ORGANIC CHEMISTRY CHM 207

1. ORGANIC CHEMISTRY CHM 207 CHAPTER 3: ALKENES NOR AKMALAZURA JANI

2. SUBTOPICS • Naming alkenes and cycloalkenes. • Physical properties of alkenes: i) boiling points and densities ii)polarity • Preparation of alkenes: i) dehydration of alcohols ii) dehydrohalogenation of haloalkanes

3. Reactions of alkenes: • i) Addition reaction: • a) Catalytic hydrogenation • b) Addition of halogens • - In inert solvent • - In water / aqueous medium • c) Addition of hydrogen halides • d) Addition reaction with concentrated sulfuric acid: hydration of • alkenes • e) Addition reaction with acidified water (H3O+): hydration of alkenes • ii) Combustion of alkenes • iii) Oxidation: • a) epoxidation • b)hydroxylation • c)Ozonolysis • iv) Polymerization

4. Unsaturation tests of alkenes: i) Reactions of alkenes with KMnO4 ii) Reactions of alkenes with bromine. • Uses of alkenes: i) PE ii) PVC iii) ethanol

5. ALKENES • Also called olefins • Contain at least one carbon-carbon double bond (C=C) • General formula, CnH2n (n=2,3,…) • Classified as unsaturated hydrocarbons (compound with double or triple carbon-carbon bonds that enable them to add hydrogen atoms. • sp2-hybridized • For example: C2H4 - ethylene

6. Naming Alkenes

7. IUPAC RULES RULE 1. Select the longest continuous carbon chain that contains a double bond. This chain contains 6 carbon atoms

8. RULE 2. Name this compound as you would an alkane, but change –ane to –ene for an alkene. This chain contains 8 carbon atoms This is the longest continuous chain. Select it as the parent compound. Name the parent compound octene.

9. RULE 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon to indicate the position of the double bond. Place this number in front of the alkene name.

10. IUPAC RULES This end of the chain is closest to the double bond. Begin numbering here.

11. IUPAC RULES The name of the parent compound is 1-octene. 4 3 2 1 5 6 7 8

12. RULE 4. Branched chains and other groups are treated as in naming alkanes. Name the substituent group, and designate its position on the parent chain with a number.

13. IUPAC RULES The ethyl group is attached to carbon 4. This is an ethyl group. 4 4 3 2 1 5 6 4-ethyl-1-octene 7 8

14. NEW IUPAC NAMES • Placing numbers (location of double bond) before the part of the name –ene. • Example:

15. A compound with more than one double bond. • Two double bond: diene • Three double bond: triene • Four double bond: tetraene * Numbers are used to specify the locations of the double bonds.

16. ALKENES AS SUBSTITUENTS • Alkenes names as substituents are called alkenyl groups. • Can be named systematically as ethenyl, propenyl, etc. or by common names such as vinyl, ally, methylene and phenyl groups.

17. CYCLOALKENES • Contains C=C in the ring • Nomenclature of cycloalkenes: • Similar to that alkenes • Number the cycloalkane so that the double bond is between C1 and C2 and so that the first substituent has as low a number as possible. • * Double bond always between C1 and C2.

18. NOMENCLATURE OF cis-trans ISOMERS • cis – two particular atoms (or groups of atoms) are adjacent to each other • trans – the two atoms (or groups of atoms) are across from each other

19. PHYSICAL PROPERTIES OF ALKENES • Boiling points and densities: - Most physical properties of alkenes are similar to those alkanes. - Example: the boiling points of 1-butene, cis-2-butene, trans-2-butene and n-butane are close to 0oC. - Densities of alkenes: around 0.6 or 0.7 g/cm3. - Boiling points of alkenes increase smoothly with molecular weight. - Increased branching leads to greater volatility and lower boiling points.

20. Polarity: • - relatively nonpolar. • - insoluble in water but soluble in non-polar solvents such as hexane, gasoline, halogenated solvents and ethers. • - slightly more polar than alkanes because: • i) electrons in the pi bond is more polarizable (contributing to instantaneous dipole moments). • ii) the vinylic bonds tend to be slightly polar (contributing to a permanent dipole moment).

21. Alkyl groups are electron donating toward double bond, helping to stabilize it. This donating slightly polarizes the vinylic bond, with small partial positive charge on the alkyl group and a small negative charge on the double bond carbon atom. • For example, propene has a small dipole moment of 0.35 D. propene, μ = 0.35 D Vinylic bonds Vector sum = propene, μ = 0.33 D cis-2-butene, bp 4oC Vector sum = 0 propene, μ = 0 trans-2-butene, bp 1oC

22. In a cis-disubstituted alkene, the vector sum of the two dipole moments is directed perpendicular to the double bond. • In a trans-disubstituted alkene, the two dipole moments tend to cancel out. If an alkene is symmetrically trans-disubstituted, the dipole moment is zero. Vector sum = propene, μ = 0.33 D cis-2-butene, bp 4oC Vector sum = 0 propene, μ = 0 trans-2-butene, bp 1oC

23. Cis- and trans-2-butene have similar van der Waals attractions, but only cis isomer has dipole-dipole attractions. • Because of its increased intermolecular attractions, cis-2-butene must be heated to a slightly higher temperature (4oC versus 1oC) before it begins to boil. Vector sum = propene, μ = 0.33 D cis-2-butene, bp 4oC Vector sum = 0 propene, μ = 0 trans-2-butene, bp 1oC

24. PREPARATION OF ALKENES • Dehydration of alcohols • Dehydrohalogenation of haloalkanes

25. PREPARATION OF ALKENES • Alkenes can be prepared in the following ways: i) Dehydration of alcohols conc. H2SO4 R-CH2-CH2-OH R-CH=CH2 + H2O ii) Dehydrohalogenation of haloalkanes NaOH/ethanol R-CH2-CH2-X R-CH=CH2 + HX reflux NaOH can be replaced by KOH

26. Saytzeff rule: - A reaction that produces an alkene would favour the formation of an alkene that has the greatest number of substituents attached to the C=C group. Dehydration of alcohols Dehydrohalogenation of haloalkanes

27. REACTIVITY OF ALKENES More reactive than alkanes because: • A carbon-carbon double bond consists of a σ and a π bond. It is easy to break the π bond while the σ bond remains intact. • The π electrons in the double bond act as a source of electrons (Lewis base). Alkenes are reactive towards electrophiles which are attracted to the negative charge of the π electrons. • π bond will broken, each carbon atom becomes an active site which can form a new covalent bond with another atom. One π bond is converted into 2 σ bonds.

28. REACTION OF ALKENES • i) Addition reaction: • a) Catalytic hydrogenation • b) Addition of halogens • - In inert solvent • - In water / aqueous medium • c) Addition of hydrogen halides • d) Addition reaction with concentrated sulfuric acid: hydration of • alkenes • e) Addition reaction with acidified water (H3O+): hydration of alkenes • ii) Combustion of alkenes • iii) Oxidation: • a) epoxidation • b)hydroxylation • c)Ozonolysis • iv) Polymerization

29. REACTIONS OF ALKENES • Catalytic hydrogenation: - hydrogenation: addition of hydrogen to a double bond and triple bond to yield saturated product. - alkenes will combine with hydrogen in the present to catalyst to form alkanes. • Plantinum (Pt) and palladium (Pd) – Catalysts • Pt and Pd: temperature 25-90oC • Nickel can also used as a catalyst, but a higher temperature of 140oC – 200oC is needed.

31. Addition of halogens: • i) In inert solvent: • - alkenes react with halogens at room temperature and in dark. • - the halogens is usually dissolved in an inert solvent such as dichloromethane (CH2Cl2) and tetrachloromethane (CCl4). • - Iodine will not react with alkenes because it is less reactive than chlorine and bromine. • - Fluorine is very reactive. The reaction will produced explosion.

32. EXAMPLES:

33. Addition of halogens: • ii) In water / aqueous medium: • - chlorine dissolves in water to form HCl and chloric (l) acid • (HOCl). • Cl2 (aq) + H2O(l) HCl(aq) + HOCl (aq) • - same as bromine • Br2 (aq) + H2O(l) HBr(aq) + HOBr(aq) • * Reaction of alkenes with halogens in water (eg. chlorine water and bromine water) produced halohydrins (an alcohol with a halogen on the adjacent carbon atom).

34. EXAMPLES:

35. Addition of hydrogen halides: • - Addition reaction with electrophilic reagents. • - Alkenes react with hydrogen halides (in gaseous state or in aqueous solution) to form addition products. • - The hydrogen and halogen atoms add across the double bond to form haloalkanes (alkyl halides). • - General equation: • Reactivity of hydrogen halides : HF < HCl < HBr < HI

36. * Reaction with HCl needs a catalyst such as AlCl3

37. MARKOVNIKOV’S RULE • There are 2 possible products when hydrogen halides react with an unsymmetrical alkene. • It is because hydrogen halide molecule can add to the C=C bond in two different ways.

38. Markovnikov’s rules: - the addition of HX to an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom (of the double bond) with the larger number of hydrogen atoms.

39. δ- δ+ Mechanism of electrophilic addition reactions: - C=C : electron rich part of the alkene molecule - Electrophiles: electron-seeking Step 1: Formation of carbocation. Attack of the pi bond on the electrophile to form carbocation. Step 2: Rapid reaction with a negative ion. The negative ion (Y-) acts as nucleophile and attacks the positively charged carbon atom to give product of the addition reaction.

40. ADDITION OF HYDROGEN HALIDES TO UNSYMMETRICAL ALKENES AND MARKOVNIKOV’S RULE

42. Addition reaction with concentrated sulfuric acid: hydration of alkenes • - the alkene is absorbed slowly when it passed through concentrated sulfuric acid in the cold (0-15oC). • - involves the addition of H atom and HSO4 group across the carbon-carbon double bond. • - follows Markovnikov’s rule.

44. H+ • Addition reaction with acidified water (H3O+): hydration of alkenes • Hydration: The addition of H atoms and –OH groups from water molecules to a multiple bond. • Reverse of the dehydration reaction. • Direct hydration of ethene: • - passing a mixture of ethene and steam over phosphoric (v) acid (H3PO4) absorbed on silica pellets at 300oC and a pressure of 60 atmospheres. • - H3PO4 is a catalyst.

45. H+ • Markovnikov’s rule is apply to the addition of a water molecule across the double bond of an unsymmetrical alkene. • For examples: H+ = catalyst

46. H+ = catalyst

47. ANTI-MARKOVNIKOV’S RULE: FREE RADICAL ADDITION OF HYDROGEN BROMIDE • When HBr is added to an alkene in the absence of peroxides it obey Markovnikov’s rule. • When HBr (not HCl or HI) reacts with unsymmetrical alkene in the presence of peroxides (compounds containing the O-O group) or oxygen, HBr adds in the opposite direction to that predicted by Markovnikov’s rule. • The product between propene and HBr under these conditions is 1-bromopropane and not 2-bromopropane.

48. Anti-Markovnikov’s addition: - peroxide-catalysed addition of HBr occurs through a free radical addition rather than a polar electrophilic addition. - also observed for the reaction between HBr and many different alkenes. - not observed with HF, HCl or HI.

49. Formation of anti-Markovnikov alcohol • Alkenes goes to hydroboration reaction to form anti-Markovnikov alcohol.

50. Combustion of alkenes: • The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water. • For example, ethene burns as follows : C2H4 + 3O2→ 2CO2 + 2H2O