Alkenes and Alkynes 1 Alkenes and alkynes are two families of organic molecules that contain...

Alkenes and Alkynes

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Alkenes and alkynes are two families of organic molecules that contain multiple bonds (unsaturated).

• Alkenes are compounds that contain a carbon− carbon double bond.

• Alkenes have the general formula CnH2n.

Alkenes and Alkynes

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• Alkynes are compounds that contain a carbon− carbon triple bond.

• Alkynes have the general formula CnH2n–2.

Alkenes and Alkynes

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• Alkenes and alkynes have low melting and boiling points, and are insoluble in water.

• Alkenes and alkynes are composed of nonpolar bonds.

• Their physical properties are like those of alkanes.

• They are called unsaturated hydrocarbons because they contain fewer than the maximum number of H atoms per C.

Alkenes and Alkynes

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• The multiple bond is always drawn in a condensed structure.

• An alkene is identified by the suffix – ene.

• An alkyne is identified by the suffix – yne.

Nomenclature of Alkenes and Alkynes

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HOW TO Name an Alkene or Alkyne

ExampleGive the IUPAC name of each alkene and alkyne.

Step [1] Find the longest chain that contains bothC atoms of the double or triple bond.

Nomenclature of Alkenes and Alkynes

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HOW TO Name an Alkene or Alkyne

4 C’s in longest chain

• Since the compound is an alkene, change the −ane ending to −ene.

butane ----> butene

6 C’s in longest chainhexane ----> hexyne

• Since the compound is an alkyne, change the −ane ending to −yne.

Nomenclature of Alkenes and Alkynes

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HOW TO Name an Alkene or Alkyne

Step [2]Number the carbon chain from the end thatgives the multiple bond the lower number.

• Name the compound using the first number assigned to the multiple bond.

1-butene 2-hexyne

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Nomenclature of Alkenes and Alkynes

HOW TO Name an Alkene or Alkyne

Step [3]

Number and name the substituents, and write the name.

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Cis–Trans IsomersStereoisomers—A New Class of Isomer

• There is restricted rotation around the C atoms of a double bond.

two CH3 groupson the same side

two CH3 groupson opposite sides

• Therefore, 2-butene has two possible arrangements:

cis isomer trans isomer

Cis–Trans IsomersStereoisomers—A New Class of Isomer

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Cis–Trans IsomersStereoisomers—A New Class of Isomer

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• Whenever the two groups on each end of a C═C are different from each other, two isomers are possible.

Cis–Trans IsomersStereoisomers—A New Class of Isomer

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• When the two groups on one end of the double bond are identical (e.g., both H or both CH3), no cis and trans isomers are possible.

Cis–Trans IsomersStereoisomers—A New Class of Isomer

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• Stereoisomers are isomers that differ only in the 3-D arrangement of atoms.

• Constitutional isomers differ in the way the atoms are bonded to each other.

Focus on Health and MedicineSaturated and Unsaturated Fatty Acids

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• Fatty acids are carboxylic acids (RCOOH) with long carbon chains of 12–20 carbon atoms.

• Naturally occurring animal fats and vegetable oils are formed from fatty acids.

• Saturated fatty acids have no double bonds in their long hydrocarbon chains.

• Unsaturated fatty acids have one or more double bonds in their long hydrocarbon chains.

Focus on Health and MedicineSaturated and Unsaturated Fatty Acids

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Reactions of Alkenes

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Alkenes undergo addition reactions wherein newgroups X and Y are added to the alkene. One bondof the double bond is broken and two new singlebonds are formed.

Reactions of AlkenesAddition of Hydrogen—Hydrogenation

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• Hydrogenation is the addition of H2 to an alkene.

• The metal catalyst (usually palladium—Pd) speeds up the rate of the reaction.

• The product of hydrogenation is an alkane.

Reactions of AlkenesAddition of Hydrogen—Hydrogenation

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• Another example of hydrogenation:

Reactions of alkenesAddition of halogens-Halogenation

1,2-Dichloroethane

Results in the formation of haloalkanes.

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C C

H

H

H

H

+ Cl Cl C C

H

H

H

H

ClCl

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Reactions of AlkenesAddition of Water—Hydration

• Hydration is the addition of water to an alkene.

• Hydration requires a strong acid, H2SO4.

• The product formed by hydration is an alcohol.

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Reactions of AlkenesAddition of Water—Hydration

• An additional example of hydration:

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•If the reactant is an asymmetrical alkene, two possible products can be formed in theory.

•These two potential products are constitutional isomers. In fact, addition only forms 2-propanol.

Reactions of AlkenesAddition of Water—Hydration

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•To determine which of the two products will actually form, we use Markovnikov’s rule.

•Markovnikov’s rule states that the H atom of H–OH will bond to the less substituted C atom in the C═C double bond.

•This means the C in the double bond with the most Hs will bond to the H atom of H–OH.

Reactions of AlkenesAddition of Water—Hydration

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•Looking at the reaction again:

C2 is bonded to 1 HC1 is bonded to 2 Hs

•C1 has more Hs, so it will bond to the H from H–OH.

•2-propanol is the only product formed.

Reactions of AlkenesAddition of Water—Hydration

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Focus on Health and MedicineMargarine or Butter?

Alkene Polymers-(Polymerization)

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Polystyrene (plastic/foam cups)

C C

H

H

Cl

H

+ C C

H

H

Cl

H

+ C C

H

H

Cl

H

C C

H

H

Cl

H

C C

H

H

Cl

H

C

H

H

C

Cl

H

Monomer (n = Polymer (n up to 2 ) 1000 and more)

Chloroethene

(Vinyl chloride)

Polyvinyl Chloride

H2C C

H

???

Phenylethene

(Styrene)

Polystyrene

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Polymers—The Fabric of Modern Society

• Polymers are large molecules made up of repeating units of smaller molecules (monomers) covalently bonded together.

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Polymers—The Fabric of Modern SocietySynthetic Polymers

• In polymerization, the weak bond that joins the two carbons of the double bond is broken and new strong carbon–carbon single bonds join the monomers together.

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Polymers—The Fabric of Modern SocietySynthetic Polymers

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Aromatic Compounds

• Aromatic compounds are compounds that contain a benzene ring.

• Each C is trigonal planar (i.e., 120°bond angles), making benzene a planar molecule.

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Aromatic Compounds

• Each of these representations has the same arrangement of atoms, but different locations of electrons.

• Each of these representations is equivalent.

• The physical properties of aromatic hydrocarbons are similar to other hydrocarbons—they have low melting points and boiling points and are water insoluble.

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• Tamoxifen, a potent anticancer drug sold under the trade name Novaldex, contains three benzene rings.

Aromatic Compounds

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Nomenclature of Benzene DerivativesMonosubstituted Benzenes

To name a benzene ring with one substituent:

• name the substituent first

• then add the word benzene at the end.

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Nomenclature of Benzene DerivativesMonosubstituted Benzenes

• Some monosubstituted benzenes have common names that you must learn.

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Nomenclature of Benzene DerivativesDisubstituted Benzenes

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Nomenclature of Benzene DerivativesDisubstituted Benzenes

• If there are two groups on the benzene ring and they are different, alphabetize the two substituent

names.

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Nomenclature of Benzene DerivativesDisubstituted Benzenes

• If one of the two substituents is part of a common root, then name the molecule as a derivative of that monosubstituted benzene.

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Nomenclature of Benzene DerivativesPolysubstituted Benzenes

1. Number to give the lowest possible numbers around the ring.

2. Alphabetize the substituent names.

3. When the substituents are part of common roots:

• Put the common root substituent at C1, but omit the “1” from the name.

• Name the molecule as a derivative of that monosubstituted benzene.

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Nomenclature of Benzene DerivativesPolysubstituted Benzenes

• Assign the lowest set of numbers.

• Alphabetize the names of all the substituents.

1-chloro-4-ethyl-3-propylbenzene

CH2CH3

CH2CH2CH3Cl

4

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Nomenclature of Benzene DerivativesPolysubstituted Benzenes

• Name the molecule as a derivative of the common root aniline.

• Assign the NH2 group to position 1, then assign the lowest possible set of numbers to the other groups.

2,5-dichloroaniline

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Focus on Health and MedicineSunscreens and Antioxidants

• A common sunscreen used contains benzene:

• A wide variety of phenols occur in nature: