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Chapter 4: Acids and Bases. Arrhenius Acids and Bases. In 1884, Svante Arrhenius proposed these definitions acid: a substance that produces H 3 O + ions in aqueous solution base: a substance that produces OH - ions in aqueous solution. Brønsted -Lowry Acids & Bases.

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arrhenius acids and bases
Arrhenius Acids and Bases
  • In 1884, Svante Arrhenius proposed these definitions
    • acid: a substance that produces H3O+ ions in aqueous solution
    • base: a substance that produces OH- ions in aqueous solution

Brønsted-Lowry Acids & Bases

  • Acid: a proton donor
  • Base: a proton acceptor
  • Brønsted-Lowry definitions do not require water as a reactant
br nsted lowry acids bases
Brønsted-Lowry Acids & Bases
  • we can use curved arrows to show the transfer of a proton from acetic acid to ammonia
conjugate acids bases
Conjugate Acids & Bases
  • conjugate base:the species formed from an acid when it donates a proton to a base
  • conjugate acid: the species formed from a base when it accepts a proton from an acid

A weak acid has always a strong conjugated base !!!

br nsted lowry acids bases6
Brønsted-Lowry Acids & Bases
  • Note the following about the conjugate acid-base pairs in the table:

1. an acid can be positively charged, neutral, or negativelycharged; examples of each type are H3O+, H2CO3, and H2PO4-

2. a base can be negatively charged or neutral; examples are OH-, Cl-, and NH3

3. acids are classified a monoprotic, diprotic, or triprotic depending on the number of protons each may give up; examples are HCl, H2CO3, and H3PO4

4. several molecules and ions appear in both the acid and conjugate base columns; that is, each can function as either an acid or a base

5. there is an inverse relationship between the strength of an acid and the strength of its conjugate base

    • the stronger the acid, the weaker its conjugate base
    • HI, for example, is the strongest acid and its conjugate base, I-, is the weakest base in the table
    • CH3COOH (acetic acid) is a stronger acid that H2CO3 (carbonic acid); conversely, CH3COO- (acetate ion) is a weaker base that HCO3- (bicarbonate ion)
acid and base strength
Acid and Base Strength
  • Strong acid: one that reacts completely or almost completely with water to form H3O+ ions
  • Strong base: one that reacts completely or almost completely with water to form OH- ions
  • Weak acid: a substance that dissociates only partially in water to produce H3O+ ions
  • Weak base: a substance that dissociates only partially in water to produce OH- ions
acid base reactions
Acid-Base Reactions
  • acetic acid is incompletely ionized in aqueous solution
  • the equation for the ionization of a weak acid, HA, is

pKa

acid dissociation constant

(pKa = -log10Ka)

acid base equilibrium
Acid-Base Equilibrium
  • Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and the weaker base
  • equilibrium lies on the side of the weaker acid and the weaker base
structure and acidity
Structure and Acidity
  • The most important factor in determining the relative acidity of an organic acid is the relative stability of the anion, A-, formed when the acid, HA, transfers a proton to a base
  • We consider three factors:
    • the electronegativity of the atom bonded to H in HA
    • resonance stabilization of A-
    • the inductive effect
structure and acidity14
Structure and Acidity
  • Electronegativity of the atom bearing the negative charge; within a period
    • the greater the electronegativity of the atom bearing the negative charge, the more strongly its electrons are held, the more stable the anion A-
    • and the greater the acidity of the acid HA
structure and acidity15
Structure and Acidity
  • Resonance delocalization of charge in A-
    • compare the acidity of an alcohol and a carboxylic acid
    • carboxylic acids are weak acids; values of pKa for most unsubstituted carboxylic acids fall within the range of 4 to 5
    • alcohols are very weak acids; values of pKa for most alcohols fall within the range of 15 to 18
structure and acidity16
Structure and Acidity
  • the greater the resonance stabilization of the anion, the more acidic the compound
  • there is no resonance stabilization in an alkoxide anion
  • we can write two equivalent contributing structures for the carboxylate anion; the negative charge is spread evenly over the two oxygen atoms
structure and acidity17
Structure and Acidity
  • The inductive effect
    • the polarization of electron density transmitted through covalent bonds by a nearby atom of higher electronegativity
lewis acids and bases
Lewis Acids and Bases
  • Lewis acid:any molecule or ion that can form a new covalent bond by accepting a pair of electrons
  • Lewis base:any molecule or ion that can form a new covalent bond by donating a pair of electrons
lewis acids and bases20
Lewis Acids and Bases
  • some organic Lewis bases and their relative strengths in proton-transfer reactions
lewis acids and bases21
Lewis Acids and Bases
  • Another type of Lewis acid we will encounter in later chapters is an organic cation in which a carbon is bonded to only three atoms and bears a positive formal charge
    • such carbon cations are called carbocations
slide25
Stereoisomers

Conformation of acyclic compounds:

slide26
Stereoisomers

Conformation of cyclic compounds:

slide27
Stereoisomers

Conformation of cyclic compounds:

slide28
Stereoisomers

Conformation of cyclic substituted compounds:

slide29
Stereoisomers

Conformation of cyclic substituted compounds:

geometric isomers
Geometric Isomers

Cis/Trans isomers in compounds with double bonds

slide31
Stereoisomers
  • Enantiomers:nonsuperposable mirror images
    • as an example of a molecule that exists as a pair of enantiomers, consider 2-butanol

Optical Isomers:

chiral center

chiral center

enantiomers
Enantiomers
  • one way to see that the mirror image of 2-butanol is not superposable on the original is to rotate the mirror image
enantiomers33
Enantiomers
  • now try to fit one molecule on top of the other so that all groups and bonds match exactly
  • the original and mirror image are not superposable
  • they are different molecules with different properties
  • they are enantiomers (nonsuperposable mirror images)
enantiomers34
Enantiomers
  • Objects that are not superposable on their mirror images are chiral (from the Greek: cheir, hand)
    • they show handedness
  • The most common cause of enantiomerism in organic molecules is the presence of a carbon with four different groups bonded to it
    • a carbon with four different groups bonded to it is called a chiral center
  • Enantiomers are optical active compounds.
optical activity
Optical Activity
  • Ordinary light: light waves vibrating in all planes perpendicular to its direction of propagation
  • Plane-polarized light: light waves vibrating only in parallel planes
  • Polarimeter: an instrument for measuring the ability of a compound to rotate the plane of plane-polarized light
  • Optically active: showing that a compound rotates the plane of plane-polarized light

Schematic diagram of a polarimeter

optical activity36
Optical Activity
  • Dextrorotatory (+): clockwise rotation of the plane of plane-polarized light
  • Levorotatory (-): counterclockwise rotation of the plane of plane-polarized light
  • Specific rotation: the observed rotation of an optically active substance at a concentration of 1 g/100 mL in a sample tube 10 cm long; for a pure liquid, concentration is in g/mL (density)
configuration of enantiomers
Configuration of Enantiomers

R,S Convention -> Arrangement of groups around a chiral atom

isomers with several chiral centers
Isomers with several chiral centers
  • For a molecule with 1 stereocenter, 21 = 2 stereoisomers are possible
  • For a molecule with 2 stereocenters, a maximum of 22 = 4 stereoisomers are possible
  • For a molecule with nstereocenters, a maximum of 2nstereoisomers are possible
isomers with several chiral centers41
Isomers with several chiral centers
  • 2,3,4-Trihydroxybutanal
    • two stereocenters; 22 = 4 stereoisomers are possible
meso compounds
Meso Compounds
  • Meso compound: an achiral compound possessing two or more stereocenters
    • tartaric acid
    • two stereocenters; 2n = 4, but only three stereoisomers exist
chirality in the biological world
Chirality in the Biological World
  • Except for inorganic salts and a few low-molecular-weight organic substances, the molecules in living systems, both plant and animal, are chiral
    • although these molecules can exist as a number of stereoisomers, almost invariably only one stereoisomer is found in nature
    • instances do occur in which more than one stereoisomer is found, but these rarely exist together in the same biological system
  • It’s a chiral world!
chirality in biomolecules
Chirality in Biomolecules
  • Enzymes (protein bio-catalysts) all have many stereocenters
    • an example is chymotrypsin, an enzyme in the intestines of animals that catalyzes the digestion of proteins
    • chymotrypsin has 251 stereocenters
    • the maximum number of stereoisomers possible is 2251!
    • only one of these stereoisomers is produced and used by any given organism
    • because enzymes are chiral substances, most either produce or react with only substances that match their stereochemical requirements
chirality in the biological world47
Chirality in the Biological World
  • Schematic diagram of the surface of an enzyme capable of distinguishing between enantiomers
chirality in biomolecules48
Chirality in Biomolecules
  • because interactions between molecules in living systems take place in a chiral environment, a molecule and its enantiomer or one of its diastereomers elicit different physiological responses
  • as we have seen, (S)-ibuprofen is active as a pain and fever reliever, whereas its R enantiomer is inactive
  • the S enantiomer of naproxen is the active pain reliever, whereas its R enantiomer is a liver toxin!
slide49
Racemic mixture:an equimolar mixture of two enantiomers
    • because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific activity is zero
  • Resolution:the separation of a racemic mixture into its enantiomers
fischer projection
Fischer projection
  • Fischer projection: a two dimensional representation for showing the configuration of a tetrahedral stereocenter.
  • It is used mainly for sugars (carbohydrates) and amino acids.
    • horizontal lines represent bonds projecting forward
    • vertical lines represent bonds projecting to the rear
    • the first and last carbons in the chain are written in full; others are indicated by the crossing of bonds

Carbon with highest oxidation state

d and l monosaccharides
D- and L-Monosaccharides
  • In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde

(R) - Glyceraldehyde

(S) - Glyceraldehyde

compounds with more chiral centers monosaccharides
Compounds with more chiral centers -Monosaccharides
  • According to the conventions proposed by Fischer
    • D-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the right
    • L-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the left
cyclic structure hemiacetal form of sugars
Cyclic Structure – Hemiacetal Form of Sugars
  • Monosaccharides have hydroxyl and carbonyl groups in the same molecule and exist almost entirely as five- and six-membered cyclic hemiacetals
    • anomeric carbon: the hemiacetal carbon of a cyclic form of a monosaccharide
    • anomers:monosaccharides that differ in configuration only at their anomeric carbons
haworth projections
Haworth Projections
  • the anomers of D-glucopyranose
fischer projection of amino acids
Fischer projection of Amino Acids
  • With the exception of glycine, all protein-derived amino acids have at least one stereocenter (the α-carbon) and are chiral
    • the vast majority have L-configuration (natural occurring)
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