ALKENES

Unsaturated chemical compound containing at least one carbon-carbon double bond, where rotation about the C=C is very difficult. To show the presence of the double bond, the –ane suffix from the alkane name is changed to –ene. Also called olefins( fat dissolving) sp2 atomic orbitals Trigonal planar, 120o degree

Geometric Isomerism Cis-trans isomerism -isomers that have same order of atom attachment but a different arrangement of their atoms in space.

CnH2n where n is the number of carbon atoms in the molecule General Formula CnH2n where n is the number of carbon atoms in the molecule

Physical Properties Physical state -The first lower member like ethene, propene and butene are colorless gases.  Density - lighter than water.   Solubility - insoluble in water and soluble in nonpolar organic solvents. more reactive than alkanes due to their double carbon-carbon bond.

Boiling point -The boiling points of alkenes gradually increase with an increase in the molecular mass.  The cis isomer ( example cis-2-butene, b.p.= 3.7°C) is higher in bpt than its trans isomers (example, trans-2-butene, b.p.= 1°C) 

Melting point  The melting points of alkenes increase with an increase in the molecular mass.

Natural Sources Isolated from petroleum. Plant material like plant hormone, like Ethylene – a natural ripening agent and Terpenes – found in essential oil.

Some Common Alkene Polymers and their Uses Ethylene H2C=CH2 Polyethene, Polythene Packaging, cable insulation, films and sheets Tetrafluoroethene F2C=CF2 Polytetrafluoroethene, PTFE, Teflon Coatings, gaskets. Chloroethene (vinyl chloride) H2C=CHCl Polyvinyl chloride, PVC, Tedlar Insulation, films, pipes Styrene H2C=CHC6H5 Polystyrene, Styron Foam for packaging etc. Vinyl acetate H2C=CHOCOCH3 poly(vinyl acetate), PVA Paints, adhesives.

Preparations of Alkenes 1. Dehydrohalogenation of Alkyl halides 2. Dehydration of Alcohol 3. Dehalogenation of Vicinal Halides 4. Reduction of Alkynes

Preparations of alkenes 1. Dehydrohalogenation of alkyl halides Preparations of alkenes 3° > 2° > 1° Example: n-butyl chloride 1-butene

Dehydrohalogenation of Alkyl Halides Is an Elimination reaction. The term "elimination" describes the fact that a small molecule is lost during the process. Different mechanisms are possible: Loss of the LG to form a carbocation,  removal of H+ and formation of C=C bond Simultaneous H+ removal, C=C bond formation and loss of the LG Removal of H+ to form a carbanion, loss of the LG and formation of C=C bond.

2. Dehydration of alcohol 3° > 2° > 1° ex.

n-butyl alcohol 1-butene 2-butene (chief product) sec-butyl alcohol 2-butene 1-butene (chief product)

Dehydration of Alcohols It is the elimination of water molecule from alcohol to convert into alkene. Lost of H and OH from adjacent carbons An acid catalyst alkene.

Mechanism of Alcohol Dehydration Step 1 : Alcohol unites with a hydrogen ion to form the protonated alcohol Step 2 : Alcohol associates into water and carbonium ion. Step 3 : The carbonium ion then loses a hydrogen ion to form alkene.

Dehalogenation of vicinal dihalides (same side) Example: 2,3- Dibromobutane 2- butene

4. Reduction of alkynes

Reduction of Alkynes Reducing Alkynes to form trans or cis Alkenes. Using Na/ NH3 Step 1: Sodium transfer an electron to the alkyne giving a radical anion. Step 2: The radical anion removes a proton from the ammonia in an acid/base reaction

Step 3: A second atom of sodium transfers another electron to the alkyne giving an anion. Step 4 : the anion removes proton from the ammonia in an acid/base reaction.

Reactions of Alkene Halogenation Hydration Hydrogenation Addition of hydrogen Halides Addition of sulfuric acid Addition of Carbenes Addition of Free Radical

Allylic Hydrogenation Dimerazation Alkylation Polymerization Hydroxylation Halohydrins formation Ozonolysis Hydroboration-oxidation Epoxidation

Reactions of Alkenes 1. Addition of Halogens (X2) Example.

Halogenations – Addition of Halogens When an alkene is treated at room temperature with a solution of bromine or chlorine in carbon tetrachloride or some other inert solvent, the halogens adds rapidly to the double bond of the alkene to give the corresponding vicinal dihalide ( two halogens attached adjacent carbons

2. Addition of Hydrogen (catalytic hydrogenation) Ex.

Hydrogenation of Alkenes The relationship between reactants and products in addition reactions can be illustrated by the hydrogenation of alkenes yield alkanes.Hydrogenation is the addition of H2 to a multiple bond.

3. Addition of hydrogen halides

Ex. The presence or absence of peroxide has no effect on the orientation of addition of HCl and HI, sulfuric acid and water.

Ex.

Addition of hydrogen halides In the addition of an acid to the C=C of an alkene, the hydrogen of the acid attaches itself to the carbon that already holds the greater number of hydrogens The reactivity of alkene, with halogen acids is in the order;. HI > HBr > HCl

4. Addition of sulfuric acid Ex.

Follows Markovnikov addition

5. Addition of water. HYDRATION

Ex. FOLLOWS MARKOVNIKOV’S RULE

Addition of Water - Hydration When heated with water in the presence of an acid catalyst, alkenes yield alcohol ROH. The process is called hydration of alkenes because it involves the addition of water across the double bond. The addition of the HOH across the double bonded carbon that bears the greater number of hydrogen atoms and the hydroxyl groups goes to the other double-bonded carbon

6. Halohydrin formation Ex.

Sterospecific:

7. Dimerization (di = two, mer = part, product contains exactly twice the # of C & H atom as the original).

Mechanism: Addition of the tert-butyl cation to iso butylene; the orientation of addition is duch to yield the more stable tertiary cation. Step(2) brings about the union of two : isobutylene units, which is of course necessary for the product.

8. Alkylation ex.

mechanism: Addition of a hydrogen ion to form carbocation Addition of a tert-butyl carbocation to isobutylene Carbocation abstracts a hydrogen atom with its pair of electrons from a molecule of alkane. This abstraction of hydride ion yields an alkane of 8 carbons and a new carbocation to continue the chain.

A carbocation may: a.) combine with a negative ion or other basic molecule b.) rearrange to a more stable cabocation c.) eliminate a hydrogen ion to form an alkene d.) add to an alkene to form a larger carbocation e.) abstract a hydride ion from an alkane

9. Oxymercuration - demercuration Oxymercuration – involves addition to the C=C of OH and HgOAc (mercuric ion) Demercuration – the HgOAc is replaced by H

Ex. Follows Markovnikov addition Undergo the process of oxymercuration, involves addition to the carbon – carbon double bond of –OH+ -HgOAc Follows Markovnikov addition Mercury acetate in the presence of water to give hydroxy-mercurial compounds which on reduction yields alcohol. Oxymercuration involves addition to C=C of OH and –HgOAc is replaced by H. Sodium borohydride used to reduced the cmp to alcohol

10. Hydroboration – oxidation With the reagent Diborane, alkenes undergo hydroboration to yield alkylboranes, which on oxidation give alcohols.

Mechanism: Hydroboration involves the addition of the double bond of BH3 w H becoming attach to one doubly bonded carbon and boron to the other. The alkylborane can then undergo oxidation in which the boron is replaced by –OH. Thus, the 2 – stage reaction process of hydroboration oxidation permits the effect. The addition to the carbon – carbon double bond of elements of H-OH.

11. Addition of free radicals

Mechanism:

Electrophilic addition : Markonikov orientation

Free – radical Addition : Anti – Markovnikov orientation

12. Polymerization of Alkenes Polymerization – the joining together of many small molecules to form very large molecules. Monomers – the simple compounds form which polymers are made. Ex. 5 processes of polymerization 1. Free - radical polymerization 2. Cationic polymerization 3. Anionic polymerization 4. Condensation polymerization 5. Coordination polymerization

Free – radical polymerization Polyvinyl chloride - use to make phonograph, records, plastic pipes, when plasticized with high boiling esters – raincoats, shower curtains and coatings for metal and upholstery fabrics. Peroxide – initiator, required in small amount in polymerization Free radical initiator Mechanism: Free radical adds to molecule of alkenes which for another free radical

This radical adds to another molecule of alkene to generate another free radical. This radical adds to another molecule of alkene to generate a still larger radical

13. Addition of Carbenes. Cycloaddition carbenes – derivative of methylene Methylene is highly reactive

Methylene exist into 2 different forms singlet methylene – unshared electrons are paired, less stable & generated first in photolysis : stereospecific addition triplet methylene – unshared electrons are not paired, free radical (diradical) : nonstereospecific addition

Cycloaddition: addition of the carbon – carbon double bond Sterospecific: (addition of methylene can occur with 2 different kinds of stereochemistry.) Photolysis of diazomethane into in liquid And in liquid

Singlet methylene Stereospecific Electrophilic addition Electron deficient and can find electrons at the C-C double bondingle Non – stereospecific: cis/trans2 – butene + both cis and trans 1,2 - dimethylcyclopropane Triplet methylene Non - Stereospecific Free radical addition ff. by addition

Methylene undergoes intersection Addition of substituted carbenes: 1,1 - elimination Potassium t-butoxide yields cycloalkane

Mechanism: Reaction involves a divalent carbon compound, a derivative of methylene: dichlorocarbene: CU2. Generated in 2 steps, initiated by attack on chloroform by the strong base tert-butoxide ion and then adds to the alkene. Because of the presence of halogen atom, the singlet form is the more stable form of dichlorocarbene and is the one adding to the double bond.

Addition of Carbenes Carbenes are intermediates of the general formula CH2:. The derivatives of methylene (CH2) are the carbenes. Methylene is formed by the photolysis of either diazomethane, CH2N2 or ketene, CH2=C=O.

14. Hydroxylation. Glycol formation Example. Oxidizing agents that bring about hydroxylation cold alkaline potassium permanganate, KMnO4 b. peroxy acids, such as peroxyformic acid HCO2OH

15. Halogenation. Allylic substitution ( same mechanism with substitution in alkenes) Ex.

Can we direct the attack to just one of these sites Can we direct the attack to just one of these sites? Yes, by our choice of rxn. Conditions. Conditions: alkenes undergo substitution by halogen at high temp. or under the influence of UV light, generally in gas phase. it can also undergo addition of halogen at low temp. in the absence of light and generally in liquid state(phase).

N – bromosuccinimide a reagent used for the specific purpose of brominating alkenes at the allylic position provides a constant low conc. of bromine.

. Ease of abstraction of hydrogen atoms: Vinylic hydrogen- hydrogens attached to C=C Alylic hydrogen – hydrogens attached to a carbon atom next to a double bond Ease of abstraction of hydrogen atoms: Allylic >3º > 2º > 1º > CH4 > Vinylic Ease of formation of free radicals: allyl > 3º > 2º > 1º > CH3. > vinyl

Example:

16. Ozonolysis (Cleavege rxn) Cleavage – a rxn in which the double bond is completely broken and the alkene molecules converted into 2 smaller molecule. Reducing agent (Zn) – prevent formation of hydrogen peroxide will not react with aldehyde and ketone (aldehyde are often converted to acid, RCOOH for ease of isolation.)

Ozonolysis – is a typical means of degradation

17. Cleavage with periodate ( cleavage with a diol) RCOOH are generally obtained instead of aldehydes, RCHO a terminal ==CH2 group is oxidized to CO2.

Example.

Cleavage of cycloalkenes

18. Epoxidation of Alkenes C=C +CH3COOH C C + CH3COH O Alkene Peroxyacetic Acid Epoxide Carboxylic acid

Example H2C=CH(CH2)9CH3 + CH3COOH CH2-CH(CH2)9CH3 + CH3COH O O 1-Dodecane Peroxyacetic acid 1,2-epoxydodecane Acetic acid