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Chapter 11Chapter 11
The Unsaturated Hydrocarbons:Alkenes, Alkynes, and Aromatics
The Unsaturated Hydrocarbons:Alkenes, Alkynes, and Aromatics
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1. Structure1. Structure
Alkenes are hydrocarbons with a double bond.
CnH2n
Alkynes are hydrocarbons with a triple bond.
CnH2n-2
• Alkenes and alkynes are unsaturated (don’t have the maximum number of hydrogens bonded to each carbon).
Alkenes are hydrocarbons with a double bond.
CnH2n
Alkynes are hydrocarbons with a triple bond.
CnH2n-2
• Alkenes and alkynes are unsaturated (don’t have the maximum number of hydrogens bonded to each carbon).
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1. Comparison1. Comparison
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1. Geometry1. Geometry
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1. Geometry [3.4 Lewis structures]1. Geometry [3.4 Lewis structures]
Four groups of electronsEthanetetrahedral
extend toward the corners of a regular tetrahedron
bond angle = 109.5o
Four groups of electronsEthanetetrahedral
extend toward the corners of a regular tetrahedron
bond angle = 109.5o
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1. Geometry [3.4 Lewis structures]1. Geometry [3.4 Lewis structures]
Three groups of electronsEtheneAll in the same planeTrigonal planar
Bond angle = 120o
Three groups of electronsEtheneAll in the same planeTrigonal planar
Bond angle = 120o
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1. Geometry [3.4 Lewis structures]1. Geometry [3.4 Lewis structures]
Two groups of electronsEthyneLinear
Bond angle = 180o
Two groups of electronsEthyneLinear
Bond angle = 180o
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1. Physical properties1. Physical properties
Name Melting point Boiling pointethene -160.1oC -103.7oC
propene -185.0oC -47.6oC1-butene -185.0oC -6.1oC
methylpropene -140.0oC -6.6oCethyne -81.8oC -84.0oC
propyne -101.5oC -23.2oC1-butyne -125.9oC 8.1oC2-butyne -32.3oC 27.0oC
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1. Physical properties1. Physical properties
In each case, the alkyne has a higher boiling point than the alkene.
Its structure is more linear.The molecules pack together more efficiently.Intermolecular forces are stronger.
In each case, the alkyne has a higher boiling point than the alkene.
Its structure is more linear.The molecules pack together more efficiently.Intermolecular forces are stronger.
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The root name is based on the longest chain that includes both carbons of the multiple bond.The –ane ending is changed to –ene for double bonds and –yne for triple bonds.
The root name is based on the longest chain that includes both carbons of the multiple bond.The –ane ending is changed to –ene for double bonds and –yne for triple bonds.
2. Nomenclature2. Nomenclature
ethene
ethyne
propenepropyne
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2. Nomenclature2. Nomenclature
The chain is numbered from the end nearest the multiple bond.
The position of the multiple bond is indicated with the lower-numbered carbon in the bond.
The chain is numbered from the end nearest the multiple bond.
The position of the multiple bond is indicated with the lower-numbered carbon in the bond.
1-butene[not 3-butene]
2-pentyne[not 3-pentyne]
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2. Nomenclature2. Nomenclature
Determine the name and number of each substituent and add in front of the name of the parent compound.Determine the name and number of each substituent and add in front of the name of the parent compound.
5-chloro-4-methyl-2-hexene
2,6-dimethyl-3-octene
5-bromo-4-ethyl-2-heptene
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2. Nomenclature2. Nomenclature
Alkenes with more than one double bond are calledalkadienes (2 double bonds)alkatrienes (3 double bonds)etc…
Each double bond is designated by its lower-numbered carbon.
Alkenes with more than one double bond are calledalkadienes (2 double bonds)alkatrienes (3 double bonds)etc…
Each double bond is designated by its lower-numbered carbon.
2,4-hexadiene
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2. Nomenclature2. Nomenclature
Cycloalkenes must be numbered so the double bond is between carbons one and two.Cycloalkenes must be numbered so the double bond is between carbons one and two.
3-chloro-cyclopentene
4-ethyl-5-methylcyclooctene
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2. Nomenclature2. Nomenclature
Name the following compounds.
CH3CH=C(CH2CH3)2
H2C=C-CH2-CH=CH2
Name the following compounds.
CH3CH=C(CH2CH3)2
H2C=C-CH2-CH=CH2
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2. Nomenclature2. Nomenclature
Name the following compounds. Name the following compounds.
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2. Nomenclature2. Nomenclature
Write a structural formula for each of the following compounds.
1-hexene1,3-dicholoro-2-butene
4-methyl-2-hexyne1,4-cyclohexadiene
Write a structural formula for each of the following compounds.
1-hexene1,3-dicholoro-2-butene
4-methyl-2-hexyne1,4-cyclohexadiene
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2. Nomenclature2. Nomenclature
Draw a structural formula for each of the following compounds:
1-bromo-3-hexyne
2-butyne
dichloroethyne
9-iodo-1-nonyne
Draw a structural formula for each of the following compounds:
1-bromo-3-hexyne
2-butyne
dichloroethyne
9-iodo-1-nonyne
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3. Geometric isomers3. Geometric isomers
Rotation around a double bond is restricted, in much the same was as rotation is restricted for the cycloalkanes.In the alkenes, geometric isomers occur when there are two different groups on each of the double-bonded carbon atoms.
Rotation around a double bond is restricted, in much the same was as rotation is restricted for the cycloalkanes.In the alkenes, geometric isomers occur when there are two different groups on each of the double-bonded carbon atoms.
1,2-dichloroethene
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3. Geometric isomers3. Geometric isomers
Time for the first Chapter 11 Journal question![Use tag “difference”]
In your own words, explain how constitutional isomers and geometric isomers are different. Be sure to consider BOTH their differences and their similarities! You might want to use examples of actual molecules.
Time for the first Chapter 11 Journal question![Use tag “difference”]
In your own words, explain how constitutional isomers and geometric isomers are different. Be sure to consider BOTH their differences and their similarities! You might want to use examples of actual molecules.
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3. Cis-trans isomers3. Cis-trans isomers
If both constituents are on the same side of the double bond, the isomer is cis-.
If the constituents are on opposite sides of the double bond, the isomer is trans-.
If both constituents are on the same side of the double bond, the isomer is cis-.
If the constituents are on opposite sides of the double bond, the isomer is trans-.
cis-1,2-dichloroethene
trans-1,2-dichloroethene
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3. Cis-trans isomers3. Cis-trans isomers
• Alkenes without substituents also may exhibit cis-trans isomerism.
• Alkenes without substituents also may exhibit cis-trans isomerism.
cis-4-octenetrans-4-octene
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3. Cis-trans isomers3. Cis-trans isomers
• In order for cis and trans isomers to exist, neither double-bonded carbon may have two identical substituents.
• In order for cis and trans isomers to exist, neither double-bonded carbon may have two identical substituents.
2-methyl-2-buteneno cis/trans isomerism
1-buteneno cis/trans isomerism
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3. Cis-trans isomers3. Cis-trans isomers
• Which of the following compounds can exist as geometric isomers?
– 1-bromo-1-chloro-2-methylpropene
– 1,1-dichloroethene
– 1,2-dibromoethene
– 3-ethyl-2-methyl-2-hexene
• Which of the following compounds can exist as geometric isomers?
– 1-bromo-1-chloro-2-methylpropene
– 1,1-dichloroethene
– 1,2-dibromoethene
– 3-ethyl-2-methyl-2-hexene
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4. Alkenes in nature4. Alkenes in nature
• Ethene (ethylene) and ripening
– Ripening agents
– Ripening bowl
• Ethene (ethylene) and ripening
– Ripening agents
– Ripening bowl
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5. Reactions of alkenes and alkynes 5. Reactions of alkenes and alkynes
• The most common reactions of alkenes and alkynes are addition reactions.
– Hydrogenation: addition of H2
– Halogenation: addition of X2
– Hydration: addition of H2O
– Hydrohalogenation: addition of HX
• The most common reactions of alkenes and alkynes are addition reactions.
– Hydrogenation: addition of H2
– Halogenation: addition of X2
– Hydration: addition of H2O
– Hydrohalogenation: addition of HX
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• A double bond consists of
– a sigma bond: two electrons concentrated on a line between the two connected atoms;
– a pi bond: two electrons concentrated in planes above and below the sigma bond.
• A double bond consists of
– a sigma bond: two electrons concentrated on a line between the two connected atoms;
– a pi bond: two electrons concentrated in planes above and below the sigma bond.
5. General addition reaction5. General addition reaction
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In an addition reaction, the pi bond is lost and its electrons become part of the single bonds to A and B.In an addition reaction, the pi bond is lost and its electrons become part of the single bonds to A and B.
5. General addition reaction5. General addition reaction
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5. General addition reaction5. General addition reaction
• For hydrogenation, halogenation, hydration, and hydrohalogenation, identify the A and B portions of what is being added to the double bond.
– hydrogenation, H2
– halogenation, X2 (where X = F, Cl, Br, or I)– hydration, H2O– hydrohalogenation, HX (where X = F, Cl, Br, or I)
• For hydrogenation, halogenation, hydration, and hydrohalogenation, identify the A and B portions of what is being added to the double bond.
– hydrogenation, H2
– halogenation, X2 (where X = F, Cl, Br, or I)– hydration, H2O– hydrohalogenation, HX (where X = F, Cl, Br, or I)
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5. Hydrogenation5. Hydrogenation
• In hydrogenation of an alkene, one molecule of hydrogen (H2) adds to one mole of double bonds.
• Reaction conditions:– platinum, palladium, or nickel catalyst– [sometimes] heat and/or pressure
• In hydrogenation of an alkene, one molecule of hydrogen (H2) adds to one mole of double bonds.
• Reaction conditions:– platinum, palladium, or nickel catalyst– [sometimes] heat and/or pressure
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5. Hydrogenation5. Hydrogenation
• In hydrogenation of an alkyne, two molecules of hydrogen (H2) add to one mole of triple bonds.
• Reaction conditions: same as for alkenes.
• In hydrogenation of an alkyne, two molecules of hydrogen (H2) add to one mole of triple bonds.
• Reaction conditions: same as for alkenes.
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5. Hydrogenation5. Hydrogenation
• Compare the products resulting from the hydrogenation of trans-2-pentene and cis-2-pentene.
• Compare the products resulting from the hydrogenation of trans-2-pentene and cis-2-pentene.
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5. Hydrogenation5. Hydrogenation
• Compare the products resulting from the hydrogenation of 1-butene and cis-2-butene.
• Compare the products resulting from the hydrogenation of 1-butene and cis-2-butene.
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5. Vegetable oil and margarine5. Vegetable oil and margarine
Why does hydrogenation make oils more solid?Why does hydrogenation make oils more solid?
MP = 13-14oC
MP = 69.6oC
MP = 62.9oC
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5. Halogenation5. Halogenation
• In halogenation of an alkene, one mole of a halogen (Cl2, Br2, I2) adds to one mole of double bonds.
• Since halogens are more reactive than hydrogen, no catalyst is needed.
• In halogenation of an alkene, one mole of a halogen (Cl2, Br2, I2) adds to one mole of double bonds.
• Since halogens are more reactive than hydrogen, no catalyst is needed.
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5. Halogenation5. Halogenation
• In halogenation of an alkyne, two moles of a halogen (Cl2, Br2, I2) add to one mole of double bonds.
• In halogenation of an alkyne, two moles of a halogen (Cl2, Br2, I2) add to one mole of double bonds.
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5. Halogenation5. Halogenation
• Draw the structure and write a balanced equation for the halogenation of each of the following compounds.
– 3-methyl-1,4-hexadiene
– 4-bromo-1,3-pentadiene
– 3-chloro-2,4-hexadiene
• Draw the structure and write a balanced equation for the halogenation of each of the following compounds.
– 3-methyl-1,4-hexadiene
– 4-bromo-1,3-pentadiene
– 3-chloro-2,4-hexadiene
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5. Halogenation5. Halogenation
• A solution of bromine in water has a reddish-orange color.
• A simple test for the presence of an alkene or alkane is to add bromine water.
– If a double or triple bond is present, the bromine will be used up in a halogenation reaction and the color will disappear.
• A solution of bromine in water has a reddish-orange color.
• A simple test for the presence of an alkene or alkane is to add bromine water.
– If a double or triple bond is present, the bromine will be used up in a halogenation reaction and the color will disappear.
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5. Hydration5. Hydration
• In hydration, one mole of water (H2O) is added to one mole of double bonds.
• A trace of acid is required as a catalyst.
• In hydration, one mole of water (H2O) is added to one mole of double bonds.
• A trace of acid is required as a catalyst.
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5. Hydration5. Hydration
• Unlike hydrogenation and halogenation, hydration is not a symmetric addition to a double bond.
• If the double bond is not symmetrically located in the molecule, there are two possible hydration products.
• Unlike hydrogenation and halogenation, hydration is not a symmetric addition to a double bond.
• If the double bond is not symmetrically located in the molecule, there are two possible hydration products.
41
5. Hydration5. Hydration
• The predominant product is determined by Markovnikov’s rule: The rich get richer.
• OR: The carbon that already has more hydrogens will get the hydrogen from the water.
• Hydration of propene:
• The predominant product is determined by Markovnikov’s rule: The rich get richer.
• OR: The carbon that already has more hydrogens will get the hydrogen from the water.
• Hydration of propene:
+ H2O
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5. Hydration5. Hydration
• Write a balanced equation for the hydration of each of the following compounds:
– 2-butene
– 2-ethyl-3-hexene
– 2,3-dimethylcyclohexene
Alkynes undergo a much more complicated hydration that you don’t need to remember at this time!
• Write a balanced equation for the hydration of each of the following compounds:
– 2-butene
– 2-ethyl-3-hexene
– 2,3-dimethylcyclohexene
Alkynes undergo a much more complicated hydration that you don’t need to remember at this time!
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5. Hydrohalogenation5. Hydrohalogenation
• Like hydration, hydrohalogenation is an asymmetric addition to a double bond.
– Hydrohalogenation also follows Markovnikov’s rule.
• Like hydration, hydrohalogenation is an asymmetric addition to a double bond.
– Hydrohalogenation also follows Markovnikov’s rule.
44
5. Hydrohalogenation5. Hydrohalogenation
2-butene + HBr ?
3-methyl-2-hexene + HCl ?
cyclopentene + HI ?
2-butene + HBr ?
3-methyl-2-hexene + HCl ?
cyclopentene + HI ?
45
5. Hydrohalogenation5. Hydrohalogenation
• Here’s your second Journal question!• [Use tag “addition”]
• Explain how hydrogenation and halogenation are different from hydration and hydrohalogenation as addition reactions. [Hint: There’s a rule involved!]
• Here’s your second Journal question!• [Use tag “addition”]
• Explain how hydrogenation and halogenation are different from hydration and hydrohalogenation as addition reactions. [Hint: There’s a rule involved!]
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6. Aromatic compounds6. Aromatic compounds
• Consider the following molecular formulas for unsaturated hydrocarbons:
– Hexane (all single bonds): C6H14
– Cyclohexane (one ring): C6H12
– Hexene (one double bond): C6H12
– Hexadiene (two double bonds): C6H10
– Cyclohexene (one ring, one double bond): C6H10
– Hexatriene (three double bonds): C6H8
– Cyclohexadiene (one ring, two double bonds): C6H8
• Consider the following molecular formulas for unsaturated hydrocarbons:
– Hexane (all single bonds): C6H14
– Cyclohexane (one ring): C6H12
– Hexene (one double bond): C6H12
– Hexadiene (two double bonds): C6H10
– Cyclohexene (one ring, one double bond): C6H10
– Hexatriene (three double bonds): C6H8
– Cyclohexadiene (one ring, two double bonds): C6H8
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6. Aromatic compounds6. Aromatic compounds
• The molecular formula for benzene is C6H6.– The structure must be highly unsaturated.– One ring, three double bonds?
• Reactions of benzene:– Benzene does not decolorize bromine solutions.– Benzene does not undergo typical addition reactions.– Benzene reacts mainly by substitution.
• The first three items are opposite from what is expected from unsaturated compounds.
• The last item is identical to what is expected for alkanes.
• The molecular formula for benzene is C6H6.– The structure must be highly unsaturated.– One ring, three double bonds?
• Reactions of benzene:– Benzene does not decolorize bromine solutions.– Benzene does not undergo typical addition reactions.– Benzene reacts mainly by substitution.
• The first three items are opposite from what is expected from unsaturated compounds.
• The last item is identical to what is expected for alkanes.
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6. Benzene structure6. Benzene structure
• The benzene ring consists of:– six carbon atoms – joined in a planar hexagonal arrangement– with each carbon bonded to one hydrogen atom.
• Two equivalent structures proposed by Kekulé are recognized today as resonance structures.
• The real benzene molecule is a hybrid with each
resonance structure contributing equally to the true structure.
• The benzene ring consists of:– six carbon atoms – joined in a planar hexagonal arrangement– with each carbon bonded to one hydrogen atom.
• Two equivalent structures proposed by Kekulé are recognized today as resonance structures.
• The real benzene molecule is a hybrid with each
resonance structure contributing equally to the true structure.
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6. Benzene structure6. Benzene structure
• Sigma and pi bonding in benzene:
• The sharing of six electrons over the entire ring gives the benzene structure extra stability.
• Removing any one of the six electrons would destroy that stability.
• Sigma and pi bonding in benzene:
• The sharing of six electrons over the entire ring gives the benzene structure extra stability.
• Removing any one of the six electrons would destroy that stability.
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6. Nomenclature6. Nomenclature
• Most single-substituent compounds are named as derivatives of benzene.
– Bromobenzene
– Ethylbenzene
• Most single-substituent compounds are named as derivatives of benzene.
– Bromobenzene
– Ethylbenzene
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6. Nomenclature6. Nomenclature
• A few “common” names have been adopted as IUPAC nomenclature.
– toluene
– phenol
– aniline
– xylene (any benzene ring with two methyl groups)
• A few “common” names have been adopted as IUPAC nomenclature.
– toluene
– phenol
– aniline
– xylene (any benzene ring with two methyl groups)
52
6. Nomenclature6. Nomenclature
• There are three ways for the methyl groups on xylene to be arranged.
– 1,2 [ortho-xylene]
– 1,3 [meta-xylene]
– 1,4 [para-xylene]
• There are three ways for the methyl groups on xylene to be arranged.
– 1,2 [ortho-xylene]
– 1,3 [meta-xylene]
– 1,4 [para-xylene]
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6. Nomenclature6. Nomenclature
• The substituent created by removing one hydrogen from the benzene ring is called phenyl-.
– 2-phenylhexane
– 3-phenylcyclopentene
• The substituent created by removing one hydrogen from the benzene ring is called phenyl-.
– 2-phenylhexane
– 3-phenylcyclopentene
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6. Nomenclature6. Nomenclature
• The substituent consisting of a –CH2 attached to a benzene ring is called benzyl-.
– Benzyl chloride
• The substituent consisting of a –CH2 attached to a benzene ring is called benzyl-.
– Benzyl chloride
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6. Polynuclear aromatic hydrocarbons6. Polynuclear aromatic hydrocarbons
• These consist of rings joined along one side.
• Good news! You don’t have to memorize these names!
• These consist of rings joined along one side.
• Good news! You don’t have to memorize these names!
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6. Reactions of benzene6. Reactions of benzene
• Because of the stability of benzene’s ring structure, only substitution reactions are characteristic.
– Halogenation: substitution of one or more halogen atoms for hydrogen atoms.
• Cl2 requires FeCl3 catalyst.• Br2 requires FeBr3 catalyst.
– Nitration: substitution of one or more nitro- (-NO2) groups for hydrogen atoms.
• Requires nitric acid and concentration sulfuric acid.
– Sulfonation: substitution of one sulfonic acid (-SO3H) group for a hydrogen atom.
• SO3 reactant and concentration sulfuric acid.
• Because of the stability of benzene’s ring structure, only substitution reactions are characteristic.
– Halogenation: substitution of one or more halogen atoms for hydrogen atoms.
• Cl2 requires FeCl3 catalyst.• Br2 requires FeBr3 catalyst.
– Nitration: substitution of one or more nitro- (-NO2) groups for hydrogen atoms.
• Requires nitric acid and concentration sulfuric acid.
– Sulfonation: substitution of one sulfonic acid (-SO3H) group for a hydrogen atom.
• SO3 reactant and concentration sulfuric acid.
57
7. Heterocyclic aromatic compounds7. Heterocyclic aromatic compounds
• Heterocyclic aromatic compounds have at least one non-carbon atom incorporated in an aromatic ring or polynuclear aromatic compound.
– Many of these compounds are biologically important.
• Components of DNA and RNA
• Components of hemoglobin and chlorophyll
• Pharmaceuticals
• Heterocyclic aromatic compounds have at least one non-carbon atom incorporated in an aromatic ring or polynuclear aromatic compound.
– Many of these compounds are biologically important.
• Components of DNA and RNA
• Components of hemoglobin and chlorophyll
• Pharmaceuticals
pyridine
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7. Heterocyclic aromatic compounds7. Heterocyclic aromatic compounds
• Final Journal question for this unit!
• [Use tag “common”]
• What do DNA, RNA, nicotine, hemoglobin, chlorophyll, and a drug used to treat ulcers have in common?
• Final Journal question for this unit!
• [Use tag “common”]
• What do DNA, RNA, nicotine, hemoglobin, chlorophyll, and a drug used to treat ulcers have in common?