ORGANIC CHEMISTRY 171 Section 201

2 Alkenes,Chapter 3

3 Unsaturated hydrocarbons can be 1-open-chain (linear and branched alkenes) 2- cyclic (cycloalkenes) Alkenes and Cycloalkenes

4 Unsaturated Hydrocarbons Unsaturated Hydrocarbons Hydrocarbons that contain at least one C=C ( alkenes) are called unsaturated hydrocarbons

5 Alkenes are acyclic unsaturated hydrocarbons that contain at least one C=C C2H4C2H4

6 Generic formula: Start with C n H 2n+2 and minus two for each C=C one C=C  e.g., C 2 H 4, C 3 H 6, etc. C2H4C2H4 C n H 2n

Nomenclature of Alkenes

8 Common Names Usually used for small molecules. Examples: =>

IUPAC Nomenclature of Alkenes 1. Find the longest continuous chain containing the double bond. 2. Name the corresponding alkane and change the “ane” ending to “ene” for alkenes. 3. Number the chain so as to give the double bond the lowest number. Place a numerical prefix in front of the parent name to indicate the position of the first carbon in the double bond. Number and name alkyl groups as with alkanes.

10 Nomenclature alkenes:parent chain contains C=C C=C gets lowest numbers position of C=C indicated by lower of the two numbers 1-butene 4-methyl-1-butene 3-bromocyclohexene

11 Nomenclature C=C and OH: alkenol higher priority group (OH) gets last suffix and lowest number 2-propen-1-ol 5-methyl-4-hexen-2-ol 2-cyclohexenol

12 Nomenclature as side groups: ethenyl (vinyl) 2-propenyl (allyl) 1-methylethenyl (isopropenyl) methylene vinyl chloride allyl alcohol isopropenyl bromide Examples 1-vinylcyclohexene methylenecyclopentane

Name the following compound: The longest continuous chain containing the double bond is 5 carbons long and is indicated in blue, below: The parent compound is derived from pentane. The parent alkene is pentene.

14 Nomenclature E-Z notation (E)-3-chloro-2-pentene(Z)-3-chloro-2-pentene 1.Determine the higher priority group on each end of the alkene. 2.If the higher priority groups are: on opposite sides: E (entgegen = opposite) on the same side: Z (zusammen = together) CH 3 > H Cl > CH 2 CH 3

15 Name These Alkenes 1-butene 2-methyl-2-butene 3-methylcyclopentene 2-sec-butyl-1,3-cyclohexadiene 3-n-propyl-1-heptene =>

16 ALKENE STRUCTURE AND BONDING

17 ALKENE STRUCTURE AND BONDING sp 2 CC R R R R SHAPE IS TRIGONAL PLANAR

18 Orbital Description Sigma bonds around C are sp 2 hybridized. Angles are approximately 120 degrees. No nonbonding electrons. Molecule is planar around the double bond. Pi bond is formed by the sideways overlap of parallel p orbitals perpendicular to the plane of the molecule. =>

19 THE BOND ANGLE OF AN ALKENE 120 o

20 Bond Lengths and Angles Hybrid orbitals have more s character. Pi overlap brings carbon atoms closer. Bond angle with pi orbitals increases. –Angle C=C-H is  –Angle H-C-H is  =>

21

22 Isomerism in alkenes Isomerism in alkenes

Cis and Trans Isomers Some alkenes can have the same connection of atoms, but have a different arrangement in three dimensional space. This is due to the lack of free rotation about the double bond. The different arrangements are geometric isomers. One of the isomers is cis- the other is trans.

24 CIS / TRANS ISOMERS cistrans substituents on the same side of main chain substituents on opposite sides of main chain CC H C C H CC C H C H

25 COMPARE cis / trans ISOMERS IN RING COMPOUNDS cistrans In alkenes and rings cis / trans isomers are called stereoisomers or geometric isomers.

2-butene mp = -139 o C CH 3 groups same side CH 3 groups opposite sides mp = -106 o C

Insert figure Geometric Isomers of 2- butene

28 Physical Properties Low boiling points, increasing with mass. Branched alkenes have lower boiling points. Less dense than water. Slightly polar –Pi bond is polarizable, so instantaneous dipole- dipole interactions occur. –Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. =>

29 Polarity Examples  = 0.33 D  = 0 =>

Preparation

I.Preparation 1.Dehydration This is intermolecular dehydration. What kinds of dehydration? Another reaction occur!

I.Preparation - Dehydration Saytzeff Rule: Hydrogen is preferably removed from the carbon with least no. of hydrogen since the alkene formed is more highly branched and is energetically more stable. How do you which one is major product?

33 Dehydration Mechanism =>

I.Preparation 2.Dehydrohalogenation Example: alcoholic KOH EtO - (ethoxide ion) in EtOH (ethanol) Press

35 Hofmann Product Bulky bases abstract the least hindered H + Least substituted alkene is major product. =>

I.Preparation 3.Dehalogenation CC XX + Zn dustCC + X 2 alcoholic reflux (vicinal dihalide) (c.f. gem-dihalide )

I.Preparation - dehalogenation (application) [O]

I.Preparation 4.Hydrogenation –This makes use of a catalyst which activity has been decreased by sulphur containing compound. E.g. Pd (palladium) in BaSO 4

Reactions of Alkenes

Chapter 840 Reactivity of C=C Electrons in pi bond are loosely held. Electrophiles are attracted to the pi electrons. Carbocation intermediate forms. Nucleophile adds to the carbocation. Net result is addition to the double bond. =>

Markownikoff ’ s rule –The more electronegative atom (or group of atoms) attached to carbon having least no. of H. 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: –3 ry C + > 2 ry C + > 1 ry C + > CH 3 +

(It undergoes addition reaction.) Electrophilic Addition Reactions 1-With HX

(Mechanism of Addition Reactions) (I) (II)

Electrophilic Addition Reactions (cont ’ d) 2-With conc. sulphuric acid ++ -- CC + H.HSO 4 CC HOSO 3 H CC HOH H 2 Oboil alkyl hydrogen sulphate Hence, this is used in preparation of alcohol.

3-Addition of halogen to alkene (Halogenation)

Proof for the formation of brominium ion The bromide ions attack carbon of the ring from the side opposite to that of the “positive” brominium ion.

Addition Reactions (NOT electrophilic) 4-Hydrogenation a.It is used analytically to find the number of mole of double bond or triple bond by the number of mole of hydrogen absorbed per mole of molecule. b.It is used in converting vegetable oil.

5-Ozonolysis

6-Oxidation a.at room temperature (Hydroxylation) (addition) CC + [O] + H 2 O CC OHOH from MnO 4 - /OH -

b.at vigorous condition (bond breaking) MnO 4 - / H + Further oxidation In acidic condition, the products will be oxidised to acid or ketone.

7-Addition Polymerization –This is a process by which simple molecules are joined up to form large molecule with same empirical formula. –condition :high temperature and pressure –with Ziegler ’ s catalyst

Free radical addition mechanism Initiation: Propagation:

Termination: nnnmn+m+2

e.g. polythene moleclar mass: 50000g melting point: 126 o C – 135 o C (having diff. Isomer) Properties: Light, inert (strong sigma bonds) and water-resistance (do not form H-bonding), tough and capable of moulding. It is a thermo-plastic since chains of hydrocarbons causes the chain to move apart and to come closer again on cooling.

Uses: 1.Make water-proof sheeting 2.Electrical cable insulator