Organic Chemistry 6th Edition - York College of …faculty.ycp.edu/~khalliga/Courses/CHM 234/Fall 2010/Lecture notes...The ground-state electronic configuration describes the orbitals

© 2011 Pearson Education, Inc. 1

Organic Chemistry6th Edition

Paula Yurkanis Bruice

Chapter 1

Electronic Structure and

Bonding

Acids and Bases


• Carbon-containing compounds were once considered “organ compounds” available only from living organisms.

• The synthesis of the simple organic compound urea in 1828 showed that organic compounds can be prepared in the laboratory from non-living material.

• Today, organic natural products are routinely synthesized in the laboratory.

Organic Chemistry


• Carbon neither gives up nor accepts electrons because it is in the center of the second periodic row.

• Consequently, carbon forms bonds with other carbons and other atoms by sharing electrons.

• The capacity of carbon to form bonds in this fashion makes it the building block of all living organisms.

Why Carbon?


Why Study Organic Chemistry?

• Since carbon is the building block of all living organisms, a knowledge of Organic Chemistry is a prerequisite to understanding Biochemistry, Medicinal Chemistry, and Pharmacology.

• Indeed, Organic Chemistry is a required course for studying Pharmacy, Medicine, and Dentistry.

• Admission into these professional programs is highly dependent on your performance in Organic Chemistry.

© 2011 Pearson Education, Inc.5

Examples of Organic Compounds Used as Drugs

Methotrexate, Anticancer Drug 5-Fluorouracil, Colon Cancer Drug

Tamiflu, Influenza DrugAZT, HIV Drug


Examples of Organic Compounds Used as Drugs

Haldol, AntipsychoticElavil, Antidepressant

Prozac, Antidepressant Viagra, TreatsErectile Dysfunction


The Structure of an Atom

• An atom consists of electrons, positively charged protons,and neutral neutrons.

• Electrons form chemical bonds.

• Atomic number: numbers of protons in its nucleus

• Mass number: the sum of the protons and neutrons of an atom

• Isotopes have the same atomic number but different mass numbers.

• The atomic weight: the average weighted mass of its atoms

• Molecular weight: the sum of the atomic weights of all the atomsin the molecule


The Distribution of Electrons in an Atom

• Quantum mechanics uses the mathematical equation of wavemotions to characterize the motion of an electron around a nucleus.

• Wave functions or orbitals tell us the energy of the electron and the volume of space around the nucleus where an electron ismost likely to be found.

• The atomic orbital closer to the nucleus has the lowest energy.

• Degenerate orbitals have the same energy.



The ground-state electronic configuration describes the orbitalsoccupied by the atom’s electrons with the lowest energy


• The Aufbau principle: an electron always goes to theavailable orbital with the lowest energy

• The Pauli exclusion principle: only two electrons can occupy one atomic orbital and the two electrons have opposite spin

• Hund’s rule: electrons will occupy empty degenerated orbitals before pairing up in the same orbital

The following principles determine which orbitalselectrons occupy:


Lewis’s theory: an atom will give up, accept, or share electrons inorder to achieve a filled outer shell or an outer shell that containseight electrons


Ionic Bonds Are Formed by the Transfer of Electrons

Attractive forces between opposite charges are called electrostatic attractions


Covalent Bonds Are Formed by Sharing Electrons



• Equal sharing of electrons: nonpolar covalent bond (e.g., H2)

• Sharing of electrons between atoms of different electronegativities: polar covalent bond (e.g., HF)


A polar covalent bond has a slight positive charge on one end and a slight negative charge on the other



A Polar Bond Has a Dipole Moment• A polar bond has a negative end and a positive end

dipole moment (D) = µ = e x d(e) : magnitude of the charge on the atom

(d) : distance between the two charges


Electrostatic Potential Maps


Lewis Structure

Formal charge = number of valence electrons –(number of lone pair electrons +1/2 number of bonding electrons)


Nitrogen has five valence electrons

Carbon has four valence electrons

Hydrogen has one valence electron and halogen hasseven


Important Bond NumbersNeutral

Cationic

Anionic


Non-Octet Species

Sulfuric Acid Periodic Acid Phosphoric Acid

• In the 3rd and 4th rows, expansion beyond the octet to 10 and 12 electrons is possible.

• Reactive species without an octet such as radicals, carbocations, carbenes, and electropositive atoms (boron, beryllium).

Nitric Oxide Radical,

Mammalian Signaling Agent

Radical Carbocation Carbene Borane



The s Orbitals

An orbital tells us the volume of space around the nucleuswhere an electron is most likely to be found


The p Orbitals


Molecular Orbitals

• Molecular orbitals belong to the whole molecule.

• σ bond: formed by overlapping of two s orbitals.

• Bond strength/bond dissociation: energy required to break a bond or energy released to form a bond.



In-phase overlap forms a bonding MO; out-of-phase overlap forms an antibonding MO:


Sigma bond (σ) is formed by end-on overlap of two p orbitals:

A σ bond is stronger than a π bond


Pi bond (π) is formed by sideways overlap of two parallel p orbitals:


Bonding in Methane


Hybridization of One s and Three p Orbitals


The orbitals used in bond formation determine the bond angles

• Tetrahedral bond angle: 109.5°

• Electron pairs spread themselves into space as far from each other as possible


The Bonds in Ethane


Hybrid Orbitals of Ethane


Bonding in Ethene: A Double Bond


Bonding in Ethyne: A Triple Bond


Bonding in the Methyl Cation


Bonding in the Methyl Radical


Bonding in the Methyl Anion


Bonding in Water


Bonding in Ammonia and in the Ammonium Ion


Bonding in Hydrogen Halides


Summary

• The shorter the bond, the stronger it is

• The greater the electron density in the region of orbital overlap, the stronger is the bond

• The more s character, the shorter and stronger is the bond

• The more s character, the larger is the bond angle


The vector sum of the magnitude and the direction of the individual bond dipole determines the overall dipole moment of a molecule

Molecular Dipole Moment


Brønsted–Lowry Acids and Bases• Acid donates a proton

• Base accepts a proton

• Strong reacts to give weak• The weaker the base, the stronger is its conjugate acid• Stable bases are weak bases


An Acid/Base Equilibrium

Ka: The acid dissociation constant.

The stronger the acid, the larger its Kavalue and the smaller its pKa value.

Ka =[H3O

+ ][ A− ][H2O][ AH ]

LogKa = pKa


The Most Common Organic Acids Are Carboxylic Acids



Protonated alcohols and protonated carboxylic acids arevery strong acids


An amine can behave as an acid or as a base



Strong Acids / Bases React to Form Weak Acids / Bases


The Structure of an Acid Affects Its Acidity

• The weaker the base, the stronger is its conjugateacid

• Stable bases are weak bases

• The more stable the base, the stronger is its conjugateacid


The stability of a base is affected by its size and its electronegativity



• When atoms are very different in size, the stronger acid will have its proton attached to the largest atom

size overrides electronegativity


• When atoms are similar in size, the stronger acid will have its proton attached to the more electronegative atom


Substituents Affect the Strength of an Acid


• Inductive electron withdrawal increases the acidity of a conjugate acid


Acetic acid is more acidic than ethanol

The delocalized electrons in acetic acid are shared by more than two atoms, thereby stabilizing the conjugated base


A Summary of the Factors That Determine Acid Strength

1. Size: As the atom attached to the hydrogen increases in size, the strength of the acid increases

2. Electronegativity


3. Hybridization

4. Inductive effect


5. Electron delocalization


• Lewis acid: non-proton-donating acid; will accept two electrons

• Lewis base: electron pair donors

Lewis Acids and Bases


Basicity and Drug Design: Methotrexate, Substituting Nitrogen for Oxygen


Acidity and Cosmetics: Skin Peel Agents

• Skin Peels: Remove old skin with an acid to expose new young-looking skin.

• The stronger the acid, the deeper the peel.• Examples of skin peel agents:

Deep Peel Agent:Mild Peel Agents:

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Organic Chemistry 6th Edition - York College of …faculty.ycp.edu/~khalliga/Courses/CHM 234/Fall 2010/Lecture notes...The ground-state electronic configuration describes the orbitals