The Scope of Organic Chemistry: An Overview Functional groups determine the reactivity of organic molecules Alkanes – No functional groups, only carbon and hydrogen. (Chapter 2) Alkane Reactions – Alkane bond strengths and reactions. (Chapter 3) Cyclic Alkanes – New properties and changes in reactivity (Chapter 4) Stereoisomerism – Same connectivity – different relative positioning of substituents in space (Chapter 5) Haloalkanes – Substitution Reactions and Elimination Reactions (Chapters 6 and 7)

CHAPTER 1 Structure and Bonding in Organic Molecules please read this chapter and try to get the idea of covalent bonds

The Scope of Organic Chemistry: An Overview 1-1 Synthesis is the making of new molecules Wöhler’s Synthesis of Urea: Synthesis – Construct complex organic chemicals from simpler, more readily available ones (Chapter 8). Reactions are the vocabulary, and mechanisms are the grammar of organic chemistry Reactants (Substrates) – Starting compounds Products Reaction Mechanism – Underlying details of a reaction Reaction Intermediate – Chemical species formed and then destroyed on the pathway between reactants and products.

Ionic and Covalent Bonds: The Octet Rule1-3 1.Covalent Bonds are based on the sharing of electrons. If the electrons are not shared equally, a polar covalent (partially ionic) bond is formed, otherwise a pure covalent bond is formed. 2.Ionic Bonds are based on the transfer of one or more electrons from one atom to another. The resulting cation and anion are electrostatically attracted to each other.

Ionic and Covalent Bonds: The Octet Rule1-3 The periodic table underlies the octet rule. Electrons in atoms occupy levels or shells of fixed capacity. The first has room for 2, the second 8, and the third 16. Noble gases have 8 valence electrons (Helium 2) and are particularly stable. Other elements lack octets in their outer electron shells and tend to form molecules in such a way as to create a stable octet arrangement.

In these and similar cases, covalent bonding occurs. Atoms share electrons to achieve a noble gas configuration. In certain cases, one atoms supplies both of the electrons in the bond: Often 4 electron (double) and 6 electron (triple) bonds are formed:

In most organic bonds, the electrons are not shared equally: polar covalent bonds. Pure covalent bonds (perfect sharing of electrons) and ionic bonds (complete transfer of electrons) are two extreme types of bonding. Most bonds lie somewhere between these extremes and are called polar covalent bonds. Each element can be assigned an electronegativity value which represents its electron accepting ability when participating in a chemical bond. The larger the difference in electronegativety between two atoms participating in a chemical bond, the more ionic is the bond. Bonds between atoms of different electronegativity are said to be polar bonds. A partial negative charge is found on the atom of higher electronegativity and an equal but positive charge on the other atom.

As a rule of thumb, electronegativity differences less than 0.3 represent pure covalent bonds, from 0.3 to 2.0 polar covalent bonds, and greater than 2.0 ionic bonds. The separation of opposite charges in polar covalent molecules results in the formation of dipoles:

Electron-Dot Model of Bonding: Lewis Structures1-4 Lewis structures are drawn by following simple rules. 1.Draw the molecular skeleton 2.Count the number of available valence electrons Add one electron for each negative charge, if an anion. Subtract one electron for each positive charge, if a cation. 3.Depict all covalent bonds by two shared electrons, giving as many atoms as possible a surrounding electron octet, except for H, which requires a duet. Elements at the right of the periodic table (non-metals) may contain lone pairs of electrons. Correct Lewis Structure Incorrect Lewis Structures

It is often necessary to use double or triple bonds to satisfy the octet rule:

4.Assign charges to atoms in the molecule. Charge = (# valence electrons in free, neutral atom) – - (# unshared electrons on the atom) – ½(# bonding electrons surrounding the atom) In molecules such as nitric acid, charges occur on individual atoms, even though the molecule itself is neutral.

Covalent bonds can be depicted by straight lines. Bonding pairs of electrons are most often represented as straight lines: single bonds as a single line, double bonds as two parallel lines, and triple bonds as three parallel lines. Lone pairs of electrons are either shown as dots or are omitted. Structures of this type are called Kekulé structures.

Resonance Forms1-5 The carbonate ion has several correct Lewis structures. Three equivalent structures must be drawn to accurately represent the carbonate ion. The only difference between these structures is the placement of electrons.

But what is its true structure? The “true” structure can be thought of as the average of all three structures which is called a resonance hybrid. The 2 negative charges are delocalized over all three oxygen atoms.

Not all resonance forms are equal. 1.Structures with a maximum of octets are most important. 2.Charges should be preferentially located on atoms with compatible electronegativity. If this conflicts with rule 1, then rule 1 takes precedence. 3.Structures with less separation of opposite charges are more important resonance contributors than those with more charge separation.

The overlap of atomic orbitals gives rise to sigma and pi bonds. When n atomic orbitals overlap, n new molecular orbitals are formed. When n is 2, one bonding orbital and one antibonding molecular orbital are formed. The energy lowering of the bonding orbital and energy raising of the antibonding molecular orbital with respect to the atomic orbitals is called the energy splitting. The energy splitting indicates the strength of the bond formed. Atomic orbitals of the same size and energy overlap to form the strongest bonds. Geometrical factors also affect the degree of overlap. Orbitals exhibiting directionality in space (p orbitals) can overlap to form sigma () bonds or pi ()bonds. All carbon-carbon single bonds contain one sigma bond. Double and triple bonds contain extra pi interactions.

Hybrid Orbitals: Bonding in Complex Molecules1-8 Mixing of atomic orbitals from the same atom results in new atomic orbitals of different energy and directionality. sp Hybrids produce linear structures. An incorrect structure for BeH 2 is predicted if 2s and 2p orbitals of Be are overlapped with the 1s orbitals of H:

sp 2 Hybrids create trigonal structures. Hybridization of a 2s and two 2p orbitals results in three new hybrid orbitals that point to the corners of an equilateral triangle. The remaining p orbital points up and down, perpendicular to each of the three hybrid orbitals. Bond angles in molecules using sp 2 hybridization are approximately 120 o The molecule, BH 3 is isoelectric with the methyl cation, CH 3 +. Both involve sp 2 hybridization about the central atom.

sp 3 Hybridizaton explains the shape of tetrahedral carbon compounds. When the 2s and all three 2p orbitals are hybridized, four hybrid orbitals called sp 3 orbitals are formed. These orbitals point to the corners of a regular tetrahedron. Bond angles in molecules using sp 3 hybridization are approximately o

Pi bonds are present in ethene (ethylene) and ethyne (acetylene). Molecules containing double or triple bonds contain unhybridized p orbitals that overlap lengthwise rather than end on.

Structures and Formulas of Organic Molecules1-9 To establish the identity of a molecule, we determine its structure. The empirical formula of a substance specifies the kinds and ratios of elements present in the substance. The empirical formula can be from an elemental analysis the substance. More that one substance can have the same empirical formula. Each of these substances will have its own set of unique physical and chemical properties, however. Substances having the same empirical formula but different connectivity of atoms are called constitutional or structural isomers.

A chemist may be able to identify an unknown substance if its properties match those of a substance already determined. New substances require other methods of identification such as x-ray crystallography, or various forms of spectroscopy. Two ways of representing the structures of know molecules are ball and stick models and space filling models.

Several types of drawings are used to represent molecular structures.

Tetrahedral carbon structures can be accurately represented in three dimensions using the dashed-wedged line notation.