Alkenes and Cycloalkenes Bettelheim, Brown Campbell and Farrell Chapter 12

Alkenes and Alkynes Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds ethylene is the simplest alkene Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds acetylene is the simplest alkyne

Review of Hybrid Orbitals Sigma bonds formed on axis between nuclei pi bonds formed between overlapping p orbitals (above and below internuclear axis)

sp orbitals Figure: 09-16

sp2 orbitals Figure: 09-18

Figure: 09-19 sp3 orbitals

Double Bond 2 sp2 orbitals (one from each atom) overlap The resulting bond is a sigma bond Additionally, the unused p orbitals overlap to form pi bond (with two lobes—one above and one below the sigma bond)

pi bonds form between overlapping p orbitals Two lobes formed--above and below internuclear axis Figure: 09-22

Figure: 09-24

Alkene pi bond Figure: 09-25

Figure: 09-27EX9.6

Alkyne has two pi bonds and one sigma bond (on axis between nuclei) Figure: 09-26

Alkenes and Alkynes Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds ethylene is the simplest alkene Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds acetylene is the simplest alkyne

Alkenes Cis-trans isomerism because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism

Alkenes Structure: VSEPR model predicts bond angles of 120° about each carbon of a double bond Actual angles in ethylene are close to 120° Angles about each carbon of the double bond in substituted alkenes may be greater than 120° because of repulsion of alkyl groups bonded to the double bond  

Alkenes - IUPAC Names To name an alkene Parent name is that of the longest chain that contains the C=C Number the chain to give the lower numbers to the carbons of the C=C Locate the C=C by the number of its first carbon Add -ene ending to show the presence of the C=C Branched-chain alkenes are named in a manner similar to alkanes. Name and locate substituents.

Alkenes - IUPAC Names Examples

Alkynes - IUPAC Names Use same rules as for alkenes, but use the ending -yne to show the presence of the triple bond

Name the following compound

Common Names Common names are still used for some alkenes and alkynes, particularly those of low molecular weight

Cycloalkenes To name a cycloalkene Number the carbon atoms in ring so that the double bond is between C1 and C2 Give lower number to the substituent encountered first Number and list substituents in alphabetical order

Dienes, Trienes, Polyenes Alkenes with more than one double bond are named as alkadienes, alkatrienes, etc. Compounds with several double bonds are referred to more generally as polyenes (Greek: poly, many)

Name the following

Name the following

Physical Properties of Alkenes and Alkynes Nonpolar compounds London dispersion forces between molecules Physical properties are similar to alkanes Insoluble in water Soluble in nonpolar organic liquids Liquid or solid have low densities Float on water Density less than 1 g/mL

Terpenes Terpene: a compound whose carbon skeleton can be divided into five-carbon units identical with the carbon skeleton of isoprene Example of an important principle of the molecular logic of living systems Small subunits are combined (and modified) to make larger molecules In nature, reactions carried out by enzymes (catalysts)

Examples of Terpenes

Chemical Properties Addition Reactions Add two atoms (or groups) to a double bond Break double bond One atom (or group) added to each carbon May require catalyst

Chemical Properties Addition Reactions

Reactions of Alkenes Most alkene addition reactions are exothermic Products are more stable (lower in energy) than the reactants Reaction rate depends on activation energy Many alkene addition reactions require a catalyst For hydrogenation (H2), Ni, Pt, or Pd catalyst used

Addition of H2 - Reduction Hydrogenation Requires metal catalyst (Pd, Pt, or Ni) Used for converting polyunsaturated oils into margarine

Addition of Cl2 and Br2 Addition takes place readily at room temperature Reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent Br2 Test useful to determine C=C double bond Br2 has a deep red color; dibromoalkanes are colorless

Addition of HX Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane H adds to one carbon of the C=C and X to the other Product is NOT symmetrical

Addition of HX, cont. If you start with a nonsymmetrical alkene: Reaction is regioselective Markovnikoff’s Rule: H adds to the less substituted carbon and X to the more substituted carbon (connected to more Cs) H adds to carbon that has more H’s Them what has, gets! (H)

Addition of HX Chemists use reaction mechanism to show how a reaction proceeds in steps Use curved arrows to show the movement of electron pairs Tail of arrow shows where the electron pair is before the electrons move (lone pair or bond) Head of the arrow shows its new position Curved arrows show which bonds break and which new ones form

Addition of HCl to 2-Butene Step 1: Reaction of the carbon-carbon double bond with H+ gives a secondary carbocation intermediate 2o = Carbon connected to 2 other Carbons Carbocation = Carbon with positive charge

Addition of HCl to 2-Butene Step 2: Reaction of the carbocation intermediate with chloride ion completes the addition

Addition of H2O Addition of water is called hydration Hydration is acid catalyzed, most commonly by H2SO4 Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon

Another example of Hydration

Mechanism for Hydration Step 1: Step 2: Step 3:

Step 3:

Polymerization Polymerization is a VERY important reaction of alkenes polymer: Greek: poly, many and meros, part monomer: Greek: mono, single and meros, part

Polymerization Use parentheses around the repeating monomer unit Subscript, n, indicates that this unit repeats n times Show that a polymer chain can be reproduced by repeating the enclosed structure in both directions Example: section of polypropene (polypropylene)

Polyethylene Low-density polyethylene (LDPE) Highly branched polymer, so chains do not pack well—weak London Force interactions Softens and melts above 115°C Primarily used for packaging for trash bags High-density polyethylene (HDPE) Little branching, so chains pack well--London dispersion forces between them are stronger Higher melting point and stronger than LDPE Used for squeezable jugs and bottles

Codes for Plastics