BRCC CHM 102 Class Notes Chapter 12 Page 1 of 8

Chapter 12 Alkenes and Alkynes

* alkenes – C=C double bonds * alkynes – CC triple bonds * aromatics or arenes – alternating double and single bonds such as in benzene

* saturated compounds – contain only single bonds (cannot add more hydrogens)

* unsaturated compounds – contain one or more multiple bonds – more hydrogen may be added to these compounds

Rules for Naming Alkenes

1. Use the rules for naming alkanes, but assign a name by adding a prefix from the table to the root –ene.

examples: CH2 = CH2 ethene common name is ethylene

CH2 = CH – CH3 propene common name is propylene

In these examples, there is no chance of isomers. What if you could have isomers?

CH2 = CH – CH2 – CH3 How would you name these?

CH3 – CH = CH – CH3

2. If a chain has 4 or more carbons, number the position of the double bond using the lowest of possible numbers.

examples: CH2 = CH – CH2 – CH3 1 – butene

CH – CH = CH – CH 2 – butene 3 3

CH3

H3CH2CCC CH2 2 – ethyl – 3 – methyl – 1 – pentene H

CH2CH3 BRCC CHM 102 Class Notes Chapter 12 Page 2 of 8

3. Side groups including halogens are named the same way as for alkanes. The double bond takes precedence over alkyl and halogen groups to start the numbering.

example: 7 1 CH3 CH3 CH3 | | | CH3 – C – CH2 – CH – C = CH – CH3 6–chloro-3,4,6–trimethyl–2–heptene | Cl

4. The parent chain is the longest continuous chain which contains both carbons of the double bond. The double bond must be in the parent chain.

example: CH3 – CH2 – CH – CH2 – CH3 2–ethyl–1–butene | | CH2

CH3 – CH2 – CH2 – C = CH – CH3 | 3–ethyl–2–hexene CH2 – CH3

5. Cyclic compounds containing double bonds are called cycloalkenes.

CH examples: Cl 3

H C 3

cyclobutene 1–chlorocyclopentene 3,4–dimethylcyclohexene

6. Compounds with 2 or more double bonds are given the suffix –diene, –triene, –tetraene, etc.

example: CH2 = CH – CH = CH2 1,3–butadiene

CH3 | CH2 = C – CH = CH – CH2 – CH = CH – CH3 2–methyl–1,3,6-octatriene

BRCC CHM 102 Class Notes Chapter 12 Page 3 of 8

Geometry and Stereoisomerism of Alkenes

* A single bond can rotate. H H H

C C

H H H

A double or triple bond cannot rotate. H Notice that this * H structure is flat, that CC is all the bonds are in H H one plane.

* Because there is no rotation, this allows stereoisomers similar to those we saw with the cyclic alkanes.

H3C H3C H CH3 CC CC

H CH3 H H trans–2–butene cis–2–butene

* The physical and chemical properties of these two compounds are different.

* For the trans isomer, the parent chain crosses the double bond.

* For the cis isomer, the parent chain is entirely on the same side of the double bond.

* In order for cis and trans isomers to exist, each C of the double bond must have 2 unlike constituents.

Cl CH3 CC no stereoisomers

Cl H 1,1-dichloropropene

H3C CH3 CC no stereoisomers

H3C H 2-methyl-2-butene BRCC CHM 102 Class Notes Chapter 12 Page 4 of 8

Physical Properties of Alkenes

* Alkanes and alkenes have similar physical properties. (see chapter 11 notes)

At room temperature, they exist as: 2 – 5 C’s gas 5 – 17 C’s liquid > 17 C’s solid

* They are insoluble in water with densities less than water.

Terpenes (naturally occurring alkenes)

* terpenes are made up of 2 or more isoprene units.

CH3 C

H2C C C CH2 CCCC H 1234 isoprene isoprene unit

* C1 is called the head and C4 is called the tail. Myrcene is found in oil of bay and consists of 2 isoprene units connected head to tail. CH2

C

H C CH HO 2 Geraniol found in

roses is similar to H2C CH2 myrcene CH myrcene, but has

an alcohol group. geraniol C H3CCH3

* Terpenes are widely distributed biological compounds. They illustrate how small molecules are used in building large molecules in nature.

Vitamin A is made up of 4 isoprene units.

BRCC CHM 102 Class Notes Chapter 12 Page 5 of 8

Chemical Properties of Alkenes

* Alkenes like alkanes may undergo combustion, yielding CO2 and water.

* Alkenes may undergo other chemical reactions as well.

Addition Reactions Something is added across the double bond.

A B CC + A – B C C

Addition of H reduction reaction 2 H H CC catalyst + H 2 C C

example:

Pt H3CHC CH2 + H2 CH3CH2CH3 propene propane

* This is also called a catalytic hydrogenation, where an unsaturated hydrocarbon (alkene) is saturated with H to become a saturated hydrocarbon (alkane).

Addition of Br & Cl 2 2 X = Br or Cl 2 2 2 X X CC

+ X2 C C

Br Br

H3CHC CHCH3 + Br2 H3CHC CHCH3 2 – butene 2,3 – dibromobutane

* The same group is added to each C of the double bond.

* Br2 is used as a test for unsaturated hydrocarbons. A drop of Br2 is added to the sample. Br2 is a red color. If the red disappears, then the sample is unsaturated.

BRCC CHM 102 Class Notes Chapter 12 Page 6 of 8

Addition of HX (Hydrogen Halides) X = Br, Cl, F, or I

H X CC + HX C C

example: H

+ HBr

Br cyclohexene bromocyclohexane

* A different group is added to each C. Sometimes 2 different products are possible. Do you make one product or both? Are they made in equal amounts or not?

example: Cl H

H3CHC CH2 H CHC CH 2 – chloropropane 3 2 + HCl

H Cl

H3CHC CH2 1 – chloropropane

* Markovnikov’s Rule – in the addition of HX, the H goes to the C that has more H’s. The rich get richer.

* Therefore you would make more 2 – chloropropane.

* This addition reaction is said to be regioselective, or rather there is a preference in which bonds are broken and formed.

Addition of H2O – called a hydration reaction.

H OH CC acid catalyst + H O 2 C C

BRCC CHM 102 Class Notes Chapter 12 Page 7 of 8

example:

H+ OH H + H O H3C CH2 2 H CCCH 3 2 CH3 CH3

* Addition of H2O follows Markovnikov’s Rule. The H goes to the C with more H’s.

Addition Polymers

* polymer – a long chain made up of repeating units.

* polymerization – process of making polymers from simple units called monomers.

* addition polymers – made by double-bonded molecules adding to each other. These are sometimes called chain-growth polymers because the stepwise addition of monomers is like adding links to a chain.

Cl Cl Cl Cl + C C C C H2C CH H2C CH H2 H H2 H vinyl chloride vinyl chloride poly vinyl chloride (PVC)

* repeating unit – the smallest molecular fragment which contains the nonredundant features of an entire chain. It is usually drawn inside parentheses followed by a subscript n to indicate how many times it is repeated.

* ethylene (ethene) is a common monomer used to produce polymers

n H2CCH2 (CH2CH2 )n ethylene polyethylene polymer

* The most common way of naming a polymer is to add the prefix “poly” to the name of the monomer.

BRCC CHM 102 Class Notes Chapter 12 Page 8 of 8

Alkynes

Naming Alkynes

* Use the same rules as for naming alkenes, except use the suffix “yne.”

examples:

IUPAC name: ethyne HC CH common name: acetylene

H3C CCH propyne

CH3

4 – methyl – 2 – pentyne

H3C C CCCH3 H

* Alkynes undergo addition reactions and follow Markovnikov’s Rule.

* Since addition to the triple bond yields a product with a double bond, a second addition reaction is possible.

example:

Br Br

HC CH + Br2 + Br2

HC CH

Br Br

HC CH

Br Br

* Alkynes have similar physical properties to alkenes and alkanes. They are nonpolar and undergo combustion reactions. With a triple bond, these combustion reactions are high energy.