Iran University of Science & Technology Organic Chemistry Sh.javanshir Faculty of Chemistry Iran University of Science & Technology

Chapter 4-2. Alkenes: continue

Oxidation of Alkenes: Hydroxylation Hydroxylation adds OH to each end of C=C Stereochemistry of addition is syn Product is a 1,2-dialcohol or diol (also called a glycol)

Syn Hydroxylation of Alkenes Two reagents: Osmium tetroxide (expensive!), followed by hydrogen peroxide or sodium bisulfate Cold, dilute aqueous potassium permanganate, followed by hydrolysis with base 4

syn -Glycol Formation syn glycols are made with OsO4 or KMnO4 OsO4 osmium tetroxide KMnO4 potassium permanganate

SYN ADDITION GIVES CIS GLYCOLS H 3 C H H 3 H O 2 O H O s 4 N a H S O 3 H O H cis C H 3 C H 3 H C H C H 2 O C H syn conformation C H 3 3 C H O H N a H S O 3 C O s 4 H O H meso cis -2-butene remember: addition is syn result is cis

Formation of Diols via Osmium Tetroxide Hydroxylation - converts to syn-diol Osmium tetroxide, then sodium bisulfate Via cyclic osmate di-ester

Mechanism Notice the transfer of 2e- onto Os = REDUCTION OXIDIZED cyclic osmate di-ester Notice the transfer of 2e- onto Os = REDUCTION OXIDIZED Both of the hydroxyl oxygens in the glycol come from OsO4 REDUCED

syn-Glycol Formation (II) potassium permanganate syn hydroxylation

Problem: Which Alkene?

Cleavage Reactions of Alkenes Ozonolysis Hot Potassium Permanganate

Oxidative Cleavage Both the pi and sigma bonds break. C=C becomes C=O. Two methods: Warm or concentrated or acidic KMnO4. Ozonolysis Used to determine the position of a double bond in an unknown.

Ozonolysis Reaction with ozone forms an ozonide. Ozonides are not isolated, but are treated with a mild reducing agent like Zn or dimethyl sulfide. Milder oxidation than permanganate. Products formed are ketones or aldehydes.

Ozone .. .. .. .. .. .. .. .. .. : - : : - electric discharge or cosmic rays .. .. .. .. : .. .. - .. + : + : .. .. - EQUIVALENT RESONANCE STRUCTURES

Ozonolysis of alkenes Ozonide Ozonides usually not isolated, but further reacted with reducing agents Formation of two molecules each containing C=O (Carbonyl) groups

Ozonolysis Example Ozonide DMSO

Examples Aldehydes 1-Butene Ketone 2,3-Dimethyl-2-butene

WORKUP PROCEDURES FOR OZONOLYSIS Two types of workup (decomposition of the ozonide) are possible : 1. OXIDATIVE Hydrogen peroxide is present Aldehydes are oxidized to carboxylic acids. Formaldehyde is oxidized to carbon dioxide, which is lost as a gas. 2. REDUCTIVE Add Zn and H2O or H3O+ METHOD A METHOD B Reduce the ozonide with Pd / H2 , and then add acid ( H3O+ ). Aldehydes survive intact and are not oxidized with reductive conditions.

AT ONE TIME OZONOLYSIS WAS WIDELY USED FOR STRUCTURE PROOF BY DEGRADATION Broken apart ( or degraded ) to simpler pieces that are easier to identify. Unknown compound “At one time” = before spectroscopy. The original structure can be deduced by reassembling the pieces.

PROBLEM TO SOLVE 1) O3 / CH2Cl2 C7H12 2) H3O+ Pd / H2 C7H14 answer

WHAT WAS THE ORIGINAL STRUCTURE ? H2O2 oxidative workup

Oxidation of Alkenes: Potassium permanganate With KMnO4 in basic solution, product is a 1,2-diol In neutral or acidic KMnO4 solution the alkene will be broken into 2 carbonyl groups: 24

Example 25

Oxidation of Alkenes: Potassium permanganate # of H atoms Type of Product 2 CO2 1 -COOH Ketone

Problem

Preparation of Alkenes: A Preview of Elimination Reactions

Alkene Synthesis Overview E2 dehydrohalogenation (-HX) E1 dehydrohalogenation (-HX) Dehalogenation of vicinal dibromides (-X2) Dehydration of alcohols (-H2O)

Removing HX via E2 Strong base abstracts H+ as X- leaves from the adjacent carbon. Tertiary and hindered secondary alkyl halides give good yields. Use a bulky base if the alkyl halide usually forms substitution products.

Some Bulky Bases (CH3CH2)3N : triethylamine

Hofmann Product Bulky bases abstract the least hindered H+ Least substituted alkene is major product.

E2: Cyclohexanes Leaving groups must be trans diaxial.

Removing HX via E1 Secondary or tertiary halides Formation of carbocation intermediate Weak nucleophile Usually have substitution products too

A Preview of Elimination Reactions Alkenes are commonly made by elimination of HX from alkyl halide: (dehydrohalogenation); using heat and KOH

E2: Vicinal Dibromides Remove Br2 from adjacent carbons. Bromines must be anti-coplanar (E2). Use NaI in acetone, or Zn in acetic acid. Br CH3 H

A Preview of Elimination Reactions elimination of H-OH from an alcohol (dehydration); requires strong acids (sulfuric acid, 50 ºC)

Prob.: Mechanism?

Solution:

Polymerization A polymer is a very large molecule consisting of repeating units of simpler molecules (monomers), formed by polymerization

Polymerization An alkene (monomer) can add to another molecule like itself to form a chain (polymer). Three methods: Cationic, a carbocation intermediate Free radical Anionic, a carbanion intermediate (rare)

Polymerization CH2=CH2 + heat, pressure  -(CH2CH2)-n n = 10,000+ polyethylene CH3CH=CH2 polymerization  -(CH2CH)-n CH3 polypropylene CH2=CHCl poly…  -(CH2CH)-n Cl polyvinyl chloride (PVC)

Cationic Polymerization Alkene is treated with an acid. Intermediate must be a stable carbocation. =>

Termination

Cationic Polymerization Dimerization of 2-methylpropene (CH3)2C CH2 monomer (C4H8) H2SO4 + two dimers (C8H16) CH3CCH CH3 C(CH3)2 CH3CCH2C CH2 3

Mechanism of Dimerization + CH3 H2C C CH3C CH3CCH2C CH3 + 3

Mechanism of Dimerization CH3CCH CH3 C(CH3)2 CH3CCH2C CH3 CH2 CH3CCH2C CH3 + 3

Free Radical Polymerization of Alkenes Alkenes combine many times to give polymer Reactivity induced by formation of free radicals

Free Radical Polymerization: Initiation Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule A bond is broken homolytically

Polymerization: Propagation Radical from intiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so on many times

Polymerization: Termination Chain propagation ends when two radical chains combine Not controlled specifically but affected by reactivity and concentration

Overall mechanism for free-radical chain polymerization •Initiation I  2R• R• + M  RM1• •Propagation RM1• + M  RM2• … RMn• + M  RMn+1• •Termination RMn• + RMm•  dead polymer

Free-Radical Polymerization of Ethylene H2C CH2 200 °C 2000 atm O2 peroxides polyethylene CH2 6

.. RO Mechanism • H2C CH2 7

.. RO: Mechanism H2C CH2 • 9

.. RO: Mechanism H2C CH2 • CH2 H2C 10

.. RO: Mechanism H2C CH2 H2C CH2 • 10

.. RO: Mechanism H2C CH2 H2C CH2 CH2 H2C • 10

.. RO: Mechanism H2C CH2 H2C CH2 H2C CH2 • 10

.. RO: Mechanism H2C CH2 H2C CH2 H2C CH2 CH2 H2C • 10

Unsymmetrical Monomers If alkene is unsymmetrical, reaction is via more highly substituted radical

Free-Radical Polymerization of Propene H2C CHCH3 polypropylene CH CH3 Uses: kitchenware food containers, fibres for making hard-wearing carpets. 6

.. RO Mechanism • H2C CHCH3 7

.. RO: Mechanism H2C CHCH3 • 9

.. RO: Mechanism H2C CHCH3 • CHCH3 H2C 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 • 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 CHCH3 H2C • 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 H2C CHCH3 • 10

.. RO: Mechanism H2C CHCH3 H2C CHCH3 H2C CHCH3 CHCH3 H2C • 10

Poly(phenylethene) or Polystyrene peroxides nC6H5-CH=CH2 (-CH-CH2-)n reflux in kerosene C6H5 Stiffer than poly(ethene), greater Van der Waals’ force due to the benzene rings. Uses: Toys, cups, refrigerator parts. Expanded polystyrene for packaging, heat and sound insulation.

Chain Branching During Polymerization During radical propagation chain can develop forks leading to branching One mechanism of branching is short chain branching in which an internal hydrogen is abstracted

Branching of the Polymer Chain

Anionic Polymerization Alkene must have an electron-withdrawing group like C=O, CN, or NO2. Initiator: Grignard or organolithium reagent. =>

Natural Rubber Natural rubber comes from the rubber tree (Hevea brasiliensis) and is a white, milky liquid called latex. Most rubber comes from Malaysia and other nations in East Asia. Latex can also be seen as the white fluid in dandelion stalks. The latex from the tree is actually a suspension of rubber particles in water. Rubber is a polymer of isoprene. Natural rubber is relatively reactive, and is especially vulnerable to oxidation. Long chains can be stretched, but then return to original structure. Chains slide past each other and can be pulled apart easily. Structure is cis-1,4-polyisoprene. n

Vulcanization Discovered accidentally by Goodyear(1839) , dropped rubber and sulfur on a hot stove. Sulfur produces cross-linking, strengthening the rubber. Cross-linking also increases the elasticity. Hardness can be controlled by varying the amount of sulfur. The optimum amount of sulfur to be added to the rubber is about 10% by weight. Adding an excess of sulfur produces a very brittle and inelastic substance called ebonite.

Synthetic Rubber With a Ziegler-Natta catalyst, a polymer of 1,3-butadiene can be produced, in which all the additions are 1,4 and the remaining double bonds are all cis. It may also be vulcanized.

Polyamides: Nylon Usually made from reaction of diacids with diamines, but may also be made from a single monomer with an amino group at one end and acid group at other.

Polyesters Dacron® and Mylar®: polymer of terephthalic acid and ethylene glycol. Made by the transesterification of the methyl ester.

Polycarbonates Esters of carbonic acid. Carbonic acid is in equilibrium with CO2 and water, but esters are stable. React phosgene with bisphenol A to obtain Lexan® for bulletproof windows.

Cycloalkenes Cyclopropene and cyclobutene have angle strain. Larger cycloalkenes, such as cyclopentene and cyclohexene, can incorporate a double bond into the ring with little or no angle strain. 13

Cycloalkenes- 3 to 7 “cis” olefins rings not large enough to accommodate trans double bonds C8 limited stability as trans

Stereoisomeric cycloalkenes cis-cyclooctene and trans-cyclooctene are stereoisomers cis-cyclooctene is 39 kJ/ mol more stable than trans-cyclooctene H H H cis-Cyclooctene trans-Cyclooctene 15

more stable configuration of double bond predominates Br KOCH2CH3 ethanol + (15%) (85%) more stable configuration of double bond predominates 11

Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability H trans-Cyclooctene 15

Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability cis-Cyclododecene trans-Cyclododecene 15