O-Chem Exam #2

B. the reaction favors reactants.

"A reaction carried out at 298 K, for which ΔH = 80.0 kJ/mol and ΔS = -277 J/mol·K." A. the reaction favors products. B. the reaction favors reactants. *Tip: Use the "Gibb's Free Energy" equation to find delta G, and then go from there.

1. Electrophiles 2. Nucleophiles

1. Electron Pair acceptors 2. Electron pair donator

1. Nucleophilic attack 2. Loss of a leaving group 3. Proton transfer 4. Carbocation Rearrangement

In the reading, we learned about 4 patterns of arrow pushing. Name all 4.

Chirality

Optical activity is a direct consequence of ___________.

1. Activation Energy 2. Temperature 3. Steric Consideration

The value of the rate constant in a reaction is dependent on three factors. What are they?

Atropisomers

What are stereoisomers that result from the hindered rotation of a single bond?

The chair confirmation has the lowest energy because it orients itself to have the lowest "flagpole" interactions between the hydrogen's electrons.

When looking at the different confirmation of cyclohexane, why does the chair confirmation have the lowest energy?

Electrophile

an electron-deficient center, is called an __________________. A ____________________ center is characterized by its ability to react with a negative charge or partial negative charge.

Specific Rotation

The _______________ ________________ for a compound is defined as the observed rotation under standard conditions.

1. Released 2. Required

1. -ΔH means energy is _____________. 2. +ΔH means energy is _____________.

1. Large 2. Small

1. A relatively fast reaction is associated with a ____________ rate constant 2. A relatively slow reaction is associated with a ____________ rate constant.

1. Spontaneous 2. Non-spontaneous

1. Any process with a negative ΔG will be _________________. 2. Any process with a positive ΔG will be ___________________.

1. Transition States 2. Intermediates

1. Green boxes represent __________________. 2. Orange Boxes represent __________________.

1. Products 2. Reactants

1. If K is greater than 1, then the reaction favors ____________. 2. If K is less than 1, then the reaction favors ____________.

1. Reactants 2. Products

1. If ΔG is positive, then the __________________ will be favored. 2. 1. If ΔG is negative, then the __________________ will be favored.

1. However, products E and F are favored by kinetics because formation of E and F involves a lower energy of activation. 2. Products C and D are favored by thermodynamics because they are lower in energy

1. Which reaction is more favored regarding kinetics? 2. 1. Which reaction is more favored regarding thermodynamics?

1. The rate of the forward and reverse reaction are equal. 2. ∆G is at a minimum

2 things happen when a reaction reaches equilibrium. What are they?

B

3) What effect does using a catalyst have on the rate of a reaction? a) Increases rate; Increases kinetic energy of the reactants b) Increases rate; Decreases the activation energy of a reaction c) Increases rate; Makes the products more thermodynamically stable d) Decreases rate; Increases the activation energy of a reaction e) Decreases rate; Makes the products less thermodynamically stable

Lower

A ____________ Ea (Activation Energy) will lead to a faster reaction.

Catalyst

A _______________ is a compound that can speed up the rate of a reaction without itself being consumed by the reaction.

Nucleophilic

A _________________ center is an electron-rich atom that is capable of donating a pair of electrons.

Lewis Base = Nucleophile

A nucleophile is like a lewis acid or base?

Reactants, because it's NOT spontaneous

A positive value of ΔG favors reactants or products?

spontaneous In order for a process to be spontaneous, the total entropy must increase.

A process that involves an increase in entropy is said to be _______________. Spontaneous/Nonspontaneous

The second and fourth molecules are.

Allenes can be chiral IF they're able to form either E/Z stereochemistry. With this in mind, which of these 4 molecules are then chiral?

Electrophiles

An _________________________ center is an electron-deficient atom that is capable of accepting a pair of electrons. Notice that this definition is very similar to the definition for a Lewis acid. Indeed, ____________________ are Lewis acids.

Nucleophile

An electron pair donor

Nucleophile

An electron-rich center, is called a ______________. A __________________ center is characterized by its ability to react with a positive charge or partial positive charge.

Loosing

An exothermic process involves the system __________ energy to the surroundings, so ΔH° is negative.

2 step reaction

An intermediate forms in a _____________ reaction. A. Concerted B. 2 Step

Decreases

As Ea increases, the number of molecules possessing enough energy to react __________________.

Cis

Cis or Trans?

Trans

Cis or Trans?

Cis

Cis or trans?

Neither, the double bond is not stereoisomeric and should not be labeled as cis or trans.

Cis or trans?

Trans

Cis or trans?

Energy diagram A has a negative ΔG, because the products are lower in free energy than the reactants. This is not the case in energy diagram C.

Compare diagrams A and C. Which has a negative ΔG?

Process A will more greatly favor products at equilibrium than process B.

Compare energy diagrams A and B. Which process will more greatly favor products at equilibrium?

Process C

Compare energy diagrams C and D. In which case will the transition state resemble products more than reactants?

Allenes

Compounds that contain two adjacent C=C bonds are called ___________. These can be chiral despite the absence of a chiral center.

B and D

Consider the following four energy diagrams: Which diagrams correspond with a two-step mechanism?

ΔS would be negative, as an acyclic compound is turned into a cyclic compound. A cyclic compound into an acyclic compound is an increase in entropy because acyclic compounds have more freedom of motion than cyclic compounds. Also acyclic compounds can adopt a larger number of conformations than cyclic compounds.

Describe the ΔS of the system, and why.

A cyclic compound is being converted into an acyclic compound. Such a process also represents an increase in entropy, because acyclic compounds have more freedom of motion than cyclic compounds. An acyclic compound can adopt a larger number of conformations than a cyclic compound, and once again, the larger number of possible states corresponds with a larger entropy.

Does this reaction result in an increase or decrease of entropy.

atropisomers

Enantiomers that lack a chiral center and differ because of hindered rotation.

Both terms contribute to a negative value of ΔG, which favors products.

For each of the following cases use the information given to determine whether or not the equilibrium will favor products over reactants: "An exothermic reaction with a positive ΔS"

Products are favored

For each of the following cases use the information given to determine whether or not the equilibrium will favor products over reactants: "A reaction with Keq = 1.2"

Nucleophilic Attack

For each of the following cases, read the curved arrows and identify which arrow pushing pattern is utilized.

Proton Transfer

For each of the following cases, read the curved arrows and identify which arrow pushing pattern is utilized.

Rearrangement

For each of the following cases, read the curved arrows and identify which arrow pushing pattern is utilized.

Constitutional Isomers

For each of the following pairs of compounds, determine the relationship between the two compounds.

Diastereomers

For each of the following pairs of compounds, determine the relationship between the two compounds.

ΔSsys is expected to be positive (a increase in entropy) because a cyclic compound is converted into an acyclic compound.

For each of the following processes predict the sign of ΔS for the reaction.

ΔSsys is expected to be positive (a increase in entropy) because one molecule is converted into two molecules.

For each of the following processes predict the sign of ΔS for the reaction.

This carbocation is already tertiary... Yet, in this case, rearrangement via a methyl shift will generate a more stable, tertiary allylic carbocation, which is resonance stabilized. *Pic On Next Slide

For the following carbocation determine if it will rearrange, and if so, select the correct carbocation rearrangement.

This carbocation is tertiary, and it cannot become more stable via a rearrangement.

For the following carbocation determine if it will rearrange, and if so, select the correct carbocation rearrangement.

A catalyst lowers activation energy.

How does a catalyst work?

Remove the products that are already made, so more products are made.

How might an equilibrium reaction be pushed to completion?

Greater

If the concentration of products is greater than the concentration of reactants, then K will be _____________ than 1.

If ΔG is negative, the products will be favored (Keq > 1). If ΔG is positive, then the reactants will be favored (Keq < 1). In order for a reaction to be useful (in order for products to dominate over reactants), ΔG must be negative; that is, Keq must be greater than 1

If the concentration of products is greater than the concentration of reactants, then K(eq) will be greater than 1. On the other hand, if the concentration of products is less than the concentration of reactants, then K(eq) will be less than 1. The term K(eq) indicates the exact position of the equilibrium

B. the reaction favors products.

In each of the following cases, use the data given to determine whether the reaction favors the reactants or products. "A reaction for which Keq = 900." A. the reaction favors reactants. B. the reaction favors products.

B. the reaction favors reactants.

In each of the following cases, use the data given to determine whether the reaction favors the reactants or products. "A reaction for which ΔG = 27.0 kJ/mol." A. the reaction favors products. B. the reaction favors reactants.

One reactant has a site of high electron density and the other reactant has a site of low electron density.

Ionic reactions occur when...?

No, because of a plane on symmetry.

Is this compound Chiral?

This molecule is NOT chiral as there are not 4 different atoms connected to it. There are 2 hydrogens attached.

Is this molecule chiral? Why?

Kinetics refers to the rate of a reaction, also referred to as the activation energy. Thermodynamics refers to the equilibrium concentrations of reactants and products, or ΔG.

Kinetics Versus Thermodynamics. How are they different?

Kinetics deals with activation energy. Thermodynamic deals with delta G, or the energy of the products.

Kinetics deals with activation energy. Thermodynamic deals with delta G, or the energy of the products.

Methylcyclopentane

Name it.

(2R,3S) - 3 - methyl - 2 - pentanol

Name this molecule.

(R)-2-Butanol

Name this molecule.

(R)-2-butanol

Name this molecule.

B. Negative

Predict the sign of ΔG for an exothermic reaction with an increase in entropy. A. positive B. negative C. no change D. cannot predict without additional information

Increase Why? Because the molecules will have more kinetic energy at a higher temperature. As a rule of thumb, raising the temperature by 10°C causes the rate to double.

Raising the temperature of a reaction will cause the rate of the reaction to _____________. Why?

One; two

Regarding a structure with 2 chiral centers... Each stereoisomer has exactly ________ enantiomer (mirror-image) and _______ diastereomers (siblings).

One; six

Regarding a structure with 3 chiral centers... Each stereoisomer has exactly ______ enantiomer (mirror-image ) and _________ diastereomers (siblings).

"R" configuration. Tip: If it's an "R" configuration, you're turning the steering whee right.

Regarding this molecule, would this be an "R" or "S" configuration?

True, because only hydrogens are present off the carbons, so all repulsion is the same.

TRUE OR FALSE? All of Ethane's eclipsed confirmations are degenerate.

True, all of ethane's staggered confirmations are degenerate.

TRUE OR FALSE? All of Ethane's staggered confirmations are degenerate.

Reactants; Transition State

The energy of activation (Ea) is determined by the difference in energy between the _____________ and the ______________________.

bond dissociation energy

The energy required to break a covalent bond via homolytic bond cleavage is called the __________________________________.

Positive

The total change in entropy (system plus surroundings) must be ____________ in order for the process to be spontaneous.

TRUE.

True or False? A stereoisomer will always have ONLY 1 enantiomer.

True! Any atom that has a lone pair OR Pi bond can be a nucleophile.

True or False? Any atom that possesses a localized lone pair can be nucleophilic.

True, you DON'T always need a chiral center for to have a chiral compound.

True or False? The most common source of molecular chirality is the presence of one or more chiral centers. However, the presence of a chiral center is not a necessary condition for a compound to be chiral.

True

True or false? Electrons achieve a lower energy state when they occupy a bonding molecular orbital than when on their own.

A transition state, as the name implies, is a state through which the reaction passes. Transition states cannot be isolated. In this high-energy state, bonds are in the process of being broken and/or formed simultaneously. Transition states occur very breifly. Intermediates have a certain, albeit short, lifetime. Intermediates, such as the one shown in Figure 6.23, are very common, and we will encounter them hundreds of times throughout this course.

What are the difference between transition states and intermediates?

homolytic bond cleavage

What does this describe "cleavage of a bond so that each fragment retains one electron, producing radicals"

Enthalpy

What does this describe? " A measure of the exchange of energy between the system and its surroundings during any process."

Electrophile

What does this describe? "An electron deficient area, that can accept lone pairs or pi bonds."

Steric Consideration, 1 of the 3 factors that can affect the rate of a reaction.

What does this describe? "The geometry of the reactants and the orientation of their collision can also have an impact on the frequency of collisions that lead to a reaction."

A. Exothermic

What does this describe? "The products are more stable than the reactants." A. Exothermic B. Endothermic

B. Endothermic

What does this describe? "The reactants are more stable than the products." A. Exothermic B. Endothermic

Nucleophile

What does this describe? "Usually has lone pairs, an electron rich region attracted to a positive atom"

Transition States

What does this describe? 1. The highest energy state of a reaction 2. Can't be observed as these occur very fast.

Entropy

What is the measure of disorder associated with a system?

Rearrangement

What kind of arrow pushing pattern is this an example of?

Heterolytic Bond Cleavage

What kind of bond cleavage does this describe?

Homolytic Bond Cleavage

What kind of bond cleavage does this describe?

K

What term can indicate the position of eqb of a reaction?

Electrophiles. The first compound exhibits an electrophilic carbon atom as a result of the inductive effects of the chlorine atom. The second example exhibits a positively charged carbon atom and is called a carbocation.

What would these be examples of?

At equilibrium

When is ∆G at a minimum?

1. Cyclopropane 2. Cylcobutane These has the highest energy because of angle and torsional strain.

Which 2 cycloalkanes have the highest energy, and why?

1. +ΔS

Which is favorable? 1. +ΔS 2. -ΔS

Each of these examples has lone pairs on an oxygen atom. Ethoxide bears a negative charge and is therefore more nucleophilic than ethanol. Nevertheless, ethanol can still function as a nucleophile (albeit weak), because the lone pairs in ethanol represent regions of high electron density.

Which molecule is more nucleophilic?

ONLY THE RATE OF THE REACTION (C). It does NOT change the energy of the reactants or products. Neither does it change the position of equilibrium.

Which of the following does a catalyst change? A. The the energy of reactants or products B. The position of equilibrium C. The rate of the reaction

C. exergonic, spontaneous

Which of the following indicates a reaction with a negative ∆G? A. endergonic, spontaneous B. endergonic, not spontaneous C. exergonic, spontaneous D. exergonic, not spontaneous

Keq > 1 when ΔG has a negative value. This is the case in energy diagrams A and B, because in each of these energy diagrams, the products are lower in free energy than the reactants.

Which processes will have a value of Keq that is greater than 1?

Notice that products C and D are thermodynamically favored over products E and F, because C and D are lower in energy. In addition, C and D are also kinetically favored over E and F because formation of C and D involves a smaller activation energy.

Which reaction is favored here? Why?

B

Which value of ΔG corresponds with Keq = 1? (a)+1 kJ/mol (b)0 kJ/mol (c)−1 kJ/mol

Steric interactions prevent the molecule from adopting a planar conformation, which is too high in energy and cannot form.

Why is the bond connecting the naphthalene rings exhibits severely restricted rotation?

Exothermic

Would this be an endothermic or exothermic process? "The transition state is closer in energy to the reactants than to the products. The structure of the transition state more closely resembles the reactants."

Endothermic

Would this be an endothermic or exothermic process? "the transition state is closer in energy to the products, and therefore, the transition state more closely resembles the products."

1 mole of reactant is converted to 2 moles of product.

ΔS of this reaction is positive. Explain.

Enantiomer. The compound and its mirror image are said to be a pair of enantiomers. The word "enantiomer" is used in speech in the same way that the word "twin" is used in speech.

When a compound is chiral, it will have one nonsuperimposable mirror image, called its ________________.

False, Chiral compounds ARE optically active.

True or False? Chiral compounds are not optically active.

1. Cis 2. Trans

1. The left molecule is ________. 2. The right molecule is _________.

1. Eclipsed methyls 2. Anti-staggered

1. Which confirmation of butane has the highest energy? 2. Which confirmation of butane has the lowest energy?

1. The right is more stable. 2. Why? When the substituent is in an axial position, there are steric interactions with the other axial H's on the same side of the ring

1. Which is more stable? 2. Why?

1. The right molecule 2. Why? Because you want less steric strain, so you place the methyl group in an equatorial position.

1. Which molecule would you find more in nature? 2. Why?

A. Chiral B. Achiral

1. __________ molecules are nonsuperimposable on their mirror images. 2. ___________ molecules are superimposable on their mirror images.

Two or more compounds with the same atom connectivity, but different spatial arrangement.

By definition, what is a stereoisomer?

No, achiral compounds DO NOT have enantiomers.

Do achiral compounds have enantiomers?

This molecule possesses a plane of symmetry. Any compound that possesses a plane of symmetry in any conformation will be achiral.

Each of these compounds has two chiral centers, but the trans isomer is chiral, while the cis isomer is not chiral. Why?

180º apart

How far apart are the methyl groups in an anti-staggered confirmation?

2^2 = 4

How many stereoisomers are possible for the following molecule?

2^8 = 256

How many stereoisomers are possible for the following molecule?

B. 0 There would be no possible cis/trans as the double bonds are located on the edges of the structures.

Identify the number of stereoisomers that are possible for a compound with the following constitution: H2C=CHCH2CH2CH2CH=CH2. A. 1 B. 0 C. 2 D. 3

When comparing these stereoisomers, both chiral centers have opposite configurations, so these compounds are enantiomers.

Identify whether each pair of compounds are enantiomers or diastereomers:

Equatorial

If you have only 1 methyl group present in a cyclohexane, the methyl group should be located in a ____________ position for the cyclohexane to have the lowest energy.

A. Staggered

In general, which usually have a lower energy? A. Staggered B. Eclipsed

True

True or False? Achiral compounds are not optically active.

Compounds with the same molecular formula, but have different atom connectivity.

What are constitutional isomers?

These are compounds that have 2+ chiral centers, but are not chiral as a result of a plane of symmetry.

What are meso-compounds?

Optically inactive compounds

What does this describe? These solutions of certain organic compounds (such as sugar) cannot rotate the plane of plane-polarized light.

Optically active compounds

What does this describe? These solutions of certain organic compounds (such as sugar) rotate the plane of plane-polarized light.

With each chair flip... 1. All of the axial hydrogens turn into equatorial hydrogens 2. All of the equatorial hydrogens turn into axial hydrogens

What happens to the axial and equatorial hydrogens during a chair flip?

Compounds that have the same molecular formula and different connectivity.

What is a constitutional isomer?

A diastereomer is 2+ molecules that are stereoisomers, but are NOT mirror images.

What is a diastereomer?

1. Enantiomers have the same physical properties 2. Diastereomers have different physical properties

What is an important difference between enantiomers and diastereomers?

Enantiomers are stereoisomers that are mirror images. Diastereomers are stereoisomers that are NOT mirror images.

What is the main difference between enantiomers and diastereomers?

- # OF STEREOISOMERS = 2^n - n refers to the number of chiral centers

What is the relationship between the number of chiral centers and the number of stereoisomers?

Guache

What kind of conformation?

1. Anti-Staggered 2. 4 Carbons

What kind of conformation? How many carbons are present?

Diastereomers

What's the relationship between these 2 compounds?

Enantiomers

What's the relationship between these 2 compounds?

More Branching = More Stable

When looking at 3 isomers, the isomer with ______ branching has a lower energy configuration, thus it's more stable.

You want the lowest priority molecule facing BACKWARDS into the paper, which has a dashed wedge.

When looking at a Cahn-Ingold-Prelog System molecule, you want to lowest priority molecule facing which direction?

The atom directly bonded to the chiral center with the highest atomic # has the highest priority. The atom with the lowest atomic # has the lowest priority.

When prioritizing molecules via Cahn-Ingold-Prelog System, how do you decide which molecule has the highest priority?

Eclipsed Methyls

Which of butane's eclipsed confirmations have the highest energy?

Number 4

Which of the following cycloalkanes has the least angle strain?

Number 1

Which of the following illustrates an equatorial OH?

4. Number 4 has the highest energy as it's eclipsed with 4 methyl groups on top of each other.

Which of the following is the highest energy conformer of the following compound?

#2

Which of the following is the lowest energy conformer of the most stable isomer of 1,4-diethylcyclohexane?

1. True. 2. True

Which of the following is true regarding enantiomers? 1. Enantiomers exhibit identical physical properties, such as melting and boiling points. 2. Enantiomers exhibit different behavior when exposed to plane-polarized light.