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

ALKENES Alkenes are also called Olefins (CnH2n) “unsaturated” hydrocarbons. Alkenes occur abundantly in nature. Ethylene (H2C=CH2) is a plant hormone that induces ripening in fruit. Functional group = carbon-carbon double bond sp2 hybridization => flat, 120o bond angles σ bond & π bond => H2C=CH2 No rotation about double bond!

ALKENES C3H6 propene C4H8 butene 1-Pentene 2-Pentene 2,4-heptadiene Examples: C3H6 propene C4H8 butene 1-Pentene 2-Pentene 2,4-heptadiene

Cis/Trans Isomerism: When disubstituted (two substituents other than hydrogen) alkenes contain two substituents on the same side of the double bond they are called cis alkenes and when substituents are attached to opposite side are called trans alkenes. Two methyl groups on the same side of double bond = Cis

Yes possible Not possible Cis–trans isomerism is not possible If one of the double-bond carbons (vinyl carbon) is attached to two identical groups. CH3CH=CHCH3 CH3CH2CH=CH2 Yes possible Not possible CH3 (CH3)2C=CHCH3 CH3CH=CCH2CH3 Not possible Yes possible

E and Z Alkenes For trisubstituted (three substituents) and tetrasubstituted (four substituents) double bonds, a more general method is needed for describing double-bond geometry. If the Higher mass ranked groups are on the same side, then alkene has Z geometry (German zusammen, meaning “together.”) If the higher-ranked groups are on opposite sides, while the alkene has E geometry (German entgegen, meaning “opposite).

E and Z Alkenes

Examples: (Z)-3-methyl-2-pentene or (3-methyl-cis-2-pentene (E)-1-bromo-1-chloropropene

Stability of Alkenes The trans isomer is more stable than the cis isomer by 2.8 kJ/mol (0.66 kcal/mol) at room temperature, corresponding to a 76;24 ratio. Cis alkenes are less stable due to steric strains between two larger substituents on the same side of the double bond.

Industrial Preparation of Alkenes Ethylene, Propene and butene 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 tertiary alcohol. 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 tertiary alcohol. 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 (means less H atoms), while the H attaches to the less highly substituted carbon (means more H atoms).

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+

Hydroboration Borane (BH3) is electron deficient is a Lewis acid. Borane adds to an alkene to give an organoborane. (Anti-MarkovniKov addition). 22

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) 23

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 24

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. 25

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.

Ozonolysis (Oxidation of Alkene) Bond Breaking) Mechanism

Alkynes Introduction—Structure and Bonding: The triple bond consists of 2  bonds and 1  bond. Each carbon is sp hybridized with a linear geometry and bond angles of 1800. Estimation of the energy of pi bonds in ethylene (one  and one  bonds) and acetylene (one  and two  bonds).

Alkynes

Alkynes 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 Alkyne (Acetylene) Alkynes Preparation of Alkyne (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 As we know 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

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 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 acidity: Terminal alkynes are more acidic than alkenes.

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

THE END