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ENV124: Chap4 Benzene

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  • 7/25/2019 ENV124: Chap4 Benzene

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    Marina Mokhtar

    ENV 123 / ENV 124

    BENZENE

    1

    2

    Benzene and Aromatic Compounds

    Benzene (C6H6) is the simplest aromatic hydrocarbon (orarene).

    Benzene has four degrees of unsaturation, making it a highlyunsaturated hydrocarbon.

    Whereas unsaturated hydrocarbons such as alkenes, alkynes anddienes readily undergo addition reactions, benzene does not.

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    Benzene does react with bromine, but only in the presence ofFeBr3(a Lewis acid), and the reaction is a substitution, not anaddition.

    Proposed structures of benzene must account for its high degreeof unsaturation and its lack of reactivity towards electrophilicaddition.

    August Kekulproposed that benzene was a rapidly equilibratingmixture of two compounds, each containing a six-memberedring with three alternating bonds.

    In the Kekul description, the bond between any two carbonatoms is sometimes a single bond and sometimes a double bond.

    4

    These structures are known asKekul structures.

    Although benzene is still drawn as a six-membered ring with alternating bonds, in reality there is no equilibrium between the two different kinds ofbenzene molecules.

    Current descriptions of benzene are based on resonance and electrondelocalization due to orbital overlap.

    In the nineteenth century, many other compounds having properties similarto those of benzene were isolated from natural sources. Since thesecompounds possessed strong and characteristic odors, they were calledaromatic compounds. It should be noted, however, that it is their chemicalproperties, and not their odor, that make them special.

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    Any structure for benzene must account for the following facts:1. It contains a six-membered ring and three additional degrees

    of unsaturation.

    2. It is planar.

    3. All CC bond lengths are equal.

    The Kekul structures satisfy the first two criteria but not thethird, because having three alternating bonds means thatbenzene should have three short double bonds alternating with

    three longer single bonds.

    6

    The resonance description of benzene consists of two equivalentLewis structures, each with three double bonds that alternatewith three single bonds.

    The true structure of benzene is a resonance hybrid of the twoLewis structures, with the dashed lines of the hybrid indicatingthe position of the bonds.

    We will use one of the two Lewis structures and not the hybridin drawing benzene. This will make it easier to keep track of theelectron pairs in the bonds (the electrons).

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    Because each bond has two electrons, benzene has six electrons.

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    To name a benzene ring with one substituent, name thesubstituent and add the word benzene.

    Nomenclature of Benzene Derivatives

    Many monosubstituted benzenes have common names whichyou must also learn.

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    There are three different ways that two groups can be attachedto a benzene ring, so a prefixortho,meta, orparacan beused to designate the relative position of the two substituents.

    ortho-dibromobenzeneor

    o-dibromobenzene

    or 1,2-dibromobenzene

    meta-dibromobenzeneor

    m-dibromobenzene

    or 1,3-dibromobenzene

    para-dibromobenzeneor

    p-dibromobenzene

    or 1,4-dibromobenzene

    10

    If the two groups on the benzene ring are different, alphabetizethe names of the substituents preceding the word benzene.

    If one substituent is part of a common root, name the moleculeas a derivative of that monosubstituted benzene.

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    For three or more substituents on a benzene ring:

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

    2. Alphabetize the substituent names.

    3. When substituents are part of common roots, name the molecule as aderivative of that monosubstituted benzene. The substituent thatcomprises the common root is located at C1.

    12

    A benzene substituent is called aphenyl group, and it can beabbreviated in a structure as Ph-.

    Therefore, benzene can be represented as PhH, and phenolwould bePhOH.

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    Thebenzyl group, another common substituent that contains abenzene ring, differs from a phenyl group.

    Substituents derived from other substituted aromatic rings arecollectively known asaryl groups.

    14

    Give the IUPAC name for each compound.

    PhCH(CH3)2 isopropylbenzene

    OH

    m-butylphenol

    Cl

    Br

    2-bromo-5-chlorotoluene

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    What is the correct name for this compound?

    A) 3-Nitrotoluene

    B) 4-Nitromethylbenzene

    C)p-Nitrotoluene

    D) (4,1)-Methylnitrobenzene

    O2N

    C)p-Nitrotoluene

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    What is the correct name for this compound?

    A) 3,5-difluoroanisole

    B) Difluoromethoxybenzene

    C) 1,5-difluoro-3-methoxybenzene

    D) 1,3-difluoro-5-methyl-O-benzene

    O

    F

    F

    A) 3,5-difluoroanisole

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    What is the correct name for this compound?

    A) 4-bromo-m-xylene.

    B) 1-bromo-2,4-dimethylbenzene.

    C)p-bromo-m-methyltoluene.

    D) o-bromo-m-methyltoluene.

    Br

    B) 1-bromo-2,4-dimethylbenzene.

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    What is the correct name?

    A) 1-fluoro-3-isopropyl-5-ethylbenzene

    B) 1-ethyl-3-isopropyl-5-fluorobenzene

    C) 1-ethyl-3-fluoro-5-isopropylbenzene

    D) 1-isopropyl-3-fluoro-5-ethylbenzene

    F

    C) 1-ethyl-3-fluoro-5-isopropylbenzene

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    CH2

    AB

    Which of these is aromatic?

    A) Is aromatic. Count the number of pi bonds in the

    outer ring. A has 5 which means 10 pi electrons,4(2)+2=10. While B has 6 pi bonds and 12 pi electrons,4(3)=12. Doesnt meet the Huckel rule requirementsfor aromaticity.

    Industry

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    Made from Coal Tar (a liquid left over when coal is

    heated) and Petroleum

    Used as a Source for Almost all Aromatic Compounds

    Make Complicated Aromatic Compounds by Taking

    Benzene and Add on Other Rings Piece by Piece

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    Industry - continued

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    Solvent for:

    Sulfur, Phosphorus and Iodine

    Gums

    Fats, Waxes and Resins

    Simple Organic Compounds

    One of the Most Commonly Used Solvents in

    Organic Chemistry

    Industry - continued

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    Aniline:

    Dye (Aniline Oil) Plastics

    Drugs (Sulfanilamide)

    Explosives

    Phenol:

    DisinfectantAntiseptic (Diluted)

    Insecticide

    Explosives

    Detergent

    Raw Material forAspirin

    Can make Aniline and Phenol

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    Industry - continued

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    Makes Toluene, a component of TNT (Trinitrotoluene)

    Makes Tear Gas

    Makes Acetone

    Toxic Nature

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    Carcinogen -- Causes Leukemia

    Makes DioxinsTwo Phenols Combine and Cl Connects to Make

    Chlorinated Dibenzo-p-dioxinDoes not Dissolve in Water, but does in Oils and FatsAccumulates in the Food Web

    Mimics HormonesDisrupts Normal Functions and Growth

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    Chemical ReactionsBenzene

    Nitration

    Halogenation

    Friedal Craft (Alkylation and Acylation)

    Alkylbenzene

    OxidationHalogenation (Free Radical and Substitution)

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    Nitration

    + conc. HNO3, conc. H2SO4 heated at 50 55 oC

    conc. HNO3, conc. H

    2SO

    4

    50 -55 C

    NO2

    + H2O

    > 55 C

    NO2

    NO2

    +

    NO2

    NO2O2N26

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    Halogenation+ Cl2 or + Br2Need catalyst AlCl3 or Fe or FeCl3 (or FeBr3)

    Cl2

    FeCl3

    Cl

    27

    Alkylation (Friedal-Craft)

    + RCl or + RBr or + RI

    Need catalyst AlCl3 , heat ()

    + CH3Cl

    AlCl3

    CH3

    + HCl

    28

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    Acylation (Friedal Craft)+ acyl chloride (R-COCl)

    Need catalyst AlCl3 and heat

    COCH3

    +AlCl

    3

    + HClii. C ClO

    CH3

    29

    Oxidation

    + KMnO4 , acid (H+) and heat ()

    CH3

    KMnO4, H+

    COOH

    + H2O + CO

    2

    C CH3CH3

    CH3

    ** no reaction

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    Halogenation (Free Radical)+ Br2 , hv (uv)

    CH3

    Br2

    h

    CH2 Br

    + HBr

    31

    Halogenation (Substitution)

    + Cl2Catalyst AlCl3 methyl (-CH3) is ortho, para directing

    CH3

    Cl2

    AlCl3

    CH3

    Cl

    +

    CH3

    Cl

    32

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