Ch. 12 Alkenes

Homework- 12.15, 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 H2

CH3CH3 CH2=CH2

800-900 oC

(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

C C

H

HH

H

sp2

120o

• 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 sp2 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

C C

CH3

HH

H3C

Cis-2-butene

C C

H

CH3H

H3C

Trans-2-butene

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 1st 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, 2n 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 H2O, insoluble in H2O

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

C C + H-Cl C C

H Cl

Hydrochlorination

C C + H2O C C

H OHHydration

C C + Br2 C C

Br BrBromination

C C + H2 C C

H HHydrogenation

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

H2C CH2 + H-Cl H2C CH2

H Cl

+ H-ClH

ClCl

H

+

Not Seen Experimentally

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 H2O 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

+ H-ClH

ClCl

H

+

Not Seen Experimentally

2 Hydrogens

1 Hydrogen

• 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 1o, 2o, and 3o just as we have done previously.

Addition of Water: Acid-Catalyzed Hydration

• In the presence of an acid catalyst, most commonly H2SO4, 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

CH3CH CH2H2SO4

H2SO4

OH

CH3CH

OH

CH2

H

+ H2O

+ H2O

Hydration reactions have a 3 step mechanism

Addition of Br2 and Cl2

• Adds on Br/Cl to each Carbon of the double bond.

• Examples:

CH3CH CHCH3 CH3CH

Br

Cl

Cl

CHCH3

Br

+ Br2

+ Cl2

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

CH3CH CHCH3

CH3CH

H

CH3CH2CH2CH3

CHCH3

H

+ H2

+ H2

or

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