Polarity and its Consequences

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1 Chapter 2, Part I: Polarity, Formal Charge and Resonance I. Bond Polarity Polarity and its Consequences Polar bonds can be identified as covalent bonds in which the atoms involved have significantly different electronegativity values (EN) rganic chemists regard bonds in which Δ EN < 0.5 to be fairly nonpolar Example: C bonds Common polar bonds in organic compounds: somewhat polar very polar C N or C = N C or C = C Cl C Br C F C Mg or C Li Partial charges (δ+ and δ-) & arrow notation II. Molecular polarity For a molecule to be polar: 1) It must contain polar bonds 2) They must be arranged unsymmetrically (shape matters) ow polar is a molecule? Dipole moment (μ) = Q (charge) x r (distance) As Δ EN and atomic radii increase, μ increases Effect of lone pairs? III. Formal charges: An electron deficiency or excess assigned to an atom (usually when normal bonding patterns are not followed) Formal charge = number of valence e- - 1/2 the number of - number of atom normally has bonding e- nonbonding e-

2 Resonance Found in molecules in which the electron distribution can be represented in more than one way, often involving the location of bonds or nonbonding electron pairs Example: C 3 N 2 Representing such molecules: Although this is often used to represent benzene: resonance forms & resonance hybrids These are more accurate, since benzene is a resonance hybrid: or An example from general chemistry: C 3 2- C C C Guidelines for understanding resonance forms for a molecule: 1. Resonance exists because the hybrid is more stable than any of the resonance forms. 2. The total number of electrons and the overall net charge on the molecule do not change. nly the placement of electrons and resulting formal charges on atoms may change. 3. Resonance forms must be valid Lewis structures and obey normal rules of valency. 4. Resonance forms do not have to be equivalent in stability; they may be neutral or contain atoms with charges.

3 Drawing resonance forms: Why? Molecules that have resonance are sometimes shown as a single resonance contributor but to understand reactivity, we must know all resonance forms They are useful for predicting regions of electron density or cationic character ow? 1. Spot common features: Look for three-atom groups X = Y Z * with double bonds and exchange bond positions Example: acetate ion 2. NLY move the electrons or bonds, never move the atoms themselves! 3. Watch direction of electron movement (often depicted by curved arrows): Move π electron pairs toward a + charge or toward another π bonded atom. Move nonbonding electron pairs toward an sp 2 -hybridized atom Example: butadiene 2 C C C C 2 2 C C C C 2 2 C C C C 2 4. bey the octet rule To avoid electron overload, don t move more electrons toward an sp 3 -hybridized atom because they usually have an octet already Example: acetic acid

4 Chapter 2 Part II: Review of Acids & Bases Bronsted-Lowry Acid: Donates + Base: Accepts + Lewis Acid: Accepts an electron pair Base: Donates an electron pair I. Acid-base reactions (B-L) A + :B A:- + B + Acid Base Conjugate Conjugate base acid II. Acid strength depends on acid dissociation constant, K a A + 2 A K a = K eq [ 2 ] = [ 3 + ] [A-] [A] pk a = - log K a Trend: The greater the pk a, the weaker the acid III. IV. Acid/base behavior and structure of organic compounds Functional groups Inductive effect of electronegative atoms Stability of conjugate base Predicting the direction of equilibrium Recall that a strong acid forms a weak conjugate base and vice versa In acid-base equilibrium, reaction favors formation of weaker acid or base V. Lewis acid-base reactions: electrons from the base are shared to form the new bond Electron flow can be depicted by using curved arrows: Example: 3 C N 2 B 3 C N B

5 Acid and Base review (cont d) 1) Try to think of acids and bases more from a Lewis perspective: Lewis bases are electron donors: they contain electron-rich atoms such as or N with lone pairs that can donate the lone pair for bonding Lewis acids are electron acceptors: they may be electron deficient with less than an octet or positively charged like + or carbocations. 2) Draw arrows from the bonding e- of the Lewis base to the target. Those electrons should form a covalent bond. 3) Lewis acid-base reactions may form only one product or more than one depending on the nature of the donor and acceptor 4) Do not randomly break other bonds that are not part of the base-to-acid transfer of electrons. Cl 3 C N C 3 Al Cl Cl Br 3 C C C 3.. N 3 3 C C 3 F 3 C N C 3 B F F

6 Some characteristic organic acids and bases Carboxylic acids behave like acids (thus the name) and donate + Ex: Acetic acid, pka = 4.76 C 3 C + 2 C 3 C Equilibrium favors reactants Weak acid strong conj. base Amines generally behave like bases and accept protons Ex: Methylamine (pk a = 40) C 3 N 2 + C 3 C C 3 N C 3 C- Weak base weak acid conj. acid conj. base Alcohols are considered neutral, but during a reaction can accept or donate a + Ex: Methanol (pk a = 15.5) In the presence of base, methanol loses a proton: C 3 + NaN 2 Na C N 3 Weak acid base strong base strong conj. acid In the presence of acid, methanol can be protonated C 3 + C 3 C C C 3 C- Weak base weak acid conj. acid conj. base

7 Following the flow of electrons: Reaction mechanisms with curved arrows Some rules to follow in the use of curved arrows 1. Always draw arrows so they point in the direction electrons are moving: from a pair of electrons in the base to the acidic : 2. You may need to move more than one e- pair in a given step remember, no atom can be left with more than an octet! If an atom with a full octet accepts an e- pair, it must donate another one. 3. Indicate any changes in charge that take place (be careful of +/- signs) a) With negatively charged bases, the atom donating the e- pair for bond formation becomes neutral after the new bond forms, so the conjugate acid is neutral b) If the base is neutral, after donating the e- pair it becomes a positively charged conjugate acid 4. Never use arrows to show where atoms are going, only where electrons are going! 5. In most reactions, electrons move in pairs, so use a double-headed arrow For free radical reactions, movement of a single electron is shown by a fishhook 6. The curved-arrow rules also apply to other types of reactions, not just traditional acid-base reactions. Electron or bond donors are called nucleophiles Electron acceptors are called electrophiles

8 Electronegativity chart of the elements Yellow = electropositive Green = intermediate Red = electronegative Polarity of bonds affects dipole moment of molecules

9 To calculate formal charges correctly, first draw the Lewis structure

10 Acid-base chemistry pka = 40

11 Skeletal or line-bond structures do not show the atoms Representing structure of organic compounds

12 Resonance forms and electron flow

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