Orgo Exam 2

Racemic Mixture

an achiral substitute or intermediate always forms equal amounts of enantiomers in an achiral environment

Markovnikov’s Addition

electrophiles react C-C pi bonds to form the most stable intermediate/carbocation (not always the most stable product)

Why are secondar carbocations more stable than primary carbocations?

filled-empty interactions; increasing the degree of substitution on the carbocation increases the stabilization of the positive energy (elaborate 1st lecture)

Hyperconjugation

filled empty interactions between empty p-orbitals and adjacent sigma bonds

Hammond Postulate

makes reaction predictions; connects thermodynamic of a reaction and the structure of a transition state

Hammond Postulate: the more exothermic a reaction is…

the earlier the transition state

Hammond Postulate: the more endothermic reaction is…

the later the transition state

I’m confused on what lecture 2 is trying to say

Early Transition State

looks more like the starting material (exothermic)

Later Transition State

looks more like the products (endothermic)

Confused on the first half of lecture three

Alkenes and Markovnikov

alkenes obey this rule for reactions with an electrophile even when the product of the reaction may not look that way

Electrophile always adds to the …

less substituted carbon (prefers primary)

Alkene Reactions

hydroboration; (update as needed)

Hydroboration Overview

Alkene → (BH3) Alkyl boronate → (NaOH/H2O2) → alcohol

Go over trialkyl borane reaction and theory (second half of lecture 3)

Water and Oxidation States

addition and loss of water does not change oxidation states

If 2 molecules are interconverted by substituting water, they are…

the same oxidation state

Halogenation involves…

F2, Cl2, X2

Halogenation Stereochem.

trans antiaddition

Addition Reaction Thermodynamics

sum of(BDE formed) - sum of (BDE broken)

What does a loss of entropy do to deltaG

makes deltaG more positive

F2 vs. I2 used for Halogenation

F2: -107kcal/mol; exothermic; way downhill
I2: -13kcal/mol; somewhat downhill; reversable; thermodynamics not favorable

Halogenation Rxn and Oxidation

halogenation is a oxidation reaction

Why is Halogenation Antiaddition

look up (answer in lecture 4 & 5?)

Which Halogenation Molecules are in the sweet spot?

Cl2 and Br2

Halogenation Definition

Alkene + X2 → antiaddition of X on each side of double bond

Halogenation Entropy

detaS < 0 (2 molecules to 1) → less favorable

Nucleophile Competition

nucleophiles compete with X- (halonium ions) (look at examples)

What does a squiggly line on a structure indicate?

represents the different stereochem. of the molecule & indicates a racemic mixture

Why are 6 membered rings the best

limited steric interactions and closer to sp3

Which rings are formed kinetically

3, 5, and 6 membered TS are formed kinetically; 3 mem. rings are thermodynamically unfavorable

Be careful of chiral centers

when looking at diastereomers vs. enantiomers

Ether Structure

an oxygen bonded to two carbons

WATCH OUT FOR…

MESO

A molecule is sometimes preferred kinetically when…

it has a less hindered transition state (why?)

What does a reaction rate describe?

how fast a chemical reaction occurs

How are reaction rates determined?

by measuring the change in concentration of reactants or products over time

First-Order Kinetics

the reaction rate depends linearly on the concentration of one reactant

First-Order Kinetics Rate Law and Units

rate law: rate = k[A]
units = s^-1

Second-Order Kinetics

the reaction rate depends on the square of the concentration of one reactant or on the concentration of two different reactants

Second-Order Kinetics Rate Law and Units

rate law: rate = k[A]² or rate = k[A][B]
units: M^-1 * s^-1

First-Order Kinetics Graph

A graph of ln[A] vs. time gives a straight line (slope = -k)

Second-Order Kinetics Graph

A graph of 1/[A] vs. times gives a straight line (slope = k)

How is the magnitude of a rate constant determined?

by the difference in energy between the reactants and the transition state of the reaction; often expressed as deltaG* = deltaG(TS) - deltaG(reactants)

Relationship between Reaction Barrier and Rate Constant

the higher the barrier to a reaction (deltaG*), the lower the rate constant

Energy needed for a reaction & molecule energy

the greater the amount of energy needed for a reaction to occur, the smallest the fraction of molecules that will have sufficient energy, and the slower the rate constant

Hammond Postulate Exothermic

the energy of the TS is closer to the reactant(s) than the products(s); makes the TS more similar to reactants

Hammond Postulate Endothermic

the energy of the TS is closer to the product(s) than the reactant(s); makes the TS more similar to products

Nucleophile Definition

are electron rich and can donate electrons; typically negatively charged and attack positive (electron deficient) species

Electrophile Definition

are electron deficient and can accept electrons; typically positive/partially positive; attacked by negative species

Nucleophile examples

Br-, NH3

Electrophile examples

BF3, H3O+

Reaction Mechanisms

used to represent the movement of electrons in chemical reaction using curved arrow pushing

Electrons move from _________ to _________

nucleophile → electrophile

Alkene Reaction Basics

typically involve a double bond acting as a nucleophile; some generate reactive intermediates such as a carbocation

Regioselectivity Definition

whether a reaction preferentially adds species to the more or less substituted carbon of the alkene

Markovnikov Addition

electronegative species → more-substituted carbon

Anti-Markovnikov Addition

electronegative species → less substituted carbon

Stereoselectivity Defintion

whether a reaction preferentially adds two species to the same or opposite face of an alkene

Syn Addition

the two species are added to the same face

Anti Addition

the two species are added to opposite faces

Catalytic Hydrogenation

reduces alkenes to alkanes by adding 2 hydrogens across the double bond; alkene → (H2, Pd/C) alkane

Catalytic Hydrogenation Regioselectivity, Stereoselectivity, & Intermediate

none; syn; none?