Two atoms share electrons to make a covalent bond. The differenceof the electronegativites of those two atoms determines how polar that bond is.

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1 Chapter 2: Polar covalent bonds; Acids and bases The reactivity of organic compounds is often defined by the polarities of the covalent bonds in the molecule. Polar covalent bonds: an intermediate between pure covalent bonds and ionic bonds Electronegativity difference Two atoms share electrons to make a covalent bond. The differenceof the electronegativites of those two atoms determines how polar that bond is. EN Difference Bond Type < > 2 Electronegativity:intrinsic ability of an atom to attract the shared electrons in a covalent bond Inductive effect: shifting of electrons in a σbond in response to the EN of nearby atoms ch2 Page 1 ch2 Page 2

2 2.2 Dipole moments An entire moleculecan be polarfrom the vector sum of the polar bonds in the molecule. Dipole moment: (μ) product of charge magnitude and distance between the charges. Unit = debye (D) 2.3 Formal charges Atoms with an abnormalnumber of bonds will usually have a high amount of + or -charge concentrated on them. The -electrons in the atoms will normally cancel the + protons in the nucleus to make the molecule neutral. Electrons have to be added or removed from the molecule to accommodate an abnormal number of bonds You can quickly calculate an atom's formal charge by starting with its original valence electrons and subtracting the electrons it "owns" (all unshared e - and one from each covalent bond). When the bonds are drawn as lines, just subtract 1 for each line. The direction of a molecule's dipole moment will usually be towards the most electronegative element and that element's lone pairs (if any). Can be difficult to predict in large molecules. ch2 Page 3 ch2 Page 4

3 Bonding patterns 2.4 Resonance Sometimes we have a choice as to where to put electrons in a structure (where do we put a double bond, for instance). C-O single bonds are longer than C=O double bonds. But experimentally, both bonds are the same length. Resonance structures: only differ by the placement of π or nonbonding valence electrons. They combineto make a single resonance hybrid. ch2 Page 5 ch2 Page 6

2.5 Rules for resonance forms Rule 1: Individual resonance structures are imaginary. The real structure is a singlehybridof all of the resonance structures.

4 2.5 Rules for resonance forms Rule 1: Individual resonance structures are imaginary. The real structure is a singlehybridof all of the resonance structures. Resonance structures Rule 4: Resonance forms obey normal rules of valency. Do not exceed the octet on C, N, O, or F! (Carbon never makes 5 bonds) Rule 2: Resonance forms only differ by the placement of πor nonbonding electrons. The atoms must never move! Use a curved arrowto show the movement of a pair of electrons from one resonance structure to another. (This is a key concept!) Rule 5: The resonance hybrid is more stable than any individual resonance form. Resonance delocalizes (spreads out) electrons over a larger distance, and makes the molecule more stable. (This is a key concept!) Rule 3: Different resonance forms of a substance don't have to be equivalent. The actual hybrid is closest to the most stable resonance structure (fewest charges, negative charges on more EN atoms) Patterns in resonance A three-atom grouping where each atom has a p orbital, will have two resonance forms. ch2 Page 7 ch2 Page 8

5 2.7 Acids and bases: the Brønsted-Lowry definition The two main definitions we will use for acids and base are the Brønsted-Lowry and Lewis definitions. (Arrhenius with hydronium and hydroxide is not very useful in organic chemistry) Brønsted-Lowry acid: donates a proton (H + ) Brønsted-Lowry base: accepts a proton 2.8 Acid and base strength Recall from general chemistry the equilibrium that is established when an acid donates its proton to water: HA + H 2 O K a = In this class, we will commonly refer to the pk a to compare acidities. pk a = An acid will produce a conjugate... A base will produce a conjugate In general: The pk a of water, 15.7, is an important value. There are important consequences for whether an acid is stronger or weaker than water. ch2 Page 9 ch2 Page 10

6 2.9 Predicting the direction of acid-base reactions Since every acid base reaction has an acid and base on each side, we can use pk a values to determine which direction the reaction will proceed spontaneously. Practical use of acid-base prediction To separate acetaminophen and ibuprofen, we need a base that will react with one but not the other. Common aqueous bases are OH -, CO 3 2-, and HCO 3-. Stronger acid/base Weaker acid/base Stronger acids produce Stronger bases produce... pk a/b Potential energy Stability Acid pk a H 2 O 15.7 HCO H 2 CO Conj. base Acid + Base Conj Base + Conj Acid Acetaminophen pk a = 9.9 Ibuprofen pk a = 4.5 ch2 Page 11 ch2 Page 12

7 2.10 Organic acids Organic acids and bases Organic acids have a positively polarized hydrogen. Organic baseshave an atom with a lone pair of electrons. The strength of an acid is determined by the stability of its conjugate base anion! Anions can be stabilized by resonanceand/or an electronegative atom that can hold a negative charge If an anion is stable, it is a relatively base and therefore its conjugate is a relatively acid Nitrogen-containing ammonia derivatives are the most common organic bases (pk b 3-5 usually). Oxygen-containing compounds can act as either acids or bases when reacted with strong bases or acids. ch2 Page 13 ch2 Page 14

8 2.11 Acids and bases: the Lewis definition Lewis acid: electron pair acceptor Lewis base: electron pair donor Lewis bases Lewis basesare important to recognize since they will often start the mechanism for most of our reactions! Use the curved-arrow notation to show a Lewis acid-base reaction. ch2 Page 15 ch2 Page 16

9 2.13 Non-covalent interactions Properties of molecular substances like boiling point, melting point, vapor pressure, viscosity, surface tension are due to the non-covalent interactionsthat exist between the molecules. Dipole-dipole forces Dispersion forces Hydrogen bonding Dipole-dipole forces: attractive forces between the opposite dipoles of polar molecules Hydrogen bonding H-bond donor:h attached to N, O, or F (EN elements make H extra positive) H-bond acceptor:lone pair of electrons on N, O, or F (these are the most concentrated lone pairs) H-bonding is almost always strongerthan dipole-dipole or dispersion forces. Dispersion forces: attraction between all moleculesdue to constantly changing electron distributions ("sloshing") Dispersion forces are stronger with higher molar mass and larger molecular surface area. ch2 Page 17 ch2 Page 18

2. Polar Covalent Bonds: Acids and Bases

2. Polar Covalent Bonds: Acids and Bases Based on McMurry s Organic Chemistry, 6 th edition, Chapter 2 2003 Ronald Kluger Department of Chemistry University of Toronto 2.1 Polar Covalent Bonds: Electronegativity!