Functional Groups, Orbitals, and Geometry. Resonance Structures.

Functional Groups, Orbitals, and Geometry

Resonance Structures

Bond Polarity - Part I A bond is polar when the charge is not

equally shared between the two atoms.

The more electronegative atom will have a partial negative charge (δ-).

The arrow shows the

dipole moment.

Here we show

partial charges.

Acids and Bases-Definitions

Arrhenius acid: A substance which dissolves in water to produce H+.

Brønsted-Lowry acid: a proton donor H+ is a proton.

Lewis acid: an electron pair acceptor

Arrhenius Acids and Bases Arrhenius acid: A substance

which dissolves in water to produce H+.

Arrhenius base: A substance which dissolves in water to produce OH-. Limited to aqueous solutions. Does not explain a reaction such as

NH3(g) + HCl(g) NH4Cl(s)

Brønsted-Lowry Acids and Bases

B-L acids are proton donors. B-L bases are proton acceptors.

The emphasis is on the transfer of the H+. This links acids and bases. A B-L acid HB has a conjugate base:

HB H+ + B:-

This is the equation for HB acting as an acid.


HB H+ + B:-

This is the equation for HB acting as an acid. B:- is the conjugate base.

B:- +H2O HB + OH-

This is the equation for B- acting as a base in water.

B:- + HA HB + A-

This is the equation for B- acting as a base with an acid other than water.

Be able to write these types of equations for any B-L acid or base.


Ammonia acting as an acid: NH3 NH2

- + H+

Ammonia acting as a base: NH3(aq) + H2O NH4

+(aq) + OH-(aq)

What is the conjugate acid and what is the conjugate base of ammonia?

Is ammonia a conjugate acid or base?

Acid Strength and pKa

HB H+ + B:-

Ka = acid dissociation constant

Ka = [H+][B-] [HB]

pKa = -log Ka

The more completely an acid dissociates in water, the stronger it is. The stronger the acid, the larger its Ka and the smaller its pKa.

Comparing Acid Strengths

Which is the stronger acid, ammonia or water?

There are two ways to find an answer: The quantitative way: compare pKa values. The qualitative way: compare the stabilities of the conjugate bases.


The quantitative way: compare pKa values.

NH3 NH2- + H+ pKa = 36

H2O(l) H+(aq) + OH-(aq) pKa = 15.7

Water is the stronger acid.

Comparing Acid Strengths The qualitative way: compare

stabilities of the conjugate bases. NH3 NH2

- + H+

H2O(l) H+(aq) + OH-(aq) The more stable the conjugate base is

in water, the stronger the acid. The amide ion is such a strong base it cannot exist in water, therefore ammonia is the weaker acid.


You will find it very helpful in studying organic chemistry to have a good idea of the relative strengths of some of the more common compounds acting as acids.

Please become VERY familiar with Table 1-5.

Comparing Acid Strengths by Comparing Structures

How does the structure of a compound affect its acid/base properties?

Look at the stability of the conjugate base. The more stable the conjugate base, the stronger its acid. Electronegativity Size/polarizability Resonance Stabilization Induction Hybrid orbital containing electrons

Comparing Acid Strengths by Comparing the Stabilities of the Conjugate Bases

Electronegativity (e.n.) A more electronegative atom holds

negative charge more easily. Many bases are anions. The more stable the anion, the weaker the base: e.n.(C) < e.n.(N)<e.n.(O)<e.n.(F) Base strength: CH3

->NH2->OH->F-

Acid strength: CH4<NH3<H2O<HF


Size A larger anion is more stable:

Size/stability: F- < Cl- < Br- < I-

Acid strength: HF < HCl < HBr < HI Base strength: F- > Cl- > Br- > I-


Resonance Stabilization An anion stabilized by resonance

has a stronger conjugate acid.


Induction Look at nearby atoms.

Electronegative atoms “pull” electron density away (induction). This can stabilize a negative charge. (Note: they must be very close to the negative charge to be effective.)

Trichloroacetic acid is stronger than acetic

acid.

more stable


Hybrid orbital containing electrons Acetylene (H-C≡C-H), believe it or

not, can act as an acid with certain really strong bases. H-C≡C-H + B:- H-C≡C:- + HB

The sp orbital is short (50% s character) and stabilizes the anion by holding the electrons closer to the nucleus.

Lewis Bases and Acids Lewis looked at acid/base behavior from

the viewpoint of the bonds that are formed instead of the transfer of a proton.

Lewis Bases and Acids Lewis bases have nonbonding electrons

that can be donated to form new bonds. Lewis bases are nucleophiles (lovers of nuclei +++).

Lewis acids accept these electrons. Lewis acids are electrophiles (lovers of electrons ---).

Two Bases Worth Knowing

NaH and NaNH2

sodium hydride

sodium ethoxide

sodium amide

sodium methoxide

Given the reactants, be able to write the products of any acid/base reaction!

Identifying Bases

NaH and NaNH2 Amines Hydroxide ion, OH-

Alkoxide ions, e.g. CH3O-

Alcohols Water

Identifying Acids

Inorganic (the seven strong acids) Carboxylic acids Phenols Alcohols Water These are pretty much in order

from strongest to weakest.

Functional Groups, Orbitals, and Geometry. Resonance Structures.

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