Synthesis and Properties of Alkene and Alkynes ORGANIC CHEMISTRY- 1 Synthesis and Properties of Alkene and Alkynes BY Dr. Ghulam Abbas Assistant Professor UNIVERSITY OF NIZWA

Industrial Preparation of Alkenes Ethylene, Propene and butane are synthesized industrially by cracking of light alkanes. The process is complex and involves radical reactions. The high-temperature reaction conditions cause spontaneous homolytic breaking of C -C and C -H bonds and as results smaller fragments are formed.

Preparations Unsymmetrical Alcohol secondary or ter. Dehydration of Alcohols Removal of water is called dehydration. Con. H2SO4 is used. The order of dehydration is ter. alcohol > sec. alcohol > prim. alcohol Unsymmetrical Alcohol secondary or ter. Saytzev: Highly substituted alkenes are formed.

Saytzeff Product: According to this rule, major product is the most substituted alkene i.e., major product is obtained by elimination of H from that β-carbon which has the least number of hydrogen. Hofmann Rule : According to this rule, major product is always least substituted alkene i.e., major product is formed from β-carbon which has maximum number of hydrogen.

By dehydrohalogenation of Alkyl halide Alcoholic solution of KOH By Dehalogenation of Vicinal dihalides Having two halogen atoms on adjacent carbon atom is called vicinal dihalides

Hydrogenation The addition of hydrogen in the presence of catalyst E.g. Pd (palladium) in BaSO4 .

Reduction of Alkenes: Hydrogenation Chemical Properties Reduction of Alkenes: Hydrogenation Addition of H-H across C=C Reduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution)

Addition is exclusively trans Halogenations Reaction: Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process. F2 is too reactive While I2 does not add. Mechanism Addition is exclusively trans

Flow of a pair of electron (Addition of HX) r.d.s. fast Mechanism: Note: r.d.s. = rate determining step

Chemical Properties (Mechanism of HX Addition Reactions) The first step is electrophilic attack of H+, leading to the formation of carbonium ion. This is a rate determining step. It is then followed by the attack at the positive carbon center by X- which is a fast step. Markownikoff’s rule means X group attaches to the more highly substituted carbon atom, while the H attaches to the less highly substituted carbon.

With conc. sulphuric acid Do you remember the Markovnikoff’s Rule? What is / are the products of the following? Alcohol Mechanism

Markownikoff’s rule 3ry C+ > 2ry C+ > 1ry C+ > CH3+ The H atom will attach to carbon having more no. of H atoms. In general, the greater the no. of alkyl grops present, or the larger is the alkyl group, the more stable is the carbonium ion. Stability of carbonium ion: 3ry C+ > 2ry C+ > 1ry C+ > CH3+

Hydration of Alkene Oxymercuration Intermediates Use mercuric acetate in THF followed by sodium borohydride The regiochemistry of the reaction corresponds to Markovnikov orientation and addition of water via mercurinium ion. 14

Hydroboration Borane (BH3) is electron deficient is a Lewis acid. Borane adds to an alkene to give an organoborane. 15

Orientation in Hydration via Hydroboration Regiochemistry is opposite to Markovnikov orientation (Anti-MarkovniKov addition) H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition) 16

Mechanism of Hydroboration Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of BH3 are added across C=C More stable carbocation is also consistent with steric preferences 17

Addition of Carbenes to Alkenes The carbene functional group is “half of an alkene” Carbenes are electrically neutral with six electrons in the outer shell They add symmetrically across double bonds to form cyclopropanes 18

Oxidation of Alkene Oxidation of Alkenes with peroxyacid results in acid and Epoxide synthesis.

Ozonolysis (Oxidation of Alkene) Bond Breaking) Mechanism

Ozonolysis Chemical Properties Products are carbonyl compounds which can be easily identified. If RHC=O aldehyde will be formed which will be further oxidised by H2O2 to organic acid. Since the resulting carbonyl compounds are used for identification, thus oxidation is an unfavorable process. In order to prevent this happens, zinc dust is added.

at vigorous condition (bond breaking) Oxidation at room temperature (Hydroxylation) (addition) C + [ O ] H 2 f r o m Mn 4 - / at vigorous condition (bond breaking) Further oxidation In acidic condition, the products will be oxidised to acid or ketone.

Alkynes Introduction—Structure and Bonding: Recall that the triple bond consists of 2  bonds and 1  bond. Each carbon is sp hybridized with a linear geometry and bond angles of 1800.

Alkynes Introduction—Structure and Bonding: Estimation of the energy of pi bonds in ethylene (one  and one  bonds) and acetylene (one  and two  bonds).

Alkynes Introduction—Structure and Bonding: Alkynes contain a carbon—carbon triple bond. Terminal alkynes have the triple bond at the end of the carbon chain so that a hydrogen atom is directly bonded to a carbon atom of the triple bond. Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond. An alkyne has the general molecular formula CnH2n-2, giving it four fewer hydrogens than the maximum possible for the number of carbons present. Thus, the triple bond introduces two degrees of unsaturation.

Alkynes Physical Properties: The physical properties of alkynes resemble those of hydrocarbons of similar shape and molecular weight. Alkynes have low melting points and boiling points. Melting point and boiling point increase as the number of carbons increases. Alkynes are soluble in organic solvents and insoluble in water.

Preparation of Alkynes: Acetylene Acetylene can be prepared from calcium carbide and water. CaC2 + HOH  H-CC-H Calcium carbide is prepared by heating coke and calcium oxide in an electric furnace (~2500o).

Alkynes Dehydrohalogenation Recall that alkynes are prepared by elimination reactions. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne through two successive E2 elimination reactions.

Alkynes Alkyne Reactions: Additions Like alkenes, alkynes undergo addition reactions because they contain relatively weak  bonds. Two sequential addition reactions can take place: addition of one equivalent of reagent forms an alkene, which can then add a second equivalent of reagent to yield a product having four new bonds.

Alkynes Hydrohalogenation: Electrophilic Addition of HX

Alkynes Hydrohalogenation: Electrophilic Addition of HX Alkynes undergo hydrohalogenation, i.e the, addition of hydrogen halides, HX (X = Cl, Br, I). Two equivalents of HX are usually used: addition of one mole forms a vinyl halide, which then reacts with a second mole of HX to form a geminal dihalide.

Alkynes Hydrohalogenation: Electrophilic Addition of HX

Alkynes are reduced to alkanes by addition of H2 over a metal catalyst Alkynes are reduced to alkanes by addition of H2 over a metal catalyst. The reaction occurs in two steps through an alkene intermediate. Pd, CaCO3/Quinoline

Oxidative Cleavage of Alkynes Alkynes are cleaved by powerful oxidizing agent such as KMnO4 or O3 .

Alkynes acidity: Terminal alkynes are more acidic than alkenes.

Alkynes Reactions of Acetylide anions: Acetylide anions react with unhindered alkyl halides to yield products of nucleophilic substitution. Because acetylides are strong nucleophiles, the mechanism of substitution is SN2, and thus the reaction is fastest with CH3X and 10 alkyl halides.

Alkynes Halogenation: Addition of Halogen Halogens X2 (X = Cl or Br) add to alkynes just as they do to alkenes. Addition of one mole of X2 forms a trans dihalide, which can then react with a second mole of X2 to yield a tetrahalide.

Alkynes Halogenation: Addition of Halogen

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