Ch. 12 Alkenes Homework , 12.17, 12.19, 12.23, 12.25, 12.27, 12.36, 12.37, 12.41,12.42, 12.43, 12.46

Alkenes The organic chemical industry world wide produces more pounds of ethylene than any other chemical Annual production in the U.S. alone exceeds 55 billion pounds Ethylene occurs only in trace amounts in nature Most is made through thermal cracking of ethane

Cracking In cracking, a saturated hydrocarbon in converted to an unsaturated hydrocarbon plus H 2 CH 3 CH 3 CH 2 =CH o C (Thermal Cracking)

Alkenes- first class of Unsaturated hydrocarbons Alkenes are compounds that contain one or more carbon-carbon double bonds The simplest alkene is ethylene

Alkynes are unsaturated hydrocarbons that contain one or more carbon-carbon triple bonds The simplest Alkyne is acetylene This is all we will say about alkynes because they are not widespread in nature and are not important in biochemistry

Structure We have already said the Carbon’s are sp 2 hybridized, have 120 degree bond angles, and are trigonal planar Ethylene is planar or flat Most importantly, there is no rotation about the C-C double-bond of ethylene or any double bond of an alkene

Cis/Trans Isomerism Because of the restricted rotation about the double bond, an alkene in which each carbon of the double bond has two different groups bonded to it shows cis/trans isomerism Cis/Trans Isomerisms-Isomers that have the same order of attachment of their atoms but a different arrangement of their atoms in space due to the presence of either a ring or a C-C double bond

Examples

IUPAC Nomenclature Alkenes are named using the IUPAC system Rules: 1.Find the longest chain that includes both carbons of the double or triple bond. Indicate the length of the parent chain by using a prefix that tell the number of carbon atoms and the suffix, -ene (drop –ane) 2.Number the chain from the end that gives the lower set of numbers to the carbons of the double bond. Designate the position of the double bond by the number of its 1 st carbon

IUPAC Nomenclature 3.Branched Alkenes are named in a manner similar to alkanes; substituent groups are located and named. Examples:

IUPAC Nomenclature For Alkynes, Follow the same rules except use the Suffix, -yne Example: Skip Common Names Section 12.3B

Cis/Trans Configuration, revisited The orientation of the carbon atoms of the parent chain determines whether an alkene is cis or trans If the carbons of the parent chain are on the SAME side of the double bond, the Alkene is cis If they are on OPPOSITE sides, it is a trans alkene

The cis/trans notation goes in front of the name Examples:

Naming Cycloalkenes In naming cycloalkenes, we number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the substituent encountered first the lower number Examples:

Dienes, Trienes, and Polyenes Name alkenes that contain more than one double bonds as alkadienes, alkatrienes, and so on We refer to those that contain several double bonds more generally as polyenes Examples:

Earlier, we said that compounds with 1 C- C double bonds that can have cis/trans isomers, has two, one cis, one trans For an Alkene with n C-C double bonds that can have cis/trans isomers, 2 n isomers are possible Example: 2,4-heptadiene

Physical Properties Alkenes and Alkynes are nonpolar compounds and the only intermolecular forces between them are London forces So they have very similar physical properties to Alkanes Example: liquid at RT, float on H 2 O, insoluble in H 2 O

Addition Reactions of Alkenes The most characteristic reaction of Alkenes is addition to the C-C double bond The double bond is broken and in its place single bonds form to two new atoms or groups of atoms Almost all addition reactions result in products that are more stable (have lower energy) than the reactants

Types of Addition Reactions

Addition of Hydrogen Halides Hydrogen Halides = H-F,H-Cl, H-Br, H-I Example When 1-propene reacts, we get only one of the two possible products

Regioselectivity Because we only see one of the possible products, we say the reaction is Regioselective Regioselective-A reaction in which one direction of bond forming or bond breaking occurs in preference to all others. This regioselectivity was noted by Markovnikov who made the following generalization to help predict the product:

Markovnikov’s Rule “In the addition of HX or H 2 O to an alkene, the hydrogen will add to the carbon of the double bond which already has the most hydrogens.” The Halogen goes to the other carbon

In these reaction of a hydrogen halide with a double bond, we see that one of the bonds in the C-C double bond is broken, the H-X bond is broken, and new bonds are made between one of the carbons and the X and between the other carbon and the Hydrogen It is useful for chemist to know how all this happens. –Does it happen all at once? –Is there a certain sequence that occurs? How reactions occur is explained by Reaction Mechanisms.

Reaction Mechanism Reaction Mechanism- A step-by-step description of how a chemical reaction occurs. In a mechanism, we use curved arrows to show the movement of electrons The arrow starts where the electrons are and points to where they are going

Reaction Mechanism Consider the 2-step mechanism for the addition of H-Cl to 2-butene

Carbocation Carbocation- a species containing a carbon atom with only 3 bonds to it and bearing a positive charge Carbocations are classified as 1 o, 2 o, and 3 o just as we have done previously.

Addition of Water: Acid-Catalyzed Hydration In the presence of an acid catalyst, most commonly H 2 SO 4, water adds to a C-C double bond to give an alcohol. This is called hydration Hydration reactions also follow Markovnikov’s Rule

Hydration Examples Hydration reactions have a 3 step mechanism

Addition of Br 2 and Cl 2 Adds on Br/Cl to each Carbon of the double bond. Examples: Not responsible for mechanism!!

Addition of Hydrogen: Reduction Adds a Hydrogen to each Carbon of the double bond Uses a Metal Catalyst such as Pt, Pd, or Ni

Polymers Polymers-Any long chain molecule synthesized by the bonding together of many single parts called Monomers Examples- Know Table 12.2 page 374 (all 9) –Molecule formula, common name, polymer name, and use