Orgo final - Book notes PDF

Preview

Summary

Dr. Rizzo...

Description

Organic Chemistry Final Exam Chapter 1 Chapter 2: Molecular Representations Table 2.1 – Examples of Functional Groups (picture) Identifying Lone Pairs: 1. Determine the appropriate number of valence electrons for the atom 2. Determine if the atom actually exhibits the appropriate number of electrons Fischer compounds are used for acyclic compounds while Haworth projections are used exclusively for cyclic compounds. RESONANCE STRUCTURES: Allyl carbocation – lowest energy MO (bonding molecular orbital) has no vertical nodes, next highest MO (nonbonding MO) has one vertical node, and the highest MO (antibonding MO) has two vertical nodes -

MO Theory suggests that the positive charge of the allyl carbocation is associated with the two ends of the system instead of one end Positive charge is spread over two locations

Resonance structures show that the positive charge is spread out over two locations The resonance hybrid is not flipping back and forth between the different resonance structures. RESONANCE STABILIZATION The spreading of electrons, called delocalization, is a stabilizing factor -

Molecules and ions are stabilized by the delocalization of electrons This stabilization is often referred to as a resonance stabilization, and the allyl cation is said to be resonance stabilized

DRAWING RESONANCE STRUCTURES There are two rules that must be followed when drawing curved arrows for resonance structures: 1. Avoid breaking a single bond 2. Never exceed an octet for second-row elements (octet rule) Recognize the following: 1. 2. 3. 4. 5.

Allylic lone pair Allylic carbocation – never place the tail of a curved arrow on a positive charge Lone pair adjacent to a C+ A pi bond between two atoms of differing electronegativity Conjugated pi bonds enclosed in a ring a. Pi bonds are conjugated when they were separated from each other with a sigma bond

Figure 2.6 (picture)

Organic Chemistry Final Exam ASSESSING THE SIGNIFICANCE OF RESONANCE STRUCTURES: 1. The most significant resonance forms have the greatest number of filled octects 2. The structure with fewer formal charges is more significant 3. The structure with the negative charge on the more electronegative element will be more significant 4. Resonance forms that have equally good Lewis structures are described as equivalent and contribute equally to the resonance hybrid Localized lone pairs do not participate in resonance. They are not allylic to a pi bond. -

When an atom has both a lone pair and a pi bond, they will not both participate in resonance.

Chapter 3: Acids and Bases Bronsted-Lowry Acid – proton donor

Bronsted-Lowry Base – proton acceptor

Mechanism of proton transfer reaction involves electrons from a base deprotonating an acid A strong acid will have a low pKa, weak acids have a high pKa -

The equilibrium will always favor formation of the weaker acid (higher pKa value) If conjugate base of acid is very stable, than the acid must be very strong

Factors Affecting the Stability of Negative Charges 1. Which atom bears the charge? a. Compare the atom that bears the charge in the conjugate base b. More electronegative = more stable larger size = more stable 2. Resonance – deprotonate acids and compare resonance (more resonance = more stable) 3. Induction – more induction withdrawing electron density away from the negatively charged deprotonated region of the structure a. Trichloric acid is more acidic 4. Orbital – a negative charge on an sp-hybridized carbon is more stable than a negative charge on an sp2-hybridized carbon a. Proton on a triple bond will be more acidic than a proton on a double bond In general, the equilibrium will always favor the more stabilized negative charge. LEVELING EFFECT Bases stronger than hydroxide cannot be used when the solvent is water. If a base stronger than OH is dissolved in water, the base reacts with water to produce hydroxide. This is called the leveling effect, and it dictates that strongest base that can be present in any solvent. -

THE BASE CANNOT BE STRONGER THAN THE CONJUGATE BASE OF THE SOLVENT Leveling effect dictates the strongest base and the strongest acid that can be present in any particular solvent

Organic Chemistry Final Exam SOLVATING EFFECTS Bulky, sterically hindered molecules are less capable of interacting with the solvent, so less acidic...