resonance stabilized

(+)

(Markovnikoff)

(+)

H

H

+

another alkene adds to the carbocation

+

etc.

there is no other nucleophile in this solution

Polystyrene

HOW DOES THE CHAIN END?

H +

The simplest way is to have a nucleophile add to the carbocation site

etc. etc.

.. - :O

O

.. S OH

H

H

O

+

Another way is to lose a hydrogen and form a double bond (ghosted). This method will be discussed in the next chapter.

POLYSTYRENE starting proton H OSO3H

addition of OSO3Hcompletes the end of the chain

n

repeating unit

CH2 CH

=

The repeating units of some other polymers are shown on the next two slides.

SOME COMMON ADDITION POLYMERS example

monomer

polymer

polyethylene

CH2

CH2

polypropylene

CH2 CH

CH2

CH3

polystyrene

CH2 CH

polyvinyl chloride CH2 CH (PVC)

Cl

CH2

CH2 CH CH3 CH2 CH

CH2 CH Cl

uses most common polymer bags, wire insulation, squeeze bottles fibers, bottles, indoor-outdoor carpet styrofoam, inexpensive molded objects: household items, toys synthetic leather, clear bottles, floor coverings, water pipe

Teflon

CF 2 CF 2

CF2 CF2

non-stick surfaces, chemically resistant items

polyacrylonitrile

CH2 CH

CH2 CH

fiber used in sweaters, blankets, carpets

(Orlon, Acrilan)

C N

C N

COMMON ADDITION POLYMERS (cont) example poly(methyl methacrylate)

monomer

polymer

CH3

CH3

CH2

C

(Lucite, Plexiglass)

CH2

(PVA)

CH2 CH

O C CH3

O C CH3 O

O

natural rubber

CH3 CH2

C CH CH2

CH3

CH2 C

neoprene rubber

Cl CH2

unbreakable glass, latex paint

CO2CH3

CO2CH3

poly(vinyl acetate) CH2 CH

C

uses

C CH CH2

Cl

CH CH2

adhesives, latex paints, textile coatings chewing gum the polymer is crosslinked with sulfur (vulcanization)

cross-linked with ZnO, resistant to gasoline and oil

CH2 C CH CH2

SUMMARY

REACTION WITH SULFURIC ACID DEPENDS ON CONDITIONS

1. Cold Concentrated Stoichiometric Sulfuric acid (18M) Only H2SO4 is present, therefore HOSO3is the most abundant nucleophile.

HOSO3- adds to the carbocation

-O

O

Formed from H2SO4

S O H

O

C C + H

2. Hydration - Dilute Aqueous Sulfuric Acid (3-6M) Water is the most abundant nucleophile (55.6 M)

H

H2O adds to the carbocation

O H C C + H

3. Polymerization - Small Amount of Concentrated H2SO4 The alkene itself is the most abundant nucleophile.

Another alkene molecule adds to the carbocation C C There is only a small amount of HOSO 3

C C + H

and a large amount of alkene.