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Alkenes and AlkynesGeometric Isomers of Alkenes

Addition Reactions

ALKENES and ALKYNES

• Alkenes area class of HYDROCARBONS which contain only carbon and hydrogen.

• UNSATURATED hydrocarbons contain either double or triple bonds. Since the compound is unsaturated with respect to hydrogen atoms, the extra electrons are shared between 2 carbon atoms forming either double bonds for alkenes or triple bonds for alkynes.

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Saturated and Unsaturated Compounds

Saturated compounds (alkanes) have the maximum number of hydrogen atoms attached to each carbon atom

Unsaturated compounds have fewer hydrogen atoms attached to the carbon chain than alkanes

Unsaturated compounds contain double or triple bonds

Typical Alkene• Ethene is the number one

organic chemical synthesized in the U. S. and the world. The small quantities of ethane, propane, and butane found in natural gas are converted into ethene. It can be produced by thermal cracking of ethane to produce ethene and a hydrogen molecule.

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Alkenes

Carbon-carbon double bonds Names end in -ene

H2C=CH2 ethene (ethylene)

H2C=CH-CH3 propene (propylene)

cyclohexene

Typical Alkyne

• Physical and chemical properties are similar to those of alkenes.

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Alkynes

Carbon-carbon triple bonds Names end in -yne

HCCH ethyne(acetylene)

HCC-CH3 propyne

ALKENE and ALKYNE NAMES

Root names give the number of carbons in the longest continuous chain. The names are formed by dropping the "ane" and if the chain is 3C or less replacing it with "ene“. If the chain is 4C or more the lowest “carbon number” of the two carbons involved with the double bond plus a hyphenated ene is added.

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Naming Alkenes and Alkynes

When the carbon chain has 4 or more C atoms, number the chain to give the lowest number to the double or triple bond.

1 2 3 4

CH2=CHCH2CH3 but-1-ene

CH3CH=CHCH3 but-2-ene

CH3CHCHCH3 but-2-yne

 No. ofCarbons

 Root Name Formula

CnH2n

 Structure

 2  ethene  C2H4  CH2=CH2

 3  propene  C3H6  CH2=CHCH3

 4 but-1-ene  C4H8  CH2=CHCH2CH3

 4 but-2-ene  C4H8  CH3CH=CHCH3

 5  pent-1-ene   C5H10  CH2=CHCH2CH2CH3

 5  pent-2-ene   C5H10  CH3CH=CHCH2CH3

Cis-Trans Isomerism

• The double bond of alkenes form a rigid structure around which the atoms cannot rotate as in alkanes. This leads to a form of isomerism referred to as cis- trans isomers. This isomerism is most readily seen in the two forms of but-2-ene

• Cis-but-2-ene• The hydrogens occur on

the same side (plane ).

• Trans-but-2-ene• The hydrogens occur

diagonally across from each other.

Properties

Physical Properties• Boiling points depend on

chain length, slightly less than alkanes.

• Non polar• Insoluble in water• Less dense than water

Chemical Reactivity• Alkenes are quite reactive

because of the presence of the double bond. Many small compounds react by addition i.e. molecules add to the alkene to form one product.

• All compounds: Combustion Reaction

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Learning Check

Write the IUPAC name for each of the following unsaturated compounds:

A.CH3CH2CCCH3

CH3

B. CH3C=CHCH3 C.

CH3

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Solutions

Write the IUPAC name for each of the following unsaturated compounds:

A.CH3CH2CH=CHCH3 2-pentyne

CH3

B. CH3C=CHCH3 C.

2-methyl-2-butene 3-methylcyclopentene

CH3

Alkene/Alkyne Addition Reactions

• The double and triple bonds present in alkenes and alkynes, respectively, make them much more reactive than alkanes.

• Alkenes and alkynes undergo addition reactions– a reaction in which parts of a reactant are added

to the two atoms that make up a double or triple bond

Alkene/Alkyne Addition Reactions

• Some of the common reactants that can add to alkenes and alkynes include:– Br2

– I2

– Cl2

– H2 (requires a transition metal catalyst)– H2O (requires a strong acid catalyst)– HBr– HCl– HI

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Hydrogenation

Adds a hydrogen atom to each carbon atom of a double bond

H H H H

Ni

H–C=C–H + H2 H–C–C–H

H H

ethene ethane

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Adding Halogens (Halogenation)

Halogens also add to the double bond of an alkene.

H2CCH2 + Cl2

H2C CH2

Cl Cl

CH3C CCH2CH3 + Br2 CH3C CCH2CH3

Br Br

Br Br

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Adding Hydrogen Halides

Hydrogen halides also add to the double bond of an alkene

H Cl

H–C=C–H + HCl H–C–C–H

H H H H

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Adding Water

Water also can be added to the double bond of an alkene

H OH

H–C=C–H + H2O H–C–C–H

H H H H

Alkene/Alkyne Addition Reactions

• Markovnikov’s Rule: The major product obtained from the addition of an unsymmetrical reagent such as H-Br, H-Cl, or H-OH to an alkene or alkyne is the one obtained when the H atom of the reagent is added to the C atom of the multiple bond that already has the greater number of H atoms.

““The rich get richer”The rich get richer”

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Example

Water also can be added to the double bond of an alkene

H H OH H

H–C = C– C–H + H2O H–C – C– C–H

H H H H H H

Alkene/Alkyne Addition Reactions

Example: Write the structure of the product that forms when propyne reacts with the following reagents (individually):

Br2 (excess)

H2 (excess) with Ni catalyst

HBr (excess)