chapter 2- polar covalent bonds; acids and bases 3 electronegativity values low values high values...
TRANSCRIPT
8/25/15
1
Chapter 2- Polar Covalent Bonds; Acids and Bases
Ashley Piekarski, Ph.D.
Why do I care, Dr. P?
• In Chapter 1, we studied valence bond theory which uses hybrid orbitals to account for the observed shapes of organic molecules
• In Chapter 2, we will study how the electrons are distributed in covalent bonds and how that distribuDon affects chemical reacDvity
8/25/15
2
Electronegativity
• What is electronegaDvity?
Electronegativity
The electronegativity value ! indicates the attraction of an atom for shared
electrons ! increases from left to right going across a period
on the periodic table ! decreases going down a group on the periodic
table ! is high for the nonmetals, with fluorine as the
highest ! is low for the metals
8/25/15
3
Electronegativity values
Low values
High values
Ionic bond
An ionic bond ! occurs between metal and nonmetal ions ! is a result of electron transfer ! has a large electronegativity difference (1.8 or more).
Examples: Atoms Electronegativity Type of Bond
Difference _____ ________ Cl–K 3.0 – 0.8 = 2.2 Ionic N–Na 3.0 – 0.9 = 2.1 Ionic S–Cs 2.5 – 0.7 = 1.8 Ionic
8/25/15
4
Nonpolar covalent bond
A nonpolar covalent bond ! occurs between nonmetals ! has an equal or almost equal sharing of electrons ! has almost no electronegativity difference (0.0 to
0.4)
Examples: Atoms Electronegativity Type of Bond
Difference _______________ N–N 3.0 – 3.0 = 0.0 Nonpolar covalent Cl–Br 3.0 – 2.8 = 0.2 Nonpolar covalent H–Si 2.1 – 1.8 = 0.3 Nonpolar covalent
Polar covalent bond
A polar covalent bond ! occurs between nonmetal atoms ! has an unequal sharing of electrons ! has a moderate electronegativity difference
(0.5 to 1.7) Examples: Atoms Electronegativity Type of Bond
Difference __________ _ O–Cl 3.5 – 3.0 = 0.5 Polar covalent Cl–C 3.0 – 2.5 = 0.5 Polar covalent O–S 3.5 – 2.5 = 1.0 Polar covalent
8/25/15
5
Electronegativity and bond types
Methanol
• Draw the structure for methanol. • What is the electronegaDvity value for oxygen?
• What is the electronegaDvity value for carbon?
8/25/15
6
Methanol- electrostatic map
• ElectrostaDc potenDal maps show calculated charge distribuDons
• Colors indicate electron-‐rich (red) and electron-‐poor (blue) regions
• Arrow indicate direcDon of bond polarity
Bond polarity
• This is the basis of organic chemistry. Understanding the polarity of a molecule helps to know the reacDvity!
Note: electrostaDc potenDal maps in textbook give you a clue to the electron-‐rich and electron-‐poor atoms in molecules
Red: negaDve Blue: posiDve
8/25/15
7
Inductive effect
• InducDve effect: the shiXing of electrons in a sigma bond in response to the electronegaDvity of nearby atoms • Metals inductively donate electrons • Non-metals inductively withdraw electrons
Learning check
• Assign δ+/δ-‐ charges to show the direcDon of expected polarity. • H3C-MgBr
• H3C-SH
• H2N-H
8/25/15
8
Dipole Moments
• Molecular polarity results from the vector summa,on of all individual bond polariDes and lone-‐pair contribuDons. • The quantity measured is called a dipole
moment, µ
µ =Q × rQ = charge
r = distance
Dipole Moments
8/25/15
9
Learning check
• An aromaDc compound, benzene, has a dipole moment of zero. Why?
Learning check
• Chloromethane, CH3Cl, has a dipole moment of 1.87. Make a three-‐dimensional drawing of methyl chloride and show the direcDon of the dipole moment.
8/25/15
10
Formal charges
• Formal charges are only electron “bookkeeping” and do NOT imply the presence of actual ionic charges • gives clues to the chemical reactivity
Formal Charge = # of VE's in free atom( )- # of bonding electrons2
⎛⎝⎜
⎞⎠⎟ − # of nonbonding electrons( )
Learning check
Calculate the formal charges for all the atoms in the acetate ion:
8/25/15
11
Resonance
• What is resonance? • it is the way we describe electron
delocalization in a compound that has pi bonding
Resonance Hybrid
• A structure with resonance forms does not alternate between the forms
• Instead, it is a hybrid of the two resonance forms
8/25/15
12
Resonance rules!
• Rule 1: Individual resonance forms are imaginary, not real.
Resonance rules!
• Rule 2: Resonance forms differ only in the placement of their pi or nonbonding electrons
8/25/15
13
Resonance rules!
• Rule 3: Different resonance forms of a substance don’t have to be equivalent.
Resonance rules!
• Rule 4: Resonance forms obey normal rules of valency
8/25/15
14
Learning check
• Draw the resonance forms for the 2,4-‐pentanedione aXer it has reacted with a strong base.
Acids and Bases
According to the Brønsted–Lowry theory, ! acids donate a proton (H+) ! bases accept a proton (H+)
8/25/15
15
Acids and Bases
In the reaction of ammonia and water, ! NH3 is the base that accepts H+
! H2O is the acid that donates H+
Learning check
• Draw the reacDon of aceDc acid reacDng with sodium hydroxide. What is the acid and what is the base? What are the products formed?
8/25/15
16
Acid and Base Strength
• What is the generic reacDon scheme for a weak acid reacDng with water?
• What is the Ka of this reacDon? • Based on this expression, if your Ka is very
large is it a strong or weak acid?
pKa
• Do you remember how to find pKa? • Would a stronger acid have a smaller or larger pKa? • For convenience, acid strengths are
expressed using pKa values • In organic chemistry it is good to start learning
the pKa of acids and their conjugate basesà gives you a clue to their reactivity
8/25/15
17
pKa Table
Predicting acid-base reactions
• pKa values are related as logarithms to equilibrium constants
• Useful for predicDng whether a given acid-‐base reacDon will take place
• The stronger base holds on the proton more Dghtly
8/25/15
18
Learning check
Organic acids
• Organic acids • characterized by the presence of a positively-
charged hydrogen atom
8/25/15
19
Organic acids
• Those that lose a proton from O-‐H, such as methanol or aceDc acid
• Those that lose a proton from C-‐H, usually from a carbon atom next to a C=O double bond
Conjugate bases- electrostatic maps
• Which element now has a substanDal amount of negaDve charge aXer deprotonaDon?
8/25/15
20
Organic bases
• Have an atom with a lone pair of electrons that can bond to H+
• Nitrogen-‐containing compounds derived from ammonia are the most common organic bases
• Oxygen-‐containing compounds can react as bases when a strong acid or as acids with strong bases
Lewis Definition
• Lewis acids are electron pair acceptors and Lewis bases are electron pair donors
• The Lewis definiDon leads to a general descripDon of many reacDon pajerns
8/25/15
21
Lewis acids
• The Lewis definiDon of acidity includes metal caDons, such as Mg2+
• They accept a pair of electrons when they form a bond to a base
• Group 3A elements, such as BF3 and AlCl3, are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases
• TransiDon-‐metal compounds, such as TiCl4, FeCl3, ZnCl2, and SnCl4, are Lewis acids
• Organic compounds that undergo addiDon reacDons with Lewis bases (discussed later) are called electrophiles and therefore Lewis Acids
• The combinaDon of a Lewis acid and a Lewis base can shown with a curved arrow from base to acid
Lewis acid-base reaction
8/25/15
22
Note: curved arrows
• A curved arrow always means that a pair of electrons move from the atom at the tail of the arrow to the atom at the head of the arrow
Lewis bases
• Lewis bases can accept protons as well as Lewis acids, therefore the definiDon encompasses that for Brønsted bases
• Most oxygen-‐ and nitrogen-‐containing organic compounds are Lewis bases because they have lone pairs of electrons
8/25/15
23
Noncovalent interactions
• Dipole-‐dipole forces • Dispersion forces • Hydrogen bonds
Dipole-Dipole
• Occur between polar molecules as a result of electrostaDc interacDons among dipoles
• Forces can be ajracDve or repulsive depending on orientaDon of the molecules
8/25/15
24
Dispersion Forces
• Occur between all neighboring molecules and arise because the electron distribuDon within molecules that are constantly changing
Hydrogen Bond Forces
• Most important noncovalent interacDon in biological molecules
• Forces are a result of ajracDve interacDon between a hydrogen bonded to an electronegaDve O or N atom and an unshared electron pair on another O or N atom
8/25/15
25
Applications
Hydrogen bonding in DNA