MCAT OCHEM

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Author:
Alem
ID:
191311
Filename:
MCAT OCHEM
Updated:
2013-01-03 22:43:38
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Alcohols
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MCAT OCHEM Alcohols
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  1. Alcohols
    • Alcohol chain is numbered in such a way that the carbon attached to the
    • hydroxyl group (-OH) receives the lowest number possible. 

    In compounds that possess a multiple bond and a hyroxyl group, numerical priority is given to the carbon attached to the -OH.
  2. Alcohols
    Alcohols may be methanol (Me), primary (1o), secondary (2o) or tertiary (3o).

  3. Diols
    Molecules with two hydroxyl groups are called diols (or Glycols) and are named with the suffix -diols. Two numbers are necessary to locate the two functional groups.
  4. Diols : Vicinal       
            : Geminal
    Diols with hydroxyl groups on adjacent carbons are referred to as vicinal

    Diols with hydroxyl groups on the same carbon are geminal. Geminal diols (also called hydrates) are not commonly observed because they spontaneously lose water (dehydrate) to produce carbonyl compounds.
  5. Properties of Alcohols
    All alcohols are capable of hydrogen bonding.

    • The hydrogen atom  of the hydroxyl group of all alcohols is acidic because the hydrogen is
    • made partially positive by the electron-withdrawing oxygen atom.

    • Because the carbon atom to which a hydroxyl group is attached is made partially
    • positive by the electron-withdrawing oxygen atom, the carbon is subject
    • to nucleophilic attack.
  6. Properties of Alcohols
    The hydrogen-bonding capacity of an alcohol increases the boiling point and melting point relative to the analogous hydrocarbon. Thus, methanol  has a higher boiling point and a higher melting point than methane.

    Branching tends to decrease van der Waals' forces, thereby decreasing the boiling point. The boiling point of alcohols with identical molecular weights would be expected to increase in the order: 3o < 2o < 1o (i.e. primary alcohols have a higher b.p)

    • Alcohol molecules are polar; they possess a permanent molecular dipole moment.
    • As a result, all alcohols are soluble in polar solvents. Solubility is  decreased by increasing chain length and by branching, since both of these effects serve to lessen the dipole moment (and, thus, the
    • polarity) of alcohol.
  7. Properties of Alcohols
    The oxygen atom of an alcohol group can serve as a base and accept protons from stronger acids.

    The result is a protonated alcohol with a positive charge on the oxygen.This makes the alcohol group into a good leaving group--water (hydroxide) is a poor leaving group-- and makes the carbon more susceptible to nucleophilic attack.

    The hydrogen of an alcohol group can serve as an acid and donate protons to stronger bases.
  8. Acidity of Alcohols
    Branching decreases molecular polarity; it also decreases acidity.

    Alcohol acidity is in the order: 3o < 2o < 1o Increasing molecular weight tends to decrease alcohol acidity.

    Electron-withdrawing groups increase alcohol acidity.

    Electron-donating groups decrease alcohol acidiy.

    An electron-withdrawing group increase alcohol acidity by stabilizing the alkoxy anion (RO-) that results from the acidic deprotonation. An electron-donating group destabilizes this anion.
  9. Dehydration Reactions: E1 and E2
    Alcohol may lose water to form the corresponding alkene via a dehydration reaction:

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