1. Synthesis and Properties of Alkene and Alkynes
ORGANIC CHEMISTRY- 1
Synthesis and Properties of Alkene and Alkynes

2. Types of Hydrocarbons Saturated: All C—C bonds are single bonds
Unsaturated: Hydrocarbons with one or more multiple bonds
The term unsaturated is used because C could bind more H (become saturated) if no multiple bonds existed.

3. Rules for Naming Alkenes
Name the longest chain that contains the double bond or double bonds. The name of the chain will end in –ene.
Number this longest chain so the C=C bond or bonds has/have the lowest number.
The first C of the C=C bond (for C=C bond to have lowest number) identifies the positional location of the double bond.
Name the attached functional groups.
Combine the names of the attached groups and longest chain, the same as you would with alkanes.
For multiple double bonds, indicate the locations of all multiple bonds, use numeric prefixes indicating number of double bonds (-diene, -triene).
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

4. Naming Alkenes
Step 1: Name the longest chain that contains the C=C bond. Use the IUPAC root and the –ene ending.
Step 2: Number the longest chain so the C=C bond gets the lowest number possible.
Step 3: Locate the C=C bond with the lower-numbered carbon.
Examples:
CH3-CH=CH-CH3
2-butene
CH3-CH2-CH2-CH=CH-CH3
2-hexene

5. Naming Alkenes (continued)
Step 4: Locate and name attached groups.
Step 5: Combine all the names.

6. 3-bromo-2-methyl-1-propene 5-ethyl-3-methyl-2-octene
Naming Alkenes
2-pentene
4-methylcyclohexene
2-ethyl-1-hexene
3-bromo-2-methyl-1-propene
5-ethyl-3-methyl-2-octene
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

7. The Geometry of Alkenes
In C=C bonds, sp2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2p orbital. A representation of sp2 hybridization of carbon.
During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged.

8. The Geometry of Alkenes (continued)
One bond (sigma, σ) is formed by overlap of two sp2 hybrids.
The second bond (pi, π) is formed by connecting the unhybridized p orbitals.
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

9. 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.

10. 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.

12. 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.

13. 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

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

15. 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)

16. 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

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

18. 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.

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

20. 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+

21. 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. 21

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

23. 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

24. 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

25. 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

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

27. Ozonolysis (Oxidation of Alkene) Bond Breaking)
Mechanism

28. 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.

29. 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.

30. 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.

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

32. 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.

33. 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.

34. 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).

35. 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.

36. 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.

37. Alkynes
Hydrohalogenation: Electrophilic Addition of HX

38. 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.

39. Alkynes
Hydrohalogenation: Electrophilic Addition of HX

40. 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

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

42. 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.

43. Alkynes acidity: Terminal alkynes are more acidic
than alkenes.

44. 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.

45. Alkynes
Halogenation: Addition of Halogen

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