Essential Origami-How To Build Dozens of Models from Just 10 Easy Bases.PDF
Fall 2004 Supplemental notes Acids and Bases “Curved...
Transcript of Fall 2004 Supplemental notes Acids and Bases “Curved...
Fall 2004 Supplemental notes
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Acids and Bases
“Curved Arrow Formalism” or Pushing Electrons
Carbon and other second row elements such as B, N, O, and F follow the octetrule, i.e. they try to have the sum of bonding electrons and electrons in lonepairs around them equal to 8. For the first row, hydrogen tries to have 2electrons.In general, NONE of these elements will have more than an octet (or duet forhydrogen).
Electron Deficient Compounds
Sometimes molecules have atoms that are short of an octet by one or moreelectron pairs – they tend to be very reactive. For example:
1. H+ has 0 electrons and it needs 2, thus it is deficient by 2.
2. BF3 is an electron deficient compound. The boron atom in boron tri-fluoridehas 6 electrons, and it needs 8. Thus it is deficient by 2 electrons. Oneadditional lone pair is needed to fill its octet.
BF F
F
B
F
FFBF F
F
3. Methyl cation has 6 electrons, and it needs 8, thus is deficient by 2.
CH H
H
C
H
HHCH H
H
Fall 2004 Supplemental notes
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Lewis Acids and Lewis Bases
F BF F
F F
BF F
FF B F
F
F
+
8 Electron Lewis Base Lewis Acid Tetrafluoroborate ion
Electron deficient compounds, which can behave a electron pair acceptors areLewis acids.
A species that donates an electron pair is a Lewis base.
The reaction above is called Lewis acid/ Lewis base association reaction.
Lewis acid ⇒ electrophile (“loves electrons“)Lewis base ⇒ nucleophile (why??)
We will see many Lewis acid-Lewis base reactions in coming months.
IT IS VERY IMPORTANT to be able to identify Lewis acids and Lewis bases.
Another example (simplified):
NH H
H + H
H
NH H
H
lonepair
ammonium ion8 e- Lewis Base 0 e- Lewis Acid 8 e-
Fall 2004 Supplemental notes
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What Do Curved Arrows Mean?
F BF3F BF3
new bond, electrons shared
sourceof electrons destination
of electrons
The curved arrow indicates the flow of electrons.
The arrow always starts at the electron donor and ends at the electron acceptor.
Here the arrow starts at the Lewis base end and ends at the electron deficientspecies (the Lewis acid).
**Note that charge is conserved.
What about the reverse reaction?
FBFF
F
FBF
F
F
+
Fall 2004 Supplemental notes
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Conjugate Acid-Base Pairs
NH H
H + H
H
NH H
HBr Br+
Conjugate Acid-Base Pairs
Base Acid Conjugateacid
Conjugatebase
1. Note that the H-Br bond is broken and NH3-H bond is formed.
2. Why is the H-Br bond broken? Because the H in HBr already had aduet and if it is to accept two electrons from ammonia, it must also losetwo.
3. When a lone pair is contributed, the formal charge on the atomcontributing the lone pair becomes more positive by one integer, andwhen a lone pair is gained, the formal charge on the atom receiving thelone pair becomes more negative by one integer.
Example:
NH H
H + H
H
NH H
HBr Br+
0 0 +1 -1
Nitrogen contributes a lone pair to form a new bond, so the charge increases by 1.
Bromine gains a lone pair when the bond is broken, so the charge decreases by one.
Note that the net charge on both side of the arrow should be the same (charge isconserved).
Fall 2004 Supplemental notes
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Examples
H O OH CH3CH3
OH
H + OCH3
CH3
Drawn to show tetrahedral geometry with lone pairsoccupying sites.
Base Acid
Conjugate Acid-Base Pairs
Conjugateacid
Conjugatebase
• Note the charges, bonds formed and bonds broken.• Note the conjugate Lewis acid pair and Lewis base pair.• Note that the arrows indicate flow of electrons.
What about:
H O OH CH3CH3
OH
H + OCH3
CH3
Base Acid
Conjugate Acid-Base Pairs
Conjugateacid
Conjugatebase
WrongReaction is not wrong, BUT use of the curved arrow is incorrect.REMEMBER: Electrons flow from tail to head!!
Also note BrØnsted-Lowry Acid and Bases:• BrØnsted Acid – A species which reacts by donating a proton (H+).• BrØnsted Base – A species that can accept a proton. BrØnsted-Lowry summary:
H X X H+ B + Bm n n+1m-1
BronstedAcid
BronstedBase Conjugate
BaseConjugateAcid
So BrØnsted-Lowry Acid-Base definition is a more limited definition thanLewis acid base.
Fall 2004 Supplemental notes
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Use of Curved Arrow Formalism to Derive Resonance Structures
Not all molecules can be described well by one Lewis structure. In many cases another structure can be derived by a shift of one or moreelectron pairs.
Both structures for benzene are equal in energy. Which structure is correct? Actually, neither is correct. The real structure of benzene is in between the twostructures above. The two structures shown above are called two limitingresonance structures. ** Extremely important: Resonance does NOT imply rapid interchangebetween structures, but rather that the actual structure is a weighted average ofthe two (or more) limiting resonance structures.
Curved arrows can help one draw resonance structures. Here the arrow describes ‘flow’ in the loose sense of the word.
circle impies 1.5 bonds between carbons
Fall 2004 Supplemental notes
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Other Examples
H3C O
O
H3C O
O
H3C O
O
curve implies 1.5 bonds between carbons
Note, like in the case of Lewis acids-Lewis bases: • The arrow represents “flow” of electron pair.• Flow "in", means formation of new bond;• Flow "out", means breaking a bond.• Atoms should not violate octet rule.• The overall charge is conserved.
CH3N
O
H3C
CH3
CH3N
O
H3C
CH3
Left structure, no charge separation.Right structure +,- so right structure is higher energy and contributes less.
CH3H2C
O
CH3H2C
O
Right structure has the minus charge on more electronegative atom.So, the right structure is lower in energy and contributes more to the actualstructure of the molecule.
Fall 2004 Supplemental notes
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Stability of Resonance Structures and Summary for Deriving Them:
• Try to satisfy octet.• Maximize the number of covalent bonds.• Minimize charge separation.• Try to place negative charges (electrons) on most electronegative
atom.• Positive charge on halogens is really bad (because they are highly
electronegative).• Fewer than four bonds to carbon is quite bad.• Charges on carbon are quite bad.• More than 8 electrons on carbon, nitrogen, or oxygen, is
unacceptable.
Fall 2004 Supplemental notes
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BrØnsted -Lowry Acid Base Equilibria
Equilibrium constants:
X H + B X + H Bn m n-1 m+1
e.g.
Cl H + H OH+ OH Cl
One can write an equilibrium expression:
Xn-1
H Bm+1
X Hn
Bm
Molarity of speciesKeq =
Keq > 1 implies reaction goes to the rightKeq < 1 implies reaction goes to the left
Keq > 1 implies that X-Hn is a stronger acid than H-Bm+1
and that :Bm is a stronger base than Xn-1
Fall 2004 Supplemental notes
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Acid-Base Equilibria in Water
In the case below where water is the solvent:
HX + H2O X- + H3O+
then:
X H3O
X HKeq =
H2O
In this case concentration of H2O = 55 M and is effectively unchanged since it ispresent in such a large excess.Then,
X H3O
X HKa = Keq[H2O] =
• Ka = dissociation constant and is a measure of acid strength.• Larger Ka implies stronger acid.• Range of Ka we may see is from 10-55 up to 107; 62 ! orders of magnitude.
Chemists use inverse log scale:pKa = -logKa the lower the pKa, the stronger the acidpH = -log[H3O+] the lower the pH, more acidic the solution
Note for:
X H + Y X + H Yn m n-1 m+1
Keq
Keq = 10-([pKa (HX)]-[pKa (HY)])
Fall 2004 Supplemental notes
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Examples pKa values
HF H2O NH3 CH4
3 16 33 50-60
HF + OH F + H2O
pKa 3 16Keq = 10-(3-16) = 10+13 So this reaction goes towards the right.
H2O + CH3 OH + CH4
pKa 16 ~50
Keq = 10-(16-50) = 1034 !!
So, pKa's are quantitative measures of acidity and allows one to make predictionsabout reactions.
Example:Conjgate Base: CH3
- > NH2- > OH- > :NH3 > H2Ö: > >>>> HF
Acid CH4 < :NH3 < H2Ö: < NH4+ < H3O+ <<< H2F+
pKa: >50 33 16 10 –2 << –10
Notice that both NH3 and H2O can be both acid and base. Such compounds aresaid to be amphoteric.
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Strengths of BrØnsted-Lowry Acids and Bases
Proton transfer reactions:Proton transfer reactions can generate ions
++ H2O OH H3OH2O
length of arrow indicates approximate position of equilibrium
hydroxide ion
hydronium ion
Hydronium ion as acid:
H3O + NH3 H2O + NH4
Hydroxide ion as base:
+OH NH3+H2O NH2
+OH CH4 +H2O CH3
+OH HF +H2O F
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Rules for Charge Stability of Ions with a Full OctetElement effect1. Negative charge is most stable on most electronegative atom.
F- > R-O- > R2N- > R3C-
Increasing Stability
2. For atoms of similar electronegativity, the negative or positive charge is morestable on the larger atom.
R-Te- > R-Se- > R-S- > R-O-
Increasing Stability
R2SH+ > R2OH+
Increasing Stability
Why? Larger atoms distribute charge over a greater volume.
3. Positive charge is most stable on least electronegative atom.R3NH+ > R2OH+
Increasing Stability
These trends in stability can be used to predict directions of the acid-base reactionshown above and others throughout the term.
LEARN THIS WELL!
Fall 2004 Supplemental notes
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Periodic table of 246a
Valence electrons
H He
Ne
Ar
Kr
Xe
F
Cl
Br
I
Li
Na
K
Be
Mg
Ca
Al Si P S
B C N O
1 2 3 4 5 6 7 8 Acidity increases dow
n column
Bond strength to H decreases dow
n column
Electronegativity increases across rowElectron affinity increases across rowAcidity increases across row
Acidity of acids in a row
CH4 < NH3 < H2O < HFpKa ~50 ~32 16 3.5
Acidity of acids in a column
HF < HCl < HBr < HIpKa 3.5 ~–6 ~–8 ~–10
Fall 2004 Supplemental notes
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F is more electronegative than I. So how do we explain this trend?
Consider reaction H-A → H+ + A:-
We can use Hess law of summation to break up reaction into pieces
1. Bond breaking H-A → H + A
2. Electron add to A e- + A → A:-
3. Ionization of H H → H+ + e-
Sum H-A → H+ + A-
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Now for comparison between acids:1. Bond Breakinga) Energy to break bonds drops dramatically down column
Bond H - F H - Cl H - Br H - IBond Dissociation Energy (BDE) (kcal/mol): 136 103 88 71
Reason: Lower orbital overlap
b) Energy to break bond doesn't vary so much across a row
Bond CH3-H NH2-H HO-H F-HBDE (kcal/mol) 105 107 119 136
2. e- +A → A:- is electron affinity, EA
a) Electron affinity doesn't vary that much down a column
Atom I Br Cl F
EA(kcal/mol) 70 78 83 78 b) Electron affinity increases dramatically across a row
Atom CH3 NH3 OH FEA(kcal/mol) 1.8 18 42 78
3. Ionization of H to H+
This is the same for each acid so it doesn't enter into comparison.
Fall 2004 Supplemental notes
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So,
For BDE HF HI(kcal/mol) 136 71 ∆BDE = 65 kcal/mol
Favors HI
For EA F I(kcal/mol) 78 70 ∆EA = 8 kcal/mol
Favors HF
But 65 >> 8 thus HI must be stronger acid than HF
So, down a column BDE dominates the strength of the acid.
In a row EA dominates the strength of the acid.
Fall 2004 Supplemental notes
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Polar or Inductive EffectsRemember that opposite charges attract -- like charges repel
No relative stabilization
Charge spread over largervolume-some stabilization
Dipole- some significant stablization
Two dipoles- more significant stablization
Dipole oriented in wrong direction-destablization
Remote dipole- weak stablization
1)
2)
3)
4)
5)
6)
All other things equal, if molecules have similar conjugate bases which experiencethese environments then (ignoring entry #2!):
pKa: 5 > 1 > 6 > 3 > 4
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ExamplesNumber of polar groups:
H
OH
O
H
H
H
OH
O
HCl
H
OH
O
Cl
Cl
Cl
OH
O
ClCl
pKa 4.73 2.86 1.26 0.064
Proximity of polar groups:
OH
O
OH
O
OH
O
OH
O
Cl
Cl
ClpKa 4.82 4.52 4.06 2.84
Resonance:
OH
OOH
pKa 18 5 13 orders of magnitudeWhy?
O
O
O
O
O
Obut also
less important
The minus charge is delocalized between the oxygens. The resonance structure onthe right inductively stabilizes oxygen (but is a minor contributor).
SUMMARY: Three major effects: Element effect (EA and BDE), Inductive effect, andResonance
Fall 2004 Supplemental notes
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LEARN THESE VALUES
ConjugateAcid
ConjugateBase
pKa
HI I- -11
HBr Br- -8
HCl Cl- -6
H3O+ H2O -2
SO3H SO3--1
F3COH
OF3C
O-
O0
H3COH
OH3C
O-
O4.76
H2S HS- 7.0
HCN CN- 9.2
NH4+ NH3 9.2
R-SH R-S- 10-12
R-OH R-O- 16-20
NH3 NH2- 32
H2 H- 35
CH4 CH3- 48
↑ ↓Increasing acidity: Increasing basicity:
UP in table DOWN in tableA strong acid makes a weak base and vice versa.
Fall 2004 Supplemental notes
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Hydrogen Bonding
A hydrogen bond is a particular type of a Lewis acid-Lewis base interaction.• It can occur between a hydrogen atom attached to a heteroatom such as O,
F, N (called the hydrogen bond donor group) and an atom that has a lonepair (typically also O, F, and N) the hydrogen bond acceptor.
• More generally any acidic hydrogen can be a hydrogen bond donor and anyLewis base can be a hydrogen bond acceptor.
• Hydrogen bonding is a special case of dipole-dipole interactions, and it isalso an example of a weak covalent bonding interaction.
OH H
O
H
H0.96Å
1.8-1.9Å
donor
acceptor
Note that the O–H---O angle is drawn to be 180°, I believe that this is thepreferred angle for hydrogen bonds.
Fall 2004 Supplemental notes
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Effects of Hydrogen Bonding
• Hydrogen bonding affects the boiling point of solvents. Thus for water andlow molecular weight alcohols, the boiling points are unusually high since inaddition to overcoming van der Waals interactions, the hydrogen bondsmust be broken in order to vaporize the solvent.
• If such interactions did not occur it is likely that water would boil belowambient temperature, which would make life on earth rather difficult.
• As we will see later, solvents capable of hydrogen bonding selectivelystabilize anions.
Fall 2004 Supplemental notes
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Importance of Hydrogen Bonding to Life on Earth Hydrogen bonds are critical to defining the base pairing in DNA. Thespecificity of the hydrogen bonding interactions in DNA is thought to becentral to its ability to replicate with high fidelity.
-O
NN
N
N N
O
HHO H
H
H
H
OP
-O
OH
H
O-
NN
NO
N
NO
H
HH
H
HOH
OP
O
O--O
O
H
HH
H
H
NN
N
OHO
OPO
-O
HH
H
H
O-
O
HOHH
H
H
H
NN
O
O
OP
O
O-H
H
Adenine
Guanine
Thymine
Cytosine
Fall 2004 Supplemental notes
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Hydrogen Bonding and Proteins
Hydrogen bonds are critical to the so-called secondary structure of proteins (ofwhich enzymes are a subset). • The primary structure is the sequence of amino acids that make up the
protein. The secondary structure is predominantly determined throughhydrogen bonding interactions. These interactions largely define the three-dimensional structure of the protein.
• The actual sequence of amino acids determines what hydrogen bonds can beformed. Much research is now devoted to understanding how to predict thethree-dimensional structure of proteins based upon the amino acid sequence.
• The three-dimensional structure of a protein determines its physical andchemical properties.
• As an example, spider silk has a specific secondary structure (known as βpleated sheets) that gives it strength in three dimensions (its strength per unitweight is greater than that of steel!)
•
O
NH
O
NHO
NH
• The reactivity of an enzyme is defined by its three -dimensional structure.