chem 112a homework c key
TRANSCRIPT
Chem 112A – Fall 2012 – Homework C – Resonance, Arrow-Pushing, and Acidity (lectures 9/4 and 9/6) 1. Draw additional resonance structures, following the instructions in each box regarding the number of structures and whether full octets are required. (Remember: electronegative atoms always need a full octet, but carbon atoms can sometimes have an unfilled orbital). Draw lone pairs and electron-pushing arrows to show the conversion from each resonance structure to the next one.
N
H
O
O N
N
O
two more resonance structures, all with filled octets
two more resonance structures, all with filled octets
two more resonance structures, all with filled octets
two more resonance structures, all with filled octets
two more resonance structures
2. For each of the following reactions, the structures of the reactants, intermediates, and products have been provided including all formal charges. Some of the reactions occur in a single step, while others require multiple steps (each step has its own reaction arrow). For each step of each reaction, complete the following tasks. The first one is completed for you as an example.
Add electron-pushing arrows to account for each bond that is broken or formed Draw all lone pairs on any heteroatoms that are reacting as nucleophiles or bases, and draw hydrogen atoms on
carbons near reactive centers as needed to keep track of them. Add one of the following labels to each reactant or intermediate, based on how they act in the next step:
o Brønsted acid (A) o Brønsted base (B) o Nucleophile (Nu) o Electrophile (E)
For each electrophile, state which type of molecular orbital (nb - empty, π*, or σ*) is involved in the reaction. (LUMO)
If the electrophile has a leaving group, circle it and label it (LG). For each nucleophile or base, state which type of molecular orbital (nb - lp, π, or σ) is involved in the reaction.
(HOMO)
Br+
Electrophilic additions to alkenes
+H
Br Br Br Br
Br
I++ H I I
H
An electrophilic aromatic substitution (EAS) reaction:
+ D Cl Cl +
OH
N
O
O
+
OH
H N
O
O
S
O
O
OHO S
O
O
OHO
OH
NO O
S
O
O
OHO+
+
DD
Cl(D = deuterium)
F
NO2
NO2
+ CH3S
FNO2
NO2
SCH3NO2
NO2
SCH3
+ F
A nucleophilic aromatic substitution (SNAr) reaction
O++ H Cl Cl
HO
H+ OH+
H
+ Cl + HClO
(B)( )
(A) (E)(nb - empty)
(N)(nb - lp)
(A) (B)(nb - lp)
A bimolecular elimination (E2) reaction:
ClCH3O O
H Cl+ +
H
+
Cl + CH3O +CH3O Cl+ +
A unimolecular elimination (E1) reaction:
OH +
HCl
H
HH
ClCH3O O Cl+ +
Cl + CH3OH + CH3OHClO
H Cl+ +O+ HCl
A bimolecular nucleophilic substitution (SN2) reaction:
A unimolecular nucleophilic substitution (SN1) reaction:
N +Br
+ BrN
A nucleophilic substitution (SN2') reaction
An acid-catalyzed keto-enol tautomerization
OS
O
O
OHOH O
S
O
O
OHO
H
H ++OH
S
O
O
OHOH
+
A base-catalyzed keto-enol tautomerization
O O+
OH+O
H + OH
O
3. Vitamin C is an essential nutrient that used by the body as a cofactor in a number of enzymatic reactions, including several reactions in the collagen synthesis pathway. A deficiency in vitamin C causes scurvy. The common chemical name for vitamin C is ascorbic acid, because it contains an acidic functional group. The structure is shown below. Indicate which hydrogen is most acidic, and justify your answer with a short explanation and one or more additional structural drawings.