Chapter 3

Alkenes and Alkynes

Chemistry 20

Hydrocarbons

Large family of organic compounds

Composed of only carbon and hydrogen

Saturated hydrocarbons

Alkanes

Unsaturated hydrocarbons

Alkenes, Alkynes & Aromatics

C - C C = C C CC

CC

C

CC

H

H

H

H

H

H

A Kekulé structureshowing all atoms

A Kekulé structureas a line-angle formula

Saturated compounds (alkanes): Have the maximum number of hydrogenatoms attached to each carbon atom.

Unsaturated compounds: Have fewer hydrogen atoms attached tothe carbon chain than alkanes.

• Containing double bond are alkenes.

CnH2n

• Containing triple bonds are alkynes.

CnH2n-2

Alkenes and Alkynes

Thermal cracking

CH3-CH3

800-900°CCH2=CH2 + H2

H

C C

H

H H

121.7°H3C

C C

H

H H

124.7°

Ethylene Propene

120°

H

C C

H

H H

H-C C-H

Acetylene(an alkyne)

Ethylene(an alkene)

180°

Using the IUPAC alkane names:

Alkene names change the end to -ene.

Alkyne names change the end to -yne

Naming Alkenes & Alkynes

Give the locationfor double and

triple bond

STEP 3Give the location and name of eachsubstituent (alphabetical order) as aprefix to the name of the main chain.

STEP 2Number the carbon atoms starting

from the end nearest a double or triple bond.

CH2= CH ─ CH2─ CH3 1-Butene

CH3─ CH=CH─ CH3 2-Butene

CH3

|CH3─ CH=C─CH3 2-Methyl-2-butene

CH3─ CC ─ CH3 2-Butyne

Naming Alkenes & Alkynes

1 2 3 4

1 2 3 4

4 3 2 1

1 2 3 4

CH3─ CH2─ CC ─ CH3 2-Pentyne

CH3

CH3─ CH2─ C=CH ─ CH3 3-Methyl-2-pentene

CH2 – CH3

CH3─ CH2─ C=CH ─ CH3 3-Ethyl-2-pentene

Naming Alkenes & Alkynes

5 4 3 2 1

5 4 3 2 1

5 4 3 2 1

Conformation

Under normal condition, no rotation is possible about the carbon-carbon double and triple bond.

Flat or planar molecules

Cis & Trans Stereoisomers

C = CHH

CH3H3C

C = C

H

H CH3

H3C

cis-2-Butene trans-2-Butene

The same molecular formula and the same connectivity of their atoms

but a different arrangement of their atoms in space.

mp & bp of cis < mp & bp of trans

C4H8 C4H8

Naming of Cycloalkenes

Number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the substituent lower number.

List substituents in alphabetical order.

No location for C = C

1 2

3

4

5

1

2

3

4

5

6

3-Methylcyclopentene(not 5-methylcyclopentene)

4-Ethyl-1-methylcyclohexene(not 5-ethyl-2-methylcyclohexene)

Dienes, Trienes, and Polyenes

Alkenes that contain more than one double bond.

Alkane name: -ne diene, triene, …

CH2=CCH=CH2

CH3

CH2=CHCH2CH=CH21,4-Pentadiene 2-Methyl-1,3-butadiene

(Isoprene)1,3-Cyclopentadiene

12 4 1 2 3

1

2 3

Chemical properties of Alkenes & Alkynes

More reactive than Alkanes

Addition of Hydrogen (Hydrogenation-Reduction)

Addition of Hydrogen Halides (Hydrohalogenation)

Addition of water (Hydration)

Addition of Bromine & Chlorine (Halogenation)

Chemical properties of Alkenes & Alkynes

Addition reactions

Double bond is broken and two new single bonds are formed.

Exothermic reactions

–C = C – – C – C–

Products are more stable (have the lower energy).

• A hydrogen atom adds to each carbon atom of a double bond.

• A catalyst such as platinum or palladium is used (Transition metals).

H H H H

│ │ Pt │ │

H–C=C–H + H2 H– C – C– H

│ │

H H

Ethene Ethane

Chemical properties

1. Hydrogenation (Reduction):

Pt

More reactive than Alkanes

Chemical properties

1. Hydrogenation (Reduction):

• A hydrogen halide (HCl, HBr, or HI) adds to alkene to

give haloalkane.

H H H H

│ │ │ │

H–C=C–H + HCl H– C – C– H

│ │

H Cl

Ethene Chloroethane

Chemical properties

2. Hydrohalogenation:More reactive than Alkanes

Chemical properties

2. Hydrohalogenation:

- reaction is regioselective.

- Markovnikov’s rule: H adds to double bonded carbon that has the greater number of H and halogen adds to the other carbon.

CH3CH=CH2 HCl CH3CH-CH2

HClCH3CH-CH2

ClH

1-Chloropropane(not formed)

2-ChloropropanePropene

+

The rich get richer!

Chemical properties

3. Hydration (addition of water):

• Water adds to C=C to give an alcohol.

• Acid catalyst (concentrated sulfuric acid).

• A regioselective reaction (Markovnikov’s rule).

CH3CH=CH2 H2OH2SO4

CH3CH-CH2

HOH

Propene 2-Propanol+

CH3C=CH2

CH3

H2OH2SO4 CH3C-CH2

CH3

HOH2-Methyl-2-propanol2-Methylpropene

+

• A halogen atom adds to each carbon atom of a double bond.

• Usually by using an inert solvent like CH2Cl2.

H H H H

│ │ │ │

CH3–C=C–CH3 + Cl2 CH3– C – C– CH3

│ │

Cl Cl

2-Butene 2,3-dichlorobutane

Chemical properties

4. Halogenation:

CH2Cl2

More reactives than Alkanes

Polymerization

Polymer: a long-chain molecule produced by bonding together many single parts called monomers.

nCH2=CH2 CH2CH2initiator

Ethylene Polyethylenen(polymerization)

The most important reactions of alkenes in industry.

CH2CH-CH2CH-CH2CH-CH2CHCH3 CH3 CH3 CH3

CH2CHCH3

The repeating unitPart of an extended polymer chain

n

monomer units shown in red

n

Polymerization

Propene

Monomer

Polymerization

Polypropene (Polypropylene)

Naming of polymers

Prefix “ploy-” + name of the monomer

Propene Polypropene

If the name of monomer consists of two words:

Its name is enclosed in parentheses.

ClCl

nPolymerization

Vinyl chloride Poly(Vinyl chloride) PVC

Polymers in our life

• Low-density polyethylene (LDPE):

– A highly branched polymer; polymer chains do not pack well and the London dispersion forces between them are weak.

– Softens and melts above 115°C.– Approximately 65% is used for the production of films (also used for

packaging and trash bags).

• High-density polyethylene (HDPE):

– Only minimal chain branching; chains pack well and the London dispersion forces between them are strong.

– It has a higher melting point than LDPE and it is stronger.– It can be blow molded to squeezable jugs and bottles.

Polyethylene

Inexpensive

3 V

5 PP

6 PS

Code Polymer Common Uses

1 PET poly(ethyleneterephthalate)

soft drink bottles, householdchemical bottles, films, textile fibers

2 HDPE high-densitypolyethylene

milk and water jugs, grocery bags, squeezable bottles

poly(vinylchloride), PVC

shampoo bottles, pipes, shower curtains, vinyl siding, wire insulation, floor tiles

4 LDPE low-densitypolyethylene

shrink wrap, trash and grocery bags, sandwich bags, squeeze bottles

polypropylene plastic lids, clothing fibers, bottle caps, toys, diaper linings

polystyrene styrofoam cups, egg cartons, disposable utensils, packaging materials, appliances

7 all other plastics various

Recycling