6_aciid_base_post

Sec. 7 - Acid-base Reactions Forsey

1

Substitution

Additions

Eliminations

Rearrangements

Reactions and their Mechanisms

-Cl on -CH3 has been substituted

with an -OH

Br2 has been added to C2H2

HBr has been eliminated or removed form C2H5Br

The carbon skeletal system, C6H12, has been

rearranged


2

Homolysis and Heterolysis of Covalent Bonds

Heterolysis – Cleavage of sigma bond occurs so that one fragment take away both electrons

Double headed arrow shows the movement of two electrons

Homolysis – Cleavage of sigma bond occurs so that each fragment takes away one electron.

Single headed arrow shows the movement of one electrons


3

Arrows are your thought process to solving reactions(-ve goes to +ve)

Heterolysis requires that the bond be polarized which is due to the different electronegativities between the atoms

Often heterolysis is assisted by a molecule with an unshared pair of electrons (a base) that can form a bond to on to the atoms. (-ve goes to +ve)




4

Acid-Base Reactions

Bronsted-Lowry acid substance that can donate a hydrogen ion (H+)

Bronsted-Lowry base substance that can accept a hydrogen ion (H+)

CH3COOH + ¯OH CH3COO ¯ + H2O

acid baseconjugate base conjugate acid

Lewis acid substance that can accept a pair of electrons

Lewis base substance that can donate a pair of electrons

FeBr3 + Br2 FeBr4 ¯ + Br +

Lewis acid Lewis base Lewis base Lewis acid


5

pH = -log[H+] pOH = -log[OH]

kw = [H+][¯OH] = 1 x 10-14

pH + pOH = 14

neutral solution [H+] = [ ¯OH] = 1 x 10-7 pH = 7

acidic solution [H+] > 1 x 10-7 pH range 0 to 7

basic solution [H+] < 1 x 10-7 pH range 7 to 14

Review

General formula for an acid HA; for a base B

HA H+ + A¯ B + H2O B+H + ¯OH

ka = [H+][A¯] kb = [¯OH][B+H] [HA] [B]

taking the log of both sidesHenderson-Hasselbalch equations

pH = pka + log[A¯] pOH = pkb + log[B+H] [HA] [B]

strong acid [H+] , ka , pka strong base [¯OH] , kb , pkb

weak acid [H+] , ka , pka weak base [¯OH] , kb , pkb relationship between conjugate acid base pairs

ka x kb = kw =1 x 10-14

pka + pkb = 14

Brønsted-Lowry acids and bases


6

Lewis Acids and Bases

Makes you think about the electrons and how bonds are formed

arrow show bond formation

arrow show bond breaking

new bondgained two electronsand becomesnegatively charged


7

Opposite Charges Attract and React

BF3 has substantial positive charge on the boron

NH3 has substantial negative charge localized at the lone pair

-ve goes to +ve


8

BF3 is sp2 hybridized The LUMO (Lowest Unoccupied Molecular Orbital) is shown to the right. Most of the volume of the LUMO corresponds to the empty p orbital

The base donates its electrons that occupy the HOMO to the LUMO of the acid to produce a new sigma bond

NH3 is sp3 hybridized. The HOMO (Highest Occupied Molecular Orbital) is shown to the left and is where the nonbonding lone pair resides.

The van der Waals surface electron density is indicated by the mesh


9

Heterolysis of Bonds to Carbons: Carbanions and Carbocations

Reaction can occur to give a carbocation or carbanion depending on the nature of Z

Carbocations have only 6 valence electrons and a positive charge


10

Carbanions have 8 valence electrons and a negative charge

Organic chemistry terms for Lewis acids and bases

Electrophiles (“electron-loving” reagents ): seek electrons to obtain a stable valence shell of electrons. They are electron-deficient themselves e.g. carbocations.Acid – accepts electrons

Nucleophiles (“nucleus-loving” reagents): seek a proton or some other positively charged center. They are electron-rich themselves e.g. carbanionsBase – donates electrons


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A mechanisms shows the flow of electrons that may occur to form your product. The pathway is guided by arrows. An arrow starts at a site of higher electron density (a covalent bond or unshared electron pair) and points to a site of electron deficiency.

In a few weeks you will have to know and understand the

mechanism given to the right. This is a SN1 reaction.


12

Why do acids have different acidities? What makes a strong acid a strong acid? An acid is strong if it’s conjugate base is stable. If the base is not stable it is reactive and will react with a proton and shift the equilibrium to the left in the following equilibrium.

HA A¯ + H+

Hence the strength of an acid is inversely related to the strength of its conjugate base. What makes the base stable? You must look at the structure of the molecule. What is the hybridization of the atom? Is the charge on an electronegative atom? Is the electronegative atom large so that the charge is spread over a larger area? Can the charge be delocalized throughout the molecule? All of these must be considered

Acidity increases with increasing anion stability

The first step to organic reactions is understanding acids and bases


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1. What atom is the charge on?

Acidity increases with increasing anion stability

The more electronegative an atom, the greater its ability to carry a negative charge. Remember, electronegativity is the measure of an elements affinity for an electron or its ability to accept an electron.

(CH3)3C-H (CH3)2N-H CH3O-H H-F

B2.0

C2.5

N3.0

O3.5

F4.0

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ga1.8

Ge2.0

As2.2

Se2.6

Br2.8

Te2.1

I2.5

increasing electronegativity of the underlined atom; increasing acid strength

increasing stability of anion

HA A¯ + H+


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1. What atom is the charge on?

A larger anion can disperse the negative charge over a larger volume and thus increases the stability of the conjugate base which is more important than electronegativity when comparing elements down a column in the periodic table.

H-F H-Cl H-Br H-I

increasing size of halogen increasing acid strengthincreasing stability of aniondecreasing electronegativity of halogen

B2.0

C2.5

N3.0

O3.5

F4.0

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ga1.8

Ge2.0

As2.2

Se2.6

Br2.8

Te2.1

I2.5

Thus use electronegativity when comparing atoms in the same row and size when comparing atoms in the same column

conjugate base

HA A¯ + H+


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2. The proximity of an electronegative atom to an acidic atom can help stabilize the conjugate base. (Induction)

Inductive effects: Electronegative atoms pull electron density through the sigma bond from adjacent carbons making the carbon more positive. If the carbon is close to the anion this carbon can then pull electron density from the region that has the negative charge to make it more stable.

CH3CH2CH2COOH CH2ClCH2CH2COOH CH3CHClCH2COOH CH3CH2CHClCOOH

pka 4.83 4.52 4.05 2.86

increasing acidityincreasing anion stability

H3COH ClCH2CH2OH F3CCH2OH

pka 15.5 14.3 12.4

increasing acidityincreasing anion stability

HA A¯ + H+


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O

O

Cl

ClCl

- ½

- ½

O

Cl

ClCl

+

-

-

-

- 1

The chlorines make the adjacent carbons electron poor. The carbon can then pull electron density from the region that has the negative

charge, and this effect will stabilize the negative charge

O

HF

H

How about these hydrogensDo the electronegative atoms

help stabilize the conjugate base

F

O

This is destabilizing. The lone pairs on the electronegative atom and the lone pair on the carbon repel each other.

+ H+

+ H+

These protons are not very acidic


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Alkyl and aryl groups are electron donating through

hyperconjugation or inductive effects which makes the

anion less stable.

What effect do alkyl groups

CC

H3C

H3CCH3

O

OH

Which compound is more acidic?

C

O

H3C OH

CC

H3C

H3CCH3

O

O

C

O

H3C O

+ H+

+ H+

H

C

O

C

HH O

Resonance

hyperconjugationthrough C-H sigmabond. This is not

resonance. We will talk more about

this later.

more acidic

more stable conjugate base

sp2

sp2

sp2sp3

only one C-H bond shown


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3. Resonance Stabilization

Resonance helps stabilize the negative charge by delocalizing or sharing the negative charge with more than one atom. For example which is more acidic methanol or acetic acid? In methanol the charge on the conjugate base is localized on a single oxygen whereas the conjugate base of acetic acid has the charge shared or delocalized over two oxygens. The conjugate base of acetic acid is more stable thus the equilibrium is shifted more to the right as compared to methanol making acetic acid more acidic. But which is more acidic acetic acid or phenol

CH3OH CH3O ¯ + H+

OH O¯ + H+

CH3C

O

OH CH3C

O

O¯ + H+

pka = 15.5

pka = 9.92

pka = 4.74

No resonance

two resonance structures helps

stabilize the negative charge and shifts the

equilibrium to the right compared to CH3OH

Less acidic than acetic acid? But doesn’t the phenoxide

have more resonance structure and should be more

stable than acetate ion


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O O O O

CH3C

O

O CH3C

O

O

CH3O ¯ + H+

+ H+

+ H+

Phenol has more resonance structures than acetic acid but acetic acid is more acidic, why?

Which atom is able to handle a negative charge better, carbon or oxygen?


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4. Hybridization

With increasing s character the lone pair of electrons are closer to the nucleus thus stabilizing the conjugate base

(CH3)3CH CH2=CH2 HCCH

pka > 70 44 26

increasing acidity

increasing anion stability

(CH3)3C¯ CH2=CH ¯ HCC ¯

sp3 sp2 sp

C¯ C¯ C¯

sp3 sp2 sp

Conjugate base


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List the following compounds in order of decreasing acidity

CH4, NH3, CH3OH, HF B2.0

C2.5

N3.0

O3.5

F4.0

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ga1.8

Ge2.0

As2.2

Se2.6

Br2.8

Te2.1

I2.5

¯CH3 ¯ NH2 CH3O¯ F¯

+ H+ + H+ + H+ + H+


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List the following compounds in order of decreasing acidity

a) HCl HBr HF HIB2.0

C2.5

N3.0

O3.5

F4.0

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ga1.8

Ge2.0

As2.2

Se2.6

Br2.8

Te2.1

I2.5

b) CH3CH3 H2C=CH2 HC≡CH

Conjugate base: CH3CH2 ¯ H2C=CH ¯ HC≡C ¯

sp3 sp2 sp

c) OHOH OH

O

> >

Two resonance structures with the negative charge on two electronegative atoms (oxygen) is more stabilizing than 3 resonance structures with the negative charge on carbons.


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OCl

B2.0

C2.5

N3.0

O3.5

F4.0

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ga1.8

Ge2.0

As2.2

Se2.6

Br2.8

Te2.1

I2.5

OH

O

OH

O

OH

O

Cl

Cl

1) 2) 3)d)

OH OH1) 2) 3) CH3SHe) 4) CH3OHCl

The electronegative chlorine is electron withdrawing and pulls electron density from the oxygen into the aromatic ring making the

conjugate base more


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Bases

The concept of base strength can sometimes be confusing to students. When there is a negative charge on different atoms it is straightforward since we can look at the electronegativity or hybridization of the atom to explain basicity. For example place the following molecules in decreasing basicity.

a) CH3O¯, CH3NH¯, CH3CH2¯, F¯

b) I¯ , F¯ , Cl¯ , Br¯

c) CH2=CH¯ , CH3CH2¯ , HCC¯

Where does CH3NH¯ fit into the trend found in part c)

The sp3 nitrogen is a weaker base than an sp2 carbon because it is more electronegative. It is also a weaker base than a sp2 carbon but not a sp hybridized carbon because the change in hybridization which gives the orbital more s character has a greater stabilizing effect than increasing electronegativity.


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Reactions from text Book

pKa+ pKb = 14

pKb = -24 pKb = -1.7

pKb = -24 pKb = -11

pKb = -24


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NH

NN

H H+

-

-

-

Order the bases from strongest to weakest.

1 2 3

HN N

NH2 A) 1 > 2 > 3B) 1 > 3 > 2C) 3 > 2 > 1D) 2 > 3 > 1

First put the electrons on the nitrogen atoms

Then think about the electrons. What is their hybridization, are they resonating?

Now which electrons are more available? The electrons in structure 2 or 3? What is the difference?

Which electrons are most basic or available?The sp3 nitrogen because it has more p character. Therefore 1 is the strongest base.

The electrons on the nitrogen in compound 3 are part of an aromatic system and are thus very stable, whereas the electrons in compound 2 are in a sp2 hybridized orbital sticking out from the aromatic system. The electrons in compound B are more basic, less stable, more reactive and more available than 3. Answer: A


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We are always looking at an equilibrium and which way it is shifted in a specific environment

HA A¯ + H+

eqKRTG ln

G° = -ve value the reaction favors the formation of products and if smaller than – 13 kJ mol-1 the is said to go to completion (>99% converted)

G° = +ve value the reaction favors the reactants and the formation of products is unfavorable.

G° = H° - TS°

Exothermic reaction H ° is -ve

Endothermic reaction H ° is +ve

We will come back to this when we look at reactions

S° is +ve - Change from a more ordered system to less a ordered system

S° is -ve - Change from a less ordered system to a more ordered system

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