Unsaturated Hydrocarbons -...

UnsaturatedHydrocarbons

Chemical Formulas and Unsaturation

C C C C C

C C C C C

H H H H H

H H H H H

H C C C C C H

H H H H H

H H H H H

H C C C C

H H H H

H H

C

H

H

H

H

H

Cn

CnH2n

CnH2n+2

CnH2n+2


C C C C C

Cn

C C C C C

Cn

C C C C C

Cn

CnH2n

H C C C C C H

H H H

H H H H H

C

C

C C

C

Cn

CnH2n

C

C

C C

C

HH

H

H

H

H H

H

H

H


Hydrocarbons

SaturatedHydrocarbons

UnsaturatedHydrocarbons

AlkanesCnH2n+2

CycloalkanesCnH2n

(one ring)

AlkenesCnH2n

(one C=C)

AlkynesCnH2n-2

(one C C)

BenzeneC6H6

CnH2n-6

Review of Carbon Bonding Patterns

Lewis Representation of Ethene

C C

H

H

H

HC

H

H C

H

H

N o t e : B o t h o f t h e s e representations picture the two components of the double bond as being identical. This is not the case.

Hybrid Orbitals - Alkanes

Recall that the four orbitals

associated with the bonding in

an alkane or cycloalkane are the result of sp3

hybridizaton.

Hybrid Orbitals - Bonding in Methane

Hybrid Orbitals - Alkenes

In the alkenes a different type of hybridization is used, sp2. The 2s and two of the 2p orbitals about the carbon atom hybridize to form three equivalent sp2 orbitals, leaving a single 2p orbital in an unhybridized state.

Hybrid Orbitals - Alkenes

Covalent Bonding in Ethene

C2H4, ethene, is formed from two sp2 hybridized carbon atoms and four hydrogen atoms:

Lateral overlap of the unhybridized p orbitals, above and below the triangular planes, results in a π bond of the double bond.

Covalent Bonding in Ethene

The two carbons and all four hydrogens all lie within the same plane. The bond angles are all 120º.

Note: Though the two components of the double bond are pictured the same, one of the bonds is actually weaker than the other.

Nomenclature of Alkenes

1. Select the longest carbon chain containing the double bond as the parent chain. Replace the “-ane” ending with “-ene.

2. Number the parent chain beginning at end closest to the double bond beginning with the first carbon in the chain.

3. Locate and name the substituents in alphabetical order.

4. Compounds with two double bonds are called dienes.

5. Cyclic compounds are named as cycloalkenes. Carbons in the double bond are always numbered first. Then the ring is numbered in the direction so as to give the lowest numbers to substituents.

2,3-dimethyl-1-pentene

Alkenes-Nomenclature Practice

2,3-dimethyl-2-hexene

**

*

**

*


2,5-dimethyl-2-heptene 3,3-dimethylcyclopentene

**

* * *

*

Isomerism in Alkenes

H2C CH CH2CH2 CH3

H3C CH CH CH2 CH3

H3C CH2 CH CH CH3

Positional Isomerism

1-pentene

2-pentene

????also

2-pentene

Geometric Isomerism (a form of stereoisomerism)

Geometric Isomerism (a form of stereoisomerism)


Exists for the following cases:

C C

X

Y

A

B

A≠B and X≠Y

C C

X

Y

A

B

C C

Y

X

A

B

A result of lack of free rotation around the double bond

X

Isomerism in Alkenes Cis/trans isomers have different physical properties


Cis/trans isomers are not possible for some alkenes.

cis-4-methyl-2-hexene


trans-5,6-dimethyl-3-heptene

*

** *

*

Chemical Reactions of Alkenes

Structure of the Double Bond in Alkenes

C C

The two components of the double bond

are pictured the same, but one of the

bonds (the π bond) is actually weaker than

the other.

The characteristic reaction of alkenes is an ADDITION REACTION.


C C C C

A

A"catalyst"

B

B

+

“A” and “B” are not necessarily single

atoms.

“A” and “B” are added to the structural and

molecular formula.

WHAT ARE “A” AND “B” ? H-H HydrogenationX-X HalogenationH-X HydrohalogenationH-OH Hydration


C C

A B

C C C C

A

A"catalyst"

B

B

C C

A B

Specific Examples

Catalytic Hydrogenation

C C C CH H

H HNi or Pt catalyst

HC

HC

H

HH C

H

C

H

H

H HH2, Ni or Pt catalyst

H3CC

H3CC

CH3

CH2CH3

H3C C

CH3

C

CH2CH3

CH3

H HH2, Ni or Pt catalyst

ethene ethane

C2H4 C2H6

2,3-dimethyl-2-pentene 2,3-dimethylpentane

C7H14

C7H16

Halogenation

C C C CX X

X XDark, no catalyst

HC

HC

H

HH C

H

C

H

H

Cl ClCl2, no catalyst

H3CC

H3CC

H

CH2CH3

H3C C

CH3

C

CH2CH3

H

Cl ClCl2, no catalyst

General Reaction

Specific Examples

ethene 1,2-dichloroethane

2-methyl-2-pentene 2,3-dichloro-2-methylpentane

C2H4 C2H4Cl2

C6H12

C6H12Cl2

Addition of Bromine (Br2)

CH3 CH CH CH2 CH3 CH3 CH CH CH2 CH3

Br BrBr2no catalyst

Br2no catalyst

BrBr

Br2no catalyst No reaction

Chemical Reactions of Alkenes Addition of Bromine (Br2)

Alkane Alkene Alkane+

Br2

Alkene+

Br2

Br2

Chemical Reactions of Alkenes Addition of Bromine (Br2)

The result can be quantitated.

Hydrohalogenation

C C C CH X

H X

or

C C

X H

+

H3C CH CH CH3H Cl

H3C CH CH CH3

H Cl

H3C CH CH CH3

Cl H

or

Specific Example;

2-chlorobutane

2-chlorobutane

C4H9Cl

C4H8

2-butene

What is the result if two isomeric products are possible ??

2-chloropropane

1-chloropropane

propene ?( )

( )C3H6

C3H7Cl

H3C CH CH2H Cl

H3C CH CH2

H Cl

H3C CH CH2

Cl Hor

H3C CH2 CH2

Cl

H3C CH CH3

Cl

MARKOVINIKOV’S RULE- When adding H-X or H-OH (H2O) to alkenes with different number of groups attached to the double bonds, the “H” portion of the reagent adds to the carbon that has the greater number of hydrogen atoms already attached.

H-Cl H-Br H-OH

Majorproduct

1-chloropropane

2-chloropropane

propene

H3C CH CH2H Cl

H3C CH CH2

H Cl

H3C CH CH2

Cl Hor

H3C CH2 CH2

Cl

H3C CH CH3

Cl

Hydration of Alkenes

C C C CH OH

H OH

or

C C

HO H

+

H3CC

H3CCCH3

CH3

H3C C

CH3

C

CH3

CH3H OH

H OH

or

H3C C

CH3

C

CH3

CH3

HO H

+

alcoholsalkenes

2,3-dimethyl-2-butene2,3-dimethyl-2-butanol

C6H12

C6H14O

Hydration of Alkenes

H3CC

H3CCH

CH3

H3C C

CH3

C

CH3

HH OH

H OH

or

H3C C

CH3

C

CH3

H

HO H

+ H3C CH

CH3

CH CH3

OH

H3C C

CH3

CH2 CH3

HO

Majorproduct

2-methyl-2-butene

3-methyl-2-butanol

2-methyl-2-butanol

C5H10

C5H12O

HHO

*

*

Addition Polymers Overall Reaction:

C=CA

B

X

YC=C

A

B

X

YC=C

A

B

X

YC=C

A

B

X

YC=C

A

B

X

Y

C CA

B

X

YC C

A

B

X

YC C

A

B

X

YC C

A

B

X

YC C

A

B

X

Y

Acid or “initiator”

C CA

B

X

Y( ) n n = number of individual units

CH2 CH2

CH2 CH

CH3

CH2 CH

Monomer Polymer Uses

ethylene

(CH2 CH2 )n

polyethylene

(CH2 CH )n

CH3propylene polypropylene

(CH2 CH )n

styrene polystyrene

bottles, toys, housewares, wire cableinsulation, plastic sheeting

outdoor carpeting, food packagingappliance housings

styrofoam containers, food packaginghairbrush handles, toys

Addition Polymers

Addition Polymers

CH2 CH

Cl

CH2 CH

CN

CF2 CF2

(CH2 CH )n

Cl

vinyl chloride polyvinylchloride (PVC)

(CH2 CH )n

CN

acrylonitrile polyacylonitrile

(CF2 CF2 )n

tetrafluoroethylene polytetrafluoroethylene

home siding, gutters, flooring, garden hose, PVC tubing

acrylic textile fibers

“Teflon”, mechanical parts, cookware, chemical resistant gaskets

Monomer Polymer Uses

Unsaturated Hydrocarbons -...

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