fall 2004 supplemental notes acids and bases “curved...

Fall 2004 Supplemental notes

-1-

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


-2-

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-


-3-

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

+


-4-

Conjugate Acid-Base Pairs

NH H

H + H

H

NH H

HBr Br+


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).


-5-

Examples

H O OH CH3CH3

OH

H + OCH3

CH3

Drawn to show tetrahedral geometry with lone pairsoccupying sites.

Base Acid


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


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.


-6-

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


-7-

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.


-8-

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.


-9-

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


-10-

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)])


-11-

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.


-12-

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


-13-

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-


R2SH+ > R2OH+


Why? Larger atoms distribute charge over a greater volume.

3. Positive charge is most stable on least electronegative atom.R3NH+ > R2OH+


These trends in stability can be used to predict directions of the acid-base reactionshown above and others throughout the term.

LEARN THIS WELL!


-14-

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


-15-

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-


-16-

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.


-17-

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.


-18-

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


-19-

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


-20-

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.


-21-

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.


-22-

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.


-23-

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


-24-

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.

fall 2004 supplemental notes acids and bases “curved...

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