The Acidity and Basicity of Organic Acids and Bases for VULA

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    Essential Organic Chemistry, Paula Bruice, Chapter 2

    Acidity and Basicity of Organic Compounds

    SW - 8

    BrnstedLowry Acids and Bases

    Lewis Acids and Bases Acidity constant Ka and pKa

    Acid strength - resonance

    - inductive effect- electronegativity

    - atomic size

    Organic acids and bases

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    BrnstedLowry Acids and Bases

    An acid is a proton donorA base is a proton acceptor

    acid base

    H2O H3OHBr Br + +

    NH3 H2O+ NH4 + OH

    acidbase

    Note that water can act as an acidora base

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    acid base conjugateacid

    conjugatebase

    Br+ +O

    H HO

    HHHH Br

    1st pair2nd pair

    Every acid-base reaction involving proton transfer has 2

    conjugate acid base-pairs.

    The remaining species after H+

    has been donated is the conjugatebase. The resulting species after H+ has been accepted is theconjugate acid.

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    B

    F

    F FCH3CH2 O CH2CH3+ B

    F

    F

    F

    O

    CH2CH3

    CH2CH3

    Adduct

    e.g. Fe3+ AlCl3 NH3 H2O Cl-

    Electron deficient

    Lewis acid

    Electron rich

    Lewis base

    Lewis Acids and Bases

    A Lewis acid is an electron pairacceptor.

    A Lewis base is an electron pairdonor.

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    The Acidity Constant, Ka

    The strength of an acid is represented by its ionizationconstant (acidity constant), Ka

    HA + H2O A H3O+

    Ka = A H3O

    HA

    Since the Ka values for various acids have such a wide range,a more manageable way to discuss acidity is to use pKa

    pKa = log (Ka)

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    Will the following reactions take place in the direction

    indicated?

    (4.72)(9.2)(i) HCN + CH3CO2 Na NaCN + CH3CO2H

    (ii) CH3CH2OH + NaCN CH3CH2O + HCN

    (16.0) (9.2)

    pKa values given in brackets

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    Acid Strength

    HA

    A1-

    A2-

    Higher energy astronger conj.base from a weakeracid

    ENE

    RGY

    ionization

    easier

    The difference between a strong acid and a weak acid can bedescribed by the stability of the conjugate base.

    Compare 2 conjugate bases A1 and A

    2

    Lower energy aweaker conj. base

    from a stronger acid

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    Acid Strength explained

    An acid will give up its H+ readily if the conjugate base whichforms upon acid dissociation can be stabilised.

    So consider structure of conjugate base of the acid to see ifthere are any factors that can stabilise it.

    1. Resonance

    2. Inductive effects

    3. Electronegativity

    4. Atomic Size

    5. Hybridization

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    1. Resonance Effects- influence the stability of the conjugate base

    Resonance results in the electrons and the charge being

    dispersed.

    A stable base is a weak base and will not share its electrons

    readily.

    Resonance stabilisation = extra stability a compound gainsfrom having delocalised electrons.

    The greater the number of relatively stable resonancecontributors, or the more stable the resonance contributorsare, the greater is the resonance stabilization.

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    Example 1Explain using appropriate equations & resonance structures,

    why ethanoic acid (pKa=4.72) is a stronger acid than ethanol(pKa = 16.00).

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    The ethanoate ion has 2

    resonance stabilizedequivalent structures(charges on differentoxygens).

    acetic acid

    acetate ion

    H3C C

    O

    O

    H

    H2OH3C C

    O

    OH3C C

    O

    O

    H3O+

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    Example 2Explain using appropriate equations and resonance structures,

    why phenol (pKa =10) is a stronger acid than cyclohexanol(pKa = +/- 17).

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    O O O O O

    The phenolate ion can be resonance stabilized.

    OOOO

    -

    More resonance structures,

    but not more stable thanacetate. Non-equivalentstructures (note charges oncarbon and oxygen).

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    2. The Inductive effect

    - influences the stability of the conjugate base

    The Inductive effect is a permanent polarisation which resultsfrom electronegativity differences between 2 bonding atoms.

    Cl C

    ++++

    - electronegative elements,F, Cl, Br, O,N, pull electron density away from C.

    Cl helps to stabilize carboxylateanion by withdrawing electrons from

    the carbonyl carbon. This makes thecarbonyl carbon slightly positivewhich stabilizes the ve charge.

    Cl CO

    O

    ++++

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    Inductive effect diminishes with distance- it extends for about 3 bonds

    C C

    O

    CCl ++++ ++++ ++++

    O

    Remember, the inductive effect works through the bondsystem, while the resonance effect works through the pisystem.

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    Example 3Explain using appropriate equations & resonance structures, why

    2-chloroacetic acid (pKa =2.85) is a stronger acid than acetic acid(pKa =4.72).

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    3. Electronegativity influences the stability of the conjugatebase

    (i) The following slide shows 3 sets of molecules.

    Each set consists of an analogous class of molecules,differing in elements in period 2.

    Consider the conjugate base of each molecule.

    The conjugate base will have a ve charge.

    Placing the -ve charge on a more electronegative element(from the same period) in the conjugate base leads to astronger acid. Why?

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    increasing electronegativity

    20 15 5

    183545

    >45 34 16 3.5

    pKa values given for each molecule

    Set 1

    Set 2

    Set 3

    RCH3 RNH2 ROH

    CH4 NH3 H2O HF

    RCCH3 RCNH2 RCOH

    O O O

    IV V VI VII

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    4. Atomic Size

    Placing the ve charge on a larger atom (same group) in theconj base leads to a stronger acid.

    1.36

    1.81

    -1.95

    -2.16

    HF 3.5 F-

    HCl -7 Cl-

    HBr -9 Br-

    HI -10 I-

    Increasing size pKa con base ionic radii

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    Basicity of amines:

    General reaction of amines with acids:

    N + H A

    amine acid

    N H + A

    salt

    Bases are characterized by their Kb or pKb values:(Note that pKa + pKb = pKw = 14)

    RNH2 + H2O RNH3 + OH

    Organic Bases

    [ ]

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    [ ][ ][ ]

    2

    3

    RH

    OHRHKb

    +

    = bb KpK log=

    stronger base [RNH2] smaller Kb larger pKb smaller

    weaker base [RNH2] larger Kb smaller pKb larger

    Base pKb

    (CH3CH2)2NH 3.06stronger

    base

    (CH3CH2)3N 3.21

    CH3CH

    2NH

    23.25

    CH3NH2 3.36

    NH3 4.74

    NH2

    9.37

    H3C

    C

    NH2

    O

    14.5 weakerbase

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    Why is aniline a weak base?

    Consider resonance forms of aniline:

    NH2 NH2NH2 NH2

    The non-bonding electron pair on N can be delocalized into the ring as shown

    above. This means the electron pair is less available to be donated to a

    proton.

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    Why is ethanamide a very weak base?

    In ethanamide (acetamide) the nitrogen is adjacent to an electron withdrawing

    carbonyl group. The non-bonding pair of electrons on the nitrogen can be

    delocalized towards the carbonyl pi system and are therefore not available for

    donation to an acid.

    H3C

    C

    O

    NH2 H3C

    C

    NH2

    O