Organic Chem Exam 2

What defines a strong nucleophile?

a value greater than 7

Unexpectedly strong nucleophiles with low values

SH, I, Br, Cl

Strong nucleophiles with higher values

CH3, NH2, OCH3, NH3

Nucleophiles that have high values but are weak

N(iPr)2, OtBu, NEt(iPr)2

Newman projections stability ranked from greatest to least

Anti staggered > Gauche staggered > Eclipsed > Fully eclipsed with largest groups overlapping

When a molecule’s sigma bond rotates…

it changes its confirmation

staggered confirmation

In the staggered conformation, the substituents of C₁ are not aligned with the substituents of C₂. They are not overlapping, they are offset.

Eclipsed Confirmation

the substituents of C₁ are aligned with the substituents of C₂. They overlap. In the eclipsed conformation, the substituents are slightly offset for visualization purposes.

To go from staggered to this specific eclipsed conformation, the sigma bond rotates…

180 degrees

In a newman projection, overlapping is ____ and it produces…

1. unstable

2. torsional strain

Define Torsional strain.

a destabilizing force that is produced by the repulsion between the pairs of electrons of two aligned bonds.

• Produced in Eclipsed molecules

Define Steric Effect.

A destabilizing force that arises from electron clouds from different atoms coming together (bulky groups bumping into each other)

• Produced in staggered confirmations

Anti Staggered

Confirmation in which the largest substituents are separated by 180 degrees

Gauche Staggered

Confirmation in which the largest substituents are separated by 60 degrees

• There is steric effect

Syn-eclipsed

Conformation in which the largest substituents are aligned.

• HIGHEST IN ENERGY

Angular Strain

A destabilizing force that occurs when bonds are pushed closer, forming angles that are smaller than their ideal arrangement angles

• Sp3 hybridized atoms contain bonds with 109.5 angles between them

Having a substituent in the axial conformation produces ___ and ____ strain… meaning that the _____ confirmation reduces steric strain

1. Steric

2. Torsional

3. Reduces

Cis conformational structure for chairs (on adjacent carbons)

• Both substituents are pointing up (wedge)

• One is axial and the other is equatorial

Trans conformational structure for chairs (on adjacent carbons)

• One substituent is pointing up and the other is down

• Can be a diequatorial structure (more stable)

Trans conformational structure for chairs (not on adjacent carbons)

• One substituent is pointing up but the other is down

• one is axial and the other is equatorial

Constitutional Configuration

Same number and types of atoms, but different connectivity

Conformational Configuration

Different 3D arrangement of the same molecule as a results of bond rotation (like staggered and eclipsed)

Configurational Configuration

Same connectivity, but different 3D arrangement that cannot be changed by rotation (like trans and cis, or S and R, which are covered in this lecture)

Enantiomers

Two non-identical mirror images

• not interconvertible by rotation

pKa of CH3

55

pKa of NH2

35

pKa of OCH3

15

pKa of NH3

10

pKa of SH

10

pKa of I

-9

pKa of Br

-8

pKa of Cl

-7

pKa of oic acid

5

pKa of CH3OH

0

SN2 —→ _____

SN1—→ ______

Concerted

Dissociative

What is and what is needed for a Concerted Reaction (SN2)

is straight substitution

• needs strong nucleophile

• accessible electrophile center (primary)

• Chart has 1 transition states

What is and what is needed for a Dissociative reaction (SN1)

Leaving group leaves, then nucleophile from compound from solution and joins electrophile

• needs weak nucleophile in solvent

• want tertiary molecules

• chart has 2 transition states

What is and Wha tis required for a E2 reaction

strong nucleophile attacks H, H goes to C center, then the leaving group leaves

• Strong nucleophile

• Inaccessible electrophile