Synthesis and Properties of Alkene and Alkynes ORGANIC CHEMISTRY- 1 Synthesis and Properties of Alkene and Alkynes By Dr. Sulaiman Al Sulaimi 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 Examples: C3H6 propylene C4H8 butylenes

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

(CH3)2C=CHCH3 CH3CH=CCH2CH3 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 No CH3 (CH3)2C=CHCH3 CH3CH=CCH2CH3 No Yes

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:

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.

Z E

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 Dehydration of Alcohols Removal of water is called dehydration. Con. H2SO4 is used. The order of dehydration is ter. alcohol > sec. alcohol > prim. alcohol

Mechanism of the acid-catalyzed dehydration of a tertiary alcohol to yield an alkene. The process is an E1 reaction and involves a carbocation intermediate.

Zaitsev’s rule: 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.

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 .

PROPERTIES OF ALKANES CHEMICAL PROPERTIES: Alkanes are called paraffins (low affinity compounds) because they are relatively uncreative. This term describes that alkanes show little chemical affinity for other substances and are chemically and biologically inert. The alkanes are inert due to factors below; a. Strong C-C bond and C-H bonds b. Absence of functional group

The reactivity of alkanes is limited for instance, Combustion needs a flame to get it started. Halogens react in the presence of heat or light. Much of the chemistry of alkanes involve free radical and chain reactions are involved.

They burn in a flame, producing carbon dioxide, water, and heat. 1- Reaction with oxygen occurs during combustion in an engine or furnace when the alkane is used as a fuel. They burn in a flame, producing carbon dioxide, water, and heat. CH4 + 2 O2 → CO2 + 2 H2O + 890 kJ/mol (213 kcal/mol) The amount of heat evolved when one mole of a hydrocarbon is burnt to carbon dioxide and water is called heat of combustion.

2- Reaction with Halogens : The reaction of halogen with alkanes is called halogenation. The reaction of an alkane with Cl2 occurs when a mixture of the two is irradiated with ultraviolet light as (UV) represented as hy, where hy is the Greek letter nu). Alkanes also react with fluorine and bromine. However reaction with Iodine is reversible.

Reaction with Halogens The reaction does not stop at this stage. Depending upon time and Cl2, the remaining three hydrogen atoms of methyl Chloride can be successively replaced by chlorine atoms.

Reaction with Halogens Bromine (Br) reacts with alkanes in a similar manner but less vigorously. Reaction with Iodine is reversible.

3- Nitration: Alkanes undergo nitration react at high temperature with the replacement of H- atom by –Nitro group (NO2) is called nitration, Possible with conc. HNO3 Example:

4- Sulphonation: It is the replacement of H- atom of Alkanes by –SO3H group. Sulphonation of alkanes occurs with fuming suphuric acid at moderate temperature.

5- Pyrolysis or Cracking: The decomposition of a compound by the action of heat alone is called pyrolysis. (pyr = fire and lysis = split up i. e cleavage by heat. The pyrolysis of alkanes is known as cracking”. Alkanes decompose into smaller molecule on heating to 500-700 oC . Ethane when heated to 500 oC in the absence of air, gives a mixture of methane, ethylene and hydrogen

Example: n-Hexane yields benzene. 6- Dehydrogenation: It is the removal of hydrogen form molecule Benzene and other aromatic hydrocarbons are prepared from petroleum hydrocarbons. Example: n-Hexane yields benzene.

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)

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 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

(Addition of HX) r.d.s. fast Mechanism: Rate-limiting step:The slowest step in a multistep reaction sequence; also called the rate-determining step. The rate-limiting step acts as a kind of bottleneck in multistep reactions. fast

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

Hydration of Alkene Oxymercuration Intermediates Use mercuric acetate in THF followed by sodium borohydride The regiochemistry (orientation) of the reaction corresponds to Markovnikov orientation and addition of water via mercurinium ion. 40

Hydroboration Borane (BH3) is electron deficient is a Lewis acid. Borane adds to an alkene to give an organoborane. (Anti-MarkovniKov addition). 41

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) 42

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 43

44

Anti-MarkovniKov Markownikov 45

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. 46

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

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)

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.

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 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

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