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    Organic Chemistry

    Chapter 1

    Introduction to Organic Moleculesand Functional Group

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    1.1 FUNCTIONAL GROUP

    A functional group is an atom or a group of atoms withcharacteristic chemical and physical properties. It is thereactive part of the molecule.

    Most organic compounds have CC and CH bonds.However, many organic molecules possess otherstructural features:

    Heteroatomsatoms other than carbon or hydrogen.

    Bonds - the most common bonds occur in CCand CO double bonds.

    These structural features distinguish one organicmolecule from another. They determine a molecules

    geometry, physical properties, and reactivity, andcomprise what is called afunctional group.

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    Heteroatoms and bonds confer reactivity on aparticular molecule.

    Heteroatoms have lone pairs and create electron-deficient sites on carbon.

    Bonds are easily broken in chemical reactions. Abond makes a molecule a base and a nucleophile.

    CC and CH bonds are important.They form the carbon backbone or skeleton to which the

    functional group is attached.

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    Ethane:This molecule has only CCand CHbonds, so ithas no functional group. Ethane has no polar bonds, no lonepairs, and no bonds, so it has no reactive sites.

    Consequently, ethane and molecules like it are veryunreactive.

    Ethanol:This molecule has an OH group attached to itsbackbone. This functional group is called a hydroxy group.Ethanol has lone pairs and polar bonds that make it reactive

    with a variety of reagents.The hydroxy group makes the properties of ethanol very

    different from the properties of ethane.

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    1.2 AN OVERVIEW OF FUNCTIONALGROUP

    Most common functional group:

    a) Hydrocarbon

    b) Compounds containing C - Z bond

    c) Compounds containing C = O group

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    a) Hydrocarbon

    Hydrocarbons are compounds made up of only the elementscarbonand hydrogen. They may be aliphatic or aromatic.

    Functional Groups: Hydrocarbon

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    Aromatic compounds are so named because many ofthe earliest known aromatic compounds had strong

    characteristic odors.The simplest aromatic hydrocarbon is benzene. The six-

    membered ring and three bonds of benzene comprisea single functional group.

    When a benzene ring is bonded to another group, it iscalled a phenyl group.

    Functional Groups: Hydrocarbon

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    Examples of Molecules Containing C-Z Bonds

    Functional Groups: C-Z Bonds

    b) C-Z Bonds

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    C-Z BondsFunctional Groups: Hydrocarbon

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    Compounds Containing the C=O Group:This group is called a carbonyl group.

    The polar CO bond makes the carbonyl carbon anelectrophile, while the lone pairs on O allow it to reactas a nucleophile and base.

    The carbonyl group also contains a bond that is moreeasily broken than a CO bond.

    Functional Groups: C=O Group

    c) C=O Group

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    Molecules Containing the C=O Functional Group

    Functional Groups: C=O Group

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    Functional Groups: C=O Group

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    Functional Groups

    Remember

    It should be noted that the importance of a functionalgroup cannot be overstated.

    A functional group determines all of the following

    properties of a molecule:

    1. bonding and shape

    2. type and strength of intermolecular forces

    3. physical properties

    4. nomenclature

    5. chemical reactivity

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    I t l l F

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    3 different types of interactions:

    a) van der Waals forces

    b) dipole-dipole interactions

    c) hydrogen bonding

    Intermolecular Forces

    strengthincreased

    I t l l F d W l F

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    a) van der Waals Forces

    Also known as London forces.

    Weak interactions caused by momentarychanges in electron density in a molecule.

    Only attractive forces present in nonpolarcompounds.

    Intermolecular Forces-van der Waals Forces

    I t l l F d W l F

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    eg:

    CH4has no net dipole

    Its electron density may not be completelysymmetrical- created temporary dipole.

    Will induce a temporary dipole in another molecule.

    Weak interaction of these temporary dipolesconstituents van der Waals forces.

    Intermolecular Forces-van der Waals Forces

    I t l l F d W l F

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    All compounds exhibit van der Waals forces.

    The surface areaof a molecule determines the strength

    of the interactions between molecules.The larger the surface area, the larger the attractive

    force between two molecules, and the stronger theintermolecular forces.

    Intermolecular Forces-van der Waals Forces

    I t l l F d W l F

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    van der Waals forces are also affected bypolarizability.

    Polarizabilityis a measure of how the electroncloud around an atom responds to changes in

    its electronic environment.

    Intermolecular Forces-van der Waals Forces

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    Intermolecular Forces dipole dipole int

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    b) Dipole-Dipole Interactions

    Dipole-dipoleinteractions are the attractive forces

    between the permanent dipoles of 2 polar molecules.Eg: Acetone (below). The dipoles in adjacent molecules

    align so that the partial +ve and partial -ve charges arein close proximity. These attractive forces causedbypermanent dipoles are muchstronger than weak vander Waals forces.

    Intermolecular Forces-dipole-dipole int.

    Intermolecular Forces hydrogen bonding

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    Hydrogen bondingtypically occurs when a

    hydrogen atom bonded to O, N, or F, iselectrostatically attracted to a lone pair ofelectrons on an O, N, or Fatom in anothermolecule.

    The strongest of them all.

    c) Hydrogen Bonding

    Intermolecular Forces-hydrogen bonding

    Intermolecular Forces

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    Note: as the polarity of an organic molecule increases, sodoes the strength of its intermolecular forces.

    Summary of Intermolecular Forces

    Intermolecular Forces

    Intermolecular Forces

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    APPLICATION-Nylon

    Nylon- is a polymer, composed fromsmall units-called monomers-to formlarge organic molecule

    Monomer are covalently bonded toeach other in repeating pattern

    Its long chain held byhydrogenbonding

    Intermolecular Forces

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    1.4 PHYSICAL PROPERTIES

    Strength of compounds intermolecular

    determine by:

    a) Boiling point

    b) Melting point

    c) Solubility

    Physical Properties: Boiling Point

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    Physical Properties: Boiling Point

    The boiling point of a compound: temperature at which liquidmolecules are converted into gas.

    In boiling, energy is needed to overcome the attractive forcesin the more ordered liquid state.

    The stronger the intermolecular forces, the higher theboiling point.

    For compounds with approximately the same molecularweight:

    a) Boiling point

    Physical Properties: Boiling Point

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    Eg 1: Note that the relative strength of the intermolecularforces increases from pentaneto butanalto 1-butanol.

    The boiling points of these compounds increase in thesame order.

    For two compounds with similar functional groups:The larger the surface area, the higher the boiling point.

    The more polarizable the atoms, the higher the boilingpoint.

    Physical Properties: Boiling Point

    Physical Properties: Boiling Point

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    Eg 2: Figure below illustrate the effect of size andpolarizability on boiling points.

    Physical Properties: Boiling Point

    Physical Properties: Boiling Point

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    Liquids with different boiling points can be separated in thelaboratory using a distillation apparatus, showed below

    Physical Properties: Boiling Point

    Physical Properties: Melting Point

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    The melting point: temperature at which a solid isconverted to its liquid phase.

    In melting, energy is needed to overcome the attractive

    forces in the more ordered crystalline solid.The stronger the intermolecular forces, the higher the

    melting point.

    Given the same functional group, the more symmetricalthe compound, the higher the melting point.

    Physical Properties: Melting Point

    b) Melting point

    Physical Properties: Melting Point

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    Because ionic compounds are held together by

    extremely strong interactions, they havevery highmelting points.

    With covalent molecules, the melting point dependsupon the identity of the functional group. For

    compounds of approximately the same molecularweight:

    Physical Properties: Melting Point

    Physical Properties: Melting Point

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    The trend in melting points of pentane, butanal, and 1-butanol parallels the trend observed in their boilingpoints.

    Physical Properties: Melting Point

    Physical Properties: Melting Point

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    Symmetry also plays a role in determining the meltingpoints of compounds having thesame functional groupandsimilar molecular weights, but verydifferent shapes.

    neopentanepacks well into a crystalline lattice whereasisopentane, which has a CH3group dangling from a four-carbon chain, does not. Thus,neopentane has a muchhigher melting point.

    Physical Properties: Melting Point

    Physical Properties: solubility

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    Solubility is the extent to which a compound(solute), dissolves in a liquid (solvent).

    In dissolving a compound, the energyneeded to break up the interactions betweenthe molecules or ions of the solute comesfrom new interactions between the solute andthe solvent.

    c) Solubility

    Physical Properties: solubility

    Physical Properties: solubility

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    Physical Properties: solubility

    Like dissolve LikePolar compounds dissolve in polar solvents.

    Nonpolar or weakly polar compounds dissolve innonpolar or weakly polar solvents.

    Physical Properties: solubility-solvent

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    Waterand organic solventsare two different

    kinds of solvents.

    Wateris very polarsince it is capable of

    hydrogen bonding with a solute.

    Many organic solventsare either nonpolar,like carbon tetrachloride (CCl

    4) and hexane

    [CH3(CH2)4CH3], or weakly polar, like diethylether (CH3CH2OCH2CH3).

    y p y

    Physical Properties: solubility-ionic comp.

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    Most ioniccompounds are soluble in water, butinsoluble in organic solvents.

    To dissolve an ionic compound, the strongion-ioninteractions must be replacedby many weakerion-dipole interactions.

    y p y p

    Physical Properties: solubility-organic comp.

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    Organic compound is soluble in organic solvent.

    An organic compoundis water soluble only if it

    contains one polar functional group (O @ N)capable ofhydrogen bonding with the solvent for every 5 Catomsit contains.

    Eg: Compare the Solubility of butane and acetone inH2O and CCl4.

    y p y g p

    Physical Properties: solubility-organic comp.

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    Since butaneand acetoneare both organic compoundshaving a CC and CH backbone, they are soluble in

    the organic solvent CCl4.Butane, which is nonpolar, is insoluble in H2O.

    Acetoneis soluble in H2Obecause it contains 3 C atomsand its O atom can hydrogen bond with an H atom of

    H2O.

    y p y g p

    Physical Properties: solubility-organic comp.

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    The size of an organic molecule with a polar functionalgroup determines its water solubility.

    A low molecular weight alcohol like ethanolis watersoluble (small carbon skeleton of 5 Catoms),compared to the size of its polar OH group.

    Cholesterolhas 27 carbon atomsand only one OH

    group. Its carbon skeleton is too large for the OH groupto solubilize by hydrogen bonding, so cholesterol isinsoluble in water.

    y p y g p

    Physical Properties: solubility

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    The nonpolar part of a molecule that is not

    attracted to H2O is said to behydrophobic.The polar part of a molecule that canhydrogen bond to H2O is said to be

    hydrophilic.In cholesterol, for example, the hydroxygroup is hydrophilic, whereas the carbonskeleton is hydrophobic.

    y p y

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    Solubility summary

    end

    Application

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    Vitamins are either lipid or water soluble.

    Application 1- Vitamins

    Application

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    Soap moleculeshave two distinctparts:

    a hydrophilic

    portion composedof ions called thepolar head

    a hydrophobiccarbon chain ofnonpolar CCand CH bonds,called thenonpolar tail.

    Application 2- Soap

    Application: cell membrane

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    Application 3 - The Cell Membrane

    Application: cell membrane

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    Transport Across a Cell Membrane:

    Polar molecules and ions are transported across cellmembranes encapsulated within molecules calledionophores.

    Ionophores are organic molecules that complex cations.They have a hydrophobic exterior that makes themsoluble in the nonpolar interior of the cell membrane, and

    a central cavity with several oxygens whose lone pairscomplex with a given ion.

    Application: cell membrane

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    Transport Across a Cell Membrane:

    Application: cell membrane

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    Several synthetic ionophores have also been prepared, including

    one group called crown ethers.Crown ethers are cyclic ethers containing several oxygen atomsthat bind specific cations depending on the size of their cavity.

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    1.5 Functional Groups and Reactivity

    Recall that:

    Functional groups create reactive sites in molecules.

    Electron-rich sites react with electron poor sites.

    All functional groups contain a heteroatom, a bond or

    both, and these features create electron-deficient (orelectrophilic) sites and electron-rich (or nucleophilic)sites in a molecule. Molecules react at these sites.

    Influence of Functional Groups on Reactivity

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    Influence of Functional Groups on Reactivity

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    An electron deficient carbon reacts with a nucleophile,

    symbolized as :Nuin reactions. An electron-richcarbon reacts with an electrophile, symbolized as E+inreactions.

    For example, alkenes contain a CC double bond, an

    electron-rich functional group with a nucleophilicbond. Thus, alkenes react with electrophiles E+, butnot with other electron rich species like OHor Br.

    Influence of Functional Groups on Reactivity

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    On the other hand, alkyl halides possess an

    electrophilic carbon atom, so they react with electron-rich nucleophiles.