Dr. Wolf's CHM 201 & Structural Formulas of Organic Molecules

Dr. Wolf's CHM 201 & ConstitutionConstitution The order in which the atoms of a molecule are connected is called its constitution or connectivity. The constitution of a molecule must be determined in order to write a Lewis structure.

Dr. Wolf's CHM 201 & Condensed structural formulas Lewis structures in which many (or all) covalent bonds and electron pairs are omitted. H O C C CHHHH HH: : H can be condensed to: CH 3 CHCH 3 OH (CH 3 ) 2 CHOH or

Dr. Wolf's CHM 201 & Bond-line formulas Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen. Atoms other than carbon and hydrogen are called heteroatoms. CH 3 CH 2 CH 2 CH 3 is shown as CH 3 CH 2 CH 2 CH 2 OH is shown as OH

Dr. Wolf's CHM 201 & Bond-line formulas Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen. Atoms other than carbon and hydrogen are called heteroatoms. HCl C C H2CH2CH2CH2C H2CH2CH2CH2C CH 2 H H is shown as Cl

Dr. Wolf's CHM 201 & Constitutional Isomers

Dr. Wolf's CHM 201 & Constitutional isomers Isomers are different compounds that have the same molecular formula. Constitutional isomers are isomers that differ in the order in which the atoms are connected. An older term for constitutional isomers is “structural isomers.”

Dr. Wolf's CHM 201 & A Historical Note In 1823 Friedrich Wöhler discovered that when ammonium cyanate was dissolved in hot water, it was converted to urea. Ammonium cyanate and urea are constitutional isomers of CH 4 N 2 O. Ammonium cyanate is “inorganic.” Urea is “organic.” Wöhler is credited with an important early contribution that helped overturn the theory of “vitalism.” NH 4 OCN Ammonium cyanate H 2 NCNH 2 OUrea

Dr. Wolf's CHM 201 & Nitromethane Methyl nitrite.. : H C O O N : :.. – + H H Examples of constitutional isomers Both have the molecular formula CH 3 NO 2 but the atoms are connected in a different order... CONOHHH.. :....

Dr. Wolf's CHM 201 & Resonance

Dr. Wolf's CHM 201 & two or more Lewis structures may be written for certain compounds (or ions) Recall from Table 1.5 ResonanceResonance

Dr. Wolf's CHM 201 & If an atom lacks an octet, use electron pairs on an adjacent atom to form a double or triple bond. Example: Nitrogen has only 6 electrons in the structure shown. Table 1.5 How to Write Lewis Structures.. CONOHHH.... :....

Dr. Wolf's CHM 201 & If an atom lacks an octet, use electron pairs on an adjacent atom to form a double or triple bond. Example: All the atoms have octets in this Lewis structure. Table 1.5 How to Write Lewis Structures.... CONOHHH.. :..

Dr. Wolf's CHM 201 & Calculate formal charges. Example: None of the atoms possess a formal charge in this Lewis structure. Table 1.5 How to Write Lewis Structures.... CONOHHH.. :..

Dr. Wolf's CHM 201 & Calculate formal charges. Example: This structure has formal charges; is less stable Lewis structure. Table 1.5 How to Write Lewis Structures.... CONOHHH.. :.. + –

Dr. Wolf's CHM 201 & same atomic positions differ in electron positions more stable Lewis structure less stable Lewis structure.... CONOHHH.. :.. + –.... CONOHHH.. :.. Resonance Structures of Methyl Nitrite

Dr. Wolf's CHM 201 & same atomic positions differ in electron positions more stable Lewis structure less stable Lewis structure.... CONOHHH.. :.. + –.... CONOHHH.. :.. Resonance Structures of Methyl Nitrite

Dr. Wolf's CHM 201 & Electrons in molecules are often delocalized between two or more atoms. Electrons in a single Lewis structure are assigned to specific atoms-a single Lewis structure is insufficient to show electron delocalization. Composite of resonance forms more accurately depicts electron distribution. Why Write Resonance Structures?

Dr. Wolf's CHM 201 & Ozone (O 3 ) Lewis structure of ozone shows one double bond and one single bond Expect: one short bond and one long bond Reality: bonds are of equal length (128 pm) ExampleExample OO O –+

Dr. Wolf's CHM 201 & Ozone (O 3 ) Lewis structure of ozone shows one double bond and one single bond Resonance: ExampleExample OO O –+ OO O –+ OO O –+

Dr. Wolf's CHM 201 & Ozone (O 3 ) Electrostatic potential map shows both end carbons are equivalent with respect to negative charge. Middle atom is positive. ExampleExample OO O –+ OO O –+

Dr. Wolf's CHM 201 & The Shapes of Some Simple Molecules

Dr. Wolf's CHM 201 & The most stable arrangement of groups attached to a central atom is the one that has the maximum separation of electron pairs (bonded or nonbonded). Valence Shell Electron Pair Repulsions

Dr. Wolf's CHM 201 & tetrahedral geometry H—C—H angle = 109.5° Table 1.6 Methane

Dr. Wolf's CHM 201 & tetrahedral geometry each H—C—H angle = 109.5° Table 1.6 Methane

Dr. Wolf's CHM 201 & bent geometry H—O—H angle = 105° but notice the tetrahedral arrangement of electron pairs O H.. H : Table 1.6 Water

Dr. Wolf's CHM 201 & trigonal pyramidal geometry H—N—H angle = 107° but notice the tetrahedral arrangement of electron pairs N H H H : Table 1.6 Ammonia

Dr. Wolf's CHM 201 & F—B—F angle = 120° trigonal planar geometry allows for maximum separation of three electron pairs Table 1.6 Boron Trifluoride

Dr. Wolf's CHM 201 & Four-electron double bonds and six-electron triple bonds are considered to be similar to a two-electron single bond in terms of their spatial requirements. Multiple Bonds

Dr. Wolf's CHM 201 & H—C—H and H—C—O angles are close to 120° trigonal planar geometry C O HH Table 1.6: Formaldehyde

Dr. Wolf's CHM 201 & O—C—O angle = 180° linear geometry OCO Table 1.6 Carbon Dioxide

Dr. Wolf's CHM 201 & Molecular Dipole Moments

Dr. Wolf's CHM 201 & —+—+—+— not polar A substance possesses a dipole moment if its centers of positive and negative charge do not coincide.  = e x d (expressed in Debye units) Dipole Moment

Dr. Wolf's CHM 201 & — + polar A substance possesses a dipole moment if its centers of positive and negative charge do not coincide.  = e x d (expressed in Debye units) Dipole Moment

Dr. Wolf's CHM 201 & molecule must have polar bonds necessary, but not sufficient need to know molecular shape because individual bond dipoles can cancel OCO ++++ ---- ---- Molecular Dipole Moments

Dr. Wolf's CHM 201 & OCO Carbon dioxide has no dipole moment;  = 0 D Molecular Dipole Moments

Dr. Wolf's CHM 201 &  = 1.62 D  = 0 D Carbon tetrachloride Dichloromethane Figure 1.7

Dr. Wolf's CHM 201 & Resultant of these two bond dipoles is  = 0 D Carbon tetrachloride has no dipole moment because all of the individual bond dipoles cancel. Resultant of these two bond dipoles is Figure 1.7

Dr. Wolf's CHM 201 & Resultant of these two bond dipoles is  = 1.62 D Resultant of these two bond dipoles is The individual bond dipoles do not cancel in dichloromethane; it has a dipole moment. Figure 1.7

Dr. Wolf's CHM 201 & Acids and Bases: The Arrhenius View

Dr. Wolf's CHM 201 & DefinitionsDefinitions Arrhenius An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions. Brønsted-Lowry An acid is a proton donor. A base is a proton acceptor. Lewis An acid is an electron pair acceptor. A base is an electron pair donor.

Dr. Wolf's CHM 201 & Arrhenius Acids and Bases An acid is a substance that ionizes to give protons when dissolved in water. A – H + H A +.. A base is a substance that ionizes to give hydroxide ions when dissolved in water. M + +OH –......M OH....

Dr. Wolf's CHM 201 & Arrhenius Acids and Bases Strong acids dissociate completely in water. Weak acids dissociate only partially. A – H + H A +.. Strong bases dissociate completely in water. Weak bases dissociate only partially. M + + OH –......M OH....

Dr. Wolf's CHM 201 & Acid Strength is Measured by pK a K a = [H + ][A – ] [HA] pK a = – log 10 K a A – H + H A +..

Dr. Wolf's CHM 201 & Brønsted-Lowry definition an acid is a proton donor a base is a proton acceptor 1.13 Acids and Bases: The Brønsted-Lowry View

Dr. Wolf's CHM 201 & H A B.. B H A –.. + A Brønsted Acid-Base Reaction A proton is transferred from the acid to the base. + + baseacid

Dr. Wolf's CHM 201 & H A B.. B H A –.. + A Brønsted Acid-Base Reaction A proton is transferred from the acid to the base. + + baseacid conjugate acid conjugate base

Dr. Wolf's CHM 201 & hydronium ion H Br O H H.... H H.. O H Br – Proton Transfer from HBr to Water baseacidconjugate conjugate acid base + +

Dr. Wolf's CHM 201 & [H 3 O + ][Br – ] [HBr] Ka =Ka =Ka =Ka = H Br OHH....HH.. O H Br– Equilibrium Constant for Proton Transfer Takes the same form as for Arrhenius K a, but H 3 O + replaces H +. H 3 O + and H + are considered equivalent, and there is no difference in K a values for Arrhenius and Brønsted acidity.

Dr. Wolf's CHM 201 & [H 3 O + ][Br – ] [HBr] Ka =Ka =Ka =Ka = H Br OHH....HH.. O H Br– pK a = – log 10 K a Equilibrium Constant for Proton Transfer

Dr. Wolf's CHM 201 & H OH N H H.... H H.. N H OH – Water as a Brønsted Acid baseacidconjugate conjugate acid base – + +

Dr. Wolf's CHM 201 & strong acids are stronger than hydronium ion weaker acid stronger acid AcidpKaConj. BaseHII – HBrBr – HCl Cl – H 3 O H 2 O H 2 SO 4 HSO 4 – Dissociation Constants (pK a ) of Acids*

Dr. Wolf's CHM 201 & The stronger the acid, the weaker the conjugate base. weaker acid stronger acid AcidpKaConj. BaseHII – HBrBr – HCl Cl – H 3 O H 2 O H 2 SO 4 HSO 4 – Important Generalization!

Dr. Wolf's CHM 201 & Dissociation Constants (pK a ) of Acids* weak acids are weaker than hydronium ion AcidpKaConj. BaseH 3 O + –1.7H 2 O HF3.5F – CH 3 CO 2 H4.6CH 3 CO 2 – NH NH 3 H 2 O15.7HO –

Dr. Wolf's CHM 201 & Dissociation Constants (pK a ) of Acids* alcohols resemble water in acidity; their conjugate bases are comparable to hydroxide ion in basicity AcidpKaConj. BaseCH 3 OHCH 3 O – CH 3 CH 2 OH~16CH 3 CH 2 O – (CH 3 ) 2 CHOH~17(CH 3 ) 2 CHO – (CH 3 ) 3 COH~18(CH 3 ) 3 CO – H 2 O15.7HO – 15.2

Dr. Wolf's CHM 201 & Dissociation Constants (pK a ) of Acids* ammonia and amines are very weak acids; their conjugate bases are very strong bases AcidpKaConj. BaseNH 3 ~36NH 2 – (CH 3 ) 2 NH~36(CH 3 ) 2 N –

Dr. Wolf's CHM 201 & Dissociation Constants (pK a ) of Acids* Most hydrocarbons are extremely weak acids. AcidpK a Conj. Base 26HCCH CH 3 CH 3 H2CH2CH2CH2C CH 2 HHHH HH HCC – HHH HH – H2CH2CH2CH2CCH– CH 3 CH 2 –