Exam 3 Flashcards
What are the two possible ways to create a buffer?
- Add conjugate acids and bases
2. Add strong acid/base as limiting reagent. Will leave leftover acid or base reactant and will generate conjugate
What should you do when you see a dilution?
Consider using M1V1 = M2V2
*look at the context
Buffer
mixture of weak acid and weak conjugate base
What must the ratio weak acid : weak base for it to be a buffer?
10:1 or lower
Major species
species present in high enough concentration to affect the pH
Order from most to least reactive carbonyl groups (general)
- Acid chloride
- Anhydride
- acid sulfide ?
- carboxylic acid (or Oxygen binded to anything besides H as well)
- amide
- deprotonated carboxylic acid (O-)
Anhydride
symmetric looking carbonyl group with two double bonded oxygens and an oxygen connecting them
Amide
carbonyl group with nitrogen
Why can a carbonyl compound be electrophilic?
the double bonded oxygen is highly electrophilic and pulls electrons towards it, creating a dipole
this leaves the carbon slightly positive and therefore, electrophilic
Mechanism
step by step sequence of elementary reactions that lead to an overall chemical change
a sequence of events where electrons are transferred
Reaction pathway
reaction coordinate diagram that plots change in free energy over the progress of the reaction
Transition state
the state just before the formation of new molecules
high in energy
making and breaking of bonds is occurring simultaneously
Intermediate
a reactive species formed during the reaction
lower in energy than the transition state
often we make tetrahedral intermediates on the central carbon in the carbonyl group
Formaldehyde
simplest neutral carbonyl compound
carbon double bonded to oxygen and 2 hydrogens
reactive because their LUMO is a pi-bond
a pi-bond is easier to break than a sigma-bond, so they are more reactive
??
LUMO
lowest energy unoccupied molecular orbital
Molecular orbital diagram difference with valence bond theory
valence bond theory just works with atoms, while the molecular orbital diagram works with ~molecules~
Constructive interference
when two orbitals are in the same phase and their waves overlap to form a bond
(two positive humps on graph)
Destructive interference
when two orbitals are in different phases and their waves do not form a bond
phases cancel eachother out
(negative and positive humps on graph)
Aufbau’s principle and the molecular orbital diagram
Aufabu tells us that the lowest level energy orbital will be filled first, so that is why we fill bonding orbitals and not antibonding orbitals
What is higher in energy antibonding or nonbonding orbital?
Antibonding
What is nonbonding orbital?
There is no interaction between the two orbitals of separate atoms
Steric hinderance
the stopping of a chemical reaction due to size or structure
Why are aldehydes more reactive than ketones?
Aldehydes have a relatively small hydrogen group and a R-group, while as ketones have two large R-groups
This makes steric hindrance less in ketones
Overall, aldehydes are more reactive than ketones
Two reasons why a carbonyl group is electrophilic
1) pi-bonds
2) polarity
Electrophile
species that can accept an electron pair
acids
Nucleophile
species that can donate an electron pair
bases
Why does a pi-bond make carbonyl groups electrophilic?
pi-bond has a low energy barrier to breaking if the species accepts more electrons
Dunitz-Burgi angle
the orientation that the nucleophile must have in order to collide with the carbonyl group
about 105º to 110º
it is very close to a tetrahedral angle
makes sense since a tetrahedral intermediate is formed
Where do electrons go when a collision between a carbonyl group and a nucleophile happen?
the pi-bond is broken
electrons are placed in the LUMO which is the π* orbital
Why is amide less of an electrophile than acid chloride?
both amide and acid chloride have a second resonance structure in the carbonyl group
however, chloride is located in the 3rd period of the periodic table and nitrogen is located in the 2nd
when 2p carbon overlaps with 3p chloride, it is not as direct as nitrogen 2p and carbon 2p
this stabilizes nitrogen through resonance more and makes less reactive, and less of an electrophile
What makes a better leaving group?
want the group to be a weaker base
weaker base = lower pKa
Do catalysts affect the equilibrium of a reaction?
No
they affect the kinetics
How does an acid catalyst speed up a reaction?
Makes the carbonyl group a better electrophile
Protonates the oxygen
Oxygen pulls electron density even more, creating a larger dipole
The carbon becomes even more slightly positive
What does a lower pKa indicate?
more acidic
greater electrophile
(in terms of leaving groups, weaker base and better)
What does a higher pKa indicate?
more basic
greater nucleophile
(in terms of leaving groups, stronger base and worse)
What is the first step of using an acid catalyst?
protonate the ester
How do you determine where to protonate the ester?
look for the site that is the most basic (nucleophilic) by comparing pKas
Steps of acid catalysts
- Protonate
- Addition
- Deprontonate (also remakes catalyst)
- Protonate (normally the leaving group)
- Departure of leaving group
- Deprotonate (remakes catalyst)
If a catalyst increases the forward reaction does it also increase the reverse reaction?
yes
What is another term for “addition”?
Nucleophilic attack
What will acid catalysts never make?
negatively charged products/intermediates
a negative charge would be immediately neutralized by the acid catalyst
What will basic catalysts never make?
positively charged products/intermediates
a positive charge would be immediately neutralized by the basic catalyst
Fisher esterification reaction
the reverse of the acid-catalyzed ester hydrolysis
What is the reverse of protonation?
Deprotonation
What is the reverse of a nucleophilic attack (addition)?
Departure of the leaving group
Fisher esterificiation steps
- Protonation
- Addition
- Deprotonate
- Protonate
- Departure of leaving group
- Deprotonate (form the catalyst)
Uphill reaction
a reaction where the products are higher in energy than the reactants
Acid-promoted reaction
requires an acid to occur, but the acid is not catalytic nature
the acid is not regenerated
Example of an acid-promoted reaction
hydrolysis of an amide in acid
How many steps do acid-promoted reactions have
6
What are acid-promoted reactions similar to?
acid-catalyzed reactions
What is specifically different about the final deprotonation step of acid-promoted reactions?
the leaving group acts as the base to deprotonate the carbonyl group
the conjugate base of the acid does not since it is not reformed
What step drives the reaction uphill in the hydrolysis of amide by an acid?
the final deprotonation step
this forms an acid that is more stable than the starting acid
HCl vs. NH4+
Carboxylate anion
carboxylic acid with a deprotonated -OH group
In a base promoted reaction what becomes the base?
the carbonyl group
In a base promoted reaction is the base conserved?
No
It is a promoter, not a catalyst
Can a buffer be a strong acid?
No
Buffers occur with weak acids and conjugate bases
Who discovered aspirin?
German scientist named Hoffman
What is the chemical name of aspirin?
acetylsalicylic acid
In anhydrides what is the leaving group?
carboxylic acid
leaving group of anhydrides will have a pKa~5
How to determine which carbonyl group will act as the electrophile and which will act as the nucleophile if there are multiple present?
- Determine the strongest electrophile by looking for the leaving group with the lowest pKa (strongest base)
- Then, determine the nucleophile by looking for the oxygen with the highest Pka (strongest base =strongest nucleophile)
Why are amides hydrolyzed at higher temperatures than esters?
There is greater delocalization between the C-N than the C-O
This stabilizes the amide
Need more energy to overcome the activation barrier
What does carboxylic acid and thionyl chloride react to form?
acid chloride
SO2
HCl
What are some of the steps of acid chloride synthesis?
- Follow dipoles (double bonded Oxygen pulls S)
- Neutralize the Sulfur
- Use Cl- to attack carbonyl
- Resonant through Oxygen to kick off Cl- and SO2
- Use Cl- to deprotonate oxygen
What would be the pH of KCl?
Neutral (7)
Because they derive from a strong acid HCl and a strong base KOH
this is the same for NaCl
What would be the pH of NaOH?
basic
What do you do if there is more OH- or H3O+ than weak acid/base in the buffer?
Use the concentration of OH- or H3O+ to calculate pH
pH=14-pOH
What is the leaving group ability of ROH?
high
corresponds to a pKa of <0 because ROH2 is a protonated oxygen which is highly acidic
Why do you need to deprotonate at the end of a base promoted reaction?
this is will stabilize the strong base that you used
this will also make the carbonyl group more stable
Big picture of acid and base-promoted reactions?
makes the products have a lower pKa to drive the reaction
Acid-catalyzed reactions
the pKa could still be the same on both sides and K=1, just happens faster
catalysts do not change equilibrium constants
Greenhouse gases
H2O CH4 (methane) Dinitrogen monoxide Sulfure hexaflouride O3 CHF3 (and other hydrofluorocarbons)
Photochemical smog
NO
NO2
O3
What is the rate of a chemical reaction?
change in concentration (M) over time (s)
Why is fluoride the most acidic of the halogens?
it is the most electronegative
pulls the electrons the most and creates largest partial positive on the carbon
Acetate
CH3COO-
Acetic acid
CH3COOH
