+ All Categories
Home > Documents > 5.Aromatic Compounds Whole

5.Aromatic Compounds Whole

Date post: 04-Apr-2018
Category:
Upload: nura-adila
View: 224 times
Download: 0 times
Share this document with a friend

of 71

Transcript
  • 7/30/2019 5.Aromatic Compounds Whole

    1/71

    Aromatic compounds

  • 7/30/2019 5.Aromatic Compounds Whole

    2/71

    Introduction

    Organic compounds

    Aliphatic cpds Aromatic cpds

    Aromatic compound an organic compound

    that contains a benzene ring in its molecule. Benzene is an example of aromatic ring

    with the molecular formula, C6H6.

  • 7/30/2019 5.Aromatic Compounds Whole

    3/71

    Aromatic compounds

    Sometimes called arenes.

  • 7/30/2019 5.Aromatic Compounds Whole

    4/71

    Benzene

    Colourless liquid with a characteristicsmell

    Highly flammable and burns with a smoky

    yellow flame Insoluble in water but soluble in all organic

    solvents

    Molecular formula: C6H6 The presence of three carbon-carbon

    double bonds per molecule

  • 7/30/2019 5.Aromatic Compounds Whole

    5/71

    Types of aromatic

    compounds

  • 7/30/2019 5.Aromatic Compounds Whole

    6/71

  • 7/30/2019 5.Aromatic Compounds Whole

    7/71

    Sidechain Substituted Compounds

    the functional group is present in thesidechain of the ring.

    Named as the phenyl derivatives of the

    corresponding aliphatic compounds.

  • 7/30/2019 5.Aromatic Compounds Whole

    8/71

    Naming Benzene Derivatives

    Only one kind of monosubstituted derivativesare possible in benzene rings as all six

    hydrogen atoms are equivalent. For example,

    there is only one toluene. It does not matterwhere the methyl group is attached because

    all the following arrangements are equivalent.

    When two same or different monovalent

    substituents, are present on a benzene ring,

    the following three arrangements are possible.

  • 7/30/2019 5.Aromatic Compounds Whole

    9/71

    For the same substituent (A)

    ortho (or, 1,2-)

    meta-(or, 1,3-)

    para- (or, 1,4)

  • 7/30/2019 5.Aromatic Compounds Whole

    10/71

    For different substituents (A and B)

    ortho (or, 1,2-)

    meta-(or, 1,3-)

    para- (or, 1,4)

  • 7/30/2019 5.Aromatic Compounds Whole

    11/71

    These arrangements are named as follows:

    The compound containing the two groups onthe adjacent sites is called 'ortho'; it is denoted

    as 'o-'. In the IUPAC system, the ortho

    position is designated as 1,2-.

    The compound containing the two groups on

    alternate sites is called 'meta': it is denoted as'm-'. In the IUPAC system,the meta position is

    designated as 1,3-.

  • 7/30/2019 5.Aromatic Compounds Whole

    12/71

    The compound containing the two groups

    diagonally opposite to each other is called'para': denoted as 'p-'. In the IUPAC system,

    the para position is designated as 1,4-.

    For example, the three xylenes are named as,

    o-xylene

    (1,2-dimethylbenzene)

  • 7/30/2019 5.Aromatic Compounds Whole

    13/71

    m-xylene

    (1,3-dimethylbenzene)

    p- xylene

    (1,3-dimethylbenzene)

  • 7/30/2019 5.Aromatic Compounds Whole

    14/71

    In the case of trisubstituted derivatives, the

    nature of the substituted groups determine the

    number of arrangements. When the three

    substituent groups are identical (say, A), three

    arrangements are possible. These are termed

    as follows.

    vicinal (vic-) unsymmetrical (unsym-)

    symmetrical (sym-)

  • 7/30/2019 5.Aromatic Compounds Whole

    15/71

    For a trisubstituted product, if the two

    substituents are identical and the third

    different, then six products are possible. When

    all the three groups are different, ten products

    are possible.

    Since, naming each individual compound is

    not possible, it was found convenient to

    indicate the position of any substituent by the

    numeral indicating the serial number of thecarbon atom in the ring, to which that

    substituent is attached.

  • 7/30/2019 5.Aromatic Compounds Whole

    16/71

    Numbering the Carbon Atoms in the Ring

    The numbering of carbon atoms in the ring (ornucleus) is done as follows.

    When there is only one substituent on thering, there is only one compound possible.

    Thus, numbering of the carbon atoms of the

    nucleus does not arises.

  • 7/30/2019 5.Aromatic Compounds Whole

    17/71

    If there are two or more substituents, then

    numbering is in the alphabetical order of the

    substituents on the carbon atoms. The

    prefixes such as 'di', 'tri', 'cyclo', 'iso', etc., are

    ignored while arranging the substituents

    alphabetically.

    When two or more functional groups are

    present, then the principal functional group is

    assigned the number 1. The order of priority ofthe functional groups is the same as done for

    aliphatic polyfunctional compounds.

  • 7/30/2019 5.Aromatic Compounds Whole

    18/71

    For the sake of convenience, the ring is

    oriented in such a way that position 1 is at the

    top and numbering is done in a clockwise or

    anticlockwise manner whichever gives lower

    numbers to the other substituents.

  • 7/30/2019 5.Aromatic Compounds Whole

    19/71

    This is illustrated through the following example.The IUPAC names are written in bold letters.

    (o-xylene) (m-xylene)

    1,2-dimethylbenzene 1,3-dimethylbenzene

    (p-xylene)

    1,4-dimethylbenzene

  • 7/30/2019 5.Aromatic Compounds Whole

    20/71

    Names of some typical aromatic compounds are

    given below:

    Aromatic hydrocarbons (arenes)

    toluene o-xylene

    Methylbenzene 1,2-dimethylbenzene

    m-xylene

    1,3-dimethylbenzene

  • 7/30/2019 5.Aromatic Compounds Whole

    21/71

    p-xylene 1,3,5-trimethylbenzene

    1,4-dimethylbenzene

    2-phenylpropane (cumene) phenylethene (styrene)

  • 7/30/2019 5.Aromatic Compounds Whole

    22/71

    Anthracene Phenonthrene

  • 7/30/2019 5.Aromatic Compounds Whole

    23/71

    Halogen derivatives

    2-chlorotoulene 1,2-dichlorobenzene

    phenyl chloromethane

  • 7/30/2019 5.Aromatic Compounds Whole

    24/71

    Hydroxy derivatives:

    Phenols and aromatic alcohols

    2-methyl phenol phenylmethanol

    1,4-dihydroxy benzene 2,4,6-trinitrophenol

  • 7/30/2019 5.Aromatic Compounds Whole

    25/71

    Aldehydes and Ketones

    Nuclear substituted

    Benzaldehyde Methyl phenyl ketone

    Diphenyl ketone

  • 7/30/2019 5.Aromatic Compounds Whole

    26/71

    Carboxylic acids

    Nuclear substituted

    benzoic acid 2-methylbenzoic acid

    2-hydroxybenzoic acid

    1,4-benzenedicarboxylic acid

  • 7/30/2019 5.Aromatic Compounds Whole

    27/71

    Acid derivatives

    Benzoyl chloride Benzamide

    Phenyl benzoate

  • 7/30/2019 5.Aromatic Compounds Whole

    28/71

    Benzoic anhydride

    Ethyl-4-bromobenzoate

  • 7/30/2019 5.Aromatic Compounds Whole

    29/71

    Alkoxy derivatives

    Methoxy benzene 4-nitrobenzene

    1-methoxy-phenoxybenzene

  • 7/30/2019 5.Aromatic Compounds Whole

    30/71

    Amines

    Aminobenzene 2-Amino toluene

    Benzyl amine

    Nit d i ti

  • 7/30/2019 5.Aromatic Compounds Whole

    31/71

    Nitro derivatives

    nitrobenzene 1,3-dinitrobenzene

    2,4,6-trinitrotoluene

    Nit il d C b l i

  • 7/30/2019 5.Aromatic Compounds Whole

    32/71

    Nitriles and Carbylamines

    Benzonitrile Phenylcarbylamine

    S l h i id

  • 7/30/2019 5.Aromatic Compounds Whole

    33/71

    Sulphonic acids

    benzenesulphonic acid

    benzene-1,3-disulphonic acid

    N i A ti C d

  • 7/30/2019 5.Aromatic Compounds Whole

    34/71

    Naming Aromatic Compounds

    Aromatic compounds are those carbocyclic

    compounds that contain at least one benzenering.

    The ring of six carbons in benzene or any of

    its derivatives is termed benzene ring or

    nucleus. A benzene ring may be represented

    in any of the following ways.

  • 7/30/2019 5.Aromatic Compounds Whole

    35/71

    Any aliphatic group attached to the benzene

    ring is termed as the sidechain as shown

    alongside.

    In this structure R may be CH3, CH2Cl, CH2OH

    or CH2NH2.

    However, the functional groups such as NO2,OH, NH2, etc., and any other non-carbon

    substituent, linked directly to the nucleus are

    called as the sidechain.

    A ti d b l ifi d

  • 7/30/2019 5.Aromatic Compounds Whole

    36/71

    Aromatic compounds may be classified as

    follows.

    Monocyclic Aromatic Compounds Monocyclic aromatic compounds are those

    that contain only one six-membered benzene

    ring.

    The following compounds are some examples

    of monocyclic aromatic compounds.

    M lti li A ti C d

  • 7/30/2019 5.Aromatic Compounds Whole

    37/71

    Multicylic Aromatic Compounds

    Aromatic compounds containing two or morebenzene rings, condensed with each other,

    are known as multicyclic aromatic compounds

    or polynuclear aromatic compounds.

    Some typical examples of polynuclear

    aromatic compounds are,

    A l G

  • 7/30/2019 5.Aromatic Compounds Whole

    38/71

    Aryl Group

    Aromatic hydrocarbons are called arenes. When one or more hydrogens are removed

    from the aromatic hydrocarbons, the residues

    left behind are termed as aryl groups (similarto alkyl groups in the case of aliphatic

    hydrocarbons). The aryl groups are

    represented by Ar.

  • 7/30/2019 5.Aromatic Compounds Whole

    39/71

    Some typical aryl groups are:

    Aromaticity: Importance Of Aromatic

  • 7/30/2019 5.Aromatic Compounds Whole

    40/71

    Aromaticity: Importance Of AromaticCompounds

    Aromatic compounds are important in industry.Key aromatic hydrocarbons of commercialinterest are benzene, toluene, ortho-xylene andpara-xylene.

    About 35 million tonnes are produced worldwideevery year.

    They are extracted from complex mixtures

    obtained by the refining of oil or by distillation ofcoal tar, and are used to produce a range ofimportant chemicals and polymers, includingstyrene, phenol, aniline, polyesterand nylon.

    http://www.servinghistory.com/topics/aromatic_hydrocarbonshttp://www.servinghistory.com/topics/benzenehttp://www.servinghistory.com/topics/toluenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/styrenehttp://www.servinghistory.com/topics/phenolhttp://www.servinghistory.com/topics/anilinehttp://www.servinghistory.com/topics/polyesterhttp://www.servinghistory.com/topics/nylonhttp://www.servinghistory.com/topics/nylonhttp://www.servinghistory.com/topics/polyesterhttp://www.servinghistory.com/topics/anilinehttp://www.servinghistory.com/topics/phenolhttp://www.servinghistory.com/topics/styrenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/xylenehttp://www.servinghistory.com/topics/toluenehttp://www.servinghistory.com/topics/benzenehttp://www.servinghistory.com/topics/aromatic_hydrocarbons
  • 7/30/2019 5.Aromatic Compounds Whole

    41/71

    Other aromatic compounds play key roles in

    the biochemistry of all living things.

    Three aromatic amino acids phenylalanine,

    tryptophan, and tyrosine, each serve as one

    of the 20 basic building blocks of proteins.

    http://www.servinghistory.com/topics/phenylalaninehttp://www.servinghistory.com/topics/tryptophanhttp://www.servinghistory.com/topics/tyrosinehttp://www.servinghistory.com/topics/tyrosinehttp://www.servinghistory.com/topics/tryptophanhttp://www.servinghistory.com/topics/phenylalanine
  • 7/30/2019 5.Aromatic Compounds Whole

    42/71

    Further, all 5 nucleotides adenine, thymine,

    cytosine, guanine, and uracil that make upthe sequence of the genetic code in DNA

    and RNA are aromatic purines or

    pyrimidines.

    As well as that, the molecule haem

    contains an aromatic system with 22 electrons. Chlorophyll also has a similar

    aromatic system.

    http://www.servinghistory.com/topics/purineshttp://www.servinghistory.com/topics/pyrimidineshttp://www.servinghistory.com/topics/haemhttp://www.servinghistory.com/topics/Chlorophyllhttp://www.servinghistory.com/topics/Chlorophyllhttp://www.servinghistory.com/topics/haemhttp://www.servinghistory.com/topics/pyrimidineshttp://www.servinghistory.com/topics/purines
  • 7/30/2019 5.Aromatic Compounds Whole

    43/71

    Reactions of Aromatic

    Compounds

  • 7/30/2019 5.Aromatic Compounds Whole

    44/71

    Unlike alkenes, which undergo addition

    reactions, aromatic compounds usually

    undergo substitution reactions.

    A group Y substitutes for one of the

    hydrogen atoms on the aromatic ring

    without changing the ring itself.

    A substitution reaction

  • 7/30/2019 5.Aromatic Compounds Whole

    45/71

    The most common reaction of aromatic

    compounds is the electrophilic aromatic

    substitution. An electrophile (E ) reacts with an

    aromatic ring

    No addition reactions of alkenes:no decolorisation of bromine or potassium

    manganate(VII) solution

    no reaction with HCl, HBr or concentratedH2SO4

    +

  • 7/30/2019 5.Aromatic Compounds Whole

    46/71

    Substitution reaction

    1. Halogenation

    2. Nitration

    3. Friedel-Crafts reactioni. Alkylation

    ii. Acylation

    4. Sulphonation

    H l i

  • 7/30/2019 5.Aromatic Compounds Whole

    47/71

    Substitution of a bromine or chlorine(Br/Cl) for a ring hydrogen occurs when

    benzene reacts with Br2 or Cl2.

    a hydrogen atom a halogen atom Catalyst: FeBr3 or FeCl3

    Bromination

    Halogenation

    BenzeneBromobenzene

  • 7/30/2019 5.Aromatic Compounds Whole

    48/71

    Chlorination

    Benzene Chlorobenzene

  • 7/30/2019 5.Aromatic Compounds Whole

    49/71

    Substitution of a nitro group ( NO2) for a

    ring hydrogen occurs when benzene

    reacts with nitric acid.

    a hydrogen atom a nitro group, NO2

    Catalyst: sulphuric acid, H2SO4

    Nitration

    Benzene Nitrobenzene

  • 7/30/2019 5.Aromatic Compounds Whole

    50/71

    i. Alkylation: the attachment of an alkyl

    group to the benzene ring.

    a hydrogen atom an alkyl group from

    haloalkane to form an alkylbenzene

    Benzene ring can be alkylated by

    reaction with an alkyl chloride.

    Catalyst: aluminium chloride

    Friedel-Crafts reaction

  • 7/30/2019 5.Aromatic Compounds Whole

    51/71

    For example:

    The Friedel- Crafts

    alkylation ofbenzene with methyl

    chloride

    MethylbenzeneBenzeneMethyl

    chloride

  • 7/30/2019 5.Aromatic Compounds Whole

    52/71

    ii.Acylation: a hydrogen atom an acyl

    group from an acyl halide to form an

    aromatic ketone

    An acyl group, -COR is introduced onto

    the ring when an aromatic compound

    reacts with a carboxylic acid chloride,

    RCOCl.

    Catalyst: AlCl3

  • 7/30/2019 5.Aromatic Compounds Whole

    53/71

    For example

    Reaction of benzene with acetyl chloride

    yields the ketone, aceptophenone.

    Benzene Acetyl chloride Aceptophenone

    AlCl3

  • 7/30/2019 5.Aromatic Compounds Whole

    54/71

    Aromatic rings can be sulphonated by

    reaction with fuming sulfuric acid, a

    mixture of H2SO4 & SO3.

    a hydrogen atom an SO3H group

    the only reversible aromatic substitution.

    Sulphonation is favored in strong acid;

    desulphonation is favored in hot, dilute

    aqueous acid.

    Sulphonation

  • 7/30/2019 5.Aromatic Compounds Whole

    55/71

    Benzenesulfonic acidBenzene H2

    SO4

    + SO3

    For example

  • 7/30/2019 5.Aromatic Compounds Whole

    56/71

    Properties of aromatic

    compounds

  • 7/30/2019 5.Aromatic Compounds Whole

    57/71

    Physical characteristic

    Benzene is a colourless liquid with a

    characteristic smell.

    Both the liquid and the vapour of benzene

    are highly poisonous.

  • 7/30/2019 5.Aromatic Compounds Whole

    58/71

    Density

    Aromatic compounds are more dense

    than other hydrocarbons, but less dense

    than water

    It has a density of 0.88g cm-3.

  • 7/30/2019 5.Aromatic Compounds Whole

    59/71

    Boiling points

    In benzene, the only attractions between

    neighbouring molecules are van der Waals

    dispersion forces. There is no permanent

    dipole on the molecule.

    Benzene boils at 80C - rather higher than

    other hydrocarbons of similar molecular

    size (pentane and hexane, for example).

  • 7/30/2019 5.Aromatic Compounds Whole

    60/71

    This is presumably due to the ease with

    which temporary dipoles can be set up

    involving the delocalised electrons

    It is highly flammable and burns with asmoky yellow flame.

  • 7/30/2019 5.Aromatic Compounds Whole

    61/71

    Melting points

    Melting point of benzene is higher thanmethylbenzene.

    Benzene melts at 5.5C while

    methylbenzene melts at -95C. Molecules must pack efficiently in the

    solid if they are to make best use of their

    intermolecular forces. Benzene is a tidy, symmetrical molecule

    and packs very efficiently.

  • 7/30/2019 5.Aromatic Compounds Whole

    62/71

    Solubility

    The arenes are insoluble in water. Benzene is quite large compared with a water

    molecule. In order for benzene to dissolve it would have

    to break lots of existing hydrogen bondsbetween water molecules.

    Energy would also needed to break the quitestrong van der Waals dispersion forcesbetween benzene molecules. Both of thesecost energy.

  • 7/30/2019 5.Aromatic Compounds Whole

    63/71

    Benzene is soluble in all organic solvents.

    It is a very good solvent for fats, organic

    compounds with low relative molecular

    mass and inorganic substances such asiodine and sulphur.

  • 7/30/2019 5.Aromatic Compounds Whole

    64/71

    Methylbenzene

    Methylbenzene is a colourless liquid which

    smells like benzene.

  • 7/30/2019 5.Aromatic Compounds Whole

    65/71

    Boiling Point

    Methylbenzene boils at 111C. It is a bigger

    molecule and so the van der Waals

    dispersion forces will be bigger.

    Methylbenzene also has a small permanentdipole, so there will be dipole-dipole

    attractions as well as dispersion forces. The

    dipole is due to the CH3

    group's tendency to

    "push" electrons away from itself. This also

    affects the reactivity of methylbenzene.

    M lti P i t

  • 7/30/2019 5.Aromatic Compounds Whole

    66/71

    Melting Point Methybenzenes melting point (-95C) is

    lower than that of benzene (5C) eventhough its relative molecular mass is

    larger than benzene.

    This because the planar molecules ofbenzene can pack close together and the

    intermolecular forces of attraction are

    therefore stronger.

  • 7/30/2019 5.Aromatic Compounds Whole

    67/71

    In contrast, the methyl group in

    methylbenzene prevents such close packing.

    Thus, the melting point of methylbenzene islower than that of benzene.

    If the molecules aren't as closely packed, theintermolecular forces don't work as well and

    so the melting point falls.

  • 7/30/2019 5.Aromatic Compounds Whole

    68/71

    Uses of aromatic

    compounds

  • 7/30/2019 5.Aromatic Compounds Whole

    69/71

    Nitration

    - Nitration of aromatic rings is a key step in

    the synthesis of explosives such as TNT

    (trinitrotoluene) & many importantpharmaceutical agents.

    - Nitrobenzene is a starting material for

    preparing many of the brightly coloureddyes used in clothing.

  • 7/30/2019 5.Aromatic Compounds Whole

    70/71

    Halogenation

    - chlorination of an aromatic ring is a step

    used in the synthesis of numerous

    pharmaceutical agents, such as theantianxiety agent diazepam, marketed as

    Valium.

  • 7/30/2019 5.Aromatic Compounds Whole

    71/71

    Sulphonation

    - aromatic sulphonic acids are valuableintermediates in the preparation of dyes

    and pharmaceuticals. Eg: the sulfa drugssuch as sulfanilamide were among the firstuseful antibiotics. These drugs areprepared commercially by a process that

    involves aromatic sulphonation as the keystep.


Recommended