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LecturePLUS Timberlake 1
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.
LecturePLUS Timberlake 3
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.
LecturePLUS Timberlake 5
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.
LecturePLUS Timberlake 7
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.
LecturePLUS Timberlake 9
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
LecturePLUS Timberlake 14
Learning Check
Write the IUPAC name for each of the following unsaturated compounds:
A.CH3CH2CCCH3
CH3
B. CH3C=CHCH3 C.
CH3
LecturePLUS Timberlake 15
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
LecturePLUS Timberlake 18
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
LecturePLUS Timberlake 19
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
LecturePLUS Timberlake 20
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
LecturePLUS Timberlake 21
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”
LecturePLUS Timberlake 23
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)