Organic Chemistry

M. R. Naimi-Jamal

Faculty of Chemistry

Iran University of Science & Technology

Polar Covalent BondsAcids and Bases

Chapter 1. Continue

Based on: McMurry’s Fundamental of Organic Chemistry, 4th edition, Chapter 1

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Polar Covalent Bonds: Electronegativity

Covalent bonds can have ionic character These are polar covalent bonds

Bonding electrons attracted more strongly by one atom than by the other

Electron distribution between atoms in not symmetrical

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Bond Polarity and Electronegativity

Electronegativity (EN): intrinsic ability of an atom to attract the shared electrons in a covalent bond

Differences in EN produce bond polarity

Arbitrary scale. As shown in next figure, electronegativities are based on an arbitrary scale

F is most electronegative (EN = 4.0), Cs is least (EN = 0.7)

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The Periodic Table and Electronegativity

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Bond Polarity and Electronegativity

Metals on left side of periodic table attract electrons weakly: lower electronegativities

Halogens and other reactive nonmetals on right side of periodic table attract electrons strongly: higher electronegativities

Electronegativity of C = 2.5

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Bond Polarity and Inductive Effect

Nonpolar Covalent Bonds: atoms with similar electronegativities

Polar Covalent Bonds: Difference in EN of atoms < 2

Ionic Bonds: Difference in electronegativities > 2 (approximately).

Other factors (solvation, lattice energy, etc) are important in ionic character.

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Bond Polarity and Inductive Effect

Bonding electrons are pulled toward the more electronegative atom in the bond

Electropositive atom acquires partial positive charge, +

Electronegative atom acquires partial negative charge, -

Inductive effect: shifting of electrons in a bond in response to the electronegativities of nearby atoms

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Electrostatic Potential Maps Electrostatic potential maps show calculated

charge distributions Colors indicate electron-rich (red) and electron-

poor (blue) regions

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Polar Covalent Bonds: Dipole Moments

Molecules as a whole are often polar, from vector summation of individual bond polarities and lone-pair contributions

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Polar Covalent Bonds: Dipole Moments

Dipole moment - Net molecular polarity, due to difference in summed charges

- magnitude of charge Q at end of molecular dipole times distance r between charges

= Q r, in debyes (D)

1 D = 3.34 1030 coulomb meter

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Dipole Moments in Water and Ammonia Large dipole moments

Electronegativities of O and N > H Both O and N have lone-pair electrons oriented away from

all nuclei

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Practice: Suggest an explanation

Ammonia (NH3) has a dipole moment of

1.46 D, while the dipole moment of NF3 is

only 0.24 D. Why?

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Absence of Dipole Moments In symmetrical molecules, the dipole moments of

each bond has one in the opposite direction The effects of the local dipoles cancel each other

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Cis- & trans-1,2-dichloroethylenes:

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Formal Charges

Sometimes it is necessary to have structures with formal charges on individual atoms

We compare the bonding of the atom in the molecule to the valence electron structure

If the atom has one more electron in the molecule, it is shown with a “-” charge

If the atom has one less electron, it is shown with a “+” charge

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Formal Charges

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Nitromethane:

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Formal Charges

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Resonance

Some molecules have structures that cannot be shown with a single Lewis representation

In these cases we draw Lewis structures that contribute to the final structure but which differ in the position of the bond(s) or lone pair(s)

Such a structure is delocalized and is represented by resonance forms

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Resonance

The resonance forms are connected by a double-headed arrow

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Resonance Hybrids

A structure with resonance forms does not alternate between the forms

Instead, it is a hybrid of the two resonance forms, so the structure is called a resonance hybrid

For example, benzene (C6H6) has two resonance

forms with alternating double and single bonds In the resonance hybrid, the actual structure,

all of the C-C bonds are equivalent, midway between double and single bonds

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Resonance Hybrids

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Resonance Hybrids

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Rules for Resonance Forms

Individual resonance forms are imaginary - the real structure is a hybrid (only by knowing the contributors can you visualize the actual structure)

Resonance forms differ only in the placement of their or nonbonding electrons

Different resonance forms of a substance don’t have to be equivalent

Resonance forms must be valid Lewis structures: the octet rule usually applies

The resonance hybrid is more stable than any individual resonance form would be

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Curved Arrows and Resonance Forms

We can imagine that electrons move in pairs to convert from one resonance form to another

A curved arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow

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Curved Arrows and Resonance Forms

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Drawing Resonance Forms

Any three-atom grouping with a multiple bond has two resonance forms

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Different Atoms in Resonance Forms

Sometimes resonance forms involve different atom types as well as locations

The resulting resonance hybrid has properties associated with both types of contributors

The types may contribute unequally

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Resonance in the acetone enolate

The “enolate” derived from acetone is a good illustration, with delocalization between carbon and oxygen.

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2,4-Pentanedione

The anion derived from 2,4-pentanedione Lone pair of electrons and a formal negative

charge on the central carbon atom, next to a C=O bond on the left and on the right

Three resonance structures result

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Practice: Draw three resonance forms:

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Solution:

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Practice: Draw three resonance forms:

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Solution:

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Solution:

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Acids and Bases: The Brønsted–Lowry Definition

Brønsted–Lowry theory defines acids and bases by their role in reactions that transfer protons (H+) between donors and acceptors.

“proton” is a synonym for H+ - loss of an electron from H leaving the bare nucleus - a proton. Protons are always covalently bonded to another atom.

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Brønsted Acids and Bases

“Brønsted-Lowry” is usually shortened to “Brønsted”

A Brønsted acid is a substance that donates a hydrogen ion, or “proton” (H+): a proton donor

A Brønsted base is a substance that accepts the H+: a proton acceptor

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The Reaction of HCl with H2O

When HCl gas dissolves in water, a Brønsted acid–base reaction occurs

HCl donates a proton to water molecule, yielding hydronium ion (H3O+) and Cl

The reverse is also a Brønsted acid–base reaction of the conjugate acid and conjugate base

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The Reaction of HCl with H2O

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Quantitative Measures of Acid Strength

The equilibrium constant (Keq) for the reaction of an acid (HA) with water to form hydronium ion and the conjugate base (A-) is a measure related to the strength of the acid

Stronger acids have larger Keq

Note that brackets [ ] indicate concentration, moles per liter, M.

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Ka – the Acidity Constant

The concentration of water as a solvent does not change significantly when it is protonated in dilute solution.

The acidity constant, Ka for HA equals Keq times 55.6 M

(leaving [water] out of the expression)

Ka ranges from 1015 for the strongest acids to very small

values (10-60) for the weakest

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Ka – the Acidity Constant

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Acid and Base Strength

The ability of a Brønsted acid to donate a proton to is sometimes referred to as the strength of the acid.

The strength of the acid can only be measured with respect to the Brønsted base that receives the proton

Water is used as a common base for the purpose of creating a scale of Brønsted acid strength

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pKa – the Acid Strength Scale

pKa = -log Ka (in the same way that

pH = -log [H+]

The free energy in an equilibrium is related to –log of Keq (G = -RT log Keq)

A larger value of pKa indicates a stronger acid and is proportional to the energy difference between products and reactants

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pKa – the Acid Strength Scale

The pKa of water is 15.74

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pKa values for some acids:

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Predicting Acid–Base Reactions frompKa Values

pKa values are related as logarithms to equilibrium constants

The difference in two pKa values is the log of the ratio of equilibrium constants, and can be used to calculate the extent of transfer

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Predicting Acid–Base Reactionsfrom pKa Values

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Predicting Acid–Base Reactions from pKa Values

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Organic Acids and Organic Bases

The reaction patterns of organic compounds often are acid-base combinations

The transfer of a proton from a strong Brønsted acid to a Brønsted base, for example, is a very fast process and will always occur along with other reactions

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Organic Acids

Those that lose a proton from O–H, such as methanol (CH3OH) and acetic acid

(CH3COOH)

Those that lose a proton from C–H, usually from a carbon atom next to a C=O double bond (O=C– C– H)

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Organic Acids

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Organic Acids:

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Carboxylic Acids:

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Conjugate Bases:

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Organic Bases

Have an atom with a lone pair of electrons that can bond to H+

Nitrogen-containing compounds derived from ammonia are the most common organic bases

Oxygen-containing compounds can react as bases when with a strong acid or as acids with strong bases

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Organic Bases

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Acids and Bases: The Lewis Definition

Lewis acids are electron pair acceptors;Lewis bases are electron pair donors

The Lewis definition leads to a general description of many reaction patterns but there is no quantitatve scale of strengths as in the Brønsted definition of pKa

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Lewis Acids and the Curved Arrow Formalism

The Lewis definition of acidity includes metal cations, such as Mg2+

They accept a pair of electrons when they form a bond to a base

Group 3A elements, such as BF3 and AlCl3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases

Transition-metal compounds, such as TiCl4, FeCl3, ZnCl2, and SnCl4, are Lewis acids

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Lewis Acids and the Curved Arrow Formalism

Organic compounds that undergo addition reactions with Lewis bases (discussed later) are called electrophiles and therefore Lewis Acids

The combination of a Lewis acid and a Lewis base can shown with a curved arrow from base to acid

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Illustration of Curved Arrows in Following Lewis Acid-Base Reactions

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Some Lewis Acids:

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Lewis Bases

Lewis bases can accept protons as well as other Lewis acids, therefore the definition encompasses that for Brønsted bases

Most oxygen- and nitrogen-containing organic compounds are Lewis bases because they have lone pairs of electrons

Some compounds can act as either acids or bases, depending on the reaction

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Some Lewis Bases

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Lewis Acid-Base Reactions

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Imidazole:

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Drawing Chemical Structures

Condensed structures: C-H and C-C and single bonds aren't shown but understood If C has 3 H’s bonded to it, write CH3

If C has 2 H’s bonded to it, write CH2; and so on.

Horizontal bonds between carbons aren't shown in condensed structures - the CH3, CH2, and CH units

are assumed to be connected horizontally by single bonds, but vertical bonds are added for clarity

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2-methylbutane Structures

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Skeletal Structures

Minimum amount of visual information but unambiguous

C’s not shown, but assumed to be at each intersection of two lines (bonds) and at end of each line

H’s bonded to C’s aren't shown – whatever number is needed will be there to fill out the four bonds to each carbon.

All atoms other than C and H are shown

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Practice: How many H’s on each carbon?

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Solution:

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Problem: How many H’s on each carbon?

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Molecular Models

We often need to visualize the shape or connections of a molecule in three dimensions

Molecular models are three dimensional objects, on a human scale, that represent the aspects of interest of the molecule’s structure (computer models also are possible)

Drawings on paper and screens are limited in what they can present to you

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Molecular Models

Framework models (ball-and-stick) are essential for seeing the relationships within and between molecules – you should own a set

Space-filling models are better for examining the crowding within a molecule

Space-filling

Framework

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Summary Organic molecules often have polar covalent

bonds as a result of unsymmetrical electron sharing caused by differences in the electronegativity of atoms

The polarity of a molecule is measured by its dipole moment, .

(+) and () indicate formal charges on atoms in molecules to keep track of valence electrons around an atom

Some substances must be shown as a resonance hybrid of two or more resonance forms that differ by the location of electrons.

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Summary

A Brønsted(–Lowry) acid donates a proton

A Brønsted(–Lowry) base accepts a proton

The strength Brønsted acid is related to the -1 times the logarithm of the acidity constant, pKa. Weaker acids have higher pKa’s

A Lewis acid has an empty orbital that can accept an electron pair

A Lewis base can donate an unshared electron pair

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Summary

In condensed structures C-C and C-H are implied

Skeletal structures show bonds and not C or H (C is shown as a junction of two lines) – other atoms are shown

Molecular models are useful for representing structures for study

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Chapter 1-2, Questions

42, 43, 44, 46, 47, 50,

54, 55, 57