Structural Formulas of OrganIc Compounds: Isomers Section 1.6 Structural Formulas of OrganIc Compounds: Isomers

Levels of Organic Structure We think about the structures of organic molecules at various levels The composition of a molecule is captured by its molecular formula The constitution (connectivity) of a molecule is captured by a structural formula showing how atoms are connected The configuration of a molecule is captured by a structural drawing showing the positions of atoms in space C4H10O Compounds can have identical structures on a broad level but differ on a more specific level; such compounds are called isomers.

Constitutional Isomers Constitutional isomers have the same composition (molecular formula) but differ in how their atoms are connected Ethanol and diethyl ether both have the molecular formula C2H6O, but they have different connectivity Constitutional isomers have different physical and chemical properties!

Condensed and Bond-line Formulas Organic chemists make use of structural shortcuts to make structure drawings easier to parse Condensed formulas are textual and list atoms in order of their connectivity Bond-line formulas omit labels for carbon atoms and omit hydrogens where they are implied (assume the octet rule is satisfied) becomes (CH3)OCH2CH3 Carbons are implied here becomes

Expanding a Bond-line Formula To expand a bond-line formula into a full Lewis structure, add in all implied carbons, hydrogens, and lone pairs Draw carbon atoms at each vertex and at the end of each line

Expanding a Bond-line Formula To expand a bond-line formula into a full Lewis structure, add in all implied carbons, hydrogens, and lone pairs Draw carbon atoms at each vertex and at the end of each line Add implicit hydrogens to the carbons such that the octet rule is satisfied (consistent with formal charges as well)

Expanding a Bond-line Formula To expand a bond-line formula into a full Lewis structure, add in all implied carbons, hydrogens, and lone pairs Draw carbon atoms at each vertex and at the end of each line Add implicit hydrogens to the carbons such that the octet rule is satisfied (consistent with formal charges as well) If missing, add implicit lone pairs on heteroatoms and/or carbons

Resonance and Curved Arrows Section 1.7 Resonance and Curved Arrows

Resonance The assumption of electron localization built into Lewis structures is not always true! Molecules containing delocalized electrons have multiple Lewis-structural representations called resonance structures For example, protonated formaldehyde has two important resonance forms The true resonance hybrid contains a partial double bond between C and O and partial positive charge on C and O. A B The compound does not convert between A and B; it is in reality a weighted sum of structures A and B!

Effects of Resonance A Lewis structure of ozone suggests that the two O–O bonds are not equivalent However, experiment confirms that they are equivalent—they have the same length and the outer oxygens have the same partial negative charge! Resonance structures reveal that the bond orders of both O–O bonds are between 1 and 2 and that both outer oxygens share negative charge The resonance hybrid Resonance represents the delocalization and stabilization of charge. Molecules with resonance are stabilized!

Rules of Resonance When can resonance be considered? Rule 1. The connectivity and positions of the atoms must remain the same in all resonance structures Rule 2. Each contributing structure must have the same number of electrons and the same net charge A and B are not resonance structures because they differ in connectivity. They are constitutional isomers. A, C, and D are resonance structures. All have a net charge of zero, although the formal charges of atoms differ between them.

Rules of Resonance When can resonance be considered? Rule 3. Each contributing structure must have the same number of unpaired electrons Rule 4. Atoms of second-row elements must not violate the octet rule F contains two unpaired electrons while A, C, and D contain none. F is not an important resonance form of A. G and H are valid resonance forms, but I is not because within it, nitrogen violates the octet rule.

Rules of Resonance Which resonance form is most important? Rule 5. Resonance structures containing more covalent bonds are typically more important Rule 6. Resonance structures with minimal separation of opposite charges are most important Structure J with a double bond is the major contributor and is more important than structure K with a single bond (see also Rule 6). Structure L lacks formal charges and is more important than structure M, which contains formal charges.

Rules of Resonance Which resonance form is most important? Rule 7. Resonance structures with negative charge on the most electronegative atom and positive charge on the most electropositive atom are most important In the major contributor N, the more electronegative oxygen atom bears the negative charge.

The Importance of Resonance Recognizing resonance and drawing valid resonance structures are important skills for at least two reasons: Resonance indicates that molecules are stabilized due to the delocalization of charge. Resonance forms containing formal charges can reveal hidden points of reactivity in organic molecules. Carbonate anion is fairly stable despite its overall charge of –2 due to delocalization of the charge over three oxygen atoms. Minor contributor K reveals that carbon in C=O can act as a Lewis acid!

Sulfur and Phosphorus-contaInIng OrganIc Compounds and the octet Rule Section 1.8 Sulfur and Phosphorus-contaInIng OrganIc Compounds and the octet Rule

Expanded Octets of Third-row Atoms Atoms of elements in the third row can bear more than eight electrons in covalent compounds due to hypervalency Compounds of phosphorus and sulfur are the most important examples of this phenomenon Important examples include PCl5, phosphates, sulfates, sulfonates, and SF4.

Examples of Expanded Octets Trimethylphosphine oxide has two important resonance forms; form G is more important (see Rule 1) Phosphates in molecules such as adenosine triphosphate (ATP) contain expanded octets at phosphorus

Section 1.9 Molecular GeometrIes

Valence Shell Electron Pair Repulsion VSEPR theory: In covalent compounds, valence electron pairs strive to be as far away from one another as possible to minimize electron-electron repulsion This effect is one basis of the geometries of molecules. For example, methane’s four bonding pairs point to the vertices of a tetrahedron We can distinguish between the electron-group arrangement of bonding pairs and lone pairs and the molecular geometry defined by the bonds and atoms only

Electron Groups Electron groups are collections of electrons situated close in space. Two types: Lone (unshared, non-bonding) electron pairs Bonding electrons – multiple bonds are treated as a single group The number of electron groups around an atom dictates the geometry at that atom Unshared electron pairs are considered “larger” and “more repulsive” than bonded pairs

A Survey of VSEPR Geometries

Molecular Dıpole Moments Section 1.10 Molecular Dıpole Moments

Applying Geometry and Polarization With knowledge of the geometry of a molecule and the polarization of its bonds, we can deduce the molecular dipole moment Individual bond dipoles are added in a vector sense to produce the overall molecular dipole vector Compounds containing polarized bonds may still be nonpolar overall!

Nonpolar and Polar Molecules Carbon tetrachloride (CCl4) is a nonpolar molecule. The four C–Cl bond dipoles add to a net molecular dipole of zero Methylene chloride (CH2Cl2) is a polar molecule. The C–H and C–Cl bond dipoles add to give a net molecular dipole

Identifying Molecular Dipoles Symmetric molecules and those with weakly polarized or unpolarized bonds are nonpolar; all others are polar The polarization of a molecule determines its chemical behavior (reactivity) and physical properties Structure Determines Properties!